New build (my first) can't draw circles.

[m_c]

Looks like the stepper on the long axis (Y?) is stalling.
What stepper drivers have you fitted, and what voltage power supply are you running?
.
Realistically could be anything from loss of step pulses, a failing driver as it heats up, or something mechanical.

[Robjampar]

I think both x&y are skipping steps;

Sounds like the power supply is hitting its limit, what psu are you using?

Check the voltages when one stepper is running, and then when two are!

[mcyoung]

Looks like the stepper on the long axis (Y?) is stalling.
What stepper drivers have you fitted, and what voltage power supply are you running?
.
Realistically could be anything from loss of step pulses, a failing driver as it heats up, or something mechanical.

I think both x&y are skipping steps;

Sounds like the power supply is hitting its limit, what psu are you using?

Check the voltages when one stepper is running, and then when two are!

Thanks for the suggestions. Having not used this kind of machine before I failed to see that both X and Y were asynchronously missing steps, it just looked like a weird pattern.

I'm using Pololu DRV8825 drivers with an aluminium heatsink on top. This was the problem. I've now replaced the heatsinks with copper ones and put them on the top AND the bottom (should have read the driver documentation better - the bottom is where they're meant to go). It now draws circles correctly. I'm guessing that the drivers were overheating cutting out, then cooling, then restarting, resulting in the bits and pieces of curves I saw.

The rest of the kit is just what came with the machine from China. Undoubtedly many other bits will turn out to be problematical but for now you've solved my problem. Thanks!