In article ,
Jon Elson wrote:

Joseph Gwinn wrote:
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.

Yes, of course, you are dead right! But, proper setup of the
tool and how it relates to the toolpost, compound swivel, etc.
can make a big difference. You mentioned in an earlier post, I
think, that interference with the chuck jaws required you to
have so much overhang, so that points out that better work
holding would reduce the problem.

I knew the setup wasn't what it should be, but was limited by the
available toolholders, which were either too short or far too long. The
hope is that the coaxial boring bar will solve this problem.


I had some real problems doing these types of operations on
Atlas/Craftsman lathes. The 5914 is a MUCH sturdier lathe with
a much heavier bed casting, so I'm surprised you were having
this sort of trouble.

So am I, and it has lead to a real research program. I certainly did
not expect that parting off would involve so much drama.


I finally traded up to a Sheldon R15-6,
which is an even heavier lathe, in the 3000 - 3500 Lb class.
You can stand on the bed with no measurable deflection (and I
have precision electronic levels that are REALLY sensitive).
It is amazing what a difference stiffness of the lathe makes.

This is about three times what the 5914 weighs. If one assumes that
this is accomplished by tripling the thickness of everything, the
stiffness will increase by the cube of three, or a factor of 27, so I'm
not surprised the a few hundred pounds on the bed has little effect.

What are the dimensions of your Sheldon? One reason I didn't go that
heavy was space limitations, and the difficulty of getting such a heavy
machine into my basement.


Now, another comment is that you may have a poor "fit up"
between your carriage and bed. I totally rescraped my bed and
carriage to get the best fit between these surfaces. In my
case, due to the hardened bed, I had to grind it with Cratex
polishing wheels and then lap it smooth with bench stones.
(A carbide scraper blade wouldn't even SCRATCH the bed.)
I then used Moglice castable way liner to make the underside of
the carriage conform to the bed.

There is some bed wear which prevents the hold-down plates from being
too tight on the underside of the bed way, but the slop is about 0.001".
I did take one 0.002" shim out from under both hold-down plates, which
both had visible wear.

One reason that self-feeding is less of a problem when the tool moves
perpendicular to the spindle axis than when the tool motion is parallel
to the axis may be that the carriage is narrower along the way than
across the way, by a factor of about two.

The ways were hardened and ground, but not scraped. But I doubt that I
will ever have the time or energy to re-scrape them.


But, after all that work, I have no problem parting-off or with
other jobs where there is a bit of overhang. I haven't done
anything with quite as much overhang as you needed on this job,
but my guess is there is somthing in your lathe that is "not
right" and allowing maybe the carriage to rock about some local
high points. Sometimes you can see this if you take the way
wipers off. You might be able to see the oil fillet between the
bed and carriage "pulse" a bit as force is applied to the
carriage. That is a dead giveaway that the carriage is rocking.

I think that it does rock slightly, as I can feel slight movement if I
put a finger spanning the gap between carriage and way. But the skin is
very sensitive to motion in such setups, and this is probably the
~0.001" slop I already know of. The problem is that this was observed
while the lathe was shaking with chatter, and everything was bouncing,
and it wasn't clear what was cause and what was effect.

A dial indicator between carriage and way and an iron bar in the boring
bar holder (to apply force while the lathe is stopped) may be useful
here, though.

More generally, I've been finding and fixing many problems, few of which
were huge, but they do add up.

Joe Gwinn