Preface This report documents the work done for a master thesis project. It has been carried out from October 2005 to April 2006 at IPL / DTU (Department of Manufacturing Engineering, Technical University of Denmark, Lyngby, Copenhagen), under supervision of Senior Researcher Mogens Arentoft and Ph.D. student Nikolas Aulin Paldan. I really want to express my gratitude to my family (Lucio, Lina and Massimo) for supporting me during the period I spent in Denmark as an Erasmus student. I’m really thankful for all the comprehension and help that they always gave me in these years of study. I want to thank my Danish supervisors Senior Researcher Mogens Arentoft for his advices, availability and for giving me the chance to carry out my master thesis at DTU. Special thanks to Nikolas Aulin Paldan who has been very helpful in all the project phases with feedback and ideas for the construction. Big thanks also to the Italian supervisor, Professor Paolo Bariani and Doctor Engineer Stefania Bruschi, who allowed me to have this wonderful experience. Thanks to Ph.D. students Guido Tosello and for his support in and out the office, to René Sobiecki for his help in the metrolab, to Rasmus Eriksen for his help with the actuator and to Christoffer Hansson for the µEDM assistance. Thanks to Alessandro, Alessandro, Paolo, Davide and all the other friends from Carmignano for their friendship. Thanks to every single friend that I have met in Denmark for the wonderful and funny experience of life. Special thanks to Daniele, Alberto, Pavle, Alexanda, Ravi and Riccardo. Thanks to all the friends that have made great the years spent at the university in Padova. In particular, thanks to the “ragazzi dell´aula elicottero”. Last thanks but not less belong to Francesco Marinello for helping me and listening to me whenever I needed. Without him, this work would have been very difficult and I would have felt lost. Lyngby, 12th April 2006

Matteo Trevisan

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Abstract In the last years the demand for microparts has been constantly increasing. So far these parts have been manufactured mainly through turning and milling operations or chemical etching, which are expensive and wasteful production methods. In such a contest, the idea of the Masmicro project was developed, whose aim is to develop new knowledge on the manufacturing of mini/micro components, convert the knowledge into new technologies and transfer them to the industry. This master thesis is focused on the RTDs 3 and 4 of the Masmicro project, where the machine system is designed including transfer system. The main aim of this project was the design, construction and test of a rig which allowed for mini-billets handling. The developed solution would be implemented into a two-steps cold forging process, in order to move billets from the billet preparation station to the first forming stage. Due to the need of high speed and simplicity, a one-dimensional linear transfer system has been chosen. A gripper is needed to hold the billet when transferred into the transfer mechanism. A gripping solution based on adhesive forces arising from surface tension has been proposed. Tests showed that adhesive forces occurring when gripping cleaned billets (i.e. with only a thin film of moisture on their surfaces) with a cylindrical-shaped bore were not sufficient to hold them inside the bore. On the other hand, adhesive forces arising when some contaminant was present (such as a thin layer of lubricant), were enough to hold the billets inside the cylindrical-shaped bore. A prototype of the transfer system was engineered and manufactured. Tests were carried out in order to verify the efficacy of the developed system. It was not possible to move the billet from the feeding bore to the bore which simulates the first forming stage, because of an error in the position of this last one. Therefore tests were performed moving the billet from the feeding to the discharge bore, whose position and dimension allowed for a proper alignment. Analysis proved that the principle of the transfer system would work satisfactorily, once introduced into the production line.

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Table of Contents Preface ................................................................................................................. 1 Abstract ................................................................................................................. 2 Table of Contents .................................................................................................. 3 1 Introduction .................................................................................................... 5

1.1 Masmicro project .................................................................................... 6 1.2 The thesis project ................................................................................... 7 1.3 Partners .................................................................................................. 7

2 Theory............................................................................................................ 8 2.1 Cold forging ............................................................................................ 8

2.1.1 The cold forging process of the component Axle no.02 ................... 9 2.2 Sticking effects in mini/micro parts handling ......................................... 11

2.2.1 Van der Waals force ...................................................................... 12 2.2.2 Electrostatic force .......................................................................... 12 2.2.3 Surface tension.............................................................................. 13 2.2.4 Comparison between the forces .................................................... 14

2.3 Gripping devices ................................................................................... 15 2.3.1 Mechanical gripper ........................................................................ 15 2.3.2 Adhesive gripper ............................................................................ 16 2.3.3 Vacuum gripper ............................................................................. 17 2.3.4 Gripper employing the Bernoulli effect ........................................... 18

3 Proposal for a transfer system solution ........................................................ 19 3.1 Transfer system functioning principle ................................................... 19 3.2 Gripper concepts .................................................................................. 26

4 Measurement of the billet diameter .............................................................. 28 4.1 Geometry description ............................................................................ 28 4.2 Verification of the billets diameter with a screw micrometer ................. 28

4.2.1 Experimental procedure and results .............................................. 30 4.3 Verification of the billets diameter with a tesa-probe ............................ 33

4.3.1 Experimental procedure and results .............................................. 34 5 Development of the testing rig ..................................................................... 38

5.1 Handling concept .................................................................................. 38 5.2 Mobile unit ............................................................................................ 42

5.2.1 Mobile plate ................................................................................... 44 5.2.2 Upper pin-holder ............................................................................ 45 5.2.3 Cylindrical pin ................................................................................ 45 5.2.4 Check pin ...................................................................................... 46 5.2.5 Thin pin .......................................................................................... 46 5.2.6 The insert ...................................................................................... 47

5.3 Stationary unit ....................................................................................... 47 5.3.1 Stationary plate.............................................................................. 48 5.3.2 The rod .......................................................................................... 49 5.3.3 The lid ............................................................................................ 50 5.3.4 The minirail .................................................................................... 51

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5.3.5 Stationary plate support ................................................................. 52 5.3.6 Alignment bar ................................................................................ 52 5.3.7 Table ............................................................................................. 53

5.4 Actuator subsystem .............................................................................. 53 5.4.1 Linear actuator .............................................................................. 54 5.4.2 Connecting flange .......................................................................... 54 5.4.3 Actuator support ............................................................................ 55

5.5 Mobile-unit and stationary-plate bores alignment ................................. 56 6 Rig tests ....................................................................................................... 61

6.1 Actuator test ......................................................................................... 61 6.1.1 Setup ............................................................................................. 61 6.1.2 Experimental procedure and results .............................................. 62

6.2 Stationary plate bores tolerance verification ......................................... 65 6.3 Billet-gripping test ................................................................................. 67 6.4 Transfer test ......................................................................................... 69

7 Summary and conclusion ............................................................................. 70 8 References .................................................................................................. 72 Appendix ............................................................................................................. 74

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1 Introduction With the development of micro electronic technology and micro-electro-mechanical-systems, microparts are in great need now. The demand of miniaturisation comes from consumers, who are wishing for more handy electronic devices and more integrated functions, and also from technical applications such as medical equipment, sensor technology and optoelectronics. All these products contain mechanical parts such as levers, connector pins, resistor caps, tiny screws, contact springs and chip lead-frames [1]. So far these parts have mainly been manufactured by chemical etching or traditional machining processes like turning and milling, which both are expensive and wasteful production methods. This calls for the development of a production system that can meet the demands of high productivity, high reliability, low cost and accurate final shapes. Cold forging in macro scale meets these demands to a great extent, but the technology cannot be directly transferred to the micro scale, because the microstructure of the material, surface topography and precision are different relative to the size of the components. Other problems such as the production of billets, handling of components and assembly also need to be overcome in order to achieve a real industrial production of micro components by bulk forming [2].

Figure 1 – Examples of micro-components [3].

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1.1 Masmicro project The need for new knowledge on the mini/micro-scale world has led to the birth of the Masmicro project. Masmicro is a European Union project looking for an "integrated miniature/micro manufacturing facility” [4]. The overall objective of the project is to develop new knowledge on the manufacturing of miniature/micro-products and afterwards convert the knowledge into new technologies and systems that can be easily integrated and transferred to UE industries. The issues that are being analyzed to promote real breakthroughs are:

o New production concepts, supporting new products, processes and services;

o Industrial systems of the future; o Transformation of the European industry into a more knowledge-based

and added value industry; o Improved competitiveness and sustainability;

The European economy will benefit for the Masmicro development, as a consequence of the reduction of manufacturing costs, increase of employment and improvement of sustainable competitiveness. The partners involved in the Masmicro consortium are 36. They provide expertise in ten disciplines: design, materials, mechanics, processing technologies, tool/machine fabrication, manufacturing automation, metrology, software development and project management. The whole project is divided into nine different areas called RTD. RTD 3 and 4 activities are described below.

• RTD 3: Forming Tools

o Study of fundamentals of miniature/micro-tools. o Development of flexible tool systems for various forming tasks. o Development of intelligent forming tools with self "in-process" error

compensation capability. o Development of new tool fabrication techniques.

• RTD 4: Miniature/Micro-Forming and Systems

o Forming process innovation. o Development of miniature/micro-forming machine systems, including

modular, flexible, intelligent machines for mass-manufacture of sheet, bulk and tubular miniature/micro-products.

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1.2 The thesis project The present master thesis work was developed in connection with the Masmicro Project. In particular this report is focused on the RTDs 3 and 4, where the machine for mass-production of mini/micro components is designed including transfer system. The main aim of this project was the design, construction and test of a rig which allowed for the billet handling. The developed solution would be implemented into a two-steps cold forging system, in order to move billets from the billet preparation station to the first forming stage. The main requirements for the billets handling device were: - high accuracy positioning; - high speed transfer; - no risk of damages or deformation for the billets or for the handling device; - 100% efficient gripping; - structural simplicity; - no or low friction and wear effects.

1.3 Partners Partner directly involved in the development, design and manufacturing of the testing rig are listed below:

• IPL – Department of Manufacturing Engineering and Management, Technical University of Denmark.

o Supervision, office and facilities.

• Pinol A/S, Gørløse. o Manufacturer of billets.

• Pascoe Engineering, Glasgow.

o Manufacturer of all the parts of the testing rig.

• Cedrat Groupe, France. o Supplier of the actuator.

