I think I've figured out a big piece of the mystery of what I
generically called chatter in the "Clausing 5914 Chatter ..." threads.

Tightening the lathe dovetails up helped a great deal, and is a good
idea in general, but didn't really solve the problem.

It came to me in the shower Wednesday morning - self-feeding would
explain just about everything seen, and that if one reverses something,
the sign of the mechanical feedback loop can be reversed. Now the
feedback is positive, yielding the vicious loop that causes visible
tilting, flake chips, and breakage. By contrast, negative feedback
causes stability.

The key experiment was to try face grooving a 2.125" diameter 6061
aluminum slug held in the 4-jaw chuck, and using the BXA-6 toolholder
with the huge overhang (~5" from cutting edge to toolpost center) but
with the tool bit held upsidedown and the lathe running in reverse.
Running backwards cases the self-feeding effect to change sign, becoming
a self-unloading effect as the toolpost tilts away from the workpiece,
reducing the bite of the tool bit.

Now I can peel very thin chips off the plate while face grooving,
although it is still necessary to use the back gear, and there was mucho
squealing. I did it dry, with too much overhang, and without properly
shaping the tool bit. But it worked anyway. The difference is
night-and-day.

So I now think it was the tendency to self-feed that caused all the
problems, even if the self-feeding effect wasn't so powerful as to cause
visible tilting of the toolpost. The squealing will go away with a
stiffer method of holding the tool bit, and perhaps a better-shaped bit.


This also applies to parting off, and specifically explains why using an
upsidedown blade coming towards the back of the workpiece, or coming
from the front with the lathe running backwards, works. (BXA-7R)

I never really believed the theory that better chip removal was why
upsidedown cutoff blades worked better, because I had problems even when
there were no chip wads to be found and the groove sidewalls were clean.
Actually, the galloping chatter tended to throw thick flake chips far
and wide, so they were everywhere but in the groove.

To summarize, there are two elements that are necessary for face
grooving, trepanning, and parting off on the Clausing 5914 lathe: Use
the back gear (for tortional rigidity), and use an upsidedown tool bit
(for stability).


As for use of a coaxial boring bar to hold a grooving/trepanning
toolbit, it turns out to be a common method. I found it mentioned in US
Patent 5,640,890 as prior art to be improved upon. The patentee is
trepanning stainless steel hydraulic hose fittings in production, and
needed a method that worked in a screw machine of some kind.


Joe Gwinn