Teething problems...

Things have not been all rosy in reprap land (what else is new?) The extruder started playing up again, and all the prints so far have come out elongated. The stretching isn't just in X or Y either, which would point to an easily addressed calibration problem - instead it was along the 45 degree line. There's only one explanation for that - my cartesian bot was a parallelogram ;_;

After some thought I figured out that I could move the belt on one end of the X carriage back by one tooth to change it's angle with respect to Y. This seems to have cured the lean mostly (although I might still move it back one more tooth) however circles are still coming out squished in X. And it's not a calibration issue, because a simple square comes out perfectly, and smaller circles are more strongly affected than larger ones.

I discovered what I believe to be the answer by watching the reprap doing the crosshatched infill on the large gear pictured at the top of this post. The left pattern in the diagram is what I should be seeing, but I was getting the right pattern instead. The bad pattern is being caused by backlash on the X axis. It might be caused by a loose grub screw on the X stepper drive shaft, or it might be a less easily corrected fault. Skeinforge has a backlash-mitigation option available; if I can't solve the problem mechanically, I'll solve it in software.

Speaking of Skeinforge, I'm slowly coming to grips with more of the obscure functions it provides. It's ridiculously powerful but it's hell trying to understand what each option actually does. Additionally, certain features are hidden in places that one wouldn't normally expect to find them. I finally found the option to shift the starting point of an object in the Multiply section - I just set it up to print one copy of the object, and I configured the other options there to automatically centre the object in the middle of the print bed. That solved the negative-axes problem once and for all.

And the extruder? I got fed up with it and tightened the springs down as far as they can sensibly go. And it stopped misbehaving. But I'm still going to replace it with a Wade's extruder as soon as possible. I've printed out a couple of parts for it already - hence the large gear :)