"X" axis ballscrew locking up at high speed


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Thread: "X" axis ballscrew locking up at high speed

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    Default "X" axis ballscrew locking up at high speed

    Has anyone had this problem. My X axis ball screw which is 60" long and 20mm dia is locking up when I run my feed rate over 100IPM. It works fine if I slow it down but just won't run faster. It should do it but when doing rapids it will lock up and then the Mach 3 will lose steps and we all know what happens next. The ballscrews are from Mr. Chen in China but the Y and Z axis work fine at high speed. There is no whipping of the X axis and it is not bent.

    Does anybody have any experance with this issue or can steer me in the correct direction??

    Thanks for the help!

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    I think you may be confusing the ballscrew locking up with the motor stalling because you are asking too much of it. Now is the ballscrew actually jamming or something or does it just quit turning? Once you exceed a certain speed/torque threshold, the stepper will stop spinning and will just vibrate wildly until the motion is finished and it is allowed to accelarate at an acceptable rate again.

    You can try reducing the acceleration rate so that it has a smoother speed up which would reduce stalling. Also make sure your bearing s and everything are free so you have no binding. If everything looks good mechanically then you just have to make do with the speeds which are reliable or buy a bigger motor to prevent exceeding its torque output at speed.

    Matt



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    My vote is for the motor, as well. A ball screw that "locked up" would leave a mechanical trace of some sort, up to and including an ugly noise and a bad smell since this would be a friction issue. This would also then preclude "working fine" at lower speeds.



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    You probably have the 5mm pitch ballscrews, which would mean that at 100ipm, your screw has to spin at 500rpm. May not be a problem for your y and z axes which are smaller and lighter, but maybe for your gantry. You could use a higher powered stepper and gear it up. Or you might havev some resonance in the gantry axis, where a resonance damper or "rattler" might help. Another option could be to use a screw with a higher lead.



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    Guys,
    Thanks for the reply's. I hate to say it but you may all be right. I have NEMA 34's with 640oz of torque which should handle the higher IPM's but sounds like I may be wrong. The same size on both Y & Z and they rip but less weight and forces to deal with. Could swap up to a larger X axis motor which may fix the problem. You are correct and my ballscrew is a 5mm pitch. I'm down to approx 80IPM's on the X axis and it sucks.

    What do you feel is the best answer. Higher pitch or stronger stepper motor?? Also what should the max RPM on my motor be. Again keep the ideas coming.
    Steve in Maine



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    Quote Originally Posted by Barefootboy4 View Post
    Guys,
    Thanks for the reply's. I hate to say it but you may all be right. I have NEMA 34's with 640oz of torque which should handle the higher IPM's but sounds like I may be wrong. The same size on both Y & Z and they rip but less weight and forces to deal with. Could swap up to a larger X axis motor which may fix the problem. You are correct and my ballscrew is a 5mm pitch. I'm down to approx 80IPM's on the X axis and it sucks.
    Look at the torque curve for the motor. While you may start at 640 oz-in of force at 0 rpm, that number will start to fall off pretty rapidly at just a few RPM. What's important when looking at a torque curve is to pay attention to the units on the X-axis. This is often given in PPS (pulses per second) and, worse, can even be shown as a logarithmic scale to smooth-out the drop in higher-rpm torque.

    At 500rpm, you are at 100,000pps with a typical 200 step/rotation stepper motor. I don't know if your motors are from Keling, but there is a 640oz-in motor on Keling's website that includes a torque curve -- of a sort (attached picture #1).

    http://www.kelinginc.net/KL34H280-45-8AT.pdf

    If you look at this graph, and overlook the excessive use of BOLD fonts, you can see that this particular motor starts out at about 4.25 N-m at around 500 PPS and then drops down to about 1.4 N-m at 6000 PPS. Those sound like big numbers until you divide by 200 steps (pulses) per revolution. In terms of RPM, this motor loses 2/3rds of its torque going from 2.5rpm to 30rpm. The response curve, if it is like most of the other published stepper motor curves, is probably going to be sigmoid in shape (elongated "S"), so the drop from 30rpm to 500rpm won't be as steep as this early drop, but it will still be a drop.