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2 Theory

2.1 Cold forging Cold forging is a widely used industrial process well suited for mass production of parts with close tolerances and good surface quality. Producing parts by cold forging has various advantages compared to cutting processes. In fact, it is possible to produce the part with little or no waste of material, stronger parts can be obtained due to the strain hardening and higher production rates can be reached. On the other hand, the equipment costs are high, but they can be brought down thanks to large product series. The forging basic principle is based on the material ductility that is the ability of the metal to deform plastically without fracturing. By heating the work piece before the forging process, it is possible to increase the ductility and therefore reduce the flow stress (hot forging). However, higher mechanical properties, better surface quality and tolerances can be obtained at room temperature (cold forging).

Figure 2 – Forging principle [5].

The forging principle is shown in Figure 2. The ductile workpiece is positioned into a hardened die and is shaped by applying compressive force. The plastic deformation is achieved when the punch is moved downwards until the die is filled. The part can be removed manually or by an ejector (not seen in the figure)

Die

Workpiece

Punch

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and the cycle can continue. The force necessary to form the workpiece is provided by a press. When dealing with complex shapes, the final component has to be forged in different steps. In this way, it is possible to control in a better way the plastic deformation: therefore the die filling can be completed more accurately. Cold forging is used for the production of micro parts too, but this technology cannot be directly transferred to the micro scale. New problems concerning material behaviour, process, tools and machines have to be overcome. At IPU, all these areas are being studied in order to develop a production line for mass manufacturing of the micro component “Axle no.2” using the cold forging technology.

2.1.1 The cold forging process of the component Axle no.02 Figure 3 shows the actual size of the component “Axle no.02” on the left and a 5 times magnification on the right. Axle no.02 is now produced through turning and milling operation by the Danish company Pinol A/S and the amount produced annually lies in the range of 50000 pieces. By producing “Axle no.02” through a cold forging process, it will be possible to increase the production rate and decrease material waste. These respectively translate into faster time to market and lower production costs.

Figure 3 – Actual size of the component “Axle no.02” (left) and a 5 times magnification (right).

As the tolerances on this part are rather strict, a closed-die forging has to be used. The cold forging process will start with a cylindrical billet. Figure 4 shows the actual size of the billet on the left and a 5 times magnification on the right.

Figure 4 - Actual size of the billet (left) and a 5 times magnification (right).

Since “Axle no.02” has so many different diameters along the length, the billet will not be directly forged into the final component. As Deform simulations have

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revealed, the die filling cannot be properly completed with a one-step cold forging process. A further step is needed in order to form the component in a more precise way. Hence the process will be divided into two consecutive steps:

o in the first forming stage, the billet will be forged into the preform o in the second forming stage the preform will be forged into the final

component “Axle no.02”. Figure 5 shows the actual size of the preform on the left and a 5 times magnification on the right.

Figure 5 - Actual size of the preform (left) and a 5 times magnification (right).

Figure 6 shows how the production system will be realized.

Figure 6 – The production system.

The whole transfer system will have to move:

o the billet from the billet preparation station to the die no.1 o the preform from the die no.1 to the die no.2 o the component “Axle no.02” from the die no.2 to the slot conveyor

A proposal for the whole transfer system is presented in chapter 3.

Die no.2 Die no.1 Billet preparation station

Billet PreformAxle No.o2

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2.2 Sticking effects in mini/micro parts handling In this paragraph a survey of the forces dealing with the micro-world is presented, with particular attention to the ones of interest for the billet handling project. For parts with masses of several grams, the gravitational force will usually dominate adhesive forces and parts will drop when the gripper opens. On the other hand, when parts to be handled are very small, adhesive forces between gripper and object can become bigger than gravitational forces. Usually, this kind of forces is never wanted during the micro handling process. In fact, as the gripper approaches the part, adhesive forces could cause the part to jump off the surface into the gripper, with an orientation that depends on initial charge distribution. When the part is leant against the arrival surface, it may adhere better to the gripper than to the substrate, preventing accurate placement (see Figure 7).

Figure 7 – Micro handling problems due to adhesive forces [6].

On the other hand, grippers using adhesive forces to pick up objects have been developed. Adhesive forces arise primarily from:

o van der Waals forces o electrostatic forces o surface tension

The balance between these forces depends on the environmental conditions, such as humidity, surrounding medium, surface condition and material [7].

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2.2.1 Van der Waals force Van der Waals force is an attractive force that arises from the instantaneous polarization of atoms and molecules into dipoles when they are set close. Even if van der Waals force is much weaker than other intermolecular forces, like ionic interactions, hydrogen bonding or permanent dipole-dipole interactions, it is able to hold together many molecules that are too stable to become an integral part, as noble gases. The van der Wasls force between a sphere and a flat gripper can be approximated by [8]:

28 zHrFvdw ⋅

=π z << r

where H is the Hamaker constant, z is the distance between the surfaces and r the radius of the sphere. This formula is assuming atomically smooth surfaces. Severe corrections need to be made for rough surfaces. In fact the van der Waals force falls off very rapidly with distance and it is only significant for gaps less than about 100 nm. Since the tolerances on the billet, as shown in the following, are Γ 6 μm, the distance between the lateral surface of the billet and a cylindrical bore cannot be kept less than 0.1 μm, except for a few billets: for this reason the van der Waals force results prevented.

2.2.2 Electrostatic force The electrostatic forces arise from charge generation (triboelectrification) or charge transfer during contact. The attractive force per unit area between two parallel plates is:

0

22

0 221

εσε⋅

== sEp

where p is the pressure in Pascal, 0ε is the permittivity of the air, E the electric field strength and sσ the surface charge density. At atmospheric pressure and centimetre-size gaps, the breakdown strength of the air (about mV /103 6⋅ ) limits the maximum surface charge density to about 25 /103 mC−⋅ . Such a value of the surface charge density sets a limit to the pressure at about 50 Pa [9]. When very small gaps in the order of 10 μm are considered, maximum fields can increase by one order of magnitude or even more. For the specific process to be performed, no relevant charging effects are considered possible or easily achievable: therefore, even if interesting and often

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suitable for handling of micro-parts, the electrostatic force was not comprised between the eligible method for billet gripping.

2.2.3 Surface tension When two objects are exposed to the environment, a thin film of water or contaminant (i.e. oil, lubricant,…) can be formed on their surfaces. When they are brought together very closely, the films touch and melt together. In this way the two objects stick because of the surface tension. This attractive force increases because of high humidity environment, large radii of curvature, long contact time and hydrophilic surfaces. The force can be calculated through the following expression [10]:

( )d

AFtens21 coscos ϑϑγ +

=

where γ is the surface tension ( mN /1029 3−⋅ for oil), A is the shared area, d is the gap between surfaces and 21 ,ϑϑ are the contact angles between the liquid and the surfaces. In the case studied within this project, billets have the following characteristics: - the diameter φ is about 1,9mm, therefore lateral surface can be evaluated as

hAl ⋅⋅= φπ and volume as 4/2 hV ⋅⋅= φπ ; - the weight can be calculated through 4/2 hVm ⋅⋅⋅=⋅= φπρρ , where aluminium density is ρ=2,7 g/cm³. - contact angle were assumed to be (cos θ1+cos θ2)=0.5; Results of computation for the billet are given in Table 1.

Evaluated Parameters Diameter 1.89mm Lateral Surface 12,64 mm² Volume 5,97mm³ Mass 0,016g

Table 1 – Billet dimensions.

By substituting the evaluated values into the surface tension equation seen before we can see that for which range of gaps, the gravity force is balanced by the surface tension. For example with a gap of 15µm we can find a Ftens of about 12 mN and for a gap of 30μm a force Ftens of about 6 mN: those are enough to compensate the weight of the billet (0,16mN).

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2.2.4 Comparison between the forces Adhesive forces and the gravitational force are compared in Figure 8. It is assumed that the object is a silicon sphere picked up by a gripper with flat jaw surfaces. Forces are expressed as a function of the object radius. For accurate gripping, adhesive forces should be an order of magnitude higher than gravitational forces. From the comparison, it can be highlighted that surface tension forces are the biggest ones. For this reason the attention was focused on these forces, considered as the most stable, repeatable and accurate. On the other hand, van der Waals forces can start to be significant when >100 µm radius spheres have to be handled. Similarly electrostatic forces can be of interest for the manipulation task when operating with parts less than 10 µm in size.

Figure 8 – Forces acting between a spherical object and a gripper [6].

From Figure 8 it can be noticed that surface tension is higher than gravity, for objects radius smaller than 1 mm (i.e. for diameters < 2mm). The comparison refers to the specific case of a silicon sphere, but fits very well our case since: - diameters smaller than 2 mm are considered: the average billet diameter will be shown to be ~1,9mm; - the system is supposed to transfer aluminium billets, whose density is 2.7g/cm³, which is very close to the silicon one (2,3g/cm³): this means the gravity force line for aluminium and silicon are pretty close; - the sphere assumption is a worst case model: in fact, while spheres contact a plane within a small area (theoretically in the single tangent point), cylinders can touch a plane along the whole generatrix: since the contact area is much higher in the case of the cylindrical shape, a much higher contact force will be expected.

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2.3 Gripping devices Gripping devices can be divided into different categories, depending on the principle used to hold the mini/micro part.

2.3.1 Mechanical gripper Mechanical grippers are based on the friction principle (see Figure 9) and they are widely used for manipulating large object. In the microworld, mechanical grippers have to deal with these problems:

o too high forces damage the object or the object may jump away and be

lost o too low forces lead to lose the object

Therefore, the force applied by the gripper has to be precisely controlled.

Figure 9 – Friction principle [12]. Many different kind of microgripper have been developed. Kim et al. [13] built an electrostatically driven gripper which has a total length of 400 µm and a thickness of 2.5 µm (see Figure 10). The gripper closes completely by applying a voltage of 45 V. The maximum force is equal to 0, 1 µN and it is reached at 50 V. The gripper has been used to pick up microscopic objects, such as 2.7 µm diameter polystyrene spheres. However, sticking problems have been observed.

Figure 10 – Electrostatic microgripper [13].

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Suzumori et al. [14] developed a pneumatically driven gripper. In such a gripper, one of the jaws is a flexible chamber, while the other is a rigid jaw. When the flexible chamber is pressurized, it bends towards the rigid jaw (see Figure 11). The prototype is 8 mm long and has a gripping force of 2 N.

