Axis scaling errors - is this just to be expected with C7 screws?

[GND]

Thanks for the initial thoughts, gents - to cover them in order;

How did I check the Mach3 setup? I have used a plunger type DTI to check that a commanded movement of 1mm equates to 1mm of actual movement. It did, with no more than 0.03mm error each time - which wasn't cumulative, and so seems to be within typical C7 expectations. So 1mm might have come out as 1.03mm actual, and 2mm might have then been 1.99mm, if you see what I mean. These are therefore spot errors of perhaps 3%, but they average out over working distances. I also checked longer distances using digital verniers, and they showed the type of errors I have described in my initial posting. But I had difficulties getting repeatable readings using these, as a degree or two of skew made the numbers change by a lot!

In terms of the machine, I have some rather poor pictures - see below - which hopefully at least show you how each axis is made up. It is an unwieldy thing, with a too-high gantry and uses unsupported rails throughout. I could make many excuses for this - but the one I'm sticking to is that it wasn't designed nor made by me - I took it on as an unfinished project. And it wasn't originally supposed to be a router - it was a welding machine! Hence not ideal, but hopefully I can do something with it, which doesn't involve starting again....

As for how the errors have come in - I measured a circular item I made right after commissioning a year back, and it turned out to have the same error in it. So it was only when my application required accuracy, that I noticed this sub-millimetre error - it has always been in there.

Cheers
Graeme