Well, I think I finally figured it out, although there is still a
mystery.

The problem has been that I could not cut a 1.25" diameter mild steel
bar off in the 3-jaw chuck without house-shaking chatter. This with a
HSS T-blade 1/8" wide in an Aloris BXA-7 holder.

I tried many things, mostly to rule out one possibility or another that
came to mind. The gibs are now all tight, and so on. The Aloris BXA
toolpost replaced the warped Dickson that came with the lathe

On the off chance that loose back hold-down plates on the carriage were
the issue, I used a C-clamp and a piece of aluminum (to protect the bed
way) to clamp carriage firmly to bedway. No effect.

Running fast and slow using the variable speed drive and/or the VFD had
some effect, but gross chatter happened even at very low speeds.

I recently bought a used slotting tool bit, consisting of a 1/16" wide
carbide blade brazed to a 3/8" square shank, the blade protruding about
1/4". Even this chattered.

Huh? How does that work? We are making a tiny groove using this 1000#
machine. This ought to be easy. Time for some reading or re-reading.
Marlow suggested cutting off (parting off) at one third the speed used
for turning, and pressing hard if it chattered. This worked for the
little slotting tool, although I did manage to break it by pressing too
hard. Well, the used bit cost me $0.75, so I happily shed 3/4 of a
tear. Clearly, we have progress here.

So, why then did the tiny slotting tool chatter no matter the speed?
Something must not be stiff enough. But what? I've tightened or
clamped or adjusted just about everything, to no avail.

I woke up the next day with the answer -- torque. I was going slow to
be sure, but was not using the back gear, and so the drive system was
not torsionally rigid enough. When I used the back gear, the chatter
went away, and I was able to part that 1.25" diameter bar off without
danger of shaking the house apart.

What also seemed to help was that I was using a mister to spray lots of
Rustlick WS-5050 emulsion right into the bottom of the deep groove.
Trying to keep the cut lubricated using a hand brush just was not
working, especially when the groove became deeper than it was wide.
However, coolant by itself did not abolish the chatter. The back gear
is essential.

But, I did notice the whole toolpost and holder tilting when I leaned
into the cut, so I'll have to track down if something is still loose and
needs to be adjusted. Or perhaps it's normal, given the forces invloved.

The other thing that happened is that the toolpost and/or compound
rotated perhaps 5 degrees under the stress of parting off, causing the
blade to drift out of perpendicular with the bar being parted. This
caused a lot of trouble until noticed and fixed.

One thing I noticed when I first got the lathe is that one of the two
5/16-18 swivel bolts (055-017) that lock the compound against rotation
had been stretched enough to visibly distort the threads, and I always
wondered why someone would apply that much force. Perhaps this creeping
is the reason. (The stretched bolt was replaced, and the newer bolts
are slightly beefier to the eye and may have been hardened.)

Joe Gwinn