Figure 11 – Pneumatic gripper.

Keller et al. [15] made a thermal gripper, based on differential thermal expansion. By pushing the elastic structure which holds the jaws, a longitudinally expanding beam element allows the opening motion (see Figure 12). It can be provided with different types of tips, depending on the application.

Figure 12 – Thermal gripper [15].

2.3.2 Adhesive gripper The disturbing sticking forces described in the paragraph 2.2 can also be used to build a gripper. Objects are picked up by means of surface tension forces. This kind of forces, arising from air humidity, can be controlled by incorporating a microheater in the gripper [16]. In the cold condition, the object can be picked up simply by touching it. To release the object, the heater evaporates the moisture layer that keeps the object glued to the gripper. Instead of using natural moisture layers, some adhesive grippers have a dispenser that forms a small drop at the gripper’s surface. When the gripper is brought in contact with the object, the surface tension centres the component to the surface of the gripper. Figure 13 shows the different phases in the operation of an adhesive gripper.

Expanding beam Elastic structure

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Figure 13 – Different phases in the operation of an adhesive gripper [17].

The disadvantage of such a gripper is that the spreading of the drop takes some time. The micro part to be handled has to be resistant to liquids too.

2.3.3 Vacuum gripper A vacuum gripper is very simple as it consists mainly of a thin tube or pipette connected to a vacuum pump. This makes this kind of gripper cheap and easy to replace. The suction principle enables the gripping of objects with different shapes, dimensions surface quality and material. The gripping process can be performed in the fluid or gas environment. Petrovic et al. [18] presented the vacuum gripper shown in Figure 14.

Figure 14 – Vacuum gripper [18]. The glass pipette is used in order to obtain a very small cavity diameter of the vacuum gripper tip. To minimize the electrostatic attraction between the gripper and the object to be handled, the surface of the vacuum gripper is made conductive by means of a titanium layer. The vacuum gripper and the working platform carrying the microparts have to be grounded in order to have the same electrical potential and avoid electrostatic loading. The vacuum polymer micro-loader is used in order to enable the mechanical flexibility between the glass pipette and the vacuum controller. While handling microparts with such a gripper,

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the tube has to be very thin and, therefore, it is easily obstructed by small particles.

2.3.4 Gripper employing the Bernoulli effect Such a gripper can lift and transport delicate silicon wafers. It mainly consists of a circular plate with a central hole through which the air is blown. It lifts the wafers by blowing gently on the wafer upper surface so that the aerodynamic lift is created via the Bernoulli effect. Thin guides around the periphery of the circular plate keep wafers in the correct position (see Figure 15).

Figure 15 – Bernoulli effect employing gripper [12]. By blowing through the central hole, the air flows radially between the circular plate and the wafer. The air high velocity induces a dynamic pressure decrease (Bernoulli effect) that leads to an attractive force between the wafer and the circular plate.

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3 Proposal for a transfer system solution The principle of the transfer system is described in this chapter. Different gripper solutions are presented, too.

3.1 Transfer system functioning principle Figure 16 shows how the production system will be realized.

Figure 16 – Production system. A two step process is needed in order to:

o reduce the total amount of the force needed to cold forge the component; o better control the material flow during plastic deformation and therefore

assure complete filling of the die; This leads to develop the production system shown in Figure 16. Die no.1 and punch no.1 forge the billet into the preform, while die no.2 and punch no.2 provide to form the preform into the final component called “Axle no.02”. Punch no.1 and punch no.2 have different external diameter because of different shape and dimension of the component they forge. After the forging step, the preform and the final component are ejected by ejectors no.2 and no.3 respectively. At the same, the ejector no.1 pushes the billet out of the billet preparation station. Ejector no.4 is added to eject the final component from the transfer system into the slot conveyor. Ejectors no.1, no.2 and no.3 are fixed to a horizontal bar, so they move all together with the same vertical stroke. See Figure 17.

Billet

Ejector no.1 Ejector no.2Ejector no.3

Preform Final

component

Punch no.1Punch no.2Ejector no.4

Slot-conveyor Die no.2 Die no.1

Billet preparation station

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Figure 17 – Ejector holder bar.

On the upper side, punches no.1, no.2 and ejector no.4 are mounted on another horizontal bar, so they move all together with the same vertical stroke. See Figure 18.

Figure 18 – Punch holder bar.

The vertical movement of the ejector holder bar and punch holder bar is performed by a press. These two bars are connected with the punch ram of the press. When the press ram is lowered, punches move towards dies in order to form the components. When the press ram is lifted, punches get out dies and allow for the ejection of the components that have just been forged. During this phase, the press ram raises the ejector holder bar too and components can be injected into the transfer system. The principle of the transfer system is shown in Figure 19.

Figure 19 – Principle of the transfer system.

Die no.1 Die no.2

Plate no.1

Ejector no.1

Ejector no.2

Ejector no.3

Punch no.1

Punch no.2

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The plate no.1 is installed on a minirail which allows for horizontal translation above the dies. The horizontal movement of the plate no.1 is performed by means of a linear actuator. Three bores are machined in the plate no.1 to make room to the billet, to the preform and to the final component “Axle no.02”. How the work pieces could be gripped is presented in the section 3.2. Ejectors no.1, no.2 and no.3 of the production system are respectively used to inject the billet, the preform and the final component into the transfer system. These three parts also have to be ejected from the transfer system once they are pushed inside, but punches no.1 and no.2 can not be directly used as ejectors because their diameters are too big. (See Figure 20, where the problem concerning the billet ejection from the transfer system is shown).

Figure 20 – Problem concerning the billet ejection from the transfer system.

As revealed by measurements (see section 4.3.1), the billet has a diameter of 1.888 ± 0.006 mm. In the transfer-system, the diameter of the bore that has to hold the billet has to be just a little bit bigger than the billet’s diameter. A possible value could be 1.905 mm. In this way, it is possible to use the sticking effect (see section 2.2) due to a very small clearance between the surfaces of the billet and the bore. The billet is vertically moved from the billet preparation station into the transfer system’s bore by the ejector no.1. The actuator is activated to position the billet above the die no.1. The billet vertical shift from the bore of the transfer system into the die no.1 cannot be performed with the punch no.1. In fact, the punch’s diameter has a value of 2 mm and it cannot go through the bore of the transfer system (see Figure 20). A thin pin has to be added to eject the billet from the transfer system. In Figure 21, a possible solution to the problem is shown.

Punch no.1

Billet

Ejector no. 2

Die no. 1

Transfer system

Interference

1.905 mm

2.000 mm

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Figure 21 –Device for the billet ejection. The principle is based on two elements, a lower and an upper one, both with cylindrical shape. The lower cylindrical item is a feature of the plate no.1, while the upper cylindrical item is the part that can slide. The two items are coaxially aligned through a slide fit between the external cylindrical surface of the lower item (surface A) and the internal cylindrical surface of the upper item (surface B) (see Figure 21 left). By using a press fit, a thin pin is coaxially fixed into the upper cylindrical item. A spring provides the force to keep the pin holder into its starting position. The plate no.2 is fixed to the plate no.1 and it has the function of stopping the pin holder into the starting position while the spring is pushing it up. The hole no.1 is machined in correspondence to the axis of the pin holder. The diameter of this hole is larger than the punch’s diameter. In this way, the billet can be ejected from the plate no.1 because:

o the punch can go through the plate no.2; o the punch can lower the pin holder; o the pin can eject the billet from the transfer system. (see Figure 21 right).

After this phase, the pin holder has to be removed from the punch trajectory in order to allow the billet forging.

Punch no.1

Lower cylindrical

item

Upper cylindrical item or pin holder

Surface B

Surface A

Ejector no.2

Billet

Pin

Plate no.2

Plate no.1

Die no.1

Hole no.1

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So: o the punch no.1 is raised o the transfer-system is moved right and holes no. 2 and no. 3 are

positioned above the die no.1 (see Figure 22). The diameter of these holes is bigger than the punch diameter. In this way the punch no.1 can go through the transfer-system

o the punch no.1 penetrates the transfer-system and forms the billet.

Figure 22 – Forging position.

The same principle is used to eject the preform and the final component from the transfer system. The starting position of the transfer system is shown in Figure 23

Figure 23 – Starting position of the transfer system.

Hole no.2

Hole no.3

Die no.1Billet

Billet prepearation station

Die no.1 Die no.2

Bore no.1 Bore no.2 Bore no.3

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In this position:

o the bore no.1 of the transfer system is coaxially aligned with the bore of the billet preparation station.

o the bore no.2 and no.3 of the transfer system are respectively aligned with the die no.1 and the die no.2.

Ejectors no.1, no.2 and no.3 are lifted all together by the press ram. In this way, the billet, the preform and the final component are injected into the transfer system. After this operation, the three ejectors are lowered and the actuator shifts the transfer system into the position X (see Figure 24).

Figure 24 – Position X of the transfer system.

In the position X: o the bore no.1 of the transfer system is coaxially aligned with the die no.1 o the bore no.2 of the transfer system is coaxially aligned with the die no.2 o the bore no.3 of the transfer system is positioned above the hole of the

slot conveyor The punch holder bar is lowered. Punches no.1, no.2 and the ejector no.4 go through the holes no.4, no.5 and no.6 of the plate no.2. They let down the three pin holders, and the three thin pins as well. Consequently:

o the billet is injected into the die no.1 o the preform is injected into the die no.2 o the final component falls into the hole of the slot conveyor

Die no.2 Die no.1

Hole no.4 Hole no.5 Hole no.6

Plate no.2

Pin holder no.1

Pin holder no.2

Punch no.2 Punch no.1

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The billet and the preform are now inside the dies no.1 and no.2, but they cannot be formed. In fact the pin holder no.1 is between the die no.1 and punch no.1 while the pin holder no.2 is between the die no.2 and punch no.2. So, the punch holder bar is lifted and the transfer system is moved into the position Y. In the position Y:

o holes no.7 and no.8 are positioned above the die no.1 o holes no.9 and no.10 are positioned above the die no.2

After this phase, the punch holder bar is completely let down. Punch no.1 can go through the transfer system, reach the die no.1 and form the billet into the preform. In the same way, punch no.2 can form the preform into the final component.

