Screw Whip

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 I 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 unless I add oil grooves and an oiler while I have it apart anyway.
Maybe this would be a better query for the DIY section on CNC zone.
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.
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    Probably OK at that speed and diameter.

    Hmm ... Bridgeport, on my BOSS-3 CNC mill had the longest axis (the X axis) mounted solidly at one end of the table, and rotated the *nut* in a pair of matched ball bearings. This was a bit over 1" diameter, and it was a stepper machine so it could not get anywhere near the 1200 RPM, but their approach sounds like a very good way to avoid leadscrew whip.

    The oiler sounds like a very good idea.

    No worries there, I think. If you were running a Bridgeport with a 24" long ballscrew at 200 IPM with servos, you might worry a bit more.
    Good luck,         DoN.
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wrote:

Don't underestimate steppers. The Taig is running 380 oz steppers at 1200 RPM. My controller automatically switches between stepping and micro stepping.
Z axis on the Max NC has to be running even faster at 1524 to get that 60 IPM. I do think I turned the Z max down to 50 on that machine. It fast for such a small machine though.
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    [ ... ]

    O.K. That is a much smaller stepper than the Bridgeport used. Hang one of those Bridegport steppers onto the crossfeed of a Taig lathe, and it would have to hang over the edge of the workbench to clear, and unless the lathe was bolted down, the weight would simply topple the thing over the edge. The one stepper weighs more than my whole Taig lathe, including the induction motor running it's spindle.
    And the servos which I am replacing the steppers with are capable of at least 3000 RPM -- much more likely to induce whip..

    O.K. Still the screws are short enough so you should not have much problem.
    Enjoy,         DoN.
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wrote:

The number you're interested in is "critical speed." There's a calculator on this page. http://www.nookindustries.com/acme/AcmeCalculators.cfm
Note that you have to specify the *root diameter* of the screw and the configuration of the end support bearings. I didn't check how well the calculator works; I'd double check with another source before trusting the result.
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    [ ... ]

    Hmm ... it does not include the configuration which my Bridgeport uses -- with is with the rotating nut and fixed only at one end. No support at all at the other.
    Enjoy,         DoN.
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Here is some reference to that.
Quoted from: http://www.thomsonlinear.com/website/com/eng/download/document/QA_NA_Ballscrews_101.pdf
Q. Where would you recommend a nut-driven ballscrew system over a screw-driven system?
A. Nut driven systems are not very common. Occasionally this design is used on extremely long length screws where the critical speed of the screw (whip) is at an RPM lower than desired but the critical speed of the nut is HIGHER. Using the rotating nut design, the screw does not rotate and therefore its critical speed is not a factor and only the ball nuts critical speed limits the RPM.
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Bob La Londe wrote:

When the nut is at a position where the unsupported length is greatest, flick the end of the screw with your finger and note the natural resonant frequency. If you can get the screw spinning at such a frequency (note the 60 X conversion from Hz to RPM) then you have a major problem. This might actually be possible on the Sherline with its TINY screw, if your Taig screw is really 1/2" diameter (I didn't remember them being that large at all) this seems unlikely. Anyway, when the rotation frequency is the same as the natural first-order bending frequency, that is the first critical speed, and the condition where vibrations can grow without bound.
Jon
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Its definitely 1/2 x 20 leads on it. I have had it for over three years, and its got atleast ten thousand hours on the first set of screws. I am due to replace them though. Was thinking about going to ball while I have it all apart.
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Using a coarser pitch means that in order to supply the same amount of static force at the actual slide, you will also need use a larger motor / drive combination.
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To some degree I am sure, but we are talking about steppers in this case. The torque falls off as the speed increases usually.
http://tinyurl.com/6njvxwn
Pick your favorite chart, but its almost universal that torque starts falling off from the just above zero. Even when there is a rise or a relative flat spot in the curve its at the low speed side of the graph.
A the same rapid speed the motor may have better torque characteristics because it is turning closer to its peak torque range. All things like this require experimentation of course.
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Sometimes I really wonder about you....
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Ok... but I do things. Sometimes things "conventional" wisdom says isn't "right", or "best" ... and yet often they work. There are taller pitch stepper machines running a lot faster than I run my little mills with lower torque motors. I've seen some pretty big router based machines only running 280 oz steppers and claiming 100IPM or faster rapids. Those gantries weigh more than the table on a Taig I guarantee. In this case I'm really not getting all that far out of the comfort zone, and of course that really has nothing to do with the original topic about screw whip.
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Lets try it this way. "Yes...... It does require more torque to move the load if you have a higher gear ratio.... but if the motor produces more useable torque at a lower speed it will atleast partially overcome that. Steppers produce higher torque at the low end of their speed range.
The problem comes in using steppers without the benefit of being able to "dyno" them and have a reference chart.
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Lets try it this way...
Load your pickup truck with firewood, drive it up a steep hill, stopping in the middle...then try and get it moving again in 4th gear...
My point being, your 20 tpi lead screw seems to me an awfully fine pitch when compared to most ballscrews
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Yes it is awfully fine pitch I feel that a tremendous amount of power is wasted by having to "over rev" the steppers because they develop much better torque at lower speeds and real world applications show that a normal ball screw with more common faster pitches do work just fine even under some pretty heavy loads. In fact these exact steppers, controller, and power supply are used in those bigger heavier applications. I'm pretty sure its not going to be an issue, and if I turn out to be wrong so be it...
But again that really has nothing to do with screw whip. I am going to go for it anyway. At the same feeds the steppers will be revolving at roughly 1/4 the speed, the ballscrew(s) I am looking at using are larger, and the existing screws do not appear to be suffering from screw whip.
Now take a Corvette and do your same experiment side by side with a pickup truck. About the same horsepower give or take.... LOL.
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Sorry, we are getting into a bit of pissing contest here. Basically when you look at the torque of a stepper its misleading. You have to look at applied torque over a range of speeds. When you do you quickly see that steppers are the normally aspirated common crank pin v-twin of automation motors. Not a lot of power, but gobs of torque at low RPM.
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wrote:

Greetings Bob, Years ago I built some stepper systems. I gave up on them because of the torque dropoff at high speeds. These days stepper drivers are much better and can get much higher speeds while still retaining enough torque. Resonance was also a problem I encountered that is now easily dealt with. Using ballscrews, even with a much coarser pitch, will probably work well for you because the friction is so much lower. When I switched from acme screws to ball screws with 1/2 the pitch (10 tpi acme tp 5 tpi ballscrew) the system performed much better. I was using Turcite nuts with the acme screws but was still amazed at how much easier the ballscrews pushed the slides compared to the acme screws. Eric
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Critical speed. Something I learned about years ago making 2 axis machines to put hot melt adhesive on the back of a package tray before edge folding.
Exceed it and your screw looks like a jump rope centered on the nut.
So as screws get longer, they get larger in diameter or you take another route which is spinning the nut and holding the screw stationary.
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