1. #1

    First post (of many I suspect).

    I am converting an adcock & shipley 1es mill to cnc. I have raised the vertical head, added a dedicated vertical drive motor. Having serviced and set the gib strips I am happy with the machine and its time to look in more detail at the CNC conversion.

    Right now I am favouring Linux CNC with MESA motion control. I feel stepper motors will suit my budget and expectations, combined with ballscrew conversion. Nema 34 for z axis and probably Nema 24 for x/y

    The z axis carries a fair bit of weight, I did some calculations based on 5mm pitch ballscrews and this showed holding torque would be ok using a nema 34 motors. However I am concerned that the axis will back drive when the machine is powered off.

    Anyone had any experience with similar problems

    What kind of holding torque might a stepper give when not powered.

    Regards Bally

  2. Counter-balance with a gas ram, go for a few kilos less than your weight to avoid backlash and free play issues,

    - Nick
    You think that's too expensive? You're not a Model Engineer are you? :D

  3. #3
    Hi Nick

    Interesting suggestion. Was thinking about counter balance options but gas struts hadn't crossed my mind.

    Better go figure out how much this thing weighs......

    Thanks andy

  4. #4
    Could use a stepper with built in brake

  5. #5
    Hi John.

    Yes fair point. I was also thinking similar lines at one point. An electromagnetic brake certainly seems to resolve the back drive issue but pushes up cost and control complexity. (my budget suits basic range components ;-))

    My other concern is the stepper sees a high load trying to lift the weight of the knee and a much lower load lowering it.
    I am therefore tending toward the counter balance solution as it means the stepper sees more equal loads in both directions. This possibly means a smaller motor, or at least less maximum load on the motor.

    A brake will be plan B if plan A doesn't come off.......may come down to actual weight and stroke details...that's my mission to find out today.

    Cheers for your post....


  6. #6
    Quote Originally Posted by Bally123 View Post
    Hi John.

    An electromagnetic brake certainly seems to resolve the back drive issue but pushes up cost and control complexity.

    I have Yaskawa sevo's on my mill with built in brake controlled by the driver. I also am building a router at the moment and using a servo with a brake. I also have Vexta stepper motors with built in brake controlled by the driver. They are industrial stuff but can be aquired relatively cheaply with plenty of Ebay searching.

    Gas struts can be quite a good solution, (need to get one with no built in damping ), c/balance weight can be a bit problematic finding room for it to travel in and also control any bouncing with fast movements. Both solutions are IMO second choice solutions.Click image for larger version. 

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    Last edited by johnsattuk; 4 Days Ago at 12:31 PM.

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