Worm driven rotary devices
There are two very important devices used on milling machines. These are to rotary table and the dividing head. what both of these have in common is that the workpiece is rotated by means of a worm and wormwheel arrangement. The advantage of this is that whatever force is applied by the cutter to the workpiece there is no way this can fed back to the drive of whatever is rotating the workpiece.
This means that a workpiece can be machined while it is being rotated.
The engagement between the worm and wormwheel can usually be adjusted. This should be done so the two can rotate freely but, at the same time, backlash is minimised.
It is seldom possible to eliminate any backlash completely but usually whilst the workpiece is rotating the backlash is taken up by the movement. Where an operation, such as drilling happens at a specific point on the workpiece then the spindle, in the case of the dividing head, or the table, in the case of the rotary table, should be locked.