Pick-Off Collets
With the increasing use of pick-off collets with both standard and extended nose lengths (EN), there is a potential problem for users.
Problem
Generally, pick-off collets grip short part lengths that only occupy the front section of the bore. With all of the clamping force on the collet angle - which is a considerable distance from the part being held - the collet begins to collapse under the angle. At this point, the bore is no longer parallel and does not squarely grip the part, which usually results in the parts running eccentric and slipping in the collet. When this happens, the natural reaction is to increase the clamping pressure. Finally, the collet prematurely fails.
Another problem that is peculiar to pick-off collets is that they are sometimes closed without a part in place. Having no internal resistance puts undue strain on the collet walls, which can contribute to premature collet failure.
Recommendations
Whenever possible, turn the front end of the ejector pin to about .006 inch under the thru-bore size. In the retracted position, the end of the pin should be approximately 1/8 inch behind the work part. With the pin inside the bore at all times, overclosing is prevented.
Ordering Information For (EN) Extended Nose Collets
Note: Many machines use the same collet in the pick-off attachment that is used in the headstock. Headstock collet bores that are 1/2 inch and larger are grooved for extra holding power, while pick-off collets generally require a smooth bore. For this reason, if you require a collet for a pick-off operation, please specify smooth bore when ordering.
