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.

The lathe is a 15x50 gearhead, no problem with stiffness or rigidity.

I'm not exactly a newbie with a lathe, but this one has me puzzled.


Well, the most common cause is the front end of the tool is not parallel
to the spindle axis. But, if the side of the tool is not at an exact right
angle to the lathe axis, then the side will rub on the cut face as the tool
goes in, and it will force the tool to one side or the other. You don't
get this with a tool that is tapered behind the cutting edge, but the
long parting-off tools have no side relief whatsoever, and will rub
and be deflected. I generally accept that this will happen, and accomodate
it.

Jon