Hoyt McKagen <batwings@i-plus.net> wrote
Mike Simard wrote:
> Some random thoughts: Ballscrew mounting is easy to take
for granted but it
> determines the final backlash you'll have. It's as important
as the nut.
Mike is correct. On mine I use a preloaded pair of ball bearings
each on
X and Y. This can be done by mounting the bearings back to back
in
housing with the right shim between either inner or outer races.
If you
shim the outers you have a pair that has considerably more overturning
moment resistance than if you shim inners. I shimmed inners to
allow for
whatever bit of misalignment might have gotten through, from bearings
to
the traveling nut. The take-up of axial slop is the same in either
case.
For long screws such as lathe it's going to be necessary to
have support
on both ends. in my case I used similar blocks on each end; the
preloaded
pair was on drive end and a single unit on dead end. The single
unit has
an axial takeup for its own play. Blocks are bolted to the bed
of the
lathe on the mounts for original lead screw and gearbox. You do
not need
precision bearings if you preload them properly and the same goes
to a
degree for the ball nuts. My ball nuts have provision for radial
set
screw for taking up slop.
It's a necessity to run cover over the lathe lead screw as
it is in
chip-falling zone. I made a cover with U section that rides between
carriage and screw; it's supported by the bearing blocks and the
mount
for the ball nut is goose-necked down and then back up under the
one edge
of the cover. This goose-neck was made by bending the end of mounting
plate, which is horizontal to carriage, to a vertical pos. I then
finish
ground the mounting surface and cut the bent end square to it.
Then this
portion had slits cut into it for the U to ride in and a hole
for the nut
carrier. The nut and carrier mount in the gooseneck, centered
on screw
and completely under the U. And making all this match up was done
by
using the vise in explicitly same pos and workpieces stopped the
same
within it as I bored each part. I know, this is not very good
description, but if Peter wants me to I'll provide snaps. And
I'm sure
others would too.
The setup has been very reliable and I've had no problems with
it unless
you count the bearing blocks moving when I overload by hitting
solid
object (still working out limit switches) but I'd rather have
them shift
than to brinnell something.
Best wishes,
Hoyt McKagen
Belfab CNC - http://www.freeyellow.com/members/belfab/belfab.html
Best MC Repair - http://www.freeyellow.com/members/batwings/best.html
Camping/Caving - http://www.freeyellow.com/members/batwings/caving.html
Please don't panic, I'm only here to help!!