RE: [CAD_CAM_EDM_DRO] Z-axis CNC conversion on a mill-drill: a challenge (long)

I have to ask a question about some of the assumptions in this message.

You are aware that preloaded, anti-backlash gears are available. If I
understand what the problem is, an anti-backlash gear could be substituted
to the standard gear against the standard rack and driven by a
stepper/servo. The anti-backlash gears I�m aware of are simply 2 gears
spring loaded against each other by using two springs in pockets in both
gears such as HYPERLINK
"https://sdp-si.com/eStore/GroupHead.asp?GroupID=205"https://sdp-si.com/eSto
re/GroupHead.asp?GroupID=205 In fact, it might be realitively easy to make
your own anti-backlash gear to replace the stock gear.

I have also made an anti-backlash rack, using two racks spring loaded
against each other. I used shoulder screws to restrain one rack against the
other and used a spring and shoulder screw on one end of the movable rack to
limit travel and provide the end loading.

I had good luck with near zero backlash although what I was after was a
horizontal motion and didn�t have a huge load to deal with.

Could something of this type be of use in driving the quill?



R. Wink

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of arquibaldo2001
Sent: Saturday, April 28, 2007 1:19 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Z-axis CNC conversion on a mill-drill: a
challenge (long)



Dear CNC gurus:

I have a design challenge for you.

I own a RF30 mill-drill clone and am planning to convert it to CNC.
I don't think the X & Y axes will be a problem - a lot of work and
expense, but it should work all right - the conversion kit sold by
Industrial Hobbies and the great step-by-step instructions on its
web page can be used as a guide, even if the kit itself might no
longer be available since IH is (was?) closing shop.

However, the Z axis conversion is a virtually unsovable
problem, or so I thought until recently: instead of the
screw/nut system of the X & Y axes (which can readily be
converted to ballscrew/ballnut or even to original-acme-
screw/backlash--compensated--nut) the Z axis uses a rack/pinion
arrangement on the quill which presents a huge backlash.
Therefore, it is useless to power the pinion side: see the
dreaded "quill drop" syndrome which has been discussed in this
and in other forums.

The only "solution" to the quill drop problem on a mill-
drill that I know of is adding a huge amount of "upwards"
preload, via an awkward system of cable, pulleys and
counterweight. I believe this system is unpractical,
besides requiring a huge motor to counteract the preload.

Now, however, I think I found a solution in the
brilliant design introduced(?-) by "WT" (see message#90489)-: simply
abandon the rack&pinion and add a ballscrew/ballnut to
the Z axis. WT has shown the basic idea and where to fit
the new system, however no details were disclosed, and this
is the challenge I would like to pose, since this design
is far above my mechanical skill.

The problem is that, instead of the usual arrangement
(ballnut fixed on the moving piece & ballscrew rotating and
attached by bearings to the fixed piece, driven by the
motor), this approach demands the opposite: ballscrew is
fixed, attached to the quill, and the ballnut sits on the
fixed piece and is rotated by the motor - it seems that
we must have a pair of angular contact bearings and a
timing pulley around the ballnut and this contraption must
be fixed on an aluminum plate which will also hold the
stepper (or servo) and its smaller timing pulley.

Can anybody suggest a design for this
rotating-ballnut--plus-timing--pulley? A drawing or rough
sketch would be nice...

Thanks a lot.

Nelson




No virus found in this incoming message.
Checked by AVG Free Edition.
Version: 7.5.467 / Virus Database: 269.6.1/778 - Release Date: 4/27/2007
1:39 PM



No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.5.467 / Virus Database: 269.6.1/778 - Release Date: 4/27/2007
1:39 PM



[Non-text portions of this message have been removed]