https://www.dropbox.com/s/omwtz0gplg0z7gd/2016-08-11%2016.08.33.jpg?dl=0

Anyone any idea why circles are distorted as in the picture?

Sorry about the link but can't work out how to post the picture here any other way.

Mach 3 on Win 7

Motor tuning, acceleration too slow.  My initial thoughts when reading your message was scaling active on one axis or another but in that picture it definitely looks like an acceleration setting issue... at least to my limited experience.

Stephen "Highspeed" Kruse

Motor tuning, acceleration too slow.  My initial thoughts when reading your message was scaling active on one axis or another but in that picture it definitely looks like an acceleration setting issue... at least to my limited experience.

Ah that makes sense, I presume that both the X and Y axes should be set to the the same values as both have similar motors and ball-screws (Nema 23 and 16mm x 5mm pitch) Max speed is set to 4500mm/min and motors driven at 2000 ppr. ?

I would say yes, they should be the same.

Highspeed

Looks like backlash issue to me.
Hood

Looks like backlash issue to me.
Hood

Do you mean mechanical backlash? if so I can rule this out completely, both axes have brand new zero backlash ballscrews! If not then can I eliminate it within Mach 3?

Yes that is what I meant, it could however be in places other  than the actual ballscrews.
For example it could be a very slack belt or a loose pulley or worn or badly adjusted bearings or even flex in brackets.
Check for backlash using a dial indicator, check the Y especially as that to me looks like your problem axis.
Hood

Many thanks Hood, I'll give it a good look over in the morning. Cheers

Also check you do not have backlash comp enabled as if you do not have physical backlash but have comp enabled it could cause a similar issue to what you see.
Hood

Many thanks Hood, you were of course right on the money here, the Y axis ball-nut mounting hadn't been tightened up properly during assembly of the machine. I remember leaving it loose so as to ensure it was aligned correctly when putting everything together. I'll have to check everything else for soundness now! Doh!