According to Dom :
This is driving me crazy. I've tried everything I can think of.

-Centre height OK.
-Blade not overextended.
-Insert OK
-Coolant on
-Reduced speed, increased feed
-Adjusted backlash on cross slide to minimum
-Saddle clamped
-Happens with different materials, and diameters

But I can't eliminate the vibration. It is really heavy, and is
causing clamping screws and the like to loosen.

Have you sharpened the parting blade? Does it have enough end
clearance?

Hmm ... what kind of bearings do you have in your headstock?
Plain bearings with shim adjustment? If so, you need to check for
looseness and take out shims until it is tight enough. (Your spindle is
lifting during the cuts.)

If it is roller bearings, is there a chance that the ring at the
back of the spindle which draws in the bearings tight has loosened so
there is end play in the spindle as well as vertical?

It's making me feel like a complete idiot. I should be able to solve
this problem, but what's causing it?

You checked almost everything else, so I would bet on play in
the spindle. Clamp the chuck on a bar, then try lifting and pressing
down on it while measuring the motion of the spindle.

Oh yes -- one other possibility -- have the jaws of the chuck
worn bell-mouthed, so it is only gripping tight towards the back of the
jaws? If so, perhaps it is time for new jaws, or to mount a set of soft
top-jaws and bore them to near the size of the workpiece so you no
longer have a bell-mouthed set of jaws.

Any suggestions, or should I just go and slash my wrists now?! g

Oh -- slash the wrist with the handle on the carriage handwheel.
That should be dull enough so you'll have time to think of other things
to check before you get through the skin. :-)

Good Luck,
DoN.

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