experimental design and investigation of a pin-type recon ...experimental design and investigation...

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Experimental design andinvestigation of a pin-typerecon�gurable clamping

system for manufacturingaerospace components

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Citation: AL-HABAIBEH, A., GINDY, N. and PARKIN, R.M., 2003. Exper-imental design and investigation of a pin-type recon�gurable clamping systemfor manufacturing aerospace components. Proceedings of the Institution of Me-chanical Engineers, Part B: Journal of Engineering Manufacture, 217(12), pp.1771-1777.

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• This is an article from the journal, Proceedings of the In-stitution of Mechanical Engineers, Part B: Journal of En-gineering Manufacture [ c© IMechE ]. It is also available at:http://dx.doi.org/10.1243/095440503772680703

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Experimental design and investigation of a pin-typerecon®gurable clamping system for manufacturingaerospace components

A Al-Habaibeh1*, N Gindy2 and R M Parkin1

1Mechatronics Research Centre, Wolfson School of Mechanical and Manufacturing Engineering, LoughboroughUniversity, Leicestershire, UK

2School of Mechanical, Materials, Manufacturing Engineering and Management, University of Nottingham, UK

Abstract: This paper presents an experimental design and evaluation of a pin-type universal clampingsystem. The clamping system is designed for holding complex-shaped aerospace components, such asturbine blades, during machining processes. The experimental investigation is performed by comparingthe pin-type clamping system with a dedicated clamping system during the machining of aluminiumand steel parts. Force signals are monitored during machining. The experimental results arepresented and compared with a dedicated clamping system. From the comparison, the performanceof the clamping system is discussed. The results provide an evaluation of the designed pin-typeclamping systems for machining operations. The results prove that the pin-type clamping systemcan be recon®gured for machining di�erent complex shaped components with performancecomparable with a dedicated clamping system.

Keywords: ®xturing, aerospace, recon®gurable clamping system, pin-type clamping system, machin-ing, low-melt alloy, phase change material

1 INTRODUCTION

The major forces in¯uencing today’s aerospace manu-facturing environments are not di�erent from othermanufacturing industries: global competition, shortenedproduct life cycle, increasing requirements for qualityand reliability, faster paced advances in increasinglycomplex technology, rapidly expanding options inmaterials and processes and increased unpredictablesurroundings. The ability of an aerospace enterprise totake advantage of these forces is the key to any successfulaerospace manufacturing strategy. The overriding goalof the manufacturing enterprise is to achieve rapid¯exible integrated design and manufacture of innovativeproducts in batch sizes that are getting smaller at a pricethe customer is prepared to pay [1]. To thrive in the emer-ging aerospace market condition it is therefore importantthat the manufacturing enterprise should be capable ofrapidly responding to market trends by utilizing intelli-gent technology within a rapid product development

environment capable of very short times to market. Tomeet such requirements, technological advances havebeen made during the past few decades with respect tomachine and cutting tools. Nevertheless, the ®xturingtechnology in machining is still lagging behind despiteits criticality and importance.

Fixturing has been considered as one of the mainproblems to improve ¯exibility, productivity and partquality, particularly in the aerospace industry. It is oneof the most direct threats to cost e�ectiveness and opera-tional e�ciency. Fixturing is used for locating andrestraining the workpiece in position and to ensure thatdimensional accuracy is maintained during manufac-turing operations. If perfection could be achieved, ®xtur-ing could be produced at a high speed and have theability to hold any complex part. Generally, in manu-facturing environments, ®xture design is based on pastexperience and trial-and-error approach. Therefore,®xturing design is expected to be a long process with pro-hibitive cost. Many ®xturing techniques have been inves-tigated during the past few decades (see, for example,references [2] to [4]). The majority were developed tomeet changes in manufacturing industry. Fixturingsystems can be divided into dedicated modular contact®xturing and pin-type ®xturing systems.

1771

SC03503 # IMechE 2003 Proc. Instn Mech. Engrs Vol. 217 Part B: J. Engineering Manufacture

The MS was received on 30 July 2003 and was accepted after revision forpublication on 3 September 2003.*Corresponding author: Mechatronics Research Centre, LoughboroughUniversity, Holywell Building, Holywell Way, Loughborough,Leicester-shire LE11 3UZ, UK.

