On Sat, 02 Aug 2003 19:28:37 -0400, Gary Coffman
wrote:
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.
You seem to be discounting the weight and friction of all the other
pieces. This is not my first rodeo.
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.
You say "near zero", I say small amount, we're taking about the same
thing. And again, you seem to be discounting the weight and friction
of all the other pieces. I have never seen a situation where fitting
ball screws to a manual machine would result in all the weight and
moving mass being transferred to the ball screws and all other
friction eliminated.
A ballscrew will work in reverse with only a few *ounces* of pressure.
Cutting forces are almost always higher than that.
I say again, you seem to be discounting the weight and friction of all
the other pieces. This is a converted manual machine. And the
operator still has his hand on the crank.
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.
I think you are overstating the amount of the friction that is derived
from the leadscrew. Imagine a properly lubricated hardened and ground
leadscrew running in a well fitted bronze nut. Losing some of the
friction is not necessarily going to be a bad thing on older machines
converted to ballscrews. Don't ignore the presence of the other
sources of friction that typically exist in older machines.
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.
How about the increased helix angle in the coarser thread? Have you
factored that in? And then there is the question of how many angels
can dance on the head of a pin.
The only machine specifics mentioned in this discussion so far has
been $600 lathes and a 7 x 10 minilathe. Those kinds of machines are
good candidates for ballscrew conversions and there are ballscrews
with finer pitches that can be used on them.
And I think we are talking in a venue where people are tinkering with
ideas, learning things, trying to make things work, and simply not
constrained by the rules and dictums that exist in the full sized CNC
machinery world.
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.
Ah! Now I get it. You were thinking of CNC machines with ballscrews
already fitted. Aren't there any other sources of friction in those?
Are there no resting forces whatsoever?
You said "You DO NOT want to use ballscrews on a manual machine." I
disagree. There is no good reason to not do that within the framework
of the discussion here.
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