Figure 25 – Position Y of the transfer system. The punch holder bar is lifted again and the transfer system is moved to the starting position. A new cycle can start.

Hole no.8 Hole no.10

Hole no.7 Hole no.9

Punch no.1

Punch no.2

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3.2 Gripper concepts Different gripper concepts are presented in the next section. Three different solutions are proposed for the billet, perform and final component gripping. Concept no. 1 (Figure 26)

Figure 26 – Cylinder-shaped bore. This gripper consists of a cylindrical bore. The billet, preform and final component are held in position by means of adhesive forces arising from moisture or oil layer on their surface. The rounded corner makes the injection of the specimen easier. Advantages:

o Simplicity. Disadvantages:

o Vibrations can negatively affect adhesive forces. Concept no. 2 (Figure 27)

Figure 27 – Cone-shaped bore.

This gripper consists of a conical bore. In this case, the specimen is fixed by two different forces:

o Friction forces on the upper side of the specimen. o Surface tension forces on the lower side of the specimen.

The rounded corner makes easier the injection of the specimen.

Rounded corner

Friction forces

Surface tension forces

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Advantages: o Simplicity. o Friction forces improve the reliability.

Disadvantages:

o Too high compressive stress in the “friction force zone” could lead to deform the upper side of the specimen

Concept no. 3 (Figure 28)

Figure 28 – Bore with a rubber band.

This gripper consists of a cylindrical bore with a rubber band. The specimen is fixed by friction forces arising from the contact between the rubber band and the surface of the specimen. The rounded corner makes easier the injection of the specimen. Advantages:

o High reliability. Disadvantages:

o Rubber band wear. o Difficulty in the machining of the groove for the rubber band. The gripper

has to be divided into two different items to allow the machining of the groove (see Figure 29). The two items can be afterwards joined with screws.

Figure 29 – Machining of the groove.

O-ring

Rounded corner

Groove

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4 Measurement of the billet diameter The set of measurements here reported was carried out in order to check the billet’s diameter. Once the diameter of the billet has been defined, it is possible to choose a suitable value for the diameter of the bore that will hold the billet by means of adhesive forces.

4.1 Geometry description The billet is an aluminium cylinder, machined through turning operation by Pinol A/S. Its nominal diameter has a value of 1.89 mm while its nominal height is equal to 2.13 mm. The tolerance on the nominal dimensions is equal to ± 10 µm (see Figure 30).

Figure 30 – 3D drawing (left) and 2D drawing (right) of the billet.

4.2 Verification of the billets diameter with a screw micrometer All the 70 billets were measured by means of a screw micrometer. A micrometer is a widely used device in mechanical engineering for precisely measuring thickness of blocks, outer and inner diameters of shafts, depths of slots with resolution down to 1 µm. The micrometer has the advantage of being easy to use and its readouts are fairly consistent. Figure 31 shows three common types of micrometers:

o external micrometer o internal micrometer o depth micrometer

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An external micrometer is typically used to measure wires, spheres, shafts and blocks. An internal micrometer is used to measure the opening of holes while a depth micrometer typically measures depths of slots and steps.

Figure 31 – External, internal and depth micrometers.

The external micrometer used for billet diameter measurement is shown in Figure 32.

Figure 32 – External micrometer used for billet diameter measurement.

The anvil and the spindle are coaxially mounted on a metal arch. The anvil is fixed to the arch and cannot move. The thimble has inner screw threads with a fine pitch. If the thimble is clockwise turned, it will move left, while if it is counter clockwise turned, it will move right. The spindle is directly connected with the thimble and it will move right or left if the thimble is turned. The measured diameter of the billet is the distance between the anvil and the spindle when they are just in contact with the sides of the billet. Because of the mechanical advantage due to the fine pitch of the screw threads that move the thimble and the spindle, it is easy to use enough force in closing the spindle on the object being measured. This leads to:

Thimble

Friction screw

Anvil

SpindleDisplay

Zero button

Metal arch

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o damages of the object being measured o damages of the micrometer o deformation of the object being measured and consequent wrong

measurement In order to avoid all these problems, the micrometer has a friction screw that set a limit to the torque used in closing the spindle on the billet being measured. A digital display is mounted on the metal arch. The display shows the spindle’s displacements and has a resolution of 1 µm. By using a display instead of a graduation scale, the displacement reading becomes easier and faster. Under the display there is the zero button. It has to be used in order to reach the micrometer zero setting. In the micrometer zero setting, the friction screw is turned until the spindle reaches the anvil. When the friction screw comes into action because the spindle’s face and the anvil’s face are touching each other, the zero button is pushed.

4.2.1 Experimental procedure and results Before using the micrometer, it is important to control that:

o the anvil and spindle faces are cleaned o the display shows zero when the anvil and spindle faces are touching

each other To test the repeatability of the instrument, ten measurements were repeated on a calibrated gauge block with a nominal height h=1,900 mm.

Calibrated block measurement

1,894

1,896

1,898

1,9

1,902

1,904

1 2 3 4 5 6 7 8 9 10

Measure number

Hei

ght [

mm

]

Figure 33 – Calibrated gauge block measurement.

As shown in Figure 33, the screw micrometer readout was the following:

o 1,900 mm for seven times

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o 1,899 mm for three times It could be therefore deduced that the micrometer had good measurement repeatability. To check the roundness, two different billets were measured, both ten times. Measurements were carried out according to the following procedure: 1 The billet was put between the faces of the anvil and spindle. 2 The faces of the anvil and spindle were moved closer until they met the

billet surface. By using the friction screw, the correct torque was applied. 3 The value of the billet’s diameter was read in the digital display. 4 The billet was released and rotated a little bit. 5 The process was repeated from the point n° 2.

Roundness test billet n 1

1,88

1,882

1,884

1,886

1,888

1,89

1 2 3 4 5 6 7 8 9 10

Measure number

Dia

met

er [m

m]

Roundness test billet n 2

1,882

1,884

1,886

1,888

1,89

1,892

1 2 3 4 5 6 7 8 9 10

Measure number

Dia

met

er [m

m]

Figure 34 – Roundness test; billet n°1 (left) and billet n° 2 (right). As shown in Figure 34, the billet diameter had not a constant value. The difference between the maximum and the minimum diameter was equal to:

o 4 µm for the billet n° 1 o 3 µm for the billet n° 2

This roundness error could come from the machining process. In fact, billets are produced through turning operation and the cutting tool’s displacement in the range of few microns can lead to a roundness error. By changing the manufacturing process (for example producing billets by drawing operation), it will be possible to increase the precision on the billet and reduce roundness errors. After this phase, the diameter of all the available billets was measured. To improve accuracy, the diameter of each billet was taken two times and the average value was considered.

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Diameter of the billets

1,861,8651,87

1,8751,88

1,8851,89

1,8951,9

0 10 20 30 40 50 60 70

Billet number

Dia

met

er [m

m]

Figure 35 – Diameter of the billets. The average value was m=1,881 mm. It was calculated through the following expression:

∑=

=n

iix

nm

1

1 (1)

where m = average value n = number of billets ix = i-value of the billet diameter The standard deviation was σ=0,007 mm. The standard deviation is a measure of how widely values are dispersed from the average value and it was estimated through the following expression:

1

)(1

2

−

−=

∑=

n

mxn

ii

σ (2)

where m = average value n = number of billets ix = i-value of the billet diameter As a result, we can in first approximation express the diameter as φ = 1,881 ± 0,007 mm. The width of the tolerance field complies with the drawing tolerance (see Figure 30) but the average value is 9 µm smaller than the requested value. The trend line in Figure 35 shows that the diameter value decreases during the measuring process.

Trend line

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This could depend on the velocity the friction screw was turned. In fact, if the friction screw is turned in a faster way, there will be an increase of the force used in closing the spindle on the object being measured, because of inertial effects. The force increase leads to a squeezing effect and, as a result, the diameter value decreases. In addition some temperature variation, due for example to micrometer handling, can affect negatively the measuring task. To avoid that a variation of the force used in closing the spindle on the billet or heating due to instrument handling could affect the measuring process, a new set of measurements was carried out using an electronic comparator, named tesa-probe.

4.3 Verification of the billets diameter with a tesa-probe The tesa-probe is an LVDT (Linear Variable Differential Transformer) transducer. An LVDT is a reliable and accurate sensing device that converts linear position or motion to a proportional electrical output. It is based on the movement of a magnetic core respect to three bobbins, one primary bobbin which is excited with an alternating current and two secondary bobbins which are connected in opposition. As the core is moved left or right, the difference in induced voltages into the secondary bobbins produces an output that is linearly proportional in magnitude to the displacement of the core. Figure 36 shows an LVDT sketch. The probe used to carry out measurements has a resolution of 1 µm.

Figure 36 – LVDT transducer [19]. Figure 37 shows the equipment used in the metro lab of IPL (Institute for Produktion og Ledelse) in building 425.

Secondary bobbins

Primary bobbin

Movable magnetic core

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Figure 37 – Lab equipment.

The fixture positions the tesa-probe perpendicularly with respect to the plane A used to lean the billets (see Figure 39). The tesa-probe is connected with a vacuum pump and a tesa-module. The vacuum pump is used to move up and down the probe’s tip. The probe’s tip displacements are shown by the tesa-module.

4.3.1 Experimental procedure and results The tesa-probe was mounted perpendicularly with respect to the plane A (see Figure 38). The vacuum pump was switched on to move up the probe’s tip. A 1.900 mm height calibrated gauge block was put between the plane A and the probe’s tip in order to define the zero point. When the vacuum pump was switched off, the probe’s tip came into contact with the upper surface of the calibrated gauge block and the zero point could be therefore defined. The display was finally set to 0,000 mm and the zero-plane was set at 1,900 mm above the plane A (see Figure 40).

Figure 38 - Setting operation.

