RE: [CAD_CAM_EDM_DRO] Re: Backlash

Hi Jon and Vince.

My mill isn't a BP, it's much bigger and stiffer, built in 1989, and has low
hours on it so no curved bed or wear in the screws.

With the screws removed on mine the table is very hard to move, I mean very
hard, a full body shove, even with the gibs loosened. The guides are all
pressure oil lubricated and it runs with oil when operating. The table is
very heavy, but not finger movement like Jons. Are you sure Jon it isn't
this loose due to the wear?



To some extent I would have thought the measurement of backlash is more
realistic with a tight bed than a loose one.



As regards screws, and I'm afraid my experience at looking at their accuracy
is only with my machine. Have worked on many others but never been bothered
with backlash.

I had a lube pipe fail to one of my screws so I stripped it to clean any
debri out, check the balls were OK, so any bad ones could be replaced before
the expensive screw was damaged and rebuilt it. I have two ball nuts that
run free on their own, but are backed up together to provide preload between
them and backlash control, a keyway sets the preload so it was reassembled
the same as dissassembled. I can push and pull on the ball nut with a 1/10
DTI against it and can't see any movement now, actually the nut assembly is
quite stiff to turn. In the machine there is a few tenths movement, this all
comes from the end radial bearings, and it is the same all the way along the
bed. On my machine the bearing runs on ground ways, I have tightened then
hand tight, I am sure if I tighten them more I can remove the backlash, but
am unsure how tight to go.

You have sent your screws back to the manuf. Who says they are OK, one
assumes this means minimal play in the ball nut, the only thing left the
manuf doesn't check is the radial bearing which you must set when you
install the screw. This must be where all your play is (assuming it's not
the saddle etc).



So this brings the question we all need to know (At least Vince and I), how
tight do we preload the radial bearings, does anyone have a rule of thumb or
other guide line?



I think that all makes sense.

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 Jon Elson
Sent: 25 March 2007 20:38
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: Backlash



Vince Endter wrote:

> The backlash is the same regardless of where in the travel the
> ballscrew/nut is. For the backlash to be caused by flexing, the
> ballscrew would have to deflect .215". I can garentee that the
> ballscrew is not flexing that much. As for twisting, the pully in the
> video rotates almost 4 degrees before the table starts to move. The
> table takes about 40 pounds of pressure to get it moving

What? Something is seriously wrong, here! No way should it
take that much force! I have a badly worn Bridgeport that has a
very definite arc-like motion due to concave wear on the bottom
of the table and convex wear on the top of the saddle. I can
easily see this if I mill a full circular track on a piece of
scrap. (I mean I can "see" it with a dial indicator.) It was
last scraped at the Bridgeport factory in 1938!

But, even with all that wear, I can get the table moving with
5-10 Lbs of linear force, and once static friction is broken, it
will glide with less than 8 Oz of force! I have only handled a
Bridgeport with really good ways once with no screws mounted,
but the table was amazingly free to move, and you could slide
the table almost clear off the saddle with one finger.
and about 5

> pounds to keep it moving. Hiwin said their ballscrew have a working
> range well over 400 pounds.

Just because the screw can deliver 400 Lbs linear force safely,
doesn't mean that substantial forces delivered through the screw
won't deflect the system. For instance, a 50 Lb force on a
manual Bridgeport's quill deflects the cutter at least .001"
This is flex in the structure that mounts the head to the base.
You think of these things as being absolutely stiff, but there
is no such material.

Jon





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