1. #1
    Dear forum.
    I have a diy gantry type router running with NEMA23 3ph steppers. 3,1Nm
    Y-axis 2 motors for gantry D16 x 10mm Lead ballscrew. Gantry running on THK 16mm rails. Estimated 40kg.
    X-axis 1 Motor for ?? x 40mm Lead ballscrew (IAI actuator) Estimated: 8kg
    Z-axis on a THK KR33 actuator 6mm ballscrew. Estimated 3 kg

    So I get fair good Rapids, but the system is tending to resonate espacially the Z-axis.

    I was looking to convert to DC servos, at a max. of 300 USD pr. motor set that's including "everything needed"

    My goal was to find a NEMA23 motor and direct driven to the 10mm lead ballscrew as that would be an easy replacement.
    I did a calculation (MotorCalcs.xls from this site) for the torque pr. motor required and for the Gantry it's 0,52Nm (Rapids) and 0,21Nm (Cutting)

    Initially i looked at at 2 pce DC Brushless 180W 0,57Nm Rated (1,71Nm Peak) servomotors for the double Y-axis, gantry drives.
    Make:Leadshine Model: BLM57180

    I have zero experience selecting a servo, and looked at DC-brushed, brushless and AC-servo. So what to select ?

    Best Regards, Bo Andersen.

  2. #2
    Quote Originally Posted by fabhund View Post
    So I get fair good Rapids, but the system is tending to resonate espacially the Z-axis.
    So how will servo's help this problem.!. . I don't understand your logic.? They will if anything make it worse as they spin much faster and don't like resonance which affects the tuning.

    Then you have to consider the other things you'll need in place to make use of there extra speed and resolution. Namely the amount of pulses required to get full speed and make use of the encoders resolution. The parallel port isn't fast enough unless you make use of electronic gearing which defeats the main advantage of servos, resolution and even then it's stretched. So an external motion control device is required which gives much higher frequency's required to use servos correctly.

    To be honest from what you describe then you don't require servos and steppers are more than good enough.
    You'd be better off spending the money on stiffening up the frame and getting rid of the resonance. Because like any thing in life you only get out what you put in and if the machines weak then often upgrades just highlight weak areas more and no gain is felt. Often they can perform worse has the in-balance exagerates the weakness and high performance components are much less tolerant of sloppy engineering.!!
    Last edited by JAZZCNC; 13-12-2014 at 01:39 AM.

  3. #3
    Quote Originally Posted by JAZZCNC View Post
    So how will servo's help this problem.!. . I don't understand your logic.? They will if anything make it worse as they spin much faster and don't like resonance which affects the tuning.
    Dear Sir. Thank you for your feedback.

    The vibration is not coming from the rotation itself but from the steppermotor steps. Especially in low rpm feeds 50-300mm/min. So my logic was to have a "step-less" motor to get a smooter rotation motion.

    I'm aware of the problem with MACH3 pulse speed and the quality of the pulses. Currently I'm running 25kHz with good results.
    So a ESS smooth stepper (or similar) is a must for servos.

    Right now MACH3 is set up to 80 steps/mm (and depending on ballsrew lead that is 800-3200 steps/rev)

    So I guess I could invest in an external motion controller, and increase the microsteps, and increase the steps/per. And that might give an overall smooter operation, without loosing speed ?

    Best Regards, Bo Andersen.

  4. #4
    Quote Originally Posted by fabhund View Post
    Right now MACH3 is set up to 80 steps/mm (and depending on ballsrew lead that is 800-3200 steps/rev)
    Ok I understand your thinking slightly better know but still don't think servo's are required.! Also not so sure why you'd want to go so slow as 50mm/min but sure you have a good reason.?

    From the comment above I'm notsure you have the machine setup very well. For instance you say Z axis is using 6mm pitch and you are set to 80 steps/per.? This would mean the drives being set at 480 micro steps which is an odd number for micro stepping and I havent ever seen any drive with this setting.!

