CNC Lathe question

OK, so when one adds CNC to a mill or lathe, it's a good idea to upgrade to ballscrews in order to eliminate backlash.....sounds reasonable to me, but
I'm a little confused about what this does for the leadscrew of a lathe. Is there a special ballscrew that can be engaged/disengaged like a T-nut? I can't quite figure out how anyone would thread with a lathe after replacing the normal leadscrew with a ballscrew....seems like manual threading would be completely out after that.
I suppose the CNC controller could perform the threading operation, but it would need to know the spindle speed and phase...how is this normally done?
Todd
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Not sure about the manual threading option.
There is an encoder mounted on the spindle that sends to the CNC controler the speed and position of the spindle. The controler then coorinates the axises drives with the speed and position of the spindle to cut a thread. When threading, it picks the same point on rotation of the work to start the cut. The canned threading cycle will then pick the right starting point to distribute the cutting load. See here for an explanation of the threading cycle http://www.manufacturingcenter.com/tooling/archives/0504/0504tooling.asp
Todd Rearick wrote:

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Todd Rearick wrote:

The only way to do it without CNC software that supports threading is to have the ballnut be bolted to the carriage in an easily-disconnected way. Unbolt to thread, put bolt back in for CNC.
CNC control that DOS do threading has an encoder on the spindle, and "electronic gearing" that links the carriage motion to the spindle rotation for a certain distance, then breaks back out of that mode for the return pass. Note that almost all hobby-level programs do NOT do electronic gearing. Every one I've seen does spindle synchronization, then just does constant-rate motion for the thread. This is just not as accurate as real gears or true electronic gearing, where the spindle rotation rate is monitored all through the threading pass. You can almost certainly tell that any program that uses a one-pulse per rev spindle detector is doing sync, not electronic gearing.
Jon
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    [ ... ]

    [ ... ]

    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.
    And normally, the CNC machine's leadscrew is not connected to the spindle with a set of gears, so that would not work anyway.

    Hmm ... while it is rather primitive in many ways, the Emco-Maier Compact-5/CNC (which is now in the hobby price range as it is now obsolete) *does* have "electronic gearing". You can hear the stepper motor on the leadscrew change step rate as you vary the spindle speed (from 200 RPM on down, as at faster spindle speeds the CPU (a poor tired 6502) is just not up to the task. :-)
    The one thing missing which I would consider to be ideal for a really good electronic gearing would be a way to recognize when you are running the spindle backwards by hand, and to step backwards to keep the tool in sync. If you rotate the spindle by hand in the actual machine, it counts the pulses as though it was going forwards, which confuses the position rather thoroughly. :-)

    The Above-mentioned Compact-5/CNC has two rings of holes on the spindle encoder disk -- one with only one hole for indexing, and the other with a large number of holes (I've never actually counted the number, but at a guess somewhere in the 30s -- perhaps 32? Perhaps 36?
    Enjoy,         DoN.
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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...)
I could almost do this with the split-nut mechanism that my lathe already has. The current design has two brass halves of a nut that are forced together when you swing the control lever down. I could mount the ball nut to something that fits into this space (for the sake of this discussion...let's just say it is a thick walled round tube. The inside of the tube is big enough to clear the leadscrew completely). The ball nut would be bolted to this tube. I'd replace the brass nut halves with something that would grab the outside of the tube securely, and the tube would be inserted inbetween these grabbers.
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.
I don't know...sounds a little messy...but with a little attention to detail it might actually work.
Todd
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    So far, so good, with one major problem not covered.
    [ ... ]

    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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