Ah ok I understand now.
Really you shouldnt have to use something like that but if it can be used on some other project at a later stage it may be worthwhile.
Have you tried testing for flex in the machine? For example you could put a dial indicator on the axis and put the plunger against something solid, then push a bit on the axis and see what kind of reading you get. Also worth testing for backlash like you have done but at the same time pushing a bit against the motion as that may show the flex up.

Hood

I am sure I could always use it somewhere else!

I have not tested flex as I was not sure how to do that. I'll try it the way you've described. I do need to get a more accurate and reliable indicator though. My current one is cheap and not very good. I'll report back once I've tested this.

Thanks for the help!

Not sure what kind of machine you have but also test for quill/Z axis play by putting the dial on it and gently pushing either way, shouldnt need too much force to see movement if there is any.
Hood

OK, thanks! I'll try that too.

Hi Dave
We have a CNC addition to a profile cutter and have noticed the same thing start to happen.  It was OK for quite a while (more than a year) and has just started to do this.
Ona 300mm diameter circle we end up with two flat spots on opposite sides of the circle - each flat spot is around 50 - 60mm long.
I also not that circles are no longer "perfect" and don't seem to match up as well as they used to.
May be the same problem as yours?
I'll try and check backlash.
Kind regards
John

John sounds like backlash and quite a lot at that. Check the bearings, mounting brackets and couplings/pulleys on the motors, if its top/bottom then its the Y axis, left/right then X axis.
Hood

Hi Hood
I'll try and spend some time tomorrow and check for backlash - hopefully something has just come a little loose!
Thanks for the tip.
Kind regards
John