Many thanks for your replies Gerry and Neale.

I have three Astrosyn motors on the machine two L709s (x&y) with an inductance of 7.7mH and one L259RE (z) with 3.5mH - both physically the same and both with the same torque (2.8 and 4.3 amps respectively). I've used Nema34 motors as that's what the original stepper motors were and these bolt straight on with no modification.

I've changed over to parallel connection and set the driver to 5.86A rms and it's much, much better. Motor heating much lower too.

I had the drivers set to 32 microsteps since this seems to give the best compromise between speed and smoothness. Just to address Neale's point about steps - the feedscrew is 4mm pitch. 200 steps = one rev/4mm travel. At x32 = 6400. 6400/4 = 1600 = 1/4 rev or 1mm travel.

Having it set at 32 microsteps means that I can't go faster than around 1000mm/min which is a bit slow for rapids, but I can live with it. I've just knocked the drivers down to 8 mcstps and set the travel at 1800mm/min and this works well except that the "sensorless stall" protection trips from time to time. I'm guessing this is down to the power supply not being up to it.

Any recommendations for a source of an unregulated power supply?

Finally, I'm keen to use the Protuner software, but my computer doesn't have an RS232 serial port - any advice on how to connect?

Thanks again David