Accurately Mounting Jaws On Three-Jaw Chucks

The most common workholding device for turning centers is the three-jaw chuck. Setup people remove and replace top tooling (jaws) during every setup, and this task can be relatively simple if quick-change chucks are used. However, the vast majority of three-jaw chucks used on turning centers are not quick-change chucks; therefore, mounting and replacing the jaws will take much longer.

With most three-jaw chucks, two socket head cap screws are used to clamp each jaw (with a long tee nut) to the chuck’s master jaw. Thus, a total of six screws are needed for the three jaws. The master jaws on the chuck have small serrations that match serrations on each top-tooling jaw. These serrations are so small that it can be difficult to place each jaw in the same serration of its master jaw.

It is also important that the jaws be mounted in such a way that they clamp near the middle of the chuck stroke and, of course, jaws must be mounted in those serrations that allow the jaws to clamp the workpiece. This can be a complicated task —especially for beginners. If the jaws are not mounted properly, the whole task of mounting the jaws must be repeated. Because the serrations are so small, the setup person won’t know something is wrong until all three jaws are mounted.

Setup people eventually get good at approximating the position of each jaw so that it will clamp at the appropriate diameter. However, this skill comes at the price of much trial-and-error. Mounting jaws can be very frustrating for entry-level setup people as they try to figure it out. If you see your setup people struggling with the task of jaw mounting, you should do something to help them.

While the following method may sound complicated, it is one way to place all three jaws in the correct serrations on the very first try. However, it takes a little up-front work. If you can determine the diameter at which the jaws must be placed so that they clamp the workpiece in the middle of the chuck stroke, you can use a pointer along with the X-axis position display to help you mount each jaw.

For example, if you have a chuck with a 0.25-inch stroke (0.5 inch diameter increase/decrease), you’re going to mount jaws with the chuck in its closed position (master jaws toward the chuck center). When mounting jaws that will clamp on an external diameter, each jaw must be placed in the master jaw in such a way that its workpiece-contacting surface is 0.25 inch smaller (in diameter) than the diameter the jaws will be clamping on. This will allow for half the jaw stroke. When the jaws are actually clamped on a workpiece, they will contact the workpiece in the middle of the chuck’s stroke.

For soft jaws, you must also allow for jaw boring. For external clamping, subtract the amount of material you’ll be removing from the jaw from the diameter just determined. For example, to clamp on a 4.0-inch diameter using soft jaws, with about 0.1 inch of material to be removed from each jaw, the mounting diameter will be 3.55 inches (4.0 inches minus 0.25-inch jaw stroke minus 0.2 inch of stock—twice the amount of stock to be removed from each jaw).

Once you have determined the diameter at which the current clamping surface of each jaw must be mounted, you can make the long boring bar point to this diameter. To do so, calibrate the boring bar’s pointing diameter with the X-axis display.

With the X-axis display calibrated, use the handwheel to bring the tip of the boring bar to your calculated clamping diameter. Then bring the tip of the boring bar up close to the chuck face. Use the tip of the boring bar to determine which serration each jaw should be mounted in. You may have to move the boring bar away in Z (but do not move it in X) in order to actually mount the jaw. Repeat this procedure for each jaw. When you’re finished, each jaw will be in the same serration.

With hard jaws, the chuck will be in the middle of its stroke when the workpiece is clamped. However, if you’re mounting soft jaws, they must be machined. When you’re finished machining them, they will clamp on the workpiece in the middle of the chuck stroke.
The same technique can be applied to internal clamping, but you may have to calculate the diameter a little differently. If you’re still mounting the jaws with the master jaws toward the spindle center, you still subtract the jaw stroke from the workpiece diameter to determine the clamping diameter. So for hard jaws, the calculation is exactly the same as it is for external clamping. However, with soft jaws, you must add twice the amount of material you’ll be removing from each jaw to the diameter just calculated.