    What do you feel is the best answer. Higher pitch or stronger stepper motor?? Also what should the max RPM on my motor be. Again keep the ideas coming.
    Setting aside the resonance issue, which should be ruled out, the "best" answer is going to be a servo motor, given available technology. The best answer given typical constraints on budgets is "it depends". One answer would be to change your chip load and to take a lighter (less deep) cut using the existing equipment. If you are lucky, you can gain more speed (proportionally) than you take away in depth (e.g., halving your DOC gives more than a 2x increase in speed). The other options may not be as good.

    If you double the size of your stepper motor, and hold the shape of the torque curve constant, you will probably get up to 200ipm before stalling assuming no change in cutting parameters.

    If you changed your ball screw to a 10mm lead, you would also get out to 200ipm before stalling, but you have to go through the hassle of re-mounting the screw and aligning it properly (a slight mis-alignment can increase the load on the nut, so you may want to spend some time checking the screw alignment just as a precaution). Changing to a 10mm lead will also halve the resolution of your machine in that axis, so that may not be desirable.

    For me, I pretty much committed to servo motors from the outset, and my current choice for my own build are the DMM-Tech units. Specifically, the big NEMA34 unit for my own X-axis. It is limited to ~1000rpm (that pesky 200ipm limitation for your machine -- again!), but the torque curve is much more favorable to cutting at higher speeds (attached picture #2).

    Motor

    If your gantry weighs significantly less than my own does/will (I'm hoping to sneak in under 80kg when fully loaded; I'm over 65kg before adding the Z-axis or motors), then you can probably happily use the 400W motor and get beyond the 200ipm limitation (critical speed will factor into this before you hit 3000rpm). The price differences are not much between the options (~$275 - $325/axis plus power supplies and cables), so suitability is the bigger criteria.

    So, the net effect is that you should check your set-up to ensure that you aren't experiencing a resonance issue or a misalignment issue that is causing the motor to stall. If it isn't either (or both) of those, more speed means that it is time to spend more money. You can double your speed by doubling your motor size or by doubling your screw lead. The first can only go so far given electrical limitations, and the second has the drawback of losing resolution. Changing motor technology can also help, potentially a lot, but at a higher cost. No matter which route you go, though, at some point you will hit your screw's critical speed and then you will go no faster.

    Attached Thumbnails Attached Thumbnails "X" axis ballscrew locking up at high speed-keling_torque_curve_640oz-jpg   "X" axis ballscrew locking up at high speed-92gm-dht-72-final-copy-jpg  


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    What do you feel is the best answer. Higher pitch or stronger stepper motor?? Also what should the max RPM on my motor be. Again keep the ideas coming.
    Depends on the machine. Any pics? What voltage are you using, and what drives? What's the screw diameter?

    You may find, that a smaller motor will be faster. Increasing the motor size, may give you more holding torque (at zero rpm), but may have even less torque at the speed you're running at.

    Speed is generally proportional to voltage. If you double the voltage to your motors, you should get roughly double the speed.

    Gerry

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    I don't have any pics due being offshore on my ship I work on. It's basicly a shopbot copy. The ballscrew is 20mm dia 5mm pitch 60" long. The motors and drives are the standard KIT from Keling. I have no complaints about the kit other than the NEMA 34 stepper is not cutting it. Running the standard Keling 50VDC power source.

    If I replace the X stepper with a Keling NEMA 34 servo @ 1200oz would that fix my problem. Do I need a seperate power source for the Servo motor. Can I still run my Y & Z on my current 50VDC and supply higher DC for the X motor? I just can't live with 80 IPM for my X axis. Will the Mach 3 program run a mix of stepper and servo motors? Any help here is great. This is good solid data from everyone.

    Also what about a Rattler. How do you make it and mount it. How much weight?
    Steve in Maine



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    Quote Originally Posted by Barefootboy4 View Post
    I don't have any pics due being offshore on my ship I work on. It's basicly a shopbot copy. The ballscrew is 20mm dia 5mm pitch 60" long. The motors and drives are the standard KIT from Keling. I have no complaints about the kit other than the NEMA 34 stepper is not cutting it. Running the standard Keling 50VDC power source.

