The chuck stops here

Setting a z datum to do chucker work on a lathe is a frequent task for many shops. This activity involves using some sort of work stop to get repeatable parts and ensure that they are clamped squarely in the jaws. Let’s look at chuck jaws that speed up this process.

In the past, there have been two methods for setting a z datum. One is to attach a work stop to a chuck. An example is a tripod arrangement that works with hard jaws and attaches to a chuck face with magnets. That’s simple enough, but it doesn’t work on a four-jaw chuck, puts limits on part geometry and restricts axial travel of cutting tools.

Using a Stop Jaw, a boring bar travels through a bearing retainer part to machine its internal features. Image courtesy Steven Taylor

The other method is the old standby of machining soft jaws, which always work. But machining them costs time and money. Soft jaws are machined to fit a specific clamping diameter and z dimension. It’s possible to accommodate a few different part geometries with one set of jaws, but shops end up investing in a library of machined soft jaws. As it grows, so does the investment. When the job changes, the jaws may have to be changed, consuming more time and money.

What’s been needed is a work stop that is accurate, versatile, quick and easy to use, plus it shouldn’t interfere with cutting tools. To fill the void, I developed Stop Jaws, hard jaws with removable hard work stops.

Each Stop Jaw has an array of slot pairs that accept work stops and allow different z settings. The work stops have an offset design to further increase the number of z settings. Thirteen different z positions are standard.

A Stop Jaw and its associated work stop and stop retainer are shown (left). An assembled Stop Jaw is displayed (right). Image courtesy of B. Taylor

A threaded hole in the jaw accepts a threaded stop retainer, which positively locks the stop in the jaw. The stop can face in or out. Note that the work stop protrudes a small distance from the clamping face to allow cutting tools to pass by the stop without interference. (See the photograph at the bottom of the page.)

Without the stops, Stop Jaws perform like other hard jaws, such as for doing bar work. Installing a work stop enables users to perform chucker work.

In operation, a work stop is inserted into a slot pair, and the retainer is inserted into the hole in the top of the jaw and threaded down with a hex key. Go from bar work to a second operation or from one z setting to another in seconds.

It’s important to recognize that Stop Jaws don’t do the work of pie jaws or any soft jaws in which a part requires a large angular contact area.

A work stop can be used on the outside of a Stop Jaw to clamp a part on an inside surface. For example, with a cylinder clamped on its ID, its entire OD can be machined.

With a work stop used on the inside of a Stop Jaw and a part clamped on an OD, machining through a part is no problem. If a part is clamped on its OD and the diameter being clamped is less than the length being clamped, using only one work stop on one jaw does the job.

Stop Jaws are available for three-, four- and six-jaw chucks 152.4 mm to 381 mm (6" to 15") in diameter.