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

You are correct that most mill drills come standard with a .020 backlash in the Z axis. The problem that causes this slop is the excessive distance between the quill rack and cross shaft pinion. I solved this by making two encentric bushings that replace the existing ones and by turning down the pinion shaft on the fine feed side. I have an article about solving the mill drill backlash problem that I could email you if you want to see it. My mill drill fitted with this set up has only .002" vertical play which is probably would you would have if you used a ballscrew.
Jeff Davis at homecnc has some nice plans for a ballscrew conversion if you are bent on using a ballscrew here is the link. http://www.homecnc.info/plans.htm
Dan Mauch
economical 3D scanner software and kits
low cost stepper and servo motors.
cases for Gecko drives
kits and assembled 3-4 axis drives
www.camtronics-cnc.com
www.seanet.com/~dmauch

----- Original Message -----
From: arquibaldo2001
To: CAD_CAM_EDM_DRO@yahoogroups.com
Sent: Friday, April 27, 2007 11:19 PM
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





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