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

[Jonathan]

I think you are asking a lot of a machine with unsupported rails on all axes. Maybe put some better rails on your Christmas list!

Yet another way to check the XY axes are perpendicular is do put some MDF on the bed and drill holes in the corners of a rectangle, then push drill bits in the holes and measure the diagonals with a digital caliper. Size the rectangle to match your biggest caliper.

You should also try measurements on different parts of the bed - e.g opposite corners. This will tell you more about the source of the error.

To make sure it's not something electrical (I doubt it too), you could try zeroing the machine at a known point on the dial indicator, then run some random g-code:

http://www.mycncuk.com/threads/5843-...g-motor-tuning

If anything will cause it to loose position, that should do it.

Edit:
A note on ballscrew accuracy - C7 is up to 0.05mm over 300mm. This doesn't mean the error is distributed over the 300mm, e.g. a screw with a 0.04mm error over 40mm and 0.01mm error over the remaining 260mm would be classed and sold as C7. This sort of error (though hopefully not as extreme as my example) is more likely than an error over a broader distance, as if that were the case the manufacturer could compensate for it similarly to how you might do so in software. That's why I'd be wary of entering anything other than the calculated value for step/mm, as it will compensate only for the portion you measure, on average. You need to be very confident of the measurement accuracy to do this.