Plane A

Upper surface of the calibrated block

Calibrated block

Fixture Tesa-module

Vacuum pump

Tesa-probe Display

Probe´s tip

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After the setting operation, the measuring process could start. Measurements were carried out in this way: 1 The vacuum pump was switched on to move up the probe’s tip. 2 A billet was put between the plane A and the probe’s tip. 3 The vacuum pump was switched off and the probe’s tip came into contact

with the billet surface (see Figure 39 ). 4 The value of the displacement from the zero-plane was read in the tesa-

module. 5 By repeating the process from the point n° 1, a new billet could be

measured.

Figure 39 – Billet measurement. The value of the billet diameter was equal to 1,900 mm minus the probe’s tip displacement from the zero-plane (see Figure 40).

Figure 40 – Position of the zero-plane.

To test the repeatability of the probe, ten measurements were repeated on the calibrated gauge block with a nominal height h=1,900 mm.

Billet

Plane A

Probe´s tip

Plane A

Zero plane

BilletCalibrated gauge block

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Calibrated block measurements

1,881,8851,89

1,8951,9

1,9051,91

1,9151,92

1 2 3 4 5 6 7 8 9 10

Measure number

Heig

ht [m

m]

Figure 41 – Calibrated gauge block measurements. As shown in Figure 41, the tesa-probe readout was always the same: 1,900 mm. It is possible to say that the tesa-probe had a good measurement repeatability. To check the roundness, two different billets were measured, both ten times.

Roundness test billet n 1

1,882

1,884

1,886

1,888

1,89

1,892

1 2 3 4 5 6 7 8 9 10

Measure number

Dia

met

er [m

m]

Roundness test billet n 2

1,882

1,884

1,886

1,888

1,89

1,892

1 2 3 4 5 6 7 8 9 10

Measure number

Dia

met

er [m

m]

Figure 42 - Roundness test; billet n°1 (left) and billet n° 2 (right).

As shown in Figure 42, the billet diameter had not a constant value. The difference between the maximum and minimum diameter was equal to:

o 3 µm for the billet n° 1 o 3 µm for the billet n° 2

A small roundness error had been found. This result confirmed the one obtained with the screw micrometer. After this phase, the diameter of all the available billets was measured. To improve accuracy, the diameter of each billet was taken two times and the average value was taken.

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Diameter of the billets

1,861,8651,87

1,8751,88

1,8851,89

1,8951,9

0 10 20 30 40 50 60 70

Billet number

Diam

eter

[mm

]

Figure 43 – Diameter of the billets.

By using the equation n° 1, it was possible to calculate the average value m. It resulted m = 1,888 mm. The standard deviation σ was calculated with the equation n° 2. It resulted σ = 0,006 mm. As a result, it was possible to express the diameter as φ =1,888 ± 0,006 mm The specifications were therefore verified. The average diameter value obtained with the probe was 7 µm bigger than the one obtained with the screw micrometer. This can be perhaps connected to the fact that the probe applies a smaller force than the one applied with the screw micrometer. This leads to a smaller deformation of the billet. The probe use can be considered the recommended way to carry out the measure of the billet diameter measurement.

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5 Development of the testing rig The testing rig has been developed in order to check if the solution of a one-dimensional linear mechanism is suitable for the billet handling. The testing rig concept is shown as a 3D ProEngineer drawing in Figure 44. This first version allows checking the billet handling. By changing the shape and dimensions of the feeding system and the diameter of the bores used as gripper in the mobile unit, it will be possible to check the handling of the preform. Further dimension and shape changes are needed in order to test the final component “Axle no.02” handling.

Figure 44 – 3D drawing of the testing rig.

5.1 Handling concept The concept comprises a translating metal board as transfer system and a bore as gripping system. The system transports the billet from the feeding bore to the bore no. 1 which simulates the first forming stage and from this one to the bore no. 2, which is the exit from the rig. See Figure 45. The stationary plate simulates the press surface where dies are located. The mobile unit is installed on a minirail which allows horizontal translation in one direction. In the mobile unit there are two cylindrical bores which work as grippers thanks to surface tension forces. A linear actuator provides for the correct alignment between the mobile unit bores and the stationary plate bores. Thin pins provide for the injection and ejection of the billet into the mobile unit. There are two pin-holders and each one has two pins. One pin-holder is mounted on the stationary plate and provides for the billet injection into the mobile unit.

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The other pin-holder is mounted in the mobile unit and provides for the billet ejection. Springs give the force to keep the two pin-holders in the starting position.

Figure 45 – Stationary plate and mobile unit.

In the next figures is shown how the billet is moved. The starting position is shown in Figure 46. The billet is inside the feeding bore on the stationary plate. The two pin-holders are kept in the starting position by the springs. The left bore of the mobile unit is aligned with the feeding bore by the actuator. By pushing up with a finger the lower pin-holder, the billet is injected into the left bore of the mobile unit. See Figure 46 (right).

Feeding bore 1st hole

2nd bore

Lower pin holder

Upper pin holder Mobile unit

Stationary unit

Right bore

Left bore

Feeding bore 1st hole

2nd bore

Lower pin holder

Upper pin holder Mobile unit

Stationary unit

Right bore

Left bore

Feeding bore 1st bore

2nd bore

Lower pin holder

Upper pin holder Mobile unit

Stationary unit

Right bore

Left bore

Bore no. 1

Bore no. 2

Spring

Spring

Stationary plate

Minirail

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Figure 46 – Starting position (left) and first step (right). The lower pin-holder is released by removing the finger. The spring provides the force to bring the pin-holder into its starting position. See Figure 47 (left). The linear actuator is activated in order to align the left bore of the mobile unit with the bore no. 1 of the stationary plate. See Figure 47 (right).

Figure 47 – 2nd step (left) and 3rd step (right). By pushing down with a finger the upper pin-holder, the billet is injected into the bore no.1 of the stationary plate. The bore no. 1 on the stationary plate stands for the die where the billet has to be positioned in order to be forged. See Figure 48. The upper pin-holder is released by removing the finger. The spring provides the force to bring the pin-holder into its starting position. See Figure 48 (right).

Figure 48 – 4th step (left) and 5th step (right).

Feeding bore

Stationary plate

Mobile unit

Left bore

Lower pin holder

Bore no .1

Upper pin holder

Billet

Left bore

Bore no.1

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The linear actuator is activated in order to align the right bore of the mobile unit with the bore no. 1 of the stationary plate. See Figure 49 (left). BY pushing up with a finger the lower pin-holder, the billet is injected into the right bore of the mobile unit. See Figure 49 (right).

Figure 49 – 6th step (left) and 7th step (right). The lower pin-holder is released by removing the finger. The spring provides the force to bring the pin-holder into its starting position. See Figure 50 (left). The linear actuator is activated in order to align the right bore of the mobile unit with the bore no. 2 of the stationary plate. See Figure 50 (right).

Figure 50 – 8th step (left) and 9th step (right). By pushing down with a finger the upper pin-holder, the billet is injected into the bore no. 2 of the stationary plate. The diameter of this bore is two times the billet diameter in order to have a gap between the two surfaces. In this way the billet can go through the stationary plate and it can fall into the underlying surface. See Figure 51.

Right bore

Bore no. 2

Bore no. 1

Right bore

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Figure 51 – 10th step. Each element of the testing rig is described below with its functions and requirements. The whole system can be divided into three subsystems:

o mobile unit o stationary unit o actuator subsystem

5.2 Mobile unit This name is referred to the whole subsystem which can translate along the rail. It can be seen as the core of the testing rig. It has to hold, transport and place within few microns tolerance the billet. For parts with size around one millimetre, the gravitational forces may become insignificant compared to adhesive forces, which are generally proportional to surface area. In this way, a bore can work as a gripper. When the billet is pushed inside the bore, adhesive forces can prevent release of the part. The subsystem is moved laterally by the actuator and the billet can be positioned above the next bore of stationary plate.

Upper pin holder

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First edition (Figure 52)

Figure 52 – First edition of the mobile unit.

Two cylindrical bores are machined in the plate no. 1. In the centre of the plate no. 1 a cylindrical pin is fixed with a nut. The plate no. 2 can slide along the cylindrical pin. Two thin pins are assembled in the plate no. 2 with a press fit. A spring provides the force to keep the plate no. 2 into its starting position. A check pin is mounted in the plate no. 2 with a press fit. The functions of the check pin are:

o to prevent the rotation of the plate no. 2 around the cylindrical pin; o stop the plate no. 2 in the starting position while the spring is pushing up;

Using two fingers, it is possible to bring down the plate no. 2 and in this way the thin pin can eject the billet from the plate no. 1. Final edition ( Figure 53)

Figure 53 – Final edition of the mobile unit.

Mobile plate

Insert

Pin holder

Check pin

Thin pin

Insert

Cylindrical pin

Spring

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Some changes are needed in order to simplify the assembly and to increase the flexibility of the mobile unit. Two inserts are mounted into the mobile plate with a press fit. By using inserts, it is possible to test bores with different shapes and dimensions without machining a new mobile plate each time the variation is needed. The cylindrical pin is mounted in the mobile plate with a press fit. In this way the cylindrical pin and the mobile plate shape become simpler and their machining cheaper. The check pin is now mounted in the cylindrical pin. The only function of the check pin is to stop the pin holder. The rotation of the pin holder is now prevented by leaving the two thin pins inside the bores of the inserts.

5.2.1 Mobile plate It is the framework for the two inserts and for the cylindrical pin used as track for the sliding pin-holder. See Figure 54. There are two holes for the inserts and one blind hole for the cylindrical pin. For the assembly of these parts, press fit is used. The tolerance on the distance between the axes of the holes for the inserts is set as narrow as possible in order to reach a precise alignment with the bores in the stationary-unit. The plate is mounted on the carriage (see Figure 55) with four bolts. The threaded holes on the vertical surface let the fastening with the flange developed for the correct alignment with the actuator’s rod.

Figure 54 – Mobile plate. A gap of few microns between the lower surface of this plate and the upper surface of the stationary plate prevents friction forces while the unit is moving. When the pin-holder is pressed down, these two surfaces meet each other. In this way no torque moments affect the minirail. See Figure 55.

Threaded holes

Bores for the inserts

Hole for the cylindrical pin

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Figure 55 – Air gap.

