Monday, March 3, 2014

General Workholding Devices

Clamps and abutments;
Face plates.

Vices are used in  a number of machining processes including milling, drilling, shaping and planing among others. These can be elementary devices consisting of a pair of jaws, a screw mechanism and a body.
A variety of jaw shapes can be used with vices. The jaws themselves can be hydraulically actuated, although a hand screw is used to move the jaws to the workpiece and the hydraulics used only to provide the clamping force.

Clamps and abutments
The use of clamps is by far the simplest and cheapest of workholding devices. The workpiece is clamped directly to the machine table and typical applications include processes such as milling, drilling, shaping and planing. Abutments are simply locating blocks attached to the machine table to prevent the workpiece moving due to the cutting forces.

 Chucks are generally used for processes that involve rotational motion. The typical applications for chucks include turning, boring, drilling and grinding. There is a huge variety of chucks available from small
drill chucks to huge lathe chucks. The most common chucks are three-jaw chucks, which are generally self-centring, that is, all jaws move simultaneously when adjusted. Four-jaw chucks are also used and the jaws on these can be adjusted individually and can therefore be used to hold odd shapes as
well as square and rectangular shapes. They are also generally used for larger and heavier workpieces as they are of heavier construction. The jaws can be manually operated or pneumatically or hydraulically operated in the case of power chucks. The type of chuck used primarily depends on the production equipment being used, the workpiece size and the cutting forces involved.

Collets are primarily used for round workpieces and tools and typical applications include turning and boring. They can also be used for use with square or hexagonal workpieces. It is basically a longitudinally split tapered bushing. There are two basic types of collet and these are: a draw-back collet, where it is pulled into the spindle, or a push-out collet, where it is pushed into the spindle by mechanical means. Collets are generally used for workpieces with small diameters, typically less than 25 mm.

The main advantage of a collet when compared to a chuck is that it grips nearly all of the circumference of the workpiece.

These are used with lathes and is the method of workholding from which the centre lathe gets its name. The centres themselves are basically tempered shanks located in taper sockets and are generally used for long workpieces or when axial alignment is crucial.

Mandrels are generally used in conjunction with centres. The mandrel is placed inside tubular workpieces and generally mounted between two centres. They are generally used with turning when the entire cylindrical surface requires to be machined or both ends require to be machined. There are three basic types of mandrel, namely the solid mandrel, the gang mandrel and a
cone mandrel.

Face plates
Face plates are generally used in turning and boring to turn a diameter which is perpendicular to an existing datum surface. They are round plates with an arrangement of slots and holes used to clamp the workpiece to it. This means that irregular shaped workpieces can be machined and the arrangement may sometimes
require to be balanced.

Part of Process Planning Articles

Process Planning by Peter Scallan

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