I'm slowly getting to grips with my Myford Super 7 CNC conversion. Initial results were promising but I have got caught up in a "problem loop" with the accuracy of the X-axis (i.e. cross slide) feed.
I have fitted a new leadscrew which is 20 tpi though I actually work in metric. For complicated reasons this is driven through a timing belt with ratio 4/7 from a standard stepper with 8 times microstepping on the driver. "Steps per mm" then works out to 2204.724. The problem I have found is inconsistent diameter turning using the turn wizard. The diameter ends up up to 0.01 mm or so different from what it should, sometimes larger and sometimes smaller than nominal.
I have set about calibrating the leadscrew pitch using a 123 block measured by micrometer to ~25.41 mm (actually measured to a micron) and using the procedure recommended in the setup instructions. I am setting the slide to a zero with a DTI against the block, the latter mounted against an angle plate on the cross-slide, then removing the block and moving the slide under manual control to zero again and looking at the movement on the DRO. The DTI is a very nice surplus indicator that has a travel of +/- 25 microns and indicates to 0.5 micron.
Apart from a small indicated error of about 0.08 mm over the travel, I get relatively inconsistent repeatability if I move the slide back 25 mm and then bring it back to zero, of up to about 0.01 mm. I can reduce this by firmly pushing the slide back against the screw thread before reversing the feed to come back to zero - then I can get to a few microns. So I was wondering if anyone could help with the following questions please?
Are my expectations of accuracy too ambitious? If turning down to a specified diameter using the wizard, what is a reasonable accuracy to expect?
Moving the slide repeatedly backwards and forwards to the same nominal position, what repeatability can be expected?
The leadscrew is lubricated with a molybdenum grease, but it seems to me that this may be forming quite a viscous film between the screw threads and the nut, the thickness of which will depend on the force on the screw - would I be better using a light oil?
I don't think I am losing steps, but I was wondering about just how repeatable microstepping is - has anyone any experience of problems from this source?
Any answers to these or other suggestions would be most welcome! Thanks in advance,
John.