Milling – dividing head – three or four jaw chucks
When using a dividing head the workpiece can be held between centers or by using a chuck. With a chuck, the workpiece can be held directly in the chuck or by holding the workpiece using a mandrel or arbor which is held in the chuck.
If the dividing head is fitted with a chuck should it be with a three jaw chuck or a four jaw chuck?
This choice is very similar to the same choice when a lathe is involved.
The prime reason for using a three jaw chuck is that it is very quick and easy to use to hold a workpiece. The main problem is that most three jaws are not always as accurate as needed.
The main reason for using a four jaw chuck is that it is possible to set it up very accurately but whether accuracy is required or not, it takes far longer to setup. It also has the advantages that it can be used to hold workpieces that are not round or it can hold round pieces that are offset.
Making backplates for dividing heads
Making a backplate for a dividing head is slightly different to making one for a lathe. When making one for a lathe the trick is to make the final surface by turning actually on the lathe it is going to be used on. This is because the surface will be flat and at right angles to the axis of the spindle.
When making a backplate for a dividing head this does not apply. However it can be machined on the milling machine. Firstly the dividing head has to be set up so the axis of its spindle is parallel to the milling table both horizontally and vertically. This is best done with a a test bar fitted to the dividing head.. When this has been done the backplate can be fitted to the dividing head. It can then be machined using an endmill. This will produce the flat surface. The round surface to fit the recess in the back of the chuck needs to be cut using an endmill with a flat cutting edge.