RE: [CAD_CAM_EDM_DRO] Backlash

Vince if your looking at the end float in the screw before the bed moves,
detecting the move with a 1 thou DTI, your dti probably will need a few
tenths of a thou before it moves the scale.

For overall backlash, I would put the tenth thou dti against the table, and
jog in steps until you see the bed move, it will probably be less.

I have mounted my cheap 1 though DTI against my 1/10 thou Baty DTI on the
same surface and the 1/10 will often see 7/10's before I can see any
movement of the 1 thou DTI. Also when you have long arms from the mag
mounts, there is going to be some spring in these, we are stuck with them
but I hate the things. I service kit where they work to microns, and the
arms on their DTI's are massive, and so much more rigid than the cheaper
ones like I have and you in your video.



On my mill I have expensive ground screws and preloaded ball nuts. My
measurements are as crude as yours, my best DTI being a Baty 0.0001 per
division. I have 3 to 4 tenths before the bed moves, on the worse axis, and
it appear all this is in the end radial bearing of the screw, not the ball
nut. I have tightened the end bearings but still have 2 to 3 tenths play. I
rebuilt one of the screws the other day, didn't seem any better afterwards,
but with the DTI bolted by it's tap in a big CNC mill vice, the play on that
axis is now less than 2 tenths, really though I wouldn't rate a 1/10 DTI for
a measurement this small.

Realistically a few tenths isn't that bad, a Bridgeport mill is listed to
have backlash at less than half a thou when new, with ground screws.

Both our machines are better than this, and I think you be lucky to find a
cutter that will know the difference within it's own cutting stability.





Regs



Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Vince Endter
Sent: 24 March 2007 16:47
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Backlash



I have been trying to chase down a backlash problem I have with my
Hiwin ballscrews for almost a year. I even sent the ballscrews back
to Hiwin and they said there is no detectable backlash in them. I
made a video of the test recommended by Hiwin. In the test I
attached a ball bearing to the end of the ball screw shaft so I am
not measuring any out of squareness in the end of the shaft. I am
using a tenths reading (.0001") indicator on the end of the ballscrew
and a thousands reading (.001") on the table. I am measuring
between .0003" and .0004" backlash in the end play of the radial
contact bearing, and .004" on the ballscrew/table movement. The gibs
are adjusted to that if I push as hard as I can anywhere on the table
I can see only .0005" deflection. The mill was recently reground and
hand scraped by a professional machine tool rebuilder.

There are only two places I can see the backlash coming from. One is
the yoke, which was supplied by Hiwin (very large cast iron, I can't
see that flexing), and the other is the ballnut/screw combination.
The ballscrew has a single nut. Hiwin says they take care of back
lash in one of two ways, either with a different pitch between the
nut and screw, or by adjusting the ball size in the nut. They won't
say which method is used in my installation.

Here is the video of the test:
http://www.flyingcr <http://www.flyingcritters.com/video/Backlash.wmv>
itters.com/video/Backlash.wmv

Any suggestion would be greatly appreciated.

Vince





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