5.2.2 Upper pin-holder It is the support for the two pins. The pin-holder is assembled in the cylindrical pin through a slide fit. As shown in Figure 56, the central hole has two different diameters. The surface of the smaller hole is used as guiding surface in the slide fit, while the bigger hole is made in order to make room for the spring. The two small holes on the sides are used to mount the thin pins. The tolerance on the perpendicularity of these holes respect to the lower surface is set as narrow as possible in order to have the correct alignment between the pin axes and the insert axes.

Figure 56 – Upper pin holder.

5.2.3 Cylindrical pin The cylindrical pin is the track for the sliding pin-holder. A press fit in the blind hole of the mobile plate is used in order to fix it perpendicularly. The chamfer is needed in order to make room to the rounded corner of the blind hole. The hole at the top of the cylindrical pin allows the assembly of the check pin. See Figure 57.

Force

No torque moments

Guiding surface

Hole for the pin Lower

surface

Stationary plate

Carriage

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Figure 57 – Cylindrical pin.

5.2.4 Check pin The check pin (Figure 58) stops the pin-holder pushed up by the spring. The punch-holder is stopped before thin pins come out from the bores of the inserts in order to avoid the punch-holder rotation around the cylindrical pin.

Figure 58 – Check pin.

5.2.5 Thin pin The function of the thin pin is to eject the billet from the bore of the insert. The thin pin is fixed to the pin-holder with a press fit because of the simplicity. In this way, it is also possible to set the free-length to the desired value. In fact the free-length of the thin pin in the upper pin-holder is different from the one in the lower pin-holder because of the different thickness between the stationary plate and the mobile plate. See Figure 59 and Figure 45.

Figure 59 – Thin pin (left) and Free length (right).

Chamfer

Hole for the check pin

Free length

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5.2.6 The insert The insert is the part which has to hold the billet while the mobile unit is moved by the actuator. The central bore of the insert has to be coaxially aligned with the feeding bore, bore no. 1 and bore no. 2 of the stationary plate to ensure correct transfer of the billet. See Figure 62. This part has been added in order to have the possibility to test different shapes and dimensions of the bore which has to hold the billet. In this way it is not necessary to machine a new mobile plate each time a change is needed. The insert is fixed to the mobile plate with a press fit.

Figure 60 – The insert.

In Figure 60, the red lines show the surface which is in contact with the billet. This surface is machined with a µEDM machine because of the high accuracy of the process (± 2 µm). In this way it is possible to be sure about the diameter, shape and position of the bore used as gripper for the billet.

5.3 Stationary unit This name is referred to all the parts built in order to have a framework for the assembly of the mobile unit and actuator subsystem. See Figure 61.

Figure 61 – Stationary unit.

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5.3.1 Stationary plate

Figure 62 – Upper surface (left) and lower surface (right). It is the support for:

o the rail o the ejector-system o the feeding-system

• The rail is fixed on the upper surface of the stationary plate (see Figure 62).

The surface A is machined in order to ensure the correct position and alignment of the rail with respect to the feeding bore, to the bore no. 1 and to the bore no. 2. The longest side of the rail is pushed against the surface A. After this operation, it is possible to fix the rail correctly with four screws.

• The ejector system is mounted on the lower surface. This system is

necessary in order to inject the billet into the mobile unit and it is made up of:

o one cylindrical pin (see Figure 57) o one check pin (see Figure 58) o one pin holder (see Figure 56) o two thin pins (see Figure 59) o one spring

These are all components used in the mobile unit. In this way costs are reduced.

• The feeding system refills the feeding bore with a new billet each time the billet is ejected from the stationary plate into the mobile unit. It consists in a straight groove which starts from the surface B and ends in the feeding hole. (see Figure 63). Twenty billets are aligned along the groove. The rod shown in Figure 63 (right) pushes on the billets along the groove. A rubber band provides the force necessary for the pushing. This system has been chosen because of its simplicity.

Surface A Rail

Feeding bore Bore no. 1 Bore no. 2

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Figure 63 – Groove (left) and Feeding system (right).

The billet has a diameter of 1.89 mm and a height of 2.13 mm. The play between the billet’s surfaces and the groove’s surfaces is a critical point because:

o if the play is too small, the billet’s sliding will result difficult and blocking problems will occur;

o if the play is too big, the billet can fall down once it is pushed inside the groove; see Figure 64

So the groove’s width and height are set respectively to 1.9 mm and 2.2 mm.

Figure 64 – Tilt problem of the billet inside a too big groove.

5.3.2 The rod The rod has to push on the billets aligned inside the groove. It slides inside the groove of the stationary plate while a rubber band provides the force for the movement (see Figure 63 ). In order to allow the sliding of the rod, it is needed play between the rod’s surfaces and groove’s surfaces. The first version of the rod was designed as a stick with a length equal to 75 mm. Its cross section had a width of 1.9 mm and a height of 2.2 mm (see Figure 65).

Feeding bore Billets

Rod Force

Groove

Feeding bore Surface B

Rubber band

Lid

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Such a shape would lead to the Euler bending when the rubber band had been mounted.

Figure 65 – First edition of the rod.

To avoid Euler bending, two “wings” are added. The side touched by the rubber band has rounded corners in order to avoid the rubber band cutting. See Figure 66.

Figure 66 – Final edition of the rod.

5.3.3 The lid The lid is necessary in order to “close” the groove obtained after the milling in the stationary plate. The groove is milled in the lower surface of the stationary plate and after the machining it results “open” (see Figure 67 ).

Rubber band

Rounded corners

”Wings”

Lower surface of the stationary plate

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Figure 67 – Groove without the lid (left) and Lid assembled into the stationary plate (right).

The lid is fixed to the stationary plate with two bolts. The hole near the corner (see Figure 68) is made in order to allow the thin pin passing. The upper corner has been chamfered in order to make room to the rounded corner of the groove.

Figure 68 – The lid.

5.3.4 The minirail The minirail (Figure 69) allows the mobile unit sliding. The rail is mounted on the stationary plate with four screws and the carriage is fixed to the mobile unit with four screws. The large number of load-bearing balls provides the carriage with high permissible load values and torque. The minirail is sold into two different accuracy grades: precision G3 and high-precision G1. Due to the need of high precision, the G1 accuracy grade has been chosen. The length of the rail is equal

Groove

Hole for the thin pin Holes for the

bolts

Thin pin

Rounded corner

Chamfered corner

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to 70 mm. Due to the G1 accuracy grade, the maximum displacement in the X direction is equal to ± 2 µm.

Figure 69 – The minirail.

5.3.5 Stationary plate support The stationary plate support (Figure 70) positions the stationary plate horizontally and vertically. It is fixed to the table with two bolts. The three threaded holes in the upper surface are used to assemble the stationary plate. The big groove is machined in order to make room to the ejector system of the stationary unit.

Figure 70 – Stationary plate support.

5.3.6 Alignment bar It provides for the correct alignment between the stationary plate support and the actuator support. Surface C of the stationary plate support and surface D of the actuator support are leant to the surface E of the alignment bar. After this operation, these three parts are fixed with bolts to the table. In this way the correct alignment is reached. See Figure 71.

Threaded holes

Groove

X

Y

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Figure 71 – Alignment bar (left) and Assembly of the alignment bar (right).

5.3.7 Table It is the rest for the actuator support, the stationary plate support and the alignment bar. The four holes near the corners are for fixing it to the granite table of the laboratory. See Figure 72.

Figure 72 – Table.

5.4 Actuator subsystem It is referred to:

o the linear actuator o the two parts developed in order to fix and align the actuator to the mobile

and stationary units.

Surface E Surface C

Surface D

Table

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5.4.1 Linear actuator The linear actuator LAL 35/025/51 (Figure 73) moves the mobile-unit horizontally along the minirail. The movement of the rod is provided by a computer controlled linear servo motor, located inside the actuator shell. The stroke length is equal to 25 mm. Acceleration, velocity and position of the rod are measured and controlled by means of a linear encoder with a resolution of 1 µm. The actuator will be tested to verify the repeatability and accuracy of the rod displacements. In the shell there are precise machined surfaces with threaded holes which allow the assembling with the support. The linear actuator LAL 35/025/51 needs the controller LAC 1. LAC 1 is a single axis controller with 8 TTL I/O channels and 3 analogue I/P channels. It allows the storage of the program which is afterwards used to enable movements of the actuator’s rod. The controller is programmed by means of a PC. The machine code is written in a text editor, such as note pad, pasted into Hyperterminal and then transferred to the LAC 1 with a RS 232 cable.

Figure 73 – Linear actuator LAL 35/025/51.

LAL 35/025/51 and LAC 1 are provided by Cedrat Groupe.

5.4.2 Connecting flange The connecting flange (Figure 74) joins the actuator’s rod with the mobile-unit. The central hole is for mounting the part with a bolt in the threaded hole of the actuator’s rod. Three peripheral holes are made in order to fix the flange with the mobile-unit.

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Figure 74 – The connecting flange. If the actuator rod was directly fixed to the mobile unit with one bore into the mobile plate, the rod axis and the bore axis would be perfectly aligned. (see Figure 75) Using the flange, a precise positioning of the rod axis with respect to the mobile-unit is not necessary because the clearance between the three peripheral holes and their respective bolts allows small deviations.

Figure 75 – Direct connection between the actuator’s rod and the mobile unit.

5.4.3 Actuator support The actuator support (Figure 76) positions the actuator horizontally and vertically with respect to the mobile unit. It is assembled in the table with one bolt. The actuator is fixed with six bolts.

Actuator´s rod

Treaded hole

Hole for the bolt to rod Holes for the bolt

for the mobile unit

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Figure 76 – Actuator support.

5.5 Mobile-unit and stationary-plate bores alignment When transferring the billet from the stationary-plate to the mobile-unit and vice versa, the precision in the coaxial alignment between the mobile-unit bores with respect to the stationary-plate bores is an essential requisite. In fact, a small deviation will prevent the billets transfer. In the step X, the bores 1 and 2 of the mobile-unit have to be coaxially aligned with respect to the bores A and B of the stationary-plate at the same time. Only in this way, the two billets can be simultaneously moved from the stationary-plate into the mobile-unit (see Figure 77).

Figure 77 – Step X.

