On Sat, 17 Mar 2012 20:26:54 +0800, "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....

Cheap allthread will have pitch errors. Over a certain distance,
decided by the maker, the movement will even out. Does this error
occur everywhere on the screw? If you program a movement of 25mm does
the machine only move22.5mm? I bet it doesn't. If the screw really
only moved .9 to .95 of what it should then the pitch would be off by
..002" to .004" per thread. This amount of error would cause the screw
to bind in the nut. Of course, it really doesn't matter that much
since the machine is CNC. You just program it to make up for error.
Many of the most accurate positioning machines do exactly this. The
machine is moved and the movement mapped. Then the map of position
error is used by the control to adjust the movement of the machine so
that it moves the desired amount.
Eric