The Four Jaw Chuck

The Four Jaw Chuck

Four-jaw chucks have four advantages over three-jaw chucks.

1-They can be used to hold irregularly shaped parts.

2- With the use of a dial indicator they can be used to center parts with a great deal of accuracy (concentricity)

3- They can be used to deliberately hold a part off-center (eccentric)

4- They can clamp the part tighter. This is a valuable asset when machining cams, crankshafts, and similar parts.

A four jaw chuck has independently adjustable jaws. It can be used to center parts that are not round. The square part shown in the video is far off center. Each jaws is adjusted accordingly and the part is verified to be on center. After a cut it can be seen that all four sides of the square are tangent to the cut.

Installation

The chuck is usually connected to the lathe spindle by one of three methods: a camlock (figure 1), a taper (figure 2), or a thread (figure3). The back side of the chuck will reveal which one. It is important to protect both taper and threads from impact, as any dings or chips in this area of the chuck will cause misalignment.

                

            Figure 1                                               Figure 2                                               Figure 3

Indicating a Part

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.

Removing and Reversing Jaws

Each jaw is independently held into the chuck so each jaw can be removed independently from the others.

When reversing jaws, be sure not to force a jaw onto the guide rails with the screw. "Wiggle" or tap them in with a rubber or lead mallet

Jaws are generally considered outside or inside depending on which way the part is located on the jaws. There are some chucks that have the ability to remove the top part of the jaw and reverse it, without removing the master jaw. This makes setups much easier.

Article resource : www.jjjtrain.com