    Higher micros stepping really helps smoothen out stepper motors and with good quality digital stepper drives then motor smoothness is excellent. Throw into this mix external motion control card and it gets even better as the smoother and higher pulse rate also helps smoothen motor action.

    By the time you have bought servos and motion control card along with probably having to use some gearing ratio to account for faster spinning servo's you'll be well outside your $300 per axis max budget.
    1 x $300 would nearly buy you 3 x digital drives and motion control card which will make big difference to machine performance if setup correctly.

    Servo's don't always make for a better machine and there is much more involved to getting the best out of servos. Often the extra resolution etc is wasted due to rest of machine not matching up.!
    Last edited by JAZZCNC; 13-12-2014 at 04:45 PM.

  5. #5
    Quote Originally Posted by JAZZCNC View Post
    Ok I understand your thinking slightly better know but still don't think servo's are required.! Also not so sure why you'd want to go so slow as 50mm/min but sure you have a good reason.?

    From the comment above I'm notsure you have the machine setup very well. For instance you say Z axis is using 6mm pitch and you are set to 80 steps/per.? This would mean the drives being set at 480 micro steps which is an odd number for micro stepping and I havent ever seen any drive with this setting.!

    Higher micros stepping really helps smoothen out stepper motors and with good quality digital stepper drives then motor smoothness is excellent. Throw into this mix external motion control card and it gets even better as the smoother and higher pulse rate also helps smoothen motor action.

    By the time you have bought servos and motion control card along with probably having to use some gearing ratio to account for faster spinning servo's you'll be well outside your $300 per axis max budget.
    1 x $300 would nearly buy you 3 x digital drives and motion control card which will make big difference to machine performance if setup correctly.

    Servo's don't always make for a better machine and there is much more involved to getting the best out of servos. Often the extra resolution etc is wasted due to rest of machine not matching up.!
    Dear Sir.
    Thank you for your feedback.

    The 50mm/min is just for touch off the tool height, so youre correct it's not so often it's used.

    I didn't want to come with all my details. I just wanted to mention that I ran it around 80 puls/unit
    X axis: 3200 steps/rev, 40mm lead = 80 puls/mm
    Y axis (+1 slave): 800 steps/rev, 10mm lead = 80 puls/mm
    Z axis: 500steps/rev, 6mm lead = 83,33333333 puls/mm

    My drives are from JMC, type: 3M660
    And i'm running it on a 48VDC switchmode 8,3A (Rated) 400W (I do have a 2000W toroidal trafo, but the inrush current blows my 13A fuse :-/

    I will start out with an external motion controller: Warp9 ESS smooth stepper ?
    And try to bump up the microstepping.

    Thanks for your comments.

    Best Regards, Bo Andersen.

  6. #6
    Update:
    Got the ESS Smooth stepper from Warp9. Connected it directly to my 2 existing BOB's. And after a few tries on the ethernet connection, it works.

    So I'm up to 416,6666 steps/mm on the Z-drive and giving a speed of 12500mm/min
    Y+1 slave axis, is 400 steps/mm and 18500 mm/min
    X axis is 160steps/mm and 25000+ mm/min.

    Most of the Z-axis resonance is gone, I guess there will always be some. But much lower noise.

    Now is left to find the Acceleration/speed trade off.
    Y axis (the worst one) is around 1200mm/s^2 I could set the rapid speed down, and increase the Acceleration.....

    Further I need to figure out some 'issues' with the smoothe stepper, but that's a different thread.

    Best Regards, Bo Andersen.

  7. #7
    Great news.!! . . . Now buy some Lead shine EM806 Digital drives and take it to the next level. They are light years better than those drives you are using now.
    Digital drives are SOOOOO much smoother than analog drives and run the motors much cooler and allow higher feed rates still.

    With Digital drives and ESS you'll be able to run much higher Micro steps which give smoother running motors. Only way to appreciate the difference is to see it for your self.!

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