Re: Ballscrew installation issues

> You may well end up using a half step
> driver for increased torque and reduced
> resonance reasons.
> A 200 step per rev motor is 400 discrete
> half steps per rev.

I may ultimately end up using half stepping, however I'd like to have
the mechanical advantage of some gear reduction. The crosslide
doesn't need to move very fast, just precisecely.

> Also note the McMaster 5/8 screws are
> 31/64" pitch; with 1:1 drive, I
> calculate: 1.8 deg per step motors, 200
> steps per rev, 400 steps per rev half
> step mode;
> 13/64" screw lead =(.203125 inch per
> rev, or 4.9230769 rev per inch)
> So: .203125 / 400 = ~1969(.2792) steps
> per inch for ~.0005 resolution.

I have heard rumors (which I cannot personally confirm) that these
screws are actually .200 (+/- .004/foot or whatever the spec is) and the
"31/64" is just a silly rounded off conversion cause someone decided
the catalog listing needed to be fractional. Regardless, it's
probably easier for a computer to keep track of a decimal than it is
to find toothed pulleys in the right ratio (they'd be huge).

Ultimately I'd rather not trust the screws anyway - I'd like to have a
good linear encoder on the axis. And even that won't ultimately be
trusted since stiffness limits and tool offsets have to be figured in.
I'll preprocess a conversion factor into my programs if that's what it
takes to make parts come out the right size when I measure them with
calipers.

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
30 Griswold Street, Cambridge, MA 02138
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