5C Collet Alignment Question

Here's the backstory: I'm building a tool and cutter grinder for my home shop and have chosen to make one of the tool-holding arbors 5C, since my shop is set up around those and I've got a full set. I've made the arbor including the drawbar and it all fits and works nicely. Simulating the left end of my South Bend 13" lathe's spindle, I made a 45 degree taper on the back end of the arbor for a mating taper on the drawbar to engage with. I went to some lengths to ensure that the nose taper and back end taper were concentric within one thou.

The question: How important to the concentricity and repeatability of the 5C workholding collet is this "backend" of the assembly? It's pretty obvious that if you were off 10 or 20 thou between the 5C taper at the nose and drawbar taper at the other end of the arbor, then the collet could be forced into a skewed position. But what about a couple of thou difference? Will the nose taper overwhelm that offsetting force?

Ed in Florida

Ed,
To the best of my knowledge that back-end OD serves as a pilot to maintain straightness of the collet as it is being drawn into the taper seat.
A good quality 5C collet will have this diameter ground concentric with the OD taper.

Yes, the collet fits closely in the end of the spindle, both on the taper and the rear of the collet body. The threads of the collet have some thousandths of an inch clearance in the drawbar threads, so the possible skew of the drawbar should have virtually no effect on accuracy.

But craftsmanship demands we do careful work and try for near perfection when we make equipment.

Larry

I suppose it depends how good you need things to be.
I found draw bar alignment important when the work piece is extended from the collet.
The clearance of the straight portion of the collet to the collet holder is enough to cock the work if the draw bar is not true, not much but still there.
The factory made components were less then what I was aiming for.
I ended up making a draw bar using a single Timken bearing for thrust when the collet is tightened.
This worked well for me. When the collet is tightened, the Timken bearing pulled the closer assembly true to the spindle.
In this photo, the Timken bearing housing is slid forward on the closer.

collet chuck 1 picture by donsmonarch10ee - Photobucket

I used the same draw bar arrangement on my cutter grinder.

Thanks to you all for your thoughts.

I did make the arbor's straight ID a close fit, maybe 2 or 3 thou radial clearance, to the straight body of the collet.

I think I'll dial the arbor in using a 4 jaw with a pre-mounted ground drill rod in the collet and see what kind of TIR I get close and far.

Thanks again, all good advice for my next one or if I have to rebuild this one.

Ed in Florida