A dedicated ®xture is a single-purpose device which isdesigned to locate and constrain a speci®c part or com-ponent. Once the manufacturing process is completed,the ®xture is then stored for later use. This traditionalapproach is costly and time consuming since it requiresa special ®xture to be manufactured for every part orcomponent. These drawbacks have motivated research-ers to develop modular ®xturing system [5, 6]. A modular®xturing system is a ®xturing system that uses a collec-tion of reusable standard components to construct acomplete variety of special-purpose work-holdingdevices. It reduces ®xture manufacturing time, makes®xture modi®cation easier and eliminates dedicated®xture storage space. Modular ®xture design wasmainly based on trial and error until the developmentof computer systems to aiding the design process [5, 7].Computer aided ®xture design ranges from expert sys-tems [8] to a kinematics approach [9] to using geneticalgorithms [10]. Contact ®xturing system utilizes mag-netic ®xturing, instant freeze chucks and phase change®xturing system [11, 12]. The main problem in usingmagnetic work-holding methodology in aerospacemanufacturing is that the ®xture should take the shapeof the component which results in the same problemsof dedicated ®xturing. Also residual magnetism couldbe a problem in cleaning the swarf o� the component.Instant freezing chucks [12] can hold any material byplacing the component on a plate covered by a thin®lm of water. When the chuck is switched on, thewater freezes, holding the component. Phase change®xturing is based on the ability of certain materials tochange from a ¯uid to a solid and back to a ¯uid again[13]. When material is in liquid phase, the part isimmersed to the required depth. Then the material’ssolid phase is introduced, providing rigid support andwork-holding force for the component. Upon ®nishingthe manufacturing procedure, the component is removed

by reversing the phase of the work-holding material. Themain advantages are holding complex shape componentswith uniformly distributed forces. The main disadvan-tage of this technique is the possible contaminationfrom the phase change material and the di�culty ofmachining the immersed parts of the component.

2 PIN-TYPE FIXTURING SYSTEMS

2.1 Overview

A pin-type ®xturing system has been documented inseveral patents during the past 30 years [14±18]. A pin-type ®xture can be used for locating and clamping of aworkpiece of irregular shape during manufacturingprocesses. The main concept of a pin-type ®xture consistsof a main body or base that contains a two-dimensionalarray of orthogonal to the base rods or pins which pro-truded upwards through the surface and can be movedupwards and downwards individually, either manuallyor by springs or by ¯uid pressure, and then clamped orlocked in position so that the tips of the rods form acradle conforming to the shape of the workpiece, inwhich the workpiece is ®xed for manufacturing opera-tions. Figure 1a presents a generic schematic diagramof the main concept of a pin-type ®xture.

Di�erent locking mechanisms have been suggested inthe literature including mechanical methods, pneumaticpressure, hydraulic pressure and phase change material.The expected advantages of a pin-type ®xture is the uni-form distribution of forces and the recon®gurable shape.

2.2 The proposed pin-type clamping system

Figure 1b presents the designed and manufactured pin-type clamping system under evaluation. The clamping

Fig. 1 (a) The main concept of a recon®gurable pin-type clamping system; (b) the designed and manufactured

pin-type clamping system

1772 A Al-HABAIBEH, N GINDY AND R M PARKIN

Proc. Instn Mech. Engrs Vol. 217 Part B: J. Engineering Manufacture SC03503 # IMechE 2003

