an overview of cnc machines

8/3/2019 An Overview of Cnc Machines

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AN OVERVIEW OF CNC MACHINES

( 1 ) Historical Perspective

The word NC which stands for numerical control refer to control of a machine

or a process using symbolic codes consisting of characters and numerals. The

word CNC came into existence in seventies when microprocessors and

microcomputers replaced integrated circuit IC based controls used for NC

machines. The development of numerical control owes much to the United

States air force. The concept of NC was proposed in the late 1940s by John

Parsons who recommended a method of automatic machine control that would

guide a milling cutter to produce a curvilinear motion in order to generate

smooth profiles on the work-pieces. In 1949, the U.S Air Force awarded

Parsons a contract to develop new type of machine tool that would be able to

speed up production methods.

Parsons sub-contracted the Massachusetts Institute of Technology (MIT) to

develop a practical implementation of his concept. Scientists and engineers at

M.I.T built a control system for a two axis milling machine that used a

perforated paper tape as the input media. This prototype was produced by

retrofitting a conventional tracer mill with numerical control servomechanismsfor the three axes of the machine. By 1955, these machines were available to

industries with some small modifications. The machine tool builders gradually

began developing their own projects to introduce commercial NC units. Also,

certain industry users, especially airframe builders, worked to devise numerical

control machines to satisfy their own particular production needs. The Air force

continued its encouragement of NC development by sponsoring additional

research at MIT to design a part programming language that could be used in

controlling N.C. machines.

In a short period of time, all the major machine tool manufacturers were

producing some machines with NC, but it was not until late 1970s that

computer-based NC became widely used. NC matured as an automation

technology when electronics industry developed new products. At first,

miniature electronic tubes were developed, but the controls were big, bulky, and

not very reliable. Then solid-state circuitry and eventually modular or integrated

circuits were developed. The control unit became smaller, more reliable, and


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less expensive circuits were developed. The control unit became smaller, more

reliable, and less expensive.

(2) Computer Numerical Control

Computer numerical control (CNC) is the numerical control system in which a

dedicated computer is built into the control to perform basic and advanced NC

functions. CNC controls are also referred to as soft-wired NC systems because

most of their control functions are implemented by the control software

programs. CNC is a computer assisted process to control general purpose

machines from instructions generated by a processor and stored in a memory

system. It is a specific form of control system where position is the principal

controlled variable. All numerical control machines manufactured since the

seventies are of CNC type. The computer allows for the following: storage of

additional programs, program editing, running of program from memory,

machine and control diagnostics, special routines, inch/metric,

incremental/absolute switchability.

CNC machines can be used as stand alone units or in a network of machines

such as flexible machine centres. The controller uses a permanent resident

program called an executive program to process the codes into the electrical

pulses that control the machine. In any CNC machine, executive program

resides in ROM and all the NC codes in RAM. The information in ROM is

written into the electronic chips and cannot be erased and they become active

whenever the machine is on. The contents in RAM are lost when the controller

is turned off. Some use special type of RAM called CMOS memory, which

retains its contents even when the power is turned off

Figure 21.1: CNC milling machine


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( 1.3 ) Direct Numerical Control

In a Direct Numerical Control system (DNC), a mainframe computer is used to

coordinate the simultaneous operations of a number NC machines as shown in

the figures 21.2 & 21.3. The main tasks performed by the computer are to program and edit part programs as well as download part programs to NC

machines. Machine tool controllers have limited memory and a part program

may contain few thousands of blocks.So the program is stored in a separate

computer and sent directly to the machine, one block at a time.

First DNC system developed was Molins System 24 in 1967 by Cincinnati

Milacron and General Electric. They are now referred to as flexible

manufacturing systems (FMS). The computers that were used at those times

were quite expensive.

Figure 21.2: DNC system

Figure 21.3: DNC system


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21.4 Advantages & Disadvantages of CNC machine tools.

Some of the dominant advantages of the CNC machines are:

CNC machines can be used continuously and only need to be switched

off for occasional maintenance.

These machines require less skilled people to operate unlike manual

lathes / milling machines etc.

CNC machines can be updated by improving the software used to drive

the machines.

Training for the use of CNC machines can be done through the use of

'virtual software'.

The manufacturing process can be simulated virtually and no need to

make a prototype or a model. This saves time and money.

Once programmed, these machines can be left and do not require anyhuman intervention, except for work loading and unloading.

These machines can manufacture several components to the required

accuracy without any fatigue as in the case of manually operated

machines.

Savings in time that could be achieved with the CNC machines are quite

significant.

Some of the disadvantages of the CNC machines are:

CNC machines are generally more expensive than manually operated

machines.

The CNC machine operator only needs basic training and skills, enough

to supervise several machines.

