Harold Hall

Workshop Processes

Soft Jaws, using
Soft Jaws, using
Soft Jaws for the 3 Jaw Chuck with added Hexagonal Jaws






Machining the base jaws is though a little more complex. Mark the outer face of the jaws with marking blue and fit them into the chuck, tightening them onto a plug, its diameter not being important. Mount a sharp pointed lathe tool onto the top slide and turning the chuck by hand use this to scribe a line across each jaw at the point they are to be bored for the hexagon jaws, Photograph 5. Remove the jaws and using a height gauge or surface gauge mark the other axis and centre punch each jaw. Accuracy is not vital as any differences will be compensated for provided that you always use the same auxiliary and main jaw together.


Mount the first jaw in the four jaw chuck and set the centre punch mark to run true using a centre finder, Photograph 6, and bore to size. The bores must be a very close fit on the auxiliary jaw bosses. Whilst still in the chuck drill and tap as per the drawing. Photograph 7 shows the jaws ready for use.


An easier option

If you consider that you are unlikely to require to remove a set of hexagonal jaws and then refit them for eventual reuse at the same diameter, then fitting them with a single screw as Sk. 6 may be worth considering. However, this is not foolproof, as when machining the jaw to the required diameter is carried out very little load will be placed on them but this will not be the case when the workpiece is finally fitted and the chuck tightened onto it. In this situation, the clearance between the screw and the hole in the jaw may under load allow the hexagonal jaw to move on the main jaw below it and as a result of this, accuracy may be compromised. Using a substantial socket head screw, say M6 minimum but preferably M8, will minimise the risk.


The problem can though be overcome by adopting the following method. Just prior to arriving at the required size loosen the chuck and remove the plug. The workpiece can then be fitted and the chuck tightened, this will force the hexagon jaws to take up the clearance between the fixing screw and the hole in the direction required. With the workpiece removed the plug can then be refitted and the jaws finally bored to size.