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Machining soft jaws by Dimac Tooling

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According to Dimac Tooling , shops making their special top jaws in-house must apply a sound methodology for analysing job requirements because the selection, installation and qualification of the jaws will have a direct bearing on the success or failure of the machining application.

A good top jaw analysis would have to include a solid understanding of the machining forces and/or any external loads imposed on the workpiece during the machining process. It also requires an understanding of how these forces affect the top jaws and other system components, which can be determined by calculation, test data and experience with other like applications.

Most importantly, a properly installed set of top jaws should be qualified in terms of accuracy and precisely formed to fit the workpiece because the fit is paramount to achieving the best grip.

The initial fitting of the top jaws to the chuck prior to machining is important to achieve repeat accuracy. For all types except on chucks with fine serrated base jaws, the jaws should be loaded radially outward to take up any fitting clearance prior to tightening the jaw screws. This applies to top jaws, which are intended to hold a workpiece externally. The jaws should load inwards for internal gripping.

The chuck must always be clamped when the jaws are being machined. The chuck is operated several times gripping a suitable size piece of material to load the top jaws back, following which the base jaws are locked in their approximate mid-stroke position.

On chucks that accept collet pads, a suitable sized ring may be placed in the collet seating diameter of the chuck's base jaws. If extreme accuracy is not required the jaws may then be turned directly to the size required to suit the gripped workpiece diameter.

On long grip faces a back taper should be allowed to ensure that the jaws initially contact the workpiece at their outer end. A back taper of 0.076mm per 25mm on jaw height is adequate in most cases.

Where very accurate concentricity is required, top jaws should be machined as usual while accepting a suitable loading ring rather than the workpiece itself. The ring should be thin axially but of sufficient radial thickness to withstand the gripping force.

A diameter is machined to receive the ring at the outer end of the jaws with the chuck in its mid stroke condition and master jaws flush with the chuck body. The ring previously fitted to the base jaw collet seating diameter is removed after first opening the chuck. The jaws are then closed to grip the loading ring in the top jaws, effectively pre-loading the top jaws in a similar manner to the loading, which occurs when the workpiece is gripped.

The top jaws are machined to hold the workpiece. Since the top jaws are fully loaded during machining it is generally unnecessary to include any back taper on the gripping face as this occurs automatically. The jaws are then opened and the loading ring removed.

If the top step of the jaw used for holding the loading ring interferes with the workpiece then it may be machined away, else left to allow the jaws to be reskimmed at a later date with the loading ring in position.

It is important to load the top jaws in an identical manner when holding the ring as when gripping the workpiece to obtain high concentricity. Consequently, the same chuck operating pressure should be used and the ring should be held either externally or internally to suit the particular case.

Concentricity between a turned diameter and the gripped diameter of .025mm TIR can be achieved if the top jaws are machined carefully. Concentricity can be improved if the chuck is positioned with a single jaw rather than two jaws at the bottom when loading the workpiece. 

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