Increase in electrical maintenance, high initial investment and high per

hour operating costs than the traditional systems.

Fewer workers are required to operate CNC machines compared to manually

operated machines. Investment in CNC machines can lead to unemployment.

CNC Coordinate Measuring Machines:

A coordinate measuring machine is a dimensional measuring device, designed

to move the measuring probe to determine the coordinates along the surface of

the work piece. Apart from dimensional measurement, these machines are also

used for profile measurement, angularity, digitizing or imaging.

A CMM consists of four main components: the machine, measuring probe,


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control system and the measuring software. The control system in a CMM

performs the function of a live interaction between various machine drives,

displacement transducers, probing systems and the peripheral devices. Control

systems can be classified according to the following groups of CMMs.

1. Manually driven CMMs.

2. Motorized CMMs with automatic probing systems

3. Direct computer controlled (DCC) CMMs

4. CMMs linked with CAD, CAM and FMS etc.

The first two methods are very common and self explanatory. In the case of

DCC CMMs, the computer control is responsible for the movement of theslides, readout from displacement transducers and data communication. CMM

are of different configurations-fixed bridge, moving bridge, cantilever arm

figure 21.5(a), horizontal arm and gantry type CMM as shown in figure 21.5(b).

Figure 21.5(a) Cantilever type CMM

Figure 21.5(b) Gantry type CMM

CNC EDM & WEDM machines:

EDM is a nontraditional machining method primarily used to machine hardmetals that could not be machined by traditional machining methods. Material


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removal will be taking place by a series of electric arcs discharging across the

gap between the electrode and the work piece. There are two main types- ram

EDM & wire cut EDM. In wire-cut EDM, a thin wire is fed through the work

piece and is constantly fed from a spool and is held between upper and lower

guides. These guides move in the x-y plane and are precisely controlled by the

CNC. Wire feed rate is also controlled by the CNC.

Figure 21.6 (a) Ram EDM Figure 21.6 (b) Wire cut EDM


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CLASSIFICATION OF CNC MACHINE TOOLS

( 1) Based on the motion type ' Point-to-point & Contouring systems

There are two main types of machine tools and the control systems required for use with them

differ because of the basic differences in the functions of the machines to be controlled. They

are known as point-to-point and contouring controls.

( 1.1) Point-to-point systems.

Some machine tools for example drilling, boring and tapping machines etc, require the cutter

and the work piece to be placed at a certain fixed relative positions at which they must remain

while the cutter does its work. These machines are known as point-to-point machines as

shown in figure 22.1 (a) and the control equipment for use with them are known as point-to-

point control equipment. Feed rates need not to be programmed. In theses machine tools,

each axis is driven separately. In a point-to-point control system, the dimensional information

that must be given to the machine tool will be a series of required position of the two slides.

Servo systems can be used to move the slides and no attempt is made to move the slide until

the cutter has been retracted back.

( 1.2) Contouring systems (Continuous path systems).

Other type of machine tools involves motion of work piece with respect to the cutter while

cutting operation is taking place. These machine tools include milling, routing machines etc.

and are known as contouring machines as shown in figure 22.1 (b) and the controls required

for their control are known as contouring control.Contouring machines can also be used as point-to-point machines, but it will be

uneconomical to use them unless the work piece also requires having a contouring operation

to be performed on it. These machines require simultaneous control of axes. In contouring

machines, relative positions of the work piece and the tool should be continuously controlled.

The control system must be able to accept information regarding velocities and positions of

the machines slides. Feed rates should be programmed.

Figure 22.1 (a) Point-to-point system Figure 22.1 (b) Contouring system


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Figure 22.1 (c) Contouring systems

22.2 Based on the control loops ' Open loop & Closed loop systems

22.2.1 Open loop systems:

Programmed instructions are fed into the controller through an input device. These

instructions are then converted to electrical pulses (signals) by the controller and sent to the

servo amplifier to energize the servo motors. The primary drawback of the open-loop system

is that there is no feedback system to check whether the program position and velocity has

been achieved. If the system performance is affected by load, temperature, humidity, or

lubrication then the actual output could deviate from the desired output. For these reasons the

open -loop system is generally used in point-to-point systems where the accuracy

requirements are not critical. Very few continuous-path systems utilize open-loop control.

Figure 22.2 (a) Open loop control system Figure 22.2 (b) Closed loop control system


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Figure 22.2 (c) Open loop system

22.2.1 Closed loop systems

The closed-loop system has a feedback subsystem to monitor the actual output and correct

any discrepancy from the programmed input. These systems use position and velocity feed

back. The feedback system could be either analog or digital. The analog systems measure the

variation of physical variables such as position and velocity in terms of voltage levels. Digitalsystems monitor output variations by means of electrical pulses. To control the dynamic

behavior and the final position of the machine slides, a variety of position transducers are

employed. Majority of CNC systems operate on servo mechanism, a closed loop principle. If

a discrepancy is revealed between where the machine element should be and where it actually

is, the sensing device signals the driving unit to make an adjustment, bringing the movable

component to the required location.

