What would cause this?

[SMG]

What would cause my SB 9 lathe to turn a 0.010" taper over a 7inch piece of bar stock using a live center?

Thanks Steve

[PeteF]

Offset tailstock.

[reggie_obe]

Twist in the bed.

[Willy]

As PeteF said offset tailstock.
Or perhaps loose bearings in your live center.

[Dr Stan]

all of the above plus an off center center hole, a very dull tool, or a too loose live center.

To help diagnose the problem you need to turn a piece between centers.

[PeteF]

To help diagnose the problem you need to turn a piece between centers.

Isn't that what he has just done?

The easiest way to check the tailstock is correctly centred is to chuck a rod with a DTI in the chuck. Bring the tailstock up to the DTI and ensure the tailstock is locked to the bed and the friction clamp on the quill is locked. Swing the DTI around the tailstock quill. The DTI should read precisely the same left to right, but will read slightly different top to bottom even on a perfectly adjusted lathe. It is the side to side reading you should be concerned about at this point. If the 2 readings aren't exactly the same you will turn a taper between centres. To adjust, unlock the base and adjust the screws on the base of the tailstock. Re-lock and check. This is an easy check and the most common reason for turning a taper between centres.

Pete

[SMG]

Isn't that what he has just done?

The easiest way to check the tailstock is correctly centred is to chuck a rod with a DTI in the chuck. Bring the tailstock up to the DTI and ensure the tailstock is locked to the bed and the friction clamp on the quill is locked. Swing the DTI around the tailstock quill. The DTI should read precisely the same left to right, but will read slightly different top to bottom even on a perfectly adjusted lathe. It is the side to side reading you should be concerned about at this point. If the 2 readings aren't exactly the same you will turn a taper between centres. To adjust, unlock the base and adjust the screws on the base of the tailstock. Re-lock and check.

Pete

Thanks Pete, I'll give it a try.

[SMG]

Pete, I think your right. The tail stock seems to be out by about 0.008. I'll try to fine tune it and see if thats the issue. thanks again. Steve

[PeteF]

Let us know how you go. I'd suggest some of the suggestions above won't in fact cause a taper, but one that may is loose bearings in the live centre. There is a certain gap in bearings, albeit small, that allow them to turn. The pressure of the tool will cause the piece to be forced back and cause a taper (again small unless the bearings are totally crap) just as if the tailstock was slightly off. If you're after ultimate precision you probably want to turn between 2 dead centres and drive the work with a dog. Personally I think it's nit-picking, but the way the lathe geometry is set up that's just the way it works.

Pete

[darryl]

-offset headstock-