Joseph Gwinn wrote:
At Clausing's suggestion, I tightened the spindle bearing take-up nut,
which adjusts the preload on the spindle bearings, by about 1/4 turn.

The headstock now warms up if the lathe is run constantly; previously,
there was no perceptible temperature change. In retrospect, the failure
to heat up after running for say 20 minutes should have been sufficient
clue that the preload wasn't sufficient.

Tightening made a huge difference. I can now just about manage
Clausing's original high-speed smooth turning test, although the lathe
is still right on the verge of chatter with the chuck.
It should be. That was the factory test to detect a latent
problem in the machine that wasn't obvious on lighter cuts.
With the 5" overhang and no support, I know this is a cut I
could do manage on my 15" Sheldon, a 3500 Lb. machine, but it
certainly isn't good for the lathe. The fact that you now CAN
do a really serious cut like this sure seems to indicate you've
fixed it!
It turns out
that higher speeds chatter less, probably because one can get above a
major resonance. The chips came off hot and blue.
Oh, you bet they will!
The depth of cut is
0.125" (0.25" diameter reduction) and the feed is 0.0026" per turn. I
think I had to back off somewhat on the depth of cut, but it was in the
range.

Then I went back to the 5C collets. Now I'm able to take a 0.250" depth
of cut (0.5" diameter reduction) out of a 1" diameter 1018 bar at 1600
rpm, in power feed (0.0026" per turn). The chip spirals off steadily,
turning dark blue as it goes, the oil emulsion coolant boils on the
workpiece, and the cut surface is a mirror.

The tool is a BXA-16N with pos-neg carbide inserts.


I was also able to cut the 1" bar off at 600 rpm in direct drive, with
the SGIH 19-2 blade upside down and the lathe in reverse, a heavy spray
of oil emulsion coolant, and a power cross-feed of 0.0013" per turn.
You generally want quite agressive infeeds on cutoff operations,
but if the work material absolutely won't work-harden, then you
can get away with slower feed.
In any event, this lathe is beginning to act like a real lathe, and I
bet a lot of things that didn't quite work right before will now
suddenly work a lot better. This looseness had to have been undermining
everything to one degree or another.
Well, the most important thing on the lathe is the spindle, of
course, so if it isn't right, then nothing can be machined well.

Jon