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becksmachine
09-04-2012, 05:02 PM
Got around to making a part for a friends bulldozer today, it involves a bit of unorthodox workholding using a 4 jaw chuck, though a 3 jaw would work nearly as well.

A question asked on this forum in the past has involved how to machine the edge/rim of relatively thin discs. Not that this disc is all that thin, but it would have been difficult to do this while chucking on the o.d. and it does serve to illustrate the principle.

Photobucket is being uncooperative and I can't get Picasa to display thumbnails, so hopefully the photos will be self-explanatory.

Sequence of operations;

Layout & drill center hole
Insert grippers in chuck jaw screw holes
Shove disc against grippers with live center
Machine away!!


https://picasaweb.google.com/114255517588912050216/Sep42012?authuser=0&authkey=Gv1sRgCLrA6tWf1Za4xgE&feat=directlink

ogre
09-04-2012, 05:46 PM
Thanks for the set up!

RussZHC
09-04-2012, 06:10 PM
Definitely creative.
A question/concern, in your opinion, how much grip do those grippers actually provide? What I mean is they don't look as sharp as some examples of "drive pins" I've seen where there was only a smaller central portion gripped. Is how well they grip somewhat dependent on the surface of the material? E.G. if close to the edge of a flame cut disc, might they not slide? For that matter, just the differences in material itself...

Could something similar be accomplished using rare earth magnets? Or would that just be too risky in terms of slippage?

Toolguy
09-04-2012, 06:11 PM
For lighter cuts you can do the same thing by pressing the part against the chuck jaws without grippers.
Probably need grippers for something heavy duty like these bulldozer parts.

Boot
09-04-2012, 07:07 PM
The way this should be done is by having a set of soft jaws bored and faced to hold the part by about .250 on the O.D.. Or held by existing jaws with jacks holding the part level in the chuck. Then reversing it to finish the rest of the O. D.
But hey if this works for you go for it. It is certainly creative. Boot.

Boot
09-04-2012, 07:28 PM
Sorry to reply again ,but Brian has in shop made tools a spyder that could be used to hold this disc up level and it could be gripped by the chuck jaws and turned. Then it could be indicated in or put in soft jaws and finish turned and faced.
Sorry to butt in like this ,but I couldn't help it. Boot

becksmachine
09-04-2012, 11:46 PM
Definitely creative.
A question/concern, in your opinion, how much grip do those grippers actually provide? What I mean is they don't look as sharp as some examples of "drive pins" I've seen where there was only a smaller central portion gripped. Is how well they grip somewhat dependent on the surface of the material? E.G. if close to the edge of a flame cut disc, might they not slide? For that matter, just the differences in material itself...

Could something similar be accomplished using rare earth magnets? Or would that just be too risky in terms of slippage?

The grippers don't penetrate very far, but I was very comfortable with a DOC of .100" with the setup shown. I jam the tailstock quill into it as hard as I can to "set" the grippers and then back off the pressure slightly to do the actual turning. One caveat here, this is on a Lodge & Shipley Powerturn lathe which has a two speed tailstock capable of pushing a 3" drill with no pilot hole. An Atlas or South Bend lathe would require a lighter touch.

Two things to keep in mind here. One, the tailstock center does contribute to resisting the forces that would otherwise tend to make the part slide on the grippers. Two, the radius from spindle centerline is adjustable so the "twisting" force can be very close to the "resisting" force, unlike a conventional face driver setup, where the drive dogs are on a fixed radius.

I have seen pictures of magnetic chucks for a lathe, but they would obviously be limited to very light cuts on ferrous materials.

Dave

Boostinjdm
09-05-2012, 01:16 AM
I've done that before, but I'm a lot more comfortable drilling a 3/8" or 1/2" hole and using an arbor. Then weld the hole shut. I've done multiple discs stacked this way too.

Ian B
09-05-2012, 03:13 AM
Handy setup!

Couple of questions:

- Where did you get the grippers (or are they home made)?
- Would it have been a bit more secure if you'd have opened the chuck jaws up, so that the grippers contacted on a larger PCD?

With a good centre hole, there was little danger of the work coming loose. Main thing is, the setup worked.

Ian

becksmachine
09-05-2012, 10:41 AM
Handy setup!

Couple of questions:
Ian


- Where did you get the grippers (or are they home made)?

Yes and no, the checkered carbide pieces are standard (?) items used in chuck jaws and other workholding applications. They are retained by the screw, visible in the photo, in a little housing that I made to fit the hex opening in the jaw screw.


- Would it have been a bit more secure if you'd have opened the chuck jaws up, so that the grippers contacted on a larger PCD?


Yes, but I was afraid of hitting the jaw step when the tool ran off the back side of the disc.


With a good centre hole, there was little danger of the work coming loose. Main thing is, the setup worked.

Exactly! :D

Dave