system consists of two platforms. Each platform has atwo-dimensional array of rods or pins (14 £ 8). The pinshave a diameter of 5 mm and the platform dimensionsare 150 mm£100mm. The pins’ matrix has dimensionsof 110 mm£60 mm. The pins end with a round tip inorder to avoid scratching or stamping the workpiece.The platforms are made of steel and the rods are machinedfrom a brass rod. A low-melt alloy (bismuth±lead±tin±cadmium) is used as the holding mechanism. The abilityof this material to change phase at a relatively low tem-perature within about 4 min makes it a suitable optionfor the design. The low-melt temperature allows the useof hot water or induction heating as the power source ofphase change. One of the main aims of the design is toproduce the ®xture as simply and as inexpensively aspossible. The rods were cut from a standard pipe. Thelow-melt alloy has provided the locking mechanism forthe pins. The aim of the clamping system is to provide anew, inexpensive and simple universal ®xture for rapidlyand easily holding or supporting many variations ofcomplex contoured parts of irregular shape duringmachining or any other manufacturing processes oractions. The clamping system is aimed at providing arigid ®xture by the use of phase change material as ameans of locking the rods’ mechanism. The inventioneliminates the using of separate ®xtures to hold multipleor complex parts. It consequently eliminates the ®xturemanufacturing time, cost, maintenance, storage and set-up costs associated with using di�erent ®xtures for di�er-ent parts. The clamping system also has the advantage ofholding parts of the same shape but with a wide toleranceby using the recon®gurability of the ®xture.

Three main issues are tested in this paper:

(a) the applicability of the pin-type clamping systemconcept (i.e. can the clamping system be used formilling operations?);

(b) the recon®gurability of the clamping system (i.e. theuse of the same clamping system to machine di�erentcomponents);

(c) the rigidity and sti�ness of the clamping systemcompared with those of a dedicated clampingsystem.

The previous points were implemented using theoptimum clamping forces which can produce maximumclamping forces without causing any distortion to theworkpiece.

3 EXPERIMENTAL SET-UP AND PROCEDURE

The experimental work in this study was performed infour stages to verify the initial design capabilities. Theexperimental investigation was performed using a con-ventional computer numerical control (CNC) millingmachine and the Variax HexacenterTM, a parallel kine-matic machine tool. Table 1 presents the main four teststages which were conducted to evaluate the clampingsystem.

3.1 Testing the applicability and recon®gurability of the

pin-type clamping system

The ®rst two tests in Table 1 are performed in orderto ensure that the clamping system can be easily usedfor holding complex parts for machining processes.Figure 2 presents the ®rst two tests: applicability andcon®gurability of the clamping system. Figure 2a pre-sents the recon®gurability of the same clamping systemto machine the root of an aluminium blade using aconventional CNC machine tool. Figure 2b presentsthe machining of an aerofoil of a generic steel blade

Table 1 The implemented four stages to test the designed clamping system

Tested criteria Description Machining parameters

Test 1: Applicability of the pin-typeclamping system

The clamping system is used to machine groves in theroot of an aluminium blade (see Fig. 2a)

Feed rate ˆ 200mm/minAxial depth of cut ˆ1 mmRadial depth of cut ˆ 10mmSpindle speedˆ 1500r/minCutter type ˆ 10 mm slot drill

Test 2: Recon®gurability of thepin-type clamping system

The clamping system is recon®gured to locate anothersteel blade for machining its pro®le (see Fig. 2b)

Feed rate ˆ 2 m/minAxial depth of cut ˆ0.5 mmRadial depth of cut ˆ 0.2 mmSpindle speedˆ 1200r/minCutter type ˆ 32 mm round insert face milling cutter

Test 3: Rigidity and sti�ness Clamping system is recon®gured to locate analuminium rectangular workpiece and to compareits surface roughness when using a dedicatedclamping system (see Fig. 3)

Feed rate ˆ 200mm/minAxial depth of cut ˆ5 mmRadial depth of cut ˆ 10 mmSpindle speedˆ 1500±4000r/minCutter type ˆ 10 mm slot drill

Test 4: Rigidity and sti�ness Clamping system is recon®gured to locate a steelrectangular workpiece and to compare its surfaceroughness when using a dedicated clamping system(see Fig. 3)

Feed rate ˆ 50 mm/minAxial depth of cut ˆ5 mmRadial depth of cut ˆSpindle speedˆ 250±1250r/minCutter type ˆ 10 mm slot drill

EXPERIMENTAL DESIGN AND INVESTIGATION OF A PIN-TYPE RECONFIGURABLE CLAMPING SYSTEM 1773


using the Variax Hexacenter. The orientations of thegeneric blade and the machined surfaces are describedin Fig. 2.