Closed-loop systems are very powerful and accurate because they are capable of monitoring

operating conditions through feedback subsystems and automatically compensating for any

variations in real-time.

Figure 22.2 (d) Closed loop system


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(3 ) Based on the number of axes ' 2, 3, 4 & 5 axes CNC machines.

( 3.1) 2& 3 axes CNC machines

CNC lathes will be coming under 2 axes machines. There will be two axes along which

motion takes place. The saddle will be moving longitudinally on the bed (Z-axis) and thecross slide. moves transversely on the saddle (along X-axis). In 3-axes machines, there will

be one more axis, perpendicular to the above two axes. By the simultaneous control of all the

3 axes, complex surfaces can be machined

( 3.2 ) 4 & 5 axes CNC machines:

4 and 5 axes CNC machines provide multi-axis machining capabilities beyond the standard 3-

axis CNC tool path movements. A 5-axis milling centre includes the three X, Y, Z axes, the

A axis which is rotary tilting of the spindle and the B-axis, which can be a rotary index table.

Importance of higher axes machining :

Reduced cycle time by machining complex components using a single setup. In addition to

time savings, improved accuracy can also be achieved as positioning errors between setups

are eliminated.

Improved surface finish and tool life by tilting the tool to maintain optimum tool to

part contact all the times.

Improved access to under cuts and deep pockets. By tilting the tool, the tool can be

made normal to the work surface and the errors may be reduced as the major

component of cutting force will be along the tool axis.

Higher axes machining has been widely used for machining sculptures surfaces in aerospace

and automobile industry.


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( 1 ) Different components related to CNC machine tools.

Any CNC machine tool essentially consists of the following parts:

( 1.1 ) Part program:

A part program is a series of coded instructions required to produce a part. It controls the

movement of the machine tool and on/off control of auxiliary functions such as spindle

rotation and coolant. The coded instructions are composed of letters, numbers and symbols.

( 1.2 ) Program input device:

The program input device is the means for part program to be entered into the CNC control.

Three commonly used program input devices are punch tape reader, magnetic tape reader,

and computer via RS-232-C communication.

( 1.3 ) Machine Control Unit:

The machine control unit (MCU) is the heart of a CNC system. It is used to perform the

following functions:

To read the coded instructions.

To decode the coded instructions.

To implement interpolations (linear, circular, and helical) to generate axis motion

commands.

To feed the axis motion commands to the amplifier circuits for driving the axis

mechanisms. To receive the feedback signals of position and speed for each drive axis.

To implement auxiliary control functions such as coolant or spindle on/off and tool

change.

( 1.4 ) Drive System:

A drive system consists of amplifier circuits, drive motors, and ball lead-screws. The MCU

feeds the control signals (position and speed) of each axis to the amplifier circuits. The

control signals are augmented to actuate drive motors which in turn rotate the ball lead-

screws to position the machine table.

( 1.5 ) Machine Tool.

CNC controls are used to control various types of machine tools. Regardless of which type

of machine tool is controlled, it always has a slide table and a spindle to control of position

and speed. The machine table is controlled in the X and Y axes, while the spindle runs along

the Z axis.

( 1.6 ) Feed Back System.

The feedback system is also referred to as the measuring system. It uses position and speed

transducers to continuously monitor the position at which the cutting tool is located at anyparticular instant. The MCU uses the difference between reference signals and feedback


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signals to generate the control signals for correcting position and speed errors.

( 2 ) Machine axes designation.

Machine axes are designated according to the "right-hand rule", When the thumb of righthand points in the direction of the positive X axis, the index finger points toward the

positive Y axis, and the middle finger toward the positive Z axis. Figure 10 shows the right-

hand rule applied to vertical machines, while Figure 23.1 applies to horizontal machines

Figure 23.1: Right hand rule for vertical and horizontal machine

CNC SYSTEMS - ELECTRICAL COMPONENTS


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(1) Power units

In machine tools, power is generally required for

For driving the main spindle

For driving the saddles and carriages. For providing power for some ancillary units.

