On Sat, 02 Aug 2003 17:46:34 -0400, Jack Erbes wrote:
On Fri, 01 Aug 2003 14:27:30 -0400, Gary Coffman
wrote:

You DO NOT want to use ballscrews on a manual machine. With normal
screws, there is enough friction and mechanical advantage to prevent the
machine from kicking back when cutting. But with ballscrews there isn't.
You need a motor drive holding them. That's why ballscrews are only used
on CNC machines. (It is also why trying to convert a CNC machine to
manual can be a big expensive problem.)

I don't think this is right, what is meant by "kicking back"?

A ballscrew has very little friction, so it won't stay where you dialed
it against forces acting on the table. In other words, it will allow the
table to drive the screw. An ordinary Acme leadscrew has plenty of
friction, and won't do this unless cutter forces are extreme.

There should not be any problems with ball screws when cutting with a
conventional feed. For climb cutting, the hand turning the feed
crank, the weight of the table, cross and compound slides, and
properly adjusted gibs, will be enough to keep the cutting forces for
overcoming all the other friction and actually turning the screw.

If it does not, uncontrolled movement would be limited to the small
amount of freeplay in the ballscrew and that eliminates the danger of
pulling a chip heavy enough to break a cutter.

There should be near *zero* lash with a ballscrew. That's not the
issue. The problem is that there is so little friction that the table can
drive the screw, regardless of whether you are up or down milling.

I can't imagine sustained cutting forces being used on a manual
machine converted to ball screws that were heavy enough to overcome
all the other friction and drag and drive the hand feed through a ball
screw.

A ballscrew will work in reverse with only a few *ounces* of pressure.
Cutting forces are almost always higher than that.

If that could happen, the operator would feel the lighter or lack of
feeding force on the handwheel. They should realize that they are not
feeding the material but instead chasing a series of continuous self
feeds. That would be a wake up call for me, time to clamp the ways
tighter or lighten the cut.

Well, yeah, you could tighten up the gibs to create more drag, but you'd
have to tighten them up *a lot* to compensate for the lack of normal
leadscrew friction. Remember that the mechanical advantage of the
screw multiplies the effects of screw friction.

It is also worth noting that ballscrews typically are half or less the pitch
of conventional Acme screws, in other words, where you'd normally
expect an 8 TPI Acme screw, the equivalent ballscrew would be 4 TPI.
So there's less of a multiplier for the little friction that the ballscrew does
have.

If you disconnect the drives on a CNC machine with ballscrews, you
can grab the table and shove it back and forth, while the ballscrew
spins madly. Try that with a table that has conventional screws and
it is doubtful you could budge it.

Gary