3.2 Testing the rigidity and sti�ness of the pin-type

clamping system

In order to test the rigidity and sti�ness of the pin-typeclamping system, a comparison between machiningforce signals and surface roughness values of a productmachined using the pin-type clamping system and adedicated clamping system is performed. The workpieceis chosen to be a rectangular shape with the followingdimensions: 100mm length, 50 mm width and 12 mmdepth. The ¯at top surface of the workpiece of width12 mm was machined for testing. The positioning andorientation of the workpiece are as shown in Fig. 3.The cutting force signals are monitored using a three-component platform dynamometer (Kistler 9257A)which is connected to one side of the evaluated clampingsystem (see Fig. 3). The dynamometer is connected to a

three-channel charge ampli®er (Kistler 5001). The signalsare monitored using a fast data acquisition card(National Instruments AT-MIO-64E-3). Figure 3 pre-sents photographs for the experimental set-up of thepin-type clamping system and the dedicated clampingsystem.

Machining of aluminium and steel workpieces areused to compare the two clamping systems. The machin-ing parameters are shown in Table 1. In addition to theclamping and machining forces, the average surfaceroughness Ra and the maximum roughness height Rymax

were measured along the y axis (see Fig. 3), to comparethe rigidity of the clamping system.

4 RESULTS AND DISCUSSION

Tests 1 and 2, as described in Table 1, showed that thepin-type clamping system is capable of holding work-pieces for machining operations. Also, the clampingsystem is successfully recon®gured to hold a di�erent

Fig. 2 The ®rst two experimental stages of the evaluation process (tests 1 and 2 of Table 1)



geometry by reheating the low-melt alloy and clamping itaround the new workpiece. The pin-type clampingsystem is successfully used to machine the root of thealuminium blade and the aerodynamic pro®le of anothersteel blade. However, in order to evaluate the rigidityand sti�ness of the clamping system, tests 3 and 4 areperformed in order to compare a dedicated clampingsystem with the pin-type clamping system and tomeasure surface roughness and forces.

Surface roughness was measured for the selectedmachining parameters. The Ra and Rymax

values weremeasured on a length of 0.8 mm on the machined surface.Figure 4 presents the results for the two clampingsystems for machining of aluminium and steel. Themaximum measured values in the y direction of Ra andRymax

of the machined surface (see Fig. 3) are compared.It has been found that the pin-type clamping system hasa higher surface roughness than the dedicated clampingsystem when machining aluminium at a high spindlespeed (r/min). However, the pin-type clamping systemhas a better characteristic when machining a steel work-piece at a lower spindle speed. However, the range of Ra

values between both clamping systems in the two tests isabout 5 mm only. The variation could be due to the nat-ural frequency of the complete ®xturing±dynamometersystem. The variation in the level of machining forces isfound to be greater for the steel workpiece (measuredas the power of the cutting forces). This could explainthe reason why the surface roughness values of thealuminium part are less than the surface roughnessvalues for steel for both clamping systems. Since Ra ismeasured along the y axis, the surface roughnessincreases with increasing power of the Fy signal, asshown in Fig. 5. Higher forces and vibration leads tohigher surface roughness which could be a result of theinsu�cient rigidity of the clamping±dynamometersystem (see Fig. 3).

5 CONCLUSIONS AND FURTHER WORK

The designed pin-type clamping system is found to be¯exible and recon®gurable to be used for holding com-plex aerospace parts during machining. The pin-type

Fig. 3 The experimental set-up to measure clamping and machining forces for the pin-type clamping system

and a dedicated clamping system (tests 3 and 4 of Table 1)



clamping system has been found to show a slightly betterresponse for low cutting speeds of steel than the dedi-cated clamping system. This is indicated in an improvedsurface roughness and lower power level of cuttingforces. However, the dedicated clamping system isfound to have a relatively better response when machin-ing aluminium at a high cutting speed. This could be due

to the natural frequency of the pin-type clamping systemwhich is expected to be lower than that of the dedicated®xturing system. However, more analysis is needed onthe basis of real manufacturing parts to evaluate its suit-ability for real machining environments. The future workwill include a simulation and experimental investigationof the design parameters, namely the number of pins, the

Fig. 4 The surface roughness measurements of (a) aluminium and (b) steel workpieces



distribution of pins, the pin diameter, the pin length andthe pin material.

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Fig. 5 The relationship between the Fy signal and surface roughness for aluminium and steel workpieces



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