Dennis wrote:

I've just test run a little CNC router / mill I've put together.

At the moment I'm using M10 x 1.5mm pitch allthread for the lead screws and
PVC blocks for the nuts.

I've calibrated the axes (Mach3) using the 1.5mm leadscrew pitch and the
known motor steps/rev.

With a dial gauge on an axis, when I do a 1mm move sometimes I see 0.90mm or
0.95, sometimes 1.0. It seems repeatable, ie if I reverse the steps I see
the same values.

What do people think? I guess this is typical for cheap bit of allthread
from the local hardware store?

ACME leadscrews are on the drawing board when finances are willing....

Yep ordinary all-thread isn't particularly precise. You can do screw
mapping to compensate for the pitch error to some extent. Backlash
compensation is more difficult so try to use an anti-backlash nut setup
and ensure solid mounting of the leadscrews. ACME screws aren't
particularly expensive from ENCO or MSC.