The motors used for CNC system are of two kinds

Electrical - AC , DC or Stepper motors

Fluid - Hydraulic or Pneumatic

Electric motors are by far the most common component to supply mechanical input to a

linear motion system. Stepper motors and servo motors are the popular choices in linear

motion machinery due to their accuracy and controllability. They exhibit favourabletorque-speed characteristics and are relatively inexpensive.`

(1.1) Stepper motors

Stepper motors convert digital pulse and direction signals into rotary motion and are easily

controlled. Although stepper motors can be used in combination with analog or digital

feedback signals, they are usually used without feedback (open loop). Stepper motors require

motor driving voltage and control electronics. The rotor of a typical hybrid stepper motor has

two soft iron cups that surround a permanent magnet which is axially magnetized. The rotor

cups have 50 teeth on their surfaces and guide the flux through the rotor- stator air gap. In

most cases, the teeth of one set are offset from the teeth of the other by one-half tooth pitch

for a two phase stepper motor

Figure 24.1 Unipolar and Bipolar Stepper Motor

The stator generally has the same number of teeth as the rotor, but can have two fewerdepending upon the motor's design. When the teeth on the stator pole are energized with


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North polarity, the corresponding teeth on the rotor with South polarity align with them.

Similarly, teeth on the stator pole energized with South polarity attract corresponding teeth on

the rotor that are energized with North polarity. By changing the polarity of neighbouring

stator teeth one after the other in a rotating sequence, the rotor begins to turn correspondingly

as its teeth try to align themselves with the stator teeth. The strength of the magnetic fieldscan be precisely controlled by the amount of current through the windings, thus the position

of the rotor can be precisely controlled by these attractive and repulsive forces.

There are many advantages to using stepper motors. Since maximum dynamic torque occurs

at low pulse rates (low speeds), stepper motors can easily accelerate a load. Stepper motors

have large holding torque and stiffness, so there is usually no need for clutches and brakes

(unless a large external load is acting, such as gravity). Stepper motors are inherently digital.

The number of pulses determines position while the pulse frequency determines velocity.

Additional advantages are that they are inexpensive, easily and accurately controlled, and

there are no brushes to maintain. Also, they offer excellent heat dissipation, and they are very

stiff motors with high holding torques for their size. The digital nature of stepper motors also

eliminates tuning parameters.

There are disadvantages associated with stepper motors. One of the largest disadvantages is

that the torque decreases as velocity is increased. Because most stepper motors operate open

loop with no position sensing devices, the motor can stall or lose position if the load torque

exceeds the motor's available torque. Open loop stepper motor systems should not be used for

high-performance or high-load applications, unless they are significantly derated. Another

drawback is that damping may be required when load inertia is very high to prevent motorshaft oscillation at resonance points. Finally, stepper motors may perform poorly in high-

speed applications. The maximum steps/sec rate of the motor and drive system should be

considered, carefully

( 1.2) Servo Motors.

Servo motors are more robust than stepper motors, but pose a more difficult control problem.

They are primarily used in applications where speed, power, noise level as well as velocity

and positional accuracy are important. Servo motors are not functional without sensor

feedback. They are designed and intended to be applied in combination with resolvers,tachometers, or encoders (closed loop). There are several types of servo motors, and three of

the more common types are described as follows. The DC brush type servo motors are most

commonly found in low-end to mid-range CNC machinery. The "brush" refers to brushes that

pass electric current to the rotor of the rotating core of the motor. The construction consists of

a magnet stator outside and a coil rotor inside. A brush DC motor has more than one coil.

Each coil is angularly displaced from one another so when the torque from one coil has

dropped off, current is automatically switched to another coil which is properly located to

produce maximum torque. The switching is accomplished mechanically by the brushes and a

commutator as shown below.


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There are distinct advantages to using DC brush servo motors. They are very inexpensive to

apply. The motor commutates itself with the brushes and it appears as a simple, two-terminal

device that is easily controlled. Among the disadvantages it is the fact that they are thermally

inefficient, because the heat must dissipate through the external magnets. This condition

reduces the torque to volume ratio, and the motor performance may suffer inefficiencies.Also, the brushed motor will require maintenance, as the brushes will wear and need

replacement. Brushed servo motors are usually operated under 5000 rpm.

The DC brushless type offers a higher level of performance. They are often referred to as

"inside out" DC motors because of their design. The windings of a brushless motor are

located in the outer portion of the motor (stator), and the rotor is constructed from permanent

magnets as shown below. DC brushless motors are typically applied to high-end CNC

machinery, but the future may see midrange machinery use brushless technology due to the

narrowing cost gap.

AC servo motors are another variety that offers high-end performance. Their physical

construction is similar to that of the brushless DC motor; however, there are no magnets in

the AC motor. Instead, both the rotor and stator are constructed from coils. Again, there are

no brushes or contacts anywhere in the motor which means they are maintenance-free. They

are capable of delivering very high torque at very high speeds; they are very light and there is

no possibility of demagnetization.

