Turning Non-precision Eccentrics in a 3 Jaw Chuck

[rgsparber]

It is common practice to place some packing between one of the jaws of a 3 jaw chuck in order to turn an eccentric feature. For those cases where precision is necessary, software tools are available to define the precise packing needed. For the rest of the cases, the accuracy achieved with a spud and a set of scribed marks is fine. The 3 jaw chuck attachment presented here is for this lower accuracy case.

If you are interested, please see

https://rick.sparber.org/NonPreciseEccentrics.pdf


Your comments are welcome. All of us are smarter than any one of us.


Thanks,

Rick

[mklotz]

Thanks, Rick, for the credits.

The ECCENT program to which Rick refers does indeed calculate the required packing thickness needed to turn a desired eccentric in the 3 jaw. I've never been comfortable with the precarious pinching hold on that packing, though. Rick's scheme removes the need for this loosely secured missile and that's a very good thing.

There is, however, another way to do eccentrics in a 3 jaw that is very safe. The extract below from the text file that accompanies the ECCENT archive describes it...

Imagine a tube bored to be a sliding fit on the parent stock. A slot of sufficient width to pass one of the three-jaw chuck jaws is milled lengthwise in this tube. The parent stock is inserted in the bore and the whole thing clamped in the three-jaw chuck such that one jaw passes through the slot to seat on the parent stock while the other two jaws seat on the circumference of the tube. Obviously the centerline of the parent stock is offset from the lathe spindle axis and turning will produce an eccentric. If we know the OD of the parent stock and the amount of the offset required, it's possible to calculate the OD of the tube needed to produce just that offset when clamped as described above. That's exactly what ECCENTUB does.

The ECCENTUB program is included in the ECCENT archive. If the packing scheme scares you or you don't have a 3 jaw with removable jaws you might want to consider this alternate method.

[rgsparber]

Thanks, Rick, for the credits.

The ECCENT program to which Rick refers does indeed calculate the required packing thickness needed to turn a desired eccentric in the 3 jaw. I've never been comfortable with the precarious pinching hold on that packing, though. Rick's scheme removes the need for this loosely secured missile and that's a very good thing.

There is, however, another way to do eccentrics in a 3 jaw that is very safe. The extract below from the text file that accompanies the ECCENT archive describes it...

Imagine a tube bored to be a sliding fit on the parent stock. A slot of sufficient width to pass one of the three-jaw chuck jaws is milled lengthwise in this tube. The parent stock is inserted in the bore and the whole thing clamped in the three-jaw chuck such that one jaw passes through the slot to seat on the parent stock while the other two jaws seat on the circumference of the tube. Obviously the centerline of the parent stock is offset from the lathe spindle axis and turning will produce an eccentric. If we know the OD of the parent stock and the amount of the offset required, it's possible to calculate the OD of the tube needed to produce just that offset when clamped as described above. That's exactly what ECCENTUB does.

The ECCENTUB program is included in the ECCENT archive. If the packing scheme scares you or you don't have a 3 jaw with removable jaws you might want to consider this alternate method.

Marv,

If my packing was flexible enough to bend a bit more than 120 degrees, it could sit between 2 jaws and the stock. Would that be equivalent without cutting slots or machining the sleeve?

Rick

[mklotz]

Marv,

If my packing was flexible enough to bend a bit more than 120 degrees, it could sit between 2 jaws and the stock. Would that be equivalent without cutting slots or machining the sleeve?

Rick

Yes, I think it would.

I have to admit that I never attempt to turn eccentrics in the 3jaw. For me, doing so is a great deal more fiddly than just using the 4jaw.
Once one learns a simple way to center in the 4jaw, e.g...

http://www.homemadetools.net/forum/c...aw-chuck-27241

setup goes very quickly.

[rgsparber]

Yes, I think it would.

I have to admit that I never attempt to turn eccentrics in the 3jaw. For me, doing so is a great deal more fiddly than just using the 4jaw.
Once one learns a simple way to center in the 4jaw, e.g...

http://www.homemadetools.net/forum/c...aw-chuck-27241

setup goes very quickly.

My 4 jaw is not worth using and my 3 jaw is a Bison so very solid. I rarely turn eccentrics but, as always, it is fun to figure things out.

Rick

[bruce.desertrat]

I wonder if that wouldn't be more stable/secure if rather than just the end of the screw bearing on the work, a shoe of sorts, similar to a moving vise jaw or c-clamp jaw to be attached to the screw in an analogous way. Essentially just making one whole jaw of the three jaw in two parts with a adjusting screw in the middle making the whole jaw longer as needed.

probably a lot more hassle and trouble than just doing it a 4 jaw like it is supposed to be :-)

[rgsparber]

Funny you should suggest this approach. I was thinking about it this afternoon but could not figure out how to make it work without hacking up the jaw support. I do think it would be a useful modification to a standard 3 jaw chuck.

On Joe Pie's YouTube channel (

), he explains the value of a 3 jaw chuck over a 4 jaw. A 3 jaw will always contact the part on all 3 jaws while a 4 jaw might not. I've learned a lot from this guy.

Rick

[Tonyg]

Hi Rick,
Thank you. Your posts are always incisive and well documented. I only use my four jaw chuck now but many years ago I used to back off the jaws and put one of them back a turn later than the other two. I made up a small set of angled parallels with a slot through the length that I could fit a small nut and bolt which I would tighten to get to the right thickness, The top angled parallel had a cutout in it to fit the shape of the jaw to ensure that it did not fly out. It worked fine while waiting to acquire the 4 jaw.
Kind regards, Tony

[rgsparber]

Hi Rick,
Thank you. Your posts are always incisive and well documented. I only use my four jaw chuck now but many years ago I used to back off the jaws and put one of them back a turn later than the other two. I made up a small set of angled parallels with a slot through the length that I could fit a small nut and bolt which I would tighten to get to the right thickness, The top angled parallel had a cutout in it to fit the shape of the jaw to ensure that it did not fly out. It worked fine while waiting to acquire the 4 jaw.
Kind regards, Tony

Tony,

what a great bit of "out of the box" thinking! Once that jaw is offset, there is plenty of room for packing or other machinery.

Thanks!

Rick