Minimum Thread Engagement on 10K Spindle

[wheels17]

I recently bought a faceplate(TPP-100N) for my 10K, literally out of a trunk in a parking lot. Today I finally got around to seeing how it fit on my lathe. I pulled my 3-jaw off, and it looked like the register(I know, I know) area was longer on the faceplate than on the 3-jaw. This made me concerned about the number of threads engaged with the spindle.

I bought the lathe with the SB 5" 3 jaw, a Skinner for South Bend 4 jaw, a dog plate, and a Bison Set-tru 5C collet chuck. So I pulled them all out and started taking rough measurements (Mitytoyo caliper). The results were somewhat disturbing:

The 3 jaw, the new face plate, and the dog plate all seemed in the same ballpark with over 4 threads of engagement. The 4 jaw only has 3.15 threads engaged, and the heavy. overhung 5C collet chuck only has 2.9 threads of engagement.

I've used the 4 jaw extensively, and it appears by the invoice to have come with the machine, so apparently 3.1 threads engaged is OK.

I can't say for sure whether the Bison has ever been used on the lathe. The 5C collets that came with the lathe were all still in plastic. It seems to me that the right answer is to take this chuck assembly (which lists for 2/3 of what I paid for the lathe and tooling) apart and face at least .0625 off the boss. I'd probably go for .125-.1875 as long as I had everything set up. It makes me nervous though, as I am not the most skilled machinist in the shop.

Where should the line be drawn for an acceptable number of engaged threads?

 

[Danny VanVoorn]

If the collet chuck has a separate back plate then remove the back plate and go at it. If your figures are correct I'd be as concerned with the register bore diameters as the amount of thread engagement. For threaded spindles I like the register bore about .001"-.002" over the spindle diameter and most of your tooling is quit a bit more. Do remember that both the face and the bore in the back plate will control run out on the chuck and whether it repeats when removed and reinstalled.
Dan

 

[iwananew10K]

I used to think the same thing...until I got a Sheldon, it had a 2 1/4-8 spindle but the register was 2.280"- .030" over nominal.

my Sheldon chucks workedas well on it as a SB, and some experimenting proved that the chuck register bore didn't matter,it could be sloppy as hell...it was the thread quality that did the centering work.

I do however believe in minimal overhang, and would face off the backplate hubs as muck as possible...I had a simple "doughnut" made right on the lathe out of a scrap of steel that would slip over my spindle, I could then screw the backplates on BACKWARDS, face off the unnecessary projection, and bore a new register, then take the plate off,remove "doughnut, and screw the plate back on the right way, take a light trying cut across the face and reinstall chuck...done deal.