Hi Jonathan,

Many thanks for your comments. As soon as I took on the machine it seemed obvious that the long supported rails under the table for the X-axis in particular were far from ideal. The gantry wallows from side to side mid-travel if you wobble it, especially due to it being too tall and with significant mass on top with the Z-axis motor. It came with NEMA-32 motors - since swapped for NEMA-23 - so it was even worse then!

So new rails sound like a really good move - I presume for the X-axis at least? Do you think I could get away with the other axes as they are for now? I do have a plan to shorten the Z-axis travel by about 100mm - thereby shortening its rails and overhang - whilst still retaining a 150mm Z-zxis movement, which I think is fine for my needs. It also loses 50mm off the gantry height, which must be a plus!

All that said, I am still mystified by the result of my MDI tests. Cut a mark - MDI for 100mm - cut another mark. And the distance as measured with a digital caliper between the marks is wrong by the percentages I have been quoting (+0.22%, -0.13%). In fact the error is clearly visible using a steel ruler! I felt the rigidity was not a factor in that test, hence I'd isolated the error to the screws. Maybe my simplistic understanding of it all is getting in the way here....!

So what kind of effects would I expect to see currently, based on your experience? Would there be dimensional errors like I have described with these unsupported rails? Reasonably repeatable ones? And can you think of a way I could run a test that proved it was a rigidity issue? I thought my CW and CCW circles did that, but perhaps not? It would be amazing to have a more definitive test that just showed up that the machine was not rigid enough, and that the errors were the direct result. I'd be ready to believe that ahead of the ballscrew errors - especially based on your comments at the end of your post.

Many thanks for the pointer to the test G-code spreadsheet. That looks like a great test - I'll give it a go at the weekend and see what happens....!

Cheers
Graeme