.However, due to the electronic commutation, they are extremely complex and expensive to

control. Perhaps the largest advantage of using servo motors is that they are used in closedloop form, which allows for very accurate position information and also allows for high

output torque to be realized at high speeds. The motor will draw the required current to

maintain the desired path, velocity, or torque, and is controlled according to the requirements

of the application rather than by the limitations of the motor. Servo motors put out enormous

peak torque at or near stall conditions. They provide smooth, quiet operation, and depending

upon the resolution of the feedback mechanism, can have very small resolutions. Among the

disadvantages of servo motors are the increased cost, the added feedback component, and the

increased control complexity. The closed loop feature can be a disadvantage for the case

when there is a physical obstacle blocking the path of motion. Rather than stalling, the servo

motor will continue to draw current to overcome the obstacle. As a result, the system

hardware, control electronics, signal amplifier and motor may become damaged unless safety

precautions are taken.

( 2 ) Encoders

An encoder is a device used to change a signal or data into a code. These encoders are used in

metrology instruments and high precision machining tools ranging from digital calipers to

CNC machine tools.

( 2.1) Incremental encoders.


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With incremental linear encoders, the current position is determined by stating a datum and

counting measuring steps. The output signals of incremental rotary encoders are evaluated by

an electronic counter in which the measured value is determined by counting "increments".

These encoders form the majority of all rotary encoders. Incremental rotary encoders with

integral couplings used for length measurement are also in the market.

The resolution of these encoders can be increased by means of electronic interpolation. There

are, of course, the precision rotary encoders specifically designed for angle measurement. If

finer resolution is required, standard rotary encoders often utilize electronic signal

interpolation. Rotary encoders for applications in dividing heads and rotary tables, with very

small measuring steps (down to 0.36 arc second) have in principle the same basic design

features as standard rotary encoders, but incorporate some overall varying construction.

( 2.2 ) Absolute encoders.

Absolute linear encoders require no previous transfer to provide the current position value.

Absolute rotary encoders provide an angular position value which is derived from the pattern

of the coded disc. The code signal is processed within a computer or in a numerical control.

After system switch-on, such as following a power interruption, the position value is

immediately available. Since these encoder types require more sophisticated optics and

electronics than incremental versions, a higher price is normally to be expected. Apart from

these two codes, a range of other codes have been employed, though they are losing their

significance since modern computer programs usually are based on the binary system for

reasons of high speed. There are many versions of absolute encoders available today, such as

single-turn or multi-stage versions to name only two, and each must be evaluated based on its

intended application.

( 2.3 ) Rotary and Linear encoders.

A linear encoder is a sensor, transducer paired with a scale that encodes position. The sensor

reads the scale in order to convert the encoded position by a digital readout (DRO). Linear

encoder technologies include capacitive, inductive, eddy current, magnetic and optical.

A rotary encoder, also called a shaft encoder, is an electro-mechanical device used to convert

the angular position of a shaft to a digital code, making it a sort of a transducer.Rotary encoders serve as measuring sensors for rotary motion, and for linear motion when

used in conjunction with mechanical measuring standards such as lead screws. There are two

main types: absolute and relative rotary encoders. Incremental rotary encoder uses a disc

attached to a shaft. The disc has several radial lines. An optical switch, such as a photodiode,

generates an electric pulse whenever one of the lines passes through its field of view. An

electronic control circuit counts the pulses to determine the angle through which the shaft has

turned.

As the present trend of machine tools evolves toward increasingly higher accuracy and

resolution, increased reliability and speeds, and more efficient working ranges, so too must


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feedback systems. Currently, linear feedback systems are available that will achieve

resolutions in the submicron range.

Figure 24.3: Exposed and sealed linear encoders.

Submicron resolutions, for example, are required in the semiconductor industry and in ultra-

precision machining. Achieving these resolutions is possible with the use of linear scaleswhich transmit displacement information directly to a digital readout. As in rotary, linear

scales operate on the same photoelectric scanning principle, but the linear scales are

comprised in an overall straight construction, and their output signals are interpolated or

digitized differently in a direct manner. One of these signals is always used by the

accompanying digital readout or numerical control to determine and establish home position

on the linear machine axis in case of a power interruption or for workpiece referencing.

Overall, there are two physical versions of a linear scale: exposed or enclosed as shown in the

figure 24.3. With an enclosed or "sealed" scale, the scanning unit is mounted on a small

carriage guided by ball bearings along the glass scale; the carriage is connected to the

machine slide by a backlash-free coupling that compensates for alignment errors between the

scale and the machine tool guide ways.

A set of sealing lips protects the scale from contamination. The typical applications for the

enclosed linear encoders are primarily machine tools. Exposed linear encoders also consist of

a glass scale and scanning unit, but the two components are physically separated. The typical

advantages of the non-contact system are easier mounting and higher traversing speeds since

no contact or friction between the scanning unit and scale exists. Exposed linear scales can be

found in coordinate measuring machines, translation stages, and material handling

equipment.

