I have an image in my own mind of how you have set this up Gary – but a photo might help.

I cannot give you a simple "It's this…" answer but clearly if the guide surfaces are linear – then movement in either the cutter or work piece is occurring.

From what you say, I don't think I'd look first at tool deflection. It seems to me most likely that the work is able to move slightly at each end of the work travel, resulting in a deeper cut in the middle of the work. In other words the cutter is unable to apply quite the same force at the ends of the work piece.

I've had something similar happen when milling long work held centrally on the vertical slide. I was able to use adjustable 'hold-ups' (e.g. small screw jacks) at each end which helped considerably. They needed to be reset after each movement of the vertical slide. Whether you can use this in your case – I don't know.

Otherwise – is it possible that there is "rock" in your vertical slide (e.g. the gib isn't tight enough – or you are not locking the slide before the cut?

Regards

IanT

Hi Gary,
Nobody so far has mentioned on this thread anything about how are you holding the milling cutter? Not in the 3 jaw, I hope. If cutter has a screwed shank it should be screwed into a proper milling chuck. If it has a plain shank with a flat then use one of those alternative “chucks” and do up tight, side screw on the flat. You must also use a drawbar to prevent holder from vibrating loose especially as its almost certainly a Morse Taper.
NOTHING ELSE WILL DO.
No drill chucks, no lathe chucks no matter how many jaws they have.
I am thinking that maybe the cutter is moving a small amount, if its in the 3 jaw and causing your problem.
Rgds.,
John.

I was never very happy with "flat" surfaces on a lathe and bought a shaper

even a well done simple facing job had issues, which were compounded if I tried to say, make a square block, or a "flat plate"

For me lathes do "round stuff" well and shapers do "flat stuff" well

Edited By Ady1 on 06/08/2015 10:21:54

Try without the angle plate. You should be able to turn the vertical slide to mount the plate on the correct angle.

+1 on clamp the job directly to the slide table. Seems like a lot of overhang with that angle plate.