In the step Y, the bores 1 and 2 of the mobile-unit have to be coaxially aligned with respect to the bores B and C of the stationary-unit. In this way, the two billets can be simultaneously moved from the mobile-unit into the stationary-plate (see Figure 78).

Bore 2 Bore 1

Bore A Bore B Bore C

Stationary-plate

Mobile-unit

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Figure 78 – Step Y.

The most critical issue is the difference between the following distances in the Y direction on the stationary-plate (see Figure 79):

o distance of the bore A centre from the Z side of the rail o distance of the bore B centre from the Z side of the rail

This difference has to be smaller than 30 µm, as explained in the following. Consider the bore 1 of the mobile unit. It has a diameter equal to 1.905 mm. Once machined, its centre has always the same distance from the Z side of the rail while the mobile-unit is moving. The same observation is valid for bore 2 of the mobile unit. In Figure 79, the green line represents the trajectory covered by the centres of the bores 1 and 2. Bores A and B have a diameter 30 µm bigger than the diameter of the bores 1 and 2. As an example, consider step Y. If the distance between the centre of bore B from the green line was bigger than 15 µm, it would be impossible to transfer the billet from the bore 1 into the bore B. The billet would hit the upper surface of the stationary-plate in the red zone highlighted in Figure 80.

Bore A Bore B Bore C

Bore 2 Bore 1

Stationary-plate

Mobile-unit

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30 µ

m

Side Z

= position of bore A= position of bore B

= position of bore 1 in the step X= position of bore 1 in the step Y

15 µ

m

15 µ

m

Figure 79 – Coaxial alignment in the Y direction.

Figure 80 – Interference zone. Orange is the bore B, green is the bore 1.

On the other hand, the position of the bore C with respect to the green line is not so critical. In fact, the bore C has a diameter ~1000 µm bigger than the bore 2 diameter. As a result, there would be problems only if the distance between the bore C centre and the green line should be bigger than 500 µm. To achieve the coaxial alignment between

o the bore 1 with respect to the bores A and B o the bore 2 with respect to the bore B

two conditions must be respected: 1 the gap between the centres of bores A and B in the Y direction has to be

smaller than 30 µm, as described on the previous page. 2 the trajectory covered by the centres of bore 1 and 2 (green line in Figure

81) has to be positioned in the middle of the gap.

X

Y

A < 15 µm

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30 µ

m

= position of bore A

= position of bore B

Figure 81 – Conditions for reaching the coaxial alignment.

Bores A and B of the stationary plate are machined through drilling operation. The precision of this process should be sufficient to fulfil the above condition no.1. On the other hand, bores 1 and 2 of the mobile-unit are machined by means of the µEDM machine because of the highest precision of the process (± 2 µm). In fact, after having measured the distances α and β between the centres of bores A and B and the rail (see Figure 82), it will be possible to machine bores 1 and 2 in the correct position with respect to the rail and fulfil the above condition no. 2. The condition no. 1 can be fulfilled by setting the tolerances on the position of bore A and B centres as narrow as possible. The condition no. 2 can be fulfilled in this way: 1 the rail is fixed on the upper surface of the stationary plate (see section

5.3.1) 2 the distances α and β between the centres of bores A and B and the rail

are measured by means of the µEDM machine (see Figure 82). In fact, the µEDM electrode, when not run by current, can be used as a measurement instrument device

3 the rail is removed from the stationary-plate and it is mounted on the mobile-unit (see Figure 82)

4 by using the µEDM machine, the bores 1 and 2 are machined in the mobile-unit. The distance between bore 1 and 2 centres and the rail has to be:

distance δ = (distance α + distance β) / 2 This procedure uses the Z side of the rail as reference surface with respect to the distances of bore A, B, 1 and 2 centres.

x

y

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Figure 82 – Stationary-plate upper view (left) and mobile-unit lower view (right).

The coaxial alignment between the mobile-unit bores with respect to the stationary-plate bores in the X direction (see Figure 83) is more easily achievable.

= position of bore A = position of bore B

Figure 83 – Coaxial alignment in the x direction.

In fact, after having measured the distances ξ between the centres of bores A and B in the stationary plate (see Figure 83), it will be possible to machine bores 1 and 2 at the same distance ξ by means of the µEDM machine.

Rail

Bore 1

Distance δ

Distance δ

Distance α

Distance β

Bore 2

Bore B

Bore C

Bore A

Z sideZ side

y

x Distance ξ

Distance ξ

Bore 1 Bore 2

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6 Rig tests

6.1 Actuator test The test described in the following section was carried out in the metro lab of IPL (Institute for Produktion og Ledelse) in building 425. The testing of the linear actuator has been done to verify the repeatability and accuracy of the rod displacements.

6.1.1 Setup Figure 84 shows the equipment used in the actuator testing. The displacements of the tip of the actuator rod are measured with an LVDT transducer, named Tesa-probe. The LVDT working principle has already been described in chapter 4.3. A Tesa-probe with a 4 mm stroke length has been chosen because of its 1 µm resolution. The probe is electrically connected with a Tesa-module. As the tip of the probe moves, the Tesa-module shows its displacements. Plane B (see Figure 84) is used as reference plane to reach the proper alignment between the probe axis and the actuator rod axis. In fact, the fixture A positions the probe axis perpendicularly with respect to the plane B while the fixture C positions the actuator rod axis perpendicularly with respect to the same plane B.

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Figure 84 – Lab equipment.

6.1.2 Experimental procedure and results The actuator was set to zero. This operation brings the tip of the actuator rod into the home-position, which represents the displacements zero point. The display of the tesa-module was set to 0,000 mm. After the zero setting operation, the test could start. To test the repeatability of the rod displacements, the actuator was programmed so that the tip of the actuator rod could move back and forward from the home-position to the set-point A. The set-point A was positioned 500 µm far from the home-position. The back and forward motion was repeated 5 times.

Repeatability test n 1

500

502

504

506

508

510

1 2 3 4 5Stoke number

Stro

ke le

ngth

[µm

]

Figure 85 – Repeatability test with a 500 µm stroke.

Tesa-probe

Fixture A

Plane B

Fixture C

Actuator rod

Actuator

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As shown in Figure 85, the tesa-probe measured:

o a 508 µm stoke length for 4 times. o a 507 µm stroke length for 1 time.

The same test was repeated other two times, after changing the position of the set-point with respect to the home-position. The two new set-points, named B and C, were respectively positioned 1000 µm and 2000 µm far from the home-position.

Repeatability test n 2

1010

1012

1014

1016

1018

1020

1 2 3 4 5

Stroke number

Stro

ke le

ngth

[µm

]

Repeatability test n 3

2030

2032

2034

2036

2038

2040

1 2 3 4 5Stroke number

Stro

ke le

nght

[µm

]

Figure 86 - Repeatability test with a 1000 µm stroke (left) and a 2000 µm stroke (right).

As shown in Figure 86, the tesa-probe measured:

o a 1018 µm stroke length for 5 times. o a 2035 µm stroke length for 5 times.

These results show that:

o the actuator has a good repeatability. In fact, the rod tip can be moved back and forward between the home-position and the set-point reaching the same point each time;

o while reaching a set-point, the accuracy is not equal to 1 µm, as declared in the actuator specifications.

To verify the actuator accuracy, a new set of measurements was carried out in a different way. The tip of the actuator rod was moved back and forward from the home-position to twenty different set-points. The gap between two consecutive set-points was equal to 100 µm. Each stroke was measured with the Tesa-probe.

Wanted stroke [µm]

Measured stroke [µm]

Difference

[µm] 0 0 0

100 102 2

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200 204 4300 305 5400 407 7500 509 9600 611 11700 713 13800 814 14900 916 16

1000 1018 181100 1120 201200 1221 211300 1324 241400 1425 251500 1527 271600 1629 291700 1731 311800 1832 321900 1934 342000 2036 36

Table 2 – Accuracy test.

As shown in Table 2, the actuator did not stop the tip of the rod in the position programmed via computer. The tip of the rod always exceeded the position that it had to reach.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Input via computer [µm]

Stro

ke [µ

m]

Wanted stroke Measured stroke

Figure 87 – Comparison between the wanted stroke and the measured stroke

Concluding on the data shown in Figure 87, the position error was linearly dependent on the stroke length and it could be expressed as a percentage of the wanted stroke by means of the following expression:

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100% ⋅−

=keWantedStro

keWantedStrorokeMeasuredSt

It resulted that the measured stroke was always equal to + 1,8 % of the wanted stroke. The accuracy of 1 µm declared in the actuator specifications was not verified. Since this is a systematic error, the problem can be overcome through a calibration task: software compensation can be therefore implemented in order to reduce or to eliminate the device inaccuracy.

6.2 Stationary plate bores tolerance verification The position of the bore A and B centres with respect to the Z side of the rail were carried out by means of the µEDM machine. In fact, the µEDM electrode, when not run by current, can be used as a measurement instrument device. The results are shown in Figure 88.

Figure 88 – Position of bore A and B centres with respect to the rail.

The gap between the centres of bores A and B in the Y direction is equal to:

69532,19601,19 =−=ygap µm As the gapy is bigger than 30 µm, the condition no. 1 described in the section 5.5 is not verified. It results impossible to reach the coaxial alignment between the bore 1 of the mobile-unit and the bores A and B of the stationary unit in both steps X and Y. As an example, if the bore 1 centre is positioned 19,586 mm far from the Z side of the rail, the coaxial alignment will be reached in step X but not in step Y (see Figure 89). On the other hand, if the bore 1 centre is positioned 19,547 mm far from the Z side of the rail, the coaxial alignment will be reached in step Y but not in step X.

Z side

x

y Bore A Bore B

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69

µm

Side Z

= position of bore A= position of bore B

= position of bore 1 in the step X= position of bore 1 in the step Y

19,6

01 m

m

19,5

32 m

m

Figure 89 – yGap -size error.