Another version of the scale and scanning unit arrangement is one that uses a metal base

rather than glass for the scale. With a metal scale, the line grating is a deposit of highly

reflective material such as gold that reflects light back to the scanning unit onto the

photovoltaic cells. The advantage of this type of scale is that it can be manufactured in

extremely great lengths, up to 30 meters, for larger machines. Glass scales are limited in

length, typically three meters. There are several mechanical considerations that need to be

understood when discussing linear encoders. It is not a simple matter to select an encoder

based just on length or dimensional profile and install the encoder onto a machine. These


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characteristic considerations include permissible traversing speeds, accuracy and resolution

requirements, thermal behaviour and mounting guidelines.

Figure 24.4: Principle of rotary and linear encoders.

( 3 ) CNC Controller.

There are two types of CNC controllers, namely closed loop and open loop controllers. These

have been discussed in details in section 22.2.

( 3.1 ) Controller Architecture:

Most of the CNC machine tools were built around proprietary architecture and could not be

changed or updated without an expensive company upgrade. This method of protecting their

market share worked well for many years when the control technology enjoyed a four-to-five

year life cycle. Now a day the controller life cycle is only eight-to-twelve months. So CNC

manufacturers are forced to find better and less expensive ways of upgrading their

controllers.

Open architecture is the less costly than the alternatives. GE Fanuc and other manufacturers

introduced control architecture with PC connectivity to allow users to take advantage of the

new information technologies that were slowly gaining acceptance on the shop floor. They

created an open platform that could easily communicate with other devices over

commercially available MS Windows operating system, while maintaining the performance

and reliability of the CNC machine tool.


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CNC SYSTEMS - MECHANICAL COMPONENTS.

The drive units of the carriages in NC machine tools are generally the screw & the nut

mechanism. There are different types of screws and nuts used on NC machine tools which

provide low wear, higher efficiency, low friction and better reliability.

(1) Recirculating ball screw.

The recirculating ball screw assembly shown in figure 25.1 has the flanged nut attached to the

moving chamber and the screw to the fixed casting. Thus the moving member will move

during rotational movement of the screw. These recirculating ball screw designs can have ball

gages of internal or external return, but all of them are based upon the "Ogival" or "Gothic

arc".

In these types of screws, balls rotate between the screw and nut and convert the sliding

friction (as in conventional nut & screw) to the rolling friction. As a consequence wear will

be reduced and reliability of the system will be increased. The traditional ACME thread used

in conventional machine tool has efficiency ranging from 20% to 30% whereas the efficiency

of ball screws may reach up to 90%.

Figure 25.1: Recirculating ball screw assembly

Figure 25.2: Preloaded recirculating ball screw

There are two types of ball screws. In the first type, balls are returned through an external

tube after few threads. In another type, the balls are returned to the start through a channel

inside the nut after only one thread. To make the carriage movement bidirectional, backlash

between the screw and nut should be minimum. One of the methods to achieve zero backlash


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is by fitting two nuts. The nuts are preloaded by an amount which exceeds the maximum

operating load. These nuts are either forced apart or squeezed together, so that the balls in one

of the nuts contact the opposite side of the threads.

These ball screws have the problem that minimum diameter of the ball (60 to 70% of the leadscrew) must be used, limiting the rate of movement of the screw.

(2) Roller screw

Figure 25.3: Roller screw.

These types of screws provide backlash-free movement and their efficiency is same as that of

ball screws. These are capable of providing more accurate position control. Cost of the roller

screws are more compared to ball screws. The thread form is triangular with an included

angle of 90 degrees. There are two types of roller screws: planetary and recirculating screws.

Planetary roller screws.

Planetary roller screws are shown in figure 25.3. The rollers are threaded with a single start

thread. Teeth are cut at the ends of the roller, which meshes with the internal tooth cut inside

the nut. The rollers are equally spaced around and are retained in their positions by spigots or

spacer rings. There is no axial movement of the rollers relative to the nut and they are capable

of transmitting high load at fast speed.

Recirculating roller screws:

The rollers in this case are not threaded and are provided with a circular groove and are

positioned circumferentially by a cage. There is some axial movement of the rollers relative

to the nut. Each roller moves by a distance equal to the pitch of the screw for each rotation ofthe screw or nut and moves into an axial recess cut inside the nut and disengage from the


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threads on the screw and the nut and the other roller provides the driving power. Rollers in

the recess are moved back by an edge cam in the nut. Recirculating roller screws are slower

in operation, but are capable of transmitting high loads with greater accuracy.

(1) Tool changing arrangements

There are two types of tool changing arrangements: manual and automatic. Machining

centres incorporate automatic tool changer (ATC). It is the automatic tool changing capability

that distinguishes CNC machining centres from CNC milling machines.

