In article ,
David Billington wrote:

Joseph Gwinn wrote:
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,

I was initially taught in junior high metal class to part at a slow
speed but later in high school machining class where the instructor was
a machinist, he said to run at the recommended speed for the material if
the machine could cope as parting was a cutting operation like any
other. Still works for me. The junior high lathe was a little Southbend
so needed running slow.

It's true that parting off is still a cutting operation, but one
difference is that a parting tool is cutting over a broad area, rather
than close to a point or small radius.

The other issue was that because the toolpost rotated, one whole side
edge of the T-blade was attempting to cut, not just the tip.


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.

You want to find out what's going on there and fix it, not good having it
move and it shouldn't be normal. On my first lathe, a Kerry 1140 (11" x
40"), the first parting tool I used was a 1/4" wide brazed carbide tool,
it was what I could get at the time, and I used that to part 1" free
cutting steel without any problems. The Kerry was fairly worn as well
but the compound and toolpost don't lean under parting loads.

I do want to get to the bottom of this for sure. Maybe the compound is
pulling up and away from the cross-slide, though I don't see how that
could happen without breaking something. Like the central pivot pin,
which shows no signs of distress.


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.)

Hopefully the new bolts have sorted the movements problem. I certainly
wouldn't expect parting loads to move the compound unless the bolts were
loose. Both my Kerry and M300 have a similar compound clamping
arrangement to the Clausing and neither needs to be done up particularly
tight to prevent movement. The Kerry uses 5/16" Whitworth IIRC and the
M300 uses M8 so about the same as your 5/16"-18 fasteners.

I replaced only the one stretched bolt. Maybe I should replace both.
It may be that Clausing discovered that the original bolts were not
quite large enough. The difference is in the head. But still I don't
see how cutting off would generate enough force to stretch a 5/16" bolt.
That stretch has to be caused by a human with a wrench, using more
muscle than brain.


Joe Gwinn