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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Z axis too heavy stepper choice
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  1. #1
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    Z axis too heavy stepper choice

    Hi
    I build a custom z axis and used quite too much material and it's quite heavy. I'm just missing the stepper, but i really wonder about it.

    Driver run at 48v and 4.5amp set of max 5a.

    The whole z axis metal parts, carriage, sfu rod... and the 2.2kw spindle weight about 70lbs

    I run a SFU1604 ballscrew and fit a 3:1 belt ratio. From stepper to the sfu.

    So is a 280oz low inductance stepper will be ok ?

    * mostly cutting wood and alu from time to time

    thanks

  2. #2
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    Re: Z axis too heavy stepper choice

    Have you measured your axis to see what the actual torque load is where the motor will mount? Right now we are dealing with a significant number of variables, from gear reduction to friction, and it will be easiest to just measure how much torque is required.

    The best way to do this is with something like a fish scale. Just tie a string to it and wrap it around the place that your motor will mount and pull it until the axis begins to move. That will give you your detent torque, and we can figure out what kind of motor you would want from there.
    Marcus Freimanis
    www.geckodrive.com

  3. #3
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    Re: Z axis too heavy stepper choice

    Quote Originally Posted by CoAMarcus View Post
    Have you measured your axis to see what the actual torque load is where the motor will mount? Right now we are dealing with a significant number of variables, from gear reduction to friction, and it will be easiest to just measure how much torque is required.

    The best way to do this is with something like a fish scale. Just tie a string to it and wrap it around the place that your motor will mount and pull it until the axis begins to move. That will give you your detent torque, and we can figure out what kind of motor you would want from there.
    I'm just quoting this because it should be said twice.

    Anti-gravity is real! It's called a spring. Or a rope over a pulley to a weight if your Z axis doesn't need to move fast.
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  4. #4
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    Re: Z axis too heavy stepper choice

    Without some fairly expensive test equipment and a way to measure all the weight, friction, drag, and so on, the cheapest and fastest way to test it is to bolt on the motor and see if you can run it at the required operating speeds without loosing steps. HIgher inductance idoesn't mean faster or more torque. If you think it will be an issue, just get a larger stepper to start with if there is any way you can.

  5. #5
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    Re: Z axis too heavy stepper choice

    Hi Indeed it looks a good way. So if I pick a fish scale and took all the z axis all together : i will have 70 lbs. But if I put over the carriage, the preload and friction of those will eat some of the weight, So i might get a value of around 55lbs . I think it might give a usefull value as well ? or it's really with the ballrod in place too ?
    http://www.nookindustries.com/Conten...eGallery_5.png
    i did get this calculation for the screw.

    But as James said: the spring approach is quite interesting. An oleo might be ok too, but not too big at same time, i'm not sure where to get it. A spring could get some decent ''weight removal.''

  6. #6
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    Re: Z axis too heavy stepper choice

    You will have quite a good mechanical advantage with that ball screw. I got a chinese Wantai NEMA23 to develop 150kg of linear force tested against a pair of trampoline springs. I could extend them 5" back and fourth several times and did not lose a step ( I was checking position with a dial gauge ).

    I was using a 1605 ballscrew.

    I lost track of whether I was running 30V or 48V in that test. The peak current was set to limit at 4A but I'm not sure whether it was getting that high at the max step rate. I got best results at 1440 mm/min on this gear. In short I think you should be able to comfortably and reliably lift the load you described.

    A spring is NOT anti-gravity since it changes drastically with extension and will not be useful. If you find it does need some help you could try a counter weight over the top of the gantry. Look at the way elevators avoid having to lift the full weight of the cage by using a counter weight.

    I would guess that you do not even need the gear reduction which would speed up your z moves and reduce slack. You will have trouble spinning that screw fast enough as it is because steppers don't like to got fast and can't deliver the torque at fast step rates. You can never be sure of the torque you will really get. Manufacturers have tricks to make the spec look better, and the static holding torque will not be same as the drop out torque you need when raising the axis at the max speed you need. Motor torques drop off badly with speed.

    When choosing a motor make sure you select a low resistance , low impedance model. A 2nd hand US made motor rated at 1.65V may be better than a chinese 5V rated one. The latter will have more and thinner windings with higher res and inductance and the torque will drop off quicker.

    Hope that helps.

  7. #7
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    Re: Z axis too heavy stepper choice

    The main problem you'll probably notice is that your spindle will dive into the table when you turn off the power, backdriving your ball screw. Ball screws have little resistance to that. A counterweight can help, but it's difficult to design a counterweight system for a moving gantry, because it has to allow so much movement, in X and Y as well as Z. It works better with a bridge and moving table design, because the bridge doesn't have to move and the counterweight only has to account for movements in Y and Z.

    I've tried constant-force springs; they worked okay until they broke. Gas springs, like the ones that hold car hoods and hatchbacks up, worked better.
    Andrew Werby
    Website

  8. #8
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    Re: Z axis too heavy stepper choice

    your spindle will dive into the table when you turn off the power,
    That is something that needs to be addressed but can easily be controlled by some light frictional resistance or a small side-load on the ballscrew bearing.

    it's difficult to design a counterweight system for a moving gantry,
    Not really in this context. With SFU1604 ballscrews it is not going thrashing back and forth like a 3D printer ! It will need constraining with guide rails but nothing difficult to conceive or make.

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