scroll chuck jaws - anyone made 'em?

[Hopefuldave]

That'd be really handy, Tony, I think I have the geometry worked out from first principles but confirmation or contradiction would be great! Assuming I've worked it out correctly, the jig should be pretty simple but would need fairly accurate construction - a couple of critical dimensions that probably need to be within a thou" or so plus a good fit to the jaw blank and some reference points to measure from should do it. If it comes out the way I think it will, it should work on a rotary table on a vertical mill or milling slide on a lathe, or at a pinch bolted to a faceplate on the lathe... If it comes out right!

Dave H. (the other one)

[krv3000]

hi this my help go to Harold hall web page their is a set of drawings and a right up on haw to mack them

[Tony Wells]

Found it. And yup krv, it was Harold Hall.

http://www.homews.co.uk/page99a.html

[Hopefuldave]

Thanks, I took a look at Harold's method, looks like basically the same but with more exacting machining for that jig (particularly for the locating pins), and a lot more reliance on measuring rather than first-principles geometry - and I trust my geometry more than my measuring skills! I've succeeded in getting my pc (with CAD on) working again, so I can draw up my version of the jig, and don't have to type on the 'phone all the time ;)

Dave H. (the other one)

[Mayhem]

CAD working = a need to see pictures!

[Hopefuldave]

OK... A couple of initial teasers - I'll upload the Proper Drawings once I've got them into "paper space" and annotated all the dimensions (particularly those that would change for other chucks and scrolls) - a brief rundown...

The main slot needs to be precisely fitted for the jaws, and the jaw blanks need their key slots machined nice and parallel with the "non-tooth" face before cutting the scroll teeth;

the Very Observant will notice that the slot is offset from the locating sockets in the base - this is a *critical* dimension, equal (math don't fail me...) to the scroll pitch divided by 2xPi, so in the case of my 3tpi jaws, 0.053052" - drop the last digit and it should still work acceptably...

there are two sockets, precisely 0.500" diameter in the underside, for a spigot on the end of a MT2 short arbor - these are fairly critical, and need to be on centres equal to the difference between the scroll inside and outside radii - this very much simplifies using the thing!

In addition to the side clamping slots, there are countersunk holes for screws to go into T-nuts in either the rotary table or faceplate (I think it'd be possible to do the job on the lathe, if there's sufficient swing)

The "leadscrew" would need to be 3n times the scroll pitch in tpi - my chuck's scroll is 3tpi so the example is 18 tpi to allow for whole turns (6 turns when cutting each jaw tooth radius, 2 turns difference on each succeeding jaw to allow for the 120* rotation of the scroll) - the leadscrew "nut" should be held with countersunk screws to be certain it returns to the same location when removed to switch radius - the posh among us could add dowel pins if they liked!

The 2MT short taper (for the centre of the rotary table or the lathe spindle nose) needs a *precisely centred* 0.4995" parallel spigot to locate the jig, with a precise 0.250" in the end for the "setting pin" (a hardened and ground dowel) - this is where the inner and outer radii would be measured from, remembering to add/subtract 0.125 when measuring between the pin and the cutter...

       

The rows of 3/8" BSF holes are for clamps to hold the jaws firmly in the slot - button-head screws recommended for clearance if it'll be spinning on the lathe faceplate! The brass shim (poking its tongue out at you) and slightly narrower than the slot is to a) allow for different depths of jaw blank and b) allow for the bottom corner radius from milling the slot - don't ask me how etc. etc.!

It could be made from a solid bar (as shown), or if necessary built from (ground?) flat stock - although that would need very careful setup to get everything parallel and correctly offset!

OK - now someone shoot it down in flames!

Dave H. (the other one)

[EdK]

Dave,

Nice drawings. What CAD program do you use.

Thanks,
Ed

[Hopefuldave]

Ed, I use TurboCAD Pro, an old copy (v16) that I picked up very cheap in a clearance sale :)
So far it does everything I need, although it's a bit slow on my old, low spec' pc!
I'm mostly self taught, had an AutoCAD course about 20 years ago, so I occasionally struggle... Another year or three and I might be up to speed!

Dave H. (the other one)

P.S. - It can save rendered images as .png files, which are a lot smaller than equivalent .jpg files...

[Rickabilly]

On the idea of softening a set of jaws from another chuck and then re-hardening after reworking the shape, well... the softening is easy, simply carefully seal them inside a steel box purged of air and replaced with some inert gas then heat soak the whole deal above the critical temperature for about an hour per inch of section, then bury in dry sand until cool probably a day or so later, the jaws will be soft and the lack of oxygen will protect them from scale formation, re-hardening will require more accurate knowledge of the metal used in manufacture but a good guess might get you by on this, once again seal in a box for hardening this time maybe pack in charcoal like when colour case hardening ( but probably use powdered wood charcoal for better protection, there was a method that used a simple wheat flour dough which turned to carbon in the furnace) then drop the entire contents into the quenching liquid the quicker the better as this will not allow time for scale to form,

Regards
Rick