Screw Whip

[Bob La Londe]

Ok, at what kind of speeds / length does screw whip start to become a problem.

My Taig as about 12-13" of X travel now with a 20 TPI screw. I've got it running rapids at about 60IPM. The floating end of the screw is unsupported at about 20" long give or take, and it does not seem to have any problems with screw whip. It’s a half inch "precision" V thread. The math is pretty simple. Its spinning 1200 RPM when it does rapids. (I had it going faster, but I think I turned it down to 60 for some safety margin.) (Bumping the kernel speed in Mach made it stable at faster speeds than before.)

Most ball screw setups support both ends. I have a spare ball screw I could use on that axis, but it would sacrifice some travel if I supported both ends. It doesn't require its full length to give it full travel if I do not support the floating end... ie remove the bearing. Its also slightly larger than the existing 1/2" screw at 16mm which should make it more rigid.

... and it has a much faster pitch at 5mm or roughly 5.08 TPI. I could run the same 60IPM at only 304.8 RPM or run it much faster. Seems to me screw whip would really not be a problem.

Since its just got basic simple ways and brass gibbs I might not want to run it a whole lot faster anyway unless I add oil grooves and an oiler while I have it apart.

I dunno. What do you think?

I've got a 0601 ballscrew with no free end support on the Z Axis of the MaxNC. Its working fine at 60 IPM and 1mm pitch, but its only got about 6 inches of travel only about 4 useable travel.

[Bubba]

Bob,
I have seen on Mfg's sites, graphs and formulas for screw whip vs length etc. I thought I had one on my drive, but can't find it now.