Ballscrew conversion

To paraphrase my earlier conversations
with Bob Campbell and Les Watts-

Machining and mounting thrust bearing
blocks, calculating the inertia of the
screw vs. resonance, whipping effects at
high speeds, etc. have seemed pretty
daunting for me, building a gantry
router. Snip... inspires me to try an
earlier idea I had about spinning the
nut, not the screw.
Picture this: Ball or lead screw is
clamped at the ends across the axis,
gantry or table (not Z) just
a fixed mount, maybe tensioned, no
intricate machining on hardened steel! 2
nuts (or one zero backlash) attached to
tube helps stability, XL sync belt
pulley on tube, tube through flange
mount bearing, flange mounted to
motor/carriage plate.
This means the only parts to make are
the motor/carriage/bearing mount and the
nut/pulley/bearing
tube. Both could be simple parts to
make, and in my conception the light
parts would be more stable than with a
spinning screw arrangement.
Reciprocating mass in the tube assy
should be low, and the dampening effect
of the
belt drive and large diameter bearing
may inhibit resonance.

This does mean that all the motors and
wiring are on the gantry (or cross
slide); This may be an advantage if
disassembly /reassembly will be required
(I'm building mine in the office where
it's warm, I'll make room in the shop
once it's running.)
What do you think? I'm interested enough
to try first it on my gantry; parts are
only ~$100 with 48" x5/8 screw and two
nuts. If the longer axis needs a 1"
screw, the same concept could still be
applied.

I noticed IMS is selling hollow steppers
for just this use!


Date: Fri, 18 Feb 2000 00:17:34 -0500
(EST)
From: stratton@... Wrote:

Snip... I got to thinking and remembered
an alternative I might have heard
suggested somewhere: What if the screw
were fixed to the cross slide
and the nut were turned instead? It
could be captured between ball
bearings and needle thrust bearings in a
housing that would bolt onto
the back of the carriage, so cutting
forces would actually put the
screw in tension. Power transmission
would be via toothed timing
belt, with a sprocket bored out and
bolted to a flange that screws
onto the ballnut threads.