    If I replace the X stepper with a Keling NEMA 34 servo @ 1200oz would that fix my problem. Do I need a seperate power source for the Servo motor. Can I still run my Y & Z on my current 50VDC and supply higher DC for the X motor? I just can't live with 80 IPM for my X axis. Will the Mach 3 program run a mix of stepper and servo motors? Any help here is great. This is good solid data from everyone.

    Also what about a Rattler. How do you make it and mount it. How much weight?
    Steve in Maine
    I think the torque curve on the larger steppers drops more than the smaller ones. You can mix servo/stepepr with mach3 but I don't know if that would be ideal. You COULD use the larger stepper and gear it UP. But I would be inclined to try a higher-lead screw. Even with a servo, you'll be spinning that ballscrew clsoe to it's max to get a decent rapids speed. I don't know the diameter of your ballscrew, but you can check Nook's online calculator to see what the max rpm you can expect, given you screw length and diameter.

    Luckily, since most people think they need fine-pitched ballscrews to achieve accuracies and tolerances that are unrealistic to hold for most, finding high-lead ballscrews, even at your length, aren't the toughest to find at a good deal on eBay.

    An obvious thing to do would be to disengage your ballnut from your gantry and move it back and forth, and check to make sure that it [the gantry] moves freely and consistantly at all points and that it is not racking when moved.

    As for the "rattle" I cannot remember where the thread is, bu tyou should be able to find it by doing a serach here on the Forum....



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    Quote Originally Posted by louieatienza View Post
    I think the torque curve on the larger steppers drops more than the smaller ones.
    In general, yes, but for good and very specific reasons. Stepper motors are, effectively, constant power devices - though you have to calculate both the power delivered by the shaft AND the back EMF from the inductor to get there. Bigger torque requires a greater magnetic field which requires larger induction. Induction and voltage are what control the shape of the torque curve.

    You can have a low torque motor with big inductance and high voltage, but don't get anything flammable near it since it will be dissipating a ton of heat (high resistance) in the process. It is definitely possible to have a low torque motor that loses a lower percentage of its power as RPM climbs, but it will start from a smaller base (lower inductance). It's a "testable hypothesis" as to which motor has more power at high RPM between a bigger motor and a smaller motor, but if there is a big disparity in holding torque, chances are that the bigger motor will still have appreciably more torque at high RPM assuming that both are well-designed and manufactured (the smaller chassis will have trouble with the higher currents used by the larger motor). It's pretty easy to find NEMA34 motors requiring more than 80+ VDC, but a small NEMA17 motor would most likely fry at those kinds of voltages.

    Stepper motor - Wikipedia, the free encyclopedia

    Conversely, a servo motor is a constant torque device, within reason. It's power is a direct scaler with RPM (simplistically: Power = Torque * rpm). At high RPM, back EMF kills the amount of torque a stepper motor can apply to a load while a servo motor still has the same torque at 10 RPM as it has an order of magnitude or three later.

    Stepper Dampers: http://www.cnczone.com/forums/steppe...er_damper.html

    Last edited by Bear5k; 07-12-2011 at 09:08 AM.


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    Quote Originally Posted by Barefootboy4 View Post
    If I replace the X stepper with a Keling NEMA 34 servo @ 1200oz would that fix my problem.
    As was mentioned, it might get worse. The bigger the motor, the slower they usually spin.

    Do I need a seperate power source for the Servo motor.
    Depends on the servo. A servos rpm is directly proportional to voltage. You'd need to know the ratings, and how fast you want it to spin.

    Can I still run my Y & Z on my current 50VDC and supply higher DC for the X motor?
    Yes.

    Will the Mach 3 program run a mix of stepper and servo motors? Any help here is great. This is good solid data from everyone.
    Yes, but depending on the encoders, you may need a higher kernel speed.

    Also what about a Rattler. How do you make it and mount it. How much weight?
    http://www.cnczone.com/forums/steppe...tml#post256639

    Gerry

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    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    It sounds cheesy, but simply drilling a hole in the center of a hockey puck and pressing it on the rear shaft of the stepper can do wonders for getting past the mid band resonance that you may be experiencing.

    There is a thread about that here somewhere , too.

    If it helps......you could spend some time making a better damper.



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"X" axis ballscrew locking up at high speed
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