To solve the problem, the bore B has to be closed and a new bore has to be machined by means for example of the more precise µEDM machine. The distance between the centre of the new bore and the side Z of the rail has to be set equal to 19,601 mm. Since the µEDM normally achieves a higher accuracy than drilling operation, low uncertainty on the bore B position with respect to the rail is expected. In this way, the condition no. 1 described in the section 5.5 can be fulfilled. Anyway, the transfer system can be tested in a simpler way (i.e. without using bore B and bore 2):

o by pushing up the lower pin holder, the billet is moved from the bore A of the stationary plate to the bore 1 of the mobile unit (step R in Figure 90);

o the bore 1 is then positioned above the bore C of the stationary plate, that corresponds to the hole for the final discharge of the billets (step S in Figure 90);

o by pushing down the upper pin holder, the billet is moved from the bore 1 to the bore C;

(see Figure 77) (see Figure 78)

Bore 1 Mobile unit

Upper pin holder

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Figure 90 – Step R (left) and step S (right). Since the nominal value of bore C diameter is as big as 3 mm, the large gap (given by the difference between bores C and 1) allows for some misalignment. Therefore it is very easy to properly position mobile unit for the billet transfer. The only left critical point is the coaxial alignment between the bore A and the bore 1. In any case this can be achieved in this way:

o the minirail is mounted on the mobile unit; o the bore 1 is machined by means of the µEDM machine; thanks to the

high accuracy of this instrument (± 2 µm), it is possible to position the bore 1 centre at the correct distance with respect to the side Z of the rail, that corresponds to 19,601 mm (see Figure 91).

Figure 91 – Correct position of the bore 1 with respect to the side Z of the rail.

6.3 Billet-gripping test Since the preform and the final component have not been manufactured yet, it was possible to test only the billet gripping. Tests have been carried out with a cylinder-shaped bore.

Bore A Bore C Stationary plate

Bore 2

Bore B

Bore 1

Lower pin holder

19,601 mm

Bore 1

Side Z

Rail

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Figure 92 – 5 times magnification of the gripping device (left) and actual size (right).

Figure 92 shows a 5 times magnification of the gripping device on the left and the actual size on the right. The bore was machined by means of the µEDM machine, an instrument achieving higher accuracy (± 2 µm) than other available technologies. Therefore, low uncertainties for the diameter and the bore shape were assured. As a first attempt, the nominal diameter of the bore has been set equal to φ = 1,905 mm. In this way, all the 70 billets could get into the bore. In fact, after measuring the 70 billets with the Tesa-probe (see section 4.3), it was found that their diameter was φ =1,888 ± 0,006 mm: with such a value, ~ 99 % of the billets are supposed to be < 1,905 mm. These values would give a gap in the range 15÷30μm: as shown in the paragraph 2.2.3, with such a gap ensures surface forces sufficient to properly hold the billet weigh. After machining, the diameter of the gripping device was measured by means of the µEDM machine. In fact, the EDM electrode, when not run by current, can be used as a measurement instrument device. The found result was φ =1,902 ± 0,002 mm. Once completed the above descried tasks, two cycles of tests were carried out. The billet-gripping test no.1 was carried out as follows:

o All the billets and the cylindrical-shaped bore were cleaned with acetone, in order to remove any possible oil contamination.

o After ~24 hours ( in such a way a thin film of moisture was formed on the billet surface), each billet was put inside the cylindrical-shaped bore.

o The gripper was shaken in order to simulate vibrations. During the vibrations simulation, the behaviour of the billets was controlled: it was found that none of the 70 billets was kept into the hole by the surface tension forces arising from the natural film of moisture. The billet-gripping test no.2 was carried out as follows:

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o All the billets surface was coated by a thin lubricant layer, named Molykote. Such a lubricant is used during the billet forging operations.

o Each billet was put inside the cylindrical-shaped bore. o The gripper was shaken in order to simulate vibrations.

At the end of this second test, it was observed that all the 70 billets were held inside the bore. Presumably adhesive forces occurring when gripping cleaned surfaces are not sufficient to hold in position the billets. On the other hand, adhesive forces arising when some contaminant is present (such as a thin layer of lubricant), are enough to hold the billet inside the cylindrical-shaped bore. This phenomenon can be used to increase the gripper efficiency in a very cheap way, since a thin lubricant layer is in any case needed during the forging operations.

6.4 Transfer test It was shown that the billet gripping device can work whenever billets present some contaminant (as lubricant) on the surface. The transfer system was therefore tested. As described in the section 6.2, a misalignment of the bore B with respect to the rail was present, because of an out of tolerance. For this reason the test was performed as described in section 6.2. The mobile unit was manually moved from step R to step S and vice versa (see Figure 90) since the stroke of the actuator available for the project was insufficient to carry on such a test. In fact the distance between the bore A axis and the bore C axis was equal to 40 mm, too big if compared with the 25 mm stroke of the actuator. Successful result (i.e. proper transfer of the billet) was achieved in 100% of cases. Even if it was not possible to move the billet from the bore A to the bore that simulates the first forming stage (i.e. the bore B), the test demonstrated that the principle works.

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7 Summary and conclusion In the last years the demand for microparts has been constantly increasing. So far these parts have been manufactured mainly through turning and milling operations or chemical etching. In such a contest, the idea of the Masmicro project was developed, whose aim is to develop new knowledge on the manufacturing of mini/micro components, convert the knowledge into new technologies and transfer them to the industry. This report is the documentation of a master thesis project, developed as a part of the work package RTD3 and 4 of the “Masmicro” EU-project, and dealing with the engineering and manufacturing of handling devices for mass-manufacture of mini- and micro-components. During this M.Sc. Project a prototype device for handling and transferring mini-billets was designed and manufactured. The device was meant to be implemented into a two-steps cold forging process. As a first step a literature survey was carried out. The state of the art on the last developments in micro-handling was analyzed, focusing on those solutions most suitable our specific need. Secondly a transfer system to be used in the production system was outlined, and different gripping principles were suggested and described. The surface

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tension forces were recognized as a preferable principle for the device realization. A campaign analysis was performed on the billets to be used in the cold forging operation. Collected data were essential in the definition of dimensions and tolerances of the billets-gripper. The prototype rig for mini-billets handling was eventually designed, manufactured and tested. Due to the need of high speed and simplicity, a one-dimensional linear transfer system has been chosen. A gripper was needed to hold the billet when transferred into the transfer mechanism. A gripping solution based on adhesive forces arising from surface tension has been proposed. In general, when two objects are exposed to the environment, a thin film of water (due to adsorption of moisture) or contaminant (i.e. oil, lubricant in a cold forging process) is formed on their surfaces. When they are brought together very closely, the films touch and melt together. In this way the two objects stick because of the surface tension. Taking advantage of this effect, a cylindrical shaped bore slightly larger than the billet, could be used to hold the same billet. Tests showed that adhesive forces occurring when gripping cleaned billets (i.e. with only a thin film of moisture on their surfaces) were not sufficient to hold them inside the cylindrical-shaped bore. On the other hand, adhesive forces arising when some contaminant was present (such as a thin layer of lubricant), were enough to hold the billet inside the cylindrical-shaped bore. Tests were carried out in order to verify the efficacy and efficiency of the developed system. Not all of the transfer steps were analyzed, because of a misalignment of one of the bores in the stationary plate. In any case analysis proved the system would work satisfactorily, once introduced into the production line. As a future development, drilling of a new bore in the stationary plate, respecting given tolerances and new tests would be recommended. Subsequently, introduction of the designed device into the production system is expected.

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8 References [1] F. Vollertsen, Z. Hu, H. Schulze Niehoff, C. Theiler: “State of the art in micro

forming and investigations into micro deep drawing” Journal of Materials Processing Technology 151, 2004.

[2] M. Geiger, M. Kleiner, R. Eckstein, N. Tiesler, U. Engel: ”Microforming” CIRP

2001. [3] Formatech [Web page]; http://www.formatech.de [Accessed 16 Feb. 2006]. [4] Masmicro, Annex 1 – “Description of work”, 2004. [5] S.Kalpakjian, S.R.Schmid: “Manufacturing process for engineering

materials”, Pearson International Edition. [6] R. S. Fearing. “Survey of sticking effects for micro parts handling”, IEEE/RSJ

International workshop on intelligent robots and systems (IROS), Pittsburgh, PA, 1995

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[7] H.Van.Brussel, J.Peirs, D.Reynaerts, A.Delchambre, G.Reinhart, N.Roth,

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[8] K.E.Drexler “Nanosystems, molecular machining, manufacturing and

computation” John Wiley and sons, page 65, 1992 [9] J. Lowell, A.C.Rose-Innes: “Contact electrification”, Advances in physics,

1980 [10] K. Bohringer, R.Fearing, K.Goldberg: “Microassembly” The handbook of

industrial robotics, second edition, editor Shimon Nof, Wiley and sons, 1998 [11] R.L.Alley, G.J.Cuan, R.T.Howe, K.Komvopoulos, ”The effect of release-etch

processing on surface microstructure stiction”, Proc. solid state sensor and actuator workshop, pages 202-207, Hilton Head Island, SC, June 1992

[12] A.Gegeckaite, ”Micro handling and assembly”, M.Sc. thesis, Technical

University of Denmark, Department of manufacturing engineering and management, 2005, April

[13] C.Kim, A.Pisano, R.Muller “Silicon-processed overhanging microgripper”,

journal of microelectromechanical systems, 1/1:31-36, 1992 [14] K.Suzumori, S.Ikura, H.Tanaka, “Flexible microactuator for miniature robots”,

Proc. IEEE MEMS workshop, 204-209, 1991 [15] C.G.Keller, R.T.Howe, “Hexil tweezers for teleoperated micro assembly”,

IEEE workshop on micro electro mechanical systems, pages 72-73, 1997 [16] F.Aray, D.Andou, T.Fakuda, “Adhesion forces reduction for micro

manipulation based on micro physic” IEEE, pages 354-359, 1996 [17] E.Westkamper, R.Schraft, C.Bark, T.Weisener, ”Adhesive gripper, a new

approach to handling MEMS” Proc. Actuator 96, pages 100-103, 1996 [18] D.Petrovic, G.Popovic, E.Chatzitheodoridis, O.Medico, “Gripping tools for

handling and assembly of microcomponents” Proc. 23rd International conferenceon microelectronics, 1:247-250, 2002

[19]Stanford synchrotron radiation laboratory [Web page]

http://www-ssrl.slac.stanford.edu [Accessed 02 Mar 2006]

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Appendix In the following are presented the workshop drawing.