(1.1) Manual tool changing arrangement:Tool changing time belongs to non-productive time. So, it should be kept as minimum as

possible. Also the tool must be located rigidly and accurately in the spindle to assure proper

machining and should maintain the same relation with the work piece each time. This is

known as the repeatability of the tool. CNC milling machines have some type of quick tool

changing systems, which generally comprises of a quick release chuck. The chuck is a

different tool holding mechanism that will be inside the spindle and is operated either

hydraulically or pneumatically. The tool holder which fits into the chuck can be released by

pressing a button which releases the hydraulically operated chuck. The advantage of manual

tool changing is that each tool can be checked manually before loading the tools and there will

be no limitation on the number of tools from which selection can be made.

(1.2) Automatic tool changing arrangement

Tooling used with an automatic tool changer should be easy to center in the spindle, each for

the tool changer to grab the tool holder and the tool changer should safely disengage the tool

holder after it is secured properly. Figure 27.1 shows a tool holder used with ATC. The tool

changer grips the tool at point A and places it in a position aligned with the spindle. The tool

changer will then insert the tool holder into the spindle. A split bushing in the spindle will

enclose the portion B. Tool changer releases the tool holder. Tool holder is drawn inside the

spindle and is tightened.


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Figure 27.1: Tool holder

( 2) Tool turrets

An advantage of using tool turrets is that the time taken for tool changing will be

only the time taken for indexing the turret. Only limited number of tools can be

held in the turret. Tool turrets shown in figure 27.2 a, b & c are generally used in

lathes. The entire turret can be removed from the machine for setting up of

tools.

Figure 27.2(a): Six station tool turret


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Figure 27.2(b): Eight station tool turret

Figure 27.2(c): Twelve station tool turret

( 3 ) Tool magazines

Tool magazines are generally found on drilling and milling machines. When compared to toolturrets, tool magazines can hold more number of tools and also more problems regarding the

tool management. Duplication of the tools is possible and a new tool of same type may be

selected when ever a particular tool has been worn off. Though a larger tool magazine can

accommodate more number of tools, but the power required to move the tool magazine will

be more. Hence, a magazine with optimum number of tool holders must be used. The

following types of tool magazines exist: circular, chain and box type.

( 3.1 ) Chain magazine

These magazines can hold large number of tools and may hold even up to 100 tools. Figures

27.3 a & b show chain magazines holding 80 and 120 tools respectively. In these chainmagazines, tools will be identified either by their location in the tool holder or by means of


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some coding on the tool holder. In the former it is followed for identifying the tool and then

the tool must be exactly placed in its location. The positioning of the magazine for the next

tool transfer will take place during the machining operation.

Figure 27.3 (a) 80-tool chain magazine Figure 27.3 (b) 120-tool chain magazine

( 3.2) Circular magazine:

Circular magazines shown in figure 27.4 will be similar to tool turrets, but in the former the

tools will be transferred from the magazine to the spindle nose. Generally these will be

holding about 30 tools. The identification of the tool will be made either by its location in the


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tool magazine or by means of some code on the tool holder. The most common type of

circular magazine is known as carousel, which is similar to a flat disc holding one row of tools

around the periphery. Geneva mechanism is used for changing the tools.

Figuure 27.4: Circular magazine

( 3.3 ) Box magazine:

In these magazines, the tools are stored in open ended compartments. The tool holder must be

removed from the spindle before loading the new tool holder. Also the spindle should move to

the tool storage location rather than the tool to the spindle. Hence, more time will be

consumed in tool changing. Box magazines are of limited use as compared to circular and

chain type of tool magazines.

( 4 ) Automatic tool changers :

Whenever controller encounters a tool change code, a signal will be sent to the control unit so

that the appropriate tool holder in the magazine comes to the transfer position. The tool holder

will then be transferred from the tool magazine to the spindle nose. This can be done by

various mechanisms. One such mechanism is a rotating arm mechanism.

Rotating arm mechanism:

Movement of the tool magazine to place the appropriate tool in the transfer position will takeplace during the machining operation. The rotating arms with grippers at both the ends rotate

to grip the tool holders in the magazine and the spindle simultaneously. Then the tool holder

clamping mechanism will be released and the arm moves axially to remove the tool holder

from the spindle. Then the arm will be rotated through 180 degrees and the arm will then

move axially inwards to place the new tool holder into the spindle and will clamped. Now the

new tool holder is placed in the spindle and the other in the magazine. Figure 27.5 and 27.6

show various stages during tool change with a rotating arm mechanism.


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Figure 27.5: Rotating arm mechanism

Figure 27.6: Rotating arm mechanism

( 5 ) Tool wear monitoring

Most of the modern CNC machines now incorporate the facility of on-line tool wear

monitoring systems, whose purpose is to keep a continuous track of the amount of tool wear

in real time. These systems may reduce the tool replacement costs and the production delays.

