Correct Geometry Of 4 Jaw Chuck Jaw

RJSakowski

RJSakowski

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Here's a question for the group. What is the correct geometry for a 4 jaw chuck jaw?

When I bought my Grizzly G0602 lathe, it came with an 8" 4 jaw chuck. Some time ago, I mounted a test bar in the chuck and adjusted it for zero runout at the jaws. When I checked the runout about 8" out it was rather significant so I set the chuck aside, figuring a poor grind on the jaws.

A recent resurfacing of a video regarding truing the jaws on a 4 jaw chuck got me down in the shop to look closely at the jaws. What I found is this. If I held a test pin by two opposing jaws, the front of the jaws would grip the pin but the back of the jaws didn't touch the pin. In fact, I could move the pin through several degrees of arc in a plane perpendicular to a plane through the center of the jaws.

I then mounted a .300" gage pin in the first section of the jaws and found that I could fit a .314" gage pin between the last section of the jaws.

I have seen several references to having a slight taper to the jaw grind to account for deflection of the jaws when loaded but this seems excessive. Here is a depiction of the condition. The angle has been exaggerated to illustrate the problem. The second picture shows the widening of the gripping surface towards the back of the jaw, consistent with a tapered surface. This drawing reflects the actual measured angle.
4 Jaw Chuck Jaw.JPG 4 Jaw Chuck Jaw 2.JPG

Checking further, I went to several machine vendor sites sites selling chucks to see if this was common. On the Shars site and the CDCO site, their 4 jaw chucks appeared to have the same taper. I wasn't able to find any clear pictures of name brand chucks to compare.

In thinking about it, when adjusting work for zero runout in a 4 jaw chuck, one would first adjust the jaws with the indicator close to the chuck and then move out some distance from the chuck and check runout. If there was runout, a few judicious taps would make a correction. Then move back to the chuck and repeat. Having the ability to pivot would assist this process.

So again, what is the correct grind for the jaw? Should it be ground so when it has a load, the jaw surface is parallel to the spindle axis or should the jaw surface be ground so the work is free to move slightly at the rear of the jaws?
 
I don't really know. If it were the reverse, bell mouthed jaws, that would be awful, as many of us find out with old, worn out chucks. Perfectly square jaws are just some usage from being bell mouthed, so that would require early replacement or repair. What you are seeing definitely seems excessive. A few thou closer than square from top to bottom of the jaws might be a good compromise. We really don't want to have to regrind the jaws every few months or annually to remove the bell mouth condition. Adjustable jaws, anyone?
 
Thanks Bob.

I checked my 3 jaw chucks, two of which were essentially new and two which had been used for many decades. The new chucks had .0015 -.002" diametric taper, larger at the rear of the jaw. The two old chucks had about .002" diametric taper, larger at the front. Assuming the old chucks were initially correct, there is some indication of wear.

Your suggestion of a few thousandths taper seems correct.

Hey guys and gals; over 120 views and no one else weighing in?
 
I find this very interesting, but do not have enough knowledge to contribute...just trying to learn something useful by watching.
 
I've never analyzed the taper to the extent you have, but I believe that it is by design that the jaws are tapered as you see. As they wear they will "correct" that taper, as Bob pointed out, so I don't think you really have anything to worry about. And it is better than having belled jaws.

If your model is accurate per your actual jaws, the serrations are NOT tapered. Is this a depiction of the actual conditions, or did you place the serrations parallel with the axis intentionally? Of course, in manufacture, the serrations are machined while the jaw is soft, and the grip surface is done after hardening. It would be easy to see a way for the taper to be there unintentionally if (as I expect they are) the jaws are ground in a fixture far from the chuck. I just can't see the factory going through the same exercise on a 4 jaw chuck as they do on 3 jaw chucks with grinding the jaws in situ.
 
Tony, thanks for the comments.

It's true that the serrations were not tapered. The grind at the rear removed more metal resulting in a shallower serration.
The diameter of the jaw grind indeed indicates that the jaws were ground on a remote fixture as the radius of curvature is greater than the through hole on the chuck.

My suspicion is that a slight taper is intentionally added to chucks by the major manufacturers and that the Asian manufacturers thought that more is better.
 
When the jaw is loaded it is going to bend and compress slightly. This should result in it gripping along part of its length rather than at just one point. This length need not extend all the way to the back in order for the grip to be adequate. It will be adequate if the angle is small enough. With wear the grip area will extend farther and farther back until it reaches the back end. With more wear it will start to lose grip at the front. Eventually you have to regrind.
 
I checked my 4 jaw, about 6 years old and has had its share of use. It looks to me like about .0005" larger at the back of the jaw. With a 1" dia test bar clamped at normal, not wrench bending pressure, a .001" feeler gauge will not slip between the bar and any of the jaws.
 
Could it also not be to take up the clearance in the slide between the jaw and the chuck body? As all the closing force from the screws (or the scroll of a 3 jaw) is at the very back of the jaw surely there is a tendency for the jaw 'rock' so that as the jaw tighten its slot is is only contacting the body of the chuck at opposite edges (top right and bottom left in relation to your drawing) and the back of the jaw effectively closes more.
 
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