The Four Jaw Chuck - 3: Indicating a Part
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
The screws that move the jaws are 20 threads per inch (T.P.I.). A complete revolution is .050". If you keep this number in mind when indicating a part, it can speed up the process of centering.

First, use the lines machined on the face of the chuck to roughly align the part concentric with the chuck. With an indicator, read the run-out. Move the jaw closest to the high or low point 30% of the total indicator reading in the proper direction.

For example, if the indicator shows a .030 run-out. 30% of .030 is approximately .010. If one revolution of the jaw feed screw is .050, then a little less than a 1/4 turn will be .010. Back the jaw out this amount and tighten the opposite jaw. DO NOT tighten the jaws beyond "snug" until the part is running within .005" T.I.R.

Repeat this process until the part runs within required specifications. Once the part is running within .002 T.I.R. it can usually be "brought in" by a final tightening of the jaws. It should also be noted that the chuck jaws are ground with a slight angle to allow the jaws to apply equal pressure to the tip and base when properly tightened. This angle amounts to less than .001" on the jaw surface.

This video shows a pre-exiting eccentric boss being aligned for a center drilling operation. It is first aligned by eye as close to the center as possible. Next an indicator is used for fine adjustment.

It is recommended to use the 30% figure above because the high point of a part will seldom line up with a jaw. Moving a jaw too much can cause extra unintentional part movement or simply moving the high point around the chuck R.S.
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label: four jaw chuck advantages, disadvantages, inricating round part, removing and reversing jaws