Re: [CAD_CAM_EDM_DRO] stepper motor accuracy

----- Original Message -----
From: <ccs@...>
> However, I wouldn't worry about it on a machine tool, and here's why.
> To get decent torque and small increments, you are going to use
> substantial gear and screw reduction. For example, 1:4 reduction
> driving a 5 tpi screw. That's effectively 20 tpi or .050 per
> revolution. Dividing into 400 half steps gives increments of .000125.
> That's the best you can hope for in terms of positioning, so this is
> clearly not a high precision machine

Aiming for such accuracy is all very laudable but, in real terms, is it
really achievable on a 'home' machine? Does anyone here actually measure
their parts to 1/10th thou or better and do they worry if they're off by one
or two? Tool and machine flexion, blunt tools, stick-slip, backlash, program
'appropximations' etc. are all likely to give errors in excess of this. From
what little I know of 'real' engineering (mainly in the watchmaking and
aircraft building industries), machines are set up by a 'successive
approximation' method where the machine is set to theoretical values, a part
is made and measured on a very expensive measuring machine and adjustments
made to the machine to compensate for errors - this process being repeated
several times until repeatedly accurate parts can be made. Even then the
watchmaking industry uses a 'grading' system where parts are sorted for
accuracy after manufacture to select those for the better grade movements,
the remainder being relegated to lower grade 'other name' watches (for
example, Seiko make Pulsar and Lorus as their 2nd and 3rd grade watches). I
am often asked what accuracy I have to work to when I make replacement parts
for watches and its a question I can't really answer as I hardly ever
measure anything - one part has to be made to fit the next exactly and so
virtually all 'measurements' are by comparison (things like 2 screw go-no go
gauges or actually feeling the fit of one part in another) or visually using
a 50x shadowgraph etc. I'm sure other people must have some views on
this...........

Ian
--
Ian W. Wright
Sheffield UK
www.iw63.freeserve.co.uk