In article aoTSc.3030$73.2401@lakeread04,
Todd Rearick wrote:

"DoN. Nichols" wrote in message
...

Very awkward, as I don't see how to make split nuts with a ball
race in them -- hmm ... maybe I *do*, but they would be quite expensive.

I've been thinking about this more and more. This is hard to describe but
here goes:

What if I could mount the ball nut in such a way that the normal split-nut
control would allow the ball nut to rotate freely when in the dis-engaged
position, but grab-on to the outside of the ball-nut when in the engaged
position. You'd also probably have to trap the ball nut along the leadscrew
direction somehow (with thrust bearings on each end or something like
that...)

So far, so good, with one major problem not covered.

[ ... ]

When I swing the control level down, the tube (and the ball-nut that is
bolted to it) would be stopped from rotating, and would then start to drive
the table. When I swing the control lever up, the ball-nut would once again
spin freely and the table would stop.

The problem that I see with this is that the ball nut would be
at different rotary positions depending on the precise time of closing
the half-clamps (let us call them that, as they are no longer nuts), and
thus your thread would be at a different starting point. You would need
some kind of key to assure that the half nuts could *only* close at one
particular point in the rotation of the half-clamp lever.

Since it is somewhat earlier tonight when I get to this, I'll
explain what crossed my mind last night, but which I would only hint at,
because it was quite late, and I was quite tired.

A normal ball nut has a spiral groove (half-round to match the
balls), with the ends coupled by a steel tube going from one end to the
other. A ball rolls through the groove, enters the tube, and is
shuttled back to the start end of the groove.

A good one has two of these grooves, one slightly offset from
being a true continuation of the other. The result is that the balls
are crowded towards one side (say headstock side) of one groove, and
towards the other side (tailstock side) of the other, thus applying a
loading force between the two grooves and cycles of balls. The size
of the balls is selected down to a ten-thousandth of an inch, so you can
adjust for *even* wear by increasing the size of the balls by a tenth or
two.

What I am picturing here is to split the continuous grooves into
half-circle grooves, with a feedback tube at the end of every
half-circle. This way, the halves could separate just as the halves of
a set of half-nuts do in a normal lathe apron.

You would need dowel pins to assure that the halves aligned
perfectly, and the grooves would have to be more than 50% deep, so there
would be lips to keep the balls from falling out as the half-nuts
separated. This would then require shallower grooves in the ball screw
to avoid interference between the nut halves and the screw. The over
50% deep would be particularly difficult to machine, since the curve
would have to continue to close, restricting access for the tooling
needed to grind the grooves.

And finally, you would need the closing lever to apply some cam
force to hold the halves *firmly* in place, because any flex there would
introduce errors.

You would also have to design a threading dial to tell you *when*
to close the half-nuts, just as in most manual lathes.

But in reality, there is no need for ball screws for the Z axis
(parallel to the spindle) on a lathe set up for manual turning. There
is plenty of torque available from the spindle through the gear train.
It is to minimize the torque from a servo or stepper motor needed to
overcome friction that ball screws are normally needed.

And a ballscrew on the Y-axis has a major problem when not
controlled by a stepper or servo motor. Cutting forces can force the
cross-slide back to turn the leadscrew, as the low friction works both
ways -- and in particular benefits the cutting force moving the
handwheels. So -- you would at the least need some kind of friction
brake applied to the handwheels if you were to fit ballscrews to a
manual machine.

It is best postponing the addition of ballscrews to a machine to
correspond to the time when you turn over control of those screws to
a computer through servos or steppers.

Enjoy,
DoN.

P.S. I'll be away from my computer for the next couple of days, as I
will be attending Iron Fever.
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