It is based on the principle that the power required for machining increases as the cutting edge

gets worn off. Extreme limits for the spindle can be set up and whenever it is reached, a sub-

program can be called to change the tool. Following figures show some typical tool wear

monitoring systems.


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Figure 27.7: ON-line tool wear monitoring system

Figure 27.8 : Graphical display of tool wear monitoring system


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28. CNC WORK HOLDING DEVICES

With the advent of CNC technology, machining cycle times were drastically

reduced and the desire to combine greater accuracy with higher productivity has

led to the reappraisal of work holding technology. Loading or unloading of the

work will be the non-productive time which needs to be minimized. So the workis usually loaded on a special work holder away from the machine and then

transferred it to the machine table. The work should be located precisely and

secured properly and should be well supported.

28.1 Turning center work holding methods:

Machining operations on turning centers or CNC lathes are carried out mostly for

axi-symmetrical components. Surfaces are generated by the simultaneous

motions of X and Z axes. For any work holding device used on a turning centre

there is a direct "trade off" between part accuracy and the flexibility of work

holding device used.

Work holding

methodsAdvantages Disadvantages

Automatic Jaw &

chuck changing

Adaptable for a range of work-

piece shapes and sizes

High cost of jaw/chuck changing

automation. Resulting in a more

complex & higher cost machine tool

Indexing chucks

Figure 28.1

Very quick loading and

unloading of the workpiece can

be achieved. Reasonable range

of work piece sizes can be

loaded automatically

Expensive optional equipment. Bar-

feeders cannot be incorporated.

Short/medium length parts only can be

incorporated. Heavy chucks.

Pneumatic/Magnetic Simple in design and relatively Limited to a range of flat parts with


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chucks

Figure 28.3

inexpensive. Part automation is

possible. No part distortion is

caused due to clamping force

little overhang. Bar-feeders cannot be

incorporated. Parts on magnetic chucks

must be ferrous. Heavy cuts must be

avoided.

Automatic Chucks

with soft jaws

Adaptable to automation. Heavy

cuts can be taken. Individual

parts can be small or large in

diameter

Jaws must be changed manually &bared, so slow part change-overs. A

range of jaw blanks required.

Expanding mandrels

& collets

Figure 28.2

Long & short parts of

reasonably large size

accommodated. Automation can

be incorporated. Clamping

forces do not distort part.

Simple in design

Limitation on part shape. Heavy cuts

should be avoided.

Dedicated Chucks

Excellent restraint & location ofa wide range of individual &

irregular -shaped parts can be

obtained.

Expensive & can only be financially

justified with either large runs or when

extremely complex & accurate parts

are required. Tool making facilities

required. Large storage space.

( 2) Work holding for Machining Centres:

Workholding methods Advantages Disadvantages

Modular Fixtures

Figure 28.6

Highly adaptable. Can be purchased in

stages to increase its sophistication.

Reasonable accuracy. Speedily

assembled. Small stores area is required.

Can be set-up to a machine more than one

part. Proven technology

Costly for a complete

system. Difficult to

automate. Skills required in

kit assembly


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Automatic Vices

Relatively inexpensive. Can be operated

by mechanical, pneumatic, or by

hydraulic control. Quick to operate with

ease of set-up. Reasonable accuracy.

Easily automated. Simplicity of design.

Using multi-vices allows many parts to bemachined. Proven Technology

Work holding limitations.

Clamping force limitations.

Jaws can become strained.

Work location problems.

Limitations on part size.

Pneumatic/Magnetic

Work holding devices

Relatively inexpensive. Reasonable

accuracy. Can machine large areas of the

work piece. Quick setups. Easily

automated. Simplicity of design. Many

parts can be machined at one set up.

Large surface area is

required. Swarf can be a

problem. Nonferrous

material limitation on

magnetic devices.

4/5 axis CNC work

holding devices

Allows complex geometric shapes to be

machined. High accuracy. Opportunity

for "one hit" machining. Easilyautomated.

Costly & limited part

geometry clamping. Part

size limitations. Usually

only one part can bemachined. Cannot be fitted

to all machines.

Dedicated Fixturing

Large & small parts are easily

accommodated. High accuracy of part

location. Easily automated. Simplicity of

design. Proven technology. Many parts

can be machine at one setup good

vibration damping capacity

Large storage space

required. No part flexibility.

Heavy fixtures. Tool

making facilities required.

Figure 28.1: Indexing chucks


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Figure 28.2: Mandrels

Figure 28.3: Magnetic chucks

Figure 28.4: Vise


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Figure 28.5(a): Pallets

Figure 28.5(b) Figure 28.5(c)


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Figure 28.6 : Modular fixture


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Figure 28.7 : Chucks

an overview of cnc machines

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