scroll chuck jaws - anyone made 'em?

[Hopefuldave]

I could use a set of soft jaws for my Pratt-Burnerd 3-jaw, and after a look at prices I'm considering making my own...
I think I have the scroll geometry worked out, so has anyone else attempted this, any suggestions for suitable materials? The 'top' jaw's no problem (could be ally or whatever's handy), but the jaw base (engaging with the scroll) needs to be hardenable yet dimensionally stable... I can't see an easy way to grind to size after hardening, so I think I'll need to machine to size and hope it stays on size after heat-treatment... Eek!

Is my best bet to make 'em in a low-carbon steel and case-harden, or use a through-hardening steel and allow for material expansion?

Any recommendations welcome!
Dave H. (the other one)

[EdK]

Usually soft jaws are used with two piece jaws but I guess there's nothing stopping you from making them for a single piece jaw chuck. Please post pictures of the project if you decide to tackle it.

Ed

[Hopefuldave]

Quote EdK:
Usually soft jaws are used with two piece jaws...
Unquote.

That was my thought, too - hard jaw bases to engage the scroll, with ANSI keyways and soft jaw tops... Then I could make up 'disposable' jaw tops as required...

But what material would be dimensionally stable enough for the bases? I can see how to mill the scroll in the bases (although with some care and a complex set-up[1])!, but after hardening I suspect all might be less than accurate, thanks to distortion - less so if the scroll jaws were case hardened?

Dave H. (the other one)

[1] surprisingly simple geometry giving a 'teardrop' shaped tooth to engage the scroll... Bugger to set up though! May need a jig / fixture... OK, *will* need...

Dave H. (the other one)

[EdK]

So you're saying you're starting out with a chuck that is not a two piece jaw chuck, is that correct? A picture would help.

Ed

[Hopefuldave]

Correct! I only have one set of jaws for the 3-jaw, one-piece outside - so I'd need to cut the 'teeth' to engage the scroll, and the slots to fit the chuck face - once they were made I could bolt on anything I liked!

Dave H. (the other one)

[TomG]

A2 is very stable and easy to harden, but a bit pricey. Your next best bet would be to case harden them, but you'll need to do it in a box or envelope to keep the air off and maintain a good surface if you aren't going to grind them.

Accurately locating the teeth on the jaws will be a pain since they are all different. You may have to make blanks, with the teeth, but no other features, and fit them to the chuck.

Tom

[stevec]

Is there any way to unharden the existing one piece jaws, machine them to take the "soft jaws" drill & tap them and then re-harden them ? If that's do-able I'd consider doing a set myself.

[Mayhem]

I like your thinking Steve.

[Hopefuldave]

Thanks gents,A2 sounds like the right sort of material.

I think I've come up with a setup (on paper, at least) that will give the right geometry - the teeth need an outside radius to match the scroll at its innermost radius, inside matching at the outermost, with both rotated to match the scroll helix at the same points - a jig with jaw centre-line offset by the scroll pitch *EDIT* SCROLL PITCH / 2PI \*END EDIT* takes care of the helix angle, if my geometry hasn't failed me... Then it'll be down to setting the cutting tool for the two radii and advancing the jaw's for each cut - as long as I use a feed screw 3n x jaw pitch in tpi, should be possible to cut the teeth for all three jaw's using the same jig, albeit with some faffing about setting each tooth surface separately... If I can get my pc up and running I'll try to draw up the jig I have in mind, so wiser eyes can condemn it!

Thanks again for the suggestions,
Dave H. (the other one)

Edited - my geometry failed me!

[Tony Wells]

I ran across an article on how to machine the scroll side. It was more complex that meets the eye. I'll see if I can locate it. It was a good, detailed how-to.