Centering aid for polygonal stock

[mklotz]

Centering polygonal stock in the 4jaw can be a trial. The centering should be done on the flat sides of the stock and not the corners. However, with the DI point bearing on the sides, one must be very careful to withdraw the DI plunger as the stock is rotated from side to side lest the plunger be 'caught' by a corner of the stock and bent, damaging the DI.

I use a dedicated DI mounted on a QCTP for aligning in the 4jaw. I simply slap it into position on the compound and the DI is automatically at center height.

To deal with the polygonal stock problem, I made a simple gizmo.

An aluminum rod clamps to the DI housing with a single setscrew. Attached to the bottom of this rod is an L-shaped piece of thin sheet steel that projects up in front of the DI plunger.

Now, as the polygonal stock is rotated, the stock corner contacts the sheet metal first and depresses the plunger as it passes, thus avoiding damage to the DI. A further advantage is that the motion of the sheet metal makes it obvious when the DI is perpendicular to the side of the stock - the position at which one wants to make the centering indication.