In article ,
Don Foreman wrote:
I have a T-shaped parting tool in an Aloris BXA7 holder that won't cut
straight. It drifts to one side or the other as the cut progresses.
The bit is .063" at the bottom, .094 at the top, .686 high. As far
as I can tell the cutting edge is ground square (parallel to the
lathe axis), the bit is vertical in the holder to within a thou or
two, and the bit was square with the workpiece at center height.

I have a thinner tapered parting bit that does cut straight. It's
.0625 at the top, .043 at the bottom, .687 high.

[ ... ]

Any suggestions about what's going on here, please?

Hmm ... one possibility is that either the compound or the
toolpost are slipping under the greater cutting forces with the wider
parting tool. This will change the angle of the tool, and will cause it
to drift. (Obviously, if it is higher than center, you have extra force
on the end of the tool, but I presum that you have eliminated that as a
cause.)

One trick which may prevent the toolpost from rotating under
pressure is to take some thin cardboard or thick paper, cut it to match
the footprint of the toolpost, with a hole to pass the clamp-down bolt.
This will deform into any small imperfections in the finish of the top
of the compound and the bottom of the toolpost, and make it harder for
it to turn in place. If it is the compound turning, instead, I will
leave it to you to figure out how to prevent it from turning.

You *do* have the carriage locked to the bed while you're doing
this, don't you?

Now I will sit back and see what others say.

Good Luck,
DoN.
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