Eccentricity Rings True: Design & Engineering

Any shop that either makes a large volume of finished parts or does prototyping knows the utter importance of having and using soft jaws to hold those parts. There are two main reasons behind this importance, a custom fit to the actual part, and not marring the surface you just spent time finishing. A vast majority of soft jaws are used on lathes, primarily of the three-jaw universal type. The versatility is unmatched, as just about every shop has a lathe with a three-jaw chuck on it. Clamping something and having it run true in the hard jaws is nearly impossible (unless you have a set-thru chuck), so the ability to machine jaws true to the spindle’s rotation and provide a solid workholding solution is a no-brainer.

Sometimes, as we all know, the actual cutting of said soft jaws is as much of a nightmare as the expensive workpiece to be held! We also know the myriad ways of clamping the jaws down to cut the contour in the jaws can be mind-boggling, so how can your shop adopt a singular way to alleviate this headache? Enter the eccentric ring. The concept isn’t necessarily new, but not everyone knows of it. It has saved our shop from having to consume bottle after bottle of migraine relief pills.

So let me share the idea with your shop, too. See Figure 1.

Essentially a doughnut-type ring, the “eccentric” part comes from the three radial arc-shaped slots whose centers are offset from that of the ring itself. This design is easily adapted to fit the needs of your specific machine, but the basic principle is the same. The OD of the ring should be at least the same diameter as your lathe chuck while not being larger than your machine’s swing, of course. The ID should start out only as large as you need for it to easily clear. See Figure 2. A word to the wise, invest the time and material to make a few of these things, along with enough pins to outfit each ring, and maybe a spare pin or two. We have found these rings to be extremely helpful in cutting down on setup times and scrapped parts from marred surfaces. They do get used up, as you may need to cut some of the ID away to fit other jobs, so having extras on hand is highly advisable.