In article ,
"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....

As others have said, allthread isn't all that precise (unless threads
are cut).

However, there is a trick from Roland's Ruling Engine (invented in 1882
to make diffraction gratings): Use a very long elastic nut, so it
averages leadscrew errors out. Rowland used oiled wood, but PTFE ought
to work.

http://books.google.com/books?id=8eI...=PA152&dq=rowl
and+ruling+engine&source=bl&ots=tE9IMY8kn4&sig=9RN phQ9npM6RwhdN13y2KHZWEO
8&hl=en&sa=X&ei=pMhkT9OUB-XE0QHJ_PXMCA&sqi=2&ved=0CFYQ6AEwCA#v=onepage&q=
rowland%20ruling%20engine&f=false

Joe Gwinn