RE: [CAD_CAM_EDM_DRO] thermal expansion

An axial float is certainly necessary on a lightweight machine, but
fixed/fixed mounts are common on more rigid machines. This is not my
personal experience, but that of my neighbor who is a former engineering
manager for Giddings & Lewis. Their standard machine stretched the ball
screws, but then their machines were commonly over 100,000 lbs and millions
of dollars. Sometimes they used hollow ballscrews and flooded the
ballscrews themselves, They always used linear feedback, not depending on
the screw for precision, just motion.

Stretching even a 3" diameter ball screw is not too hard when the cross
sectional area of the iron frame supporting the end points is 12 sq feet or
more.

GTJ

-----Original Message-----
From: Les Watts [mailto:leswatts@...]
Sent: Tuesday, January 28, 2003 6:15 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] thermal expansion

The real issue is certainly ballscrew heating on the
larger gantry machines. We had a pretty good thread
a while back, so I won't rehash it. It can't really be
ignored on longer screws. The bottom line is:
with fixed- fixed end bearings one end must be able
to float axially a little. A duplex pair of bearings with
a push fit in their housing is what I use. It is very
lightly spring preloaded. The forces generated
by a tensioned fixed mounting are absolutely huge
even with a few degrees temperature rise. Kind of like
ice forming in rock cracks and splitting them...

We have some numbers on the phenomenon in the
thread.

Les


Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus cnc for sale:
http://www.alltel.net/~leswatts/forsale.html