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28. CNC WORK HOLDING DEVICESWith the advent of CNC technology, machining cycle times were drastically reducedand the desire to combine greater accuracy with higher productivity has led to thereappraisal of work holding technology. Loading or unloading of the work will be thenon-productive time which needs to be minimized. So the work is usually loaded on

a special work holder away from the machine and then transferred it to the machinetable. The work should be located precisely and secured properly and should be wellsupported.

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 Xand 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 holdingmethods

Advantages Disadvantages

Automatic Jaw &chuck changing

Adaptable for a range ofwork-piece shapes andsizes

High cost of jaw/chuck changing

automation. Resulting in a morecomplex & higher cost machinetool

Indexing chucksFigure 28.1

Very quick loading andunloading of the workpiececan be achieved.Reasonable range of workpiece sizes can be loadedautomatically

Expensive optional equipment.Bar-feeders cannot beincorporated. Short/mediumlength parts only can beincorporated. Heavy chucks.

Pneumatic/Magnetic

chucksFigure 28.3

Simple in design and

relatively inexpensive. Partautomation is possible. No

Limited to a range of flat parts

with little overhang. Bar-feederscannot be incorporated. Parts on

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Small stores area is required. Canbe set-up to a machine more thanone part. Proven technology

Automatic Vices

Relatively inexpensive. Can beoperated by mechanical, pneumatic,or by hydraulic control. Quick tooperate with ease of set-up.Reasonable accuracy. Easilyautomated. Simplicity of design.Using multi-vices allows many partsto be machined. Proven Technology

Work holding limitations.Clamping forcelimitations. Jaws canbecome strained. Worklocation problems.Limitations on part size.

Pneumatic/MagneticWork holding devices

Relatively inexpensive. Reasonableaccuracy. Can machine large areasof the work piece. Quick setups.Easily automated. Simplicity of

design. Many parts can bemachined at one set up.

Large surface area isrequired. Swarf can be aproblem. Nonferrous

material limitation onmagnetic devices.

4/5 axis CNC workholding devices

Allows complex geometric shapesto be machined. High accuracy.Opportunity for "one hit" machining.Easily automated.

Costly & limited partgeometry clamping. Partsize limitations. Usuallyonly one part can bemachined. Cannot befitted to all machines.

Dedicated Fixturing

Large & small parts are easilyaccommodated. High accuracy of

part location. Easily automated.Simplicity of design. Proventechnology. Many parts can bemachine at one setup goodvibration damping capacity

Large storage space

required. No partflexibility. Heavyfixtures. Tool makingfacilities 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)

Figure 28.6 : Modular fixture

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Figure 28.7 : Chucks