An Unusual (to me) Chatter Problem

Hi all, seems very quiet but hopefully someone is around to help me
with a small problem with my S7 lathe.

If I try to take a cut of more than .010" I get some "chatter" but not
over the entire length of the cut. It happens at all diameters and most
reasonable speeds. If I chuck say a 3.5" long piece of 1" diameter
(mild steel) in the (new) backplate mounted 3 jaw chuck and take a cut
over .010" with a CCMT 10mm sq tool, the cut starts OK for about 1"
then when about 1.5" from the chuck it starts to chatter this happens
for approx .5" and then clears again as it gets nearer to the chuck
jaws. It happens with different chucks but is slightly better with a
threaded body Bernerd as you would expect although it doesn't clear
completely.

The spindle has been adjusted and if anything is a little tight as it
warms up a touch at high speed, the saddle is adjusted OK and the
topslide and crosslide are locked. As far as I can measure the bed is
within .0005" thickness/width over the area. I have clocked on the top
of a test bar to see if the centre height is changing but it appears to
be within .0005" from about 5" out. The speed is about 1000 but it also
happens at a faster or slower speed unless I slow it right down to less
than 350ish. I've tried a new tip .02 and .04 rad, newly sharpened HSS
tools and a brand new carbide but they all do the same. The tool is set
spot on centre height and it is the same if set a little low. If set
high I get the intermittent banding as the tool goes in/out cut. I've
tried a larger .5" tool without the Dixon QCTP but it is exactly the
same.

To be honest it is driving me mad so any help/advice/experience you can
offer would be much appreciated.

Thanks in advance, best regards

Keith

.

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