8mm Lead Screw + 1.8° Stepper = 0.04mm min elevator movement?



  • Hi all,

    I can appreciate that micro stepping makes the systems smoother while the axis are moving. Can steppers hold position on a microstep that isn't one of the full steps?

    Following this same logic I would have a minimum positional accuracy of 0.16mm in X and Y when I've 100 steps per mm with 16x microstepping?

    I can see a good case for finer pitch lead screw and 0.9° steppers on the X & Y axis!



  • Have you been looking at the RepRap forums? It's just that there was a discussion on there recently.
    No doubt David will be along soon but my understanding is that steppers can hold position on microsteps providing that the motor torque is sufficient. But that is resolution - not accuracy. As you point out, a typical stepper has an accuracy of 5% non-accumulative which works out at about 1 full step so the positional accuracy with a 1.8 degree motor, 16 tooth pulley with 2mm pitch belt would be 32/200 = 0.16mm and that won't alter with micro stepping. 0.9 degree motors would give you half that error but they tend to have less torque. 2:1 gearing would have the same effect providing it can be done without introducing backlash. TBH, 0.16mm is the worst case and there are very many other factors which will have more of an effect on accuracy. If you want better accuracy then maybe our robot controlled hot melt glue guns aren't the best tool to use IMO.


  • administrators

    It's generally reckoned that for X and Y, 80 microsteps/mm @ x16 microstepping is sufficient.

    For Z on deltas where only a lightweight effector and hot end is generally moving, some designs use 80 steps/mm too, but 200 is better.

    For Z on a cartesian or CoreXY machine where much more mass is being lifted, finer steps are better. The torque supplied by a motor depends on how far from its commanded position it is, which means that the more torque is needed, the finer the x16 microsteps need to be to ensure accurate positioning. Most designs I have seen use at least 800 steps/mm on the Z axis @ x16 microstepping.



  • Thanks.

    So the system accurately holds positions between the full motor steps?


  • administrators

    How accurately the system holds position depends on how much load there is on it and how much torque the motors provide. If the Z stepper motor(s) is/are supporting a heavy bed, then the torque needed to maintain position of you use belts or ball screws may be considerable.

    Also, resolution is not the same as accuracy. Even if each individual 1/16 microstep causes movement, the system may not be accurate to within a single 1/16 microstep because the positioning error on a stepper motor may be as much as +/-5% of one full step out at the full step positions, and more than that at microstep positions. This generally doesn't matter on the X and Y axes, but does matter on the Z axis because it affects layer height.

    So I suggest you make the steps/mm high enough so that there are at least 5 full steps per layer. Then the error reduces to a maximum of +/1% per layer. If you use x16 microstepping and a layer height of 0.2mm, this means you need at least 5 * 16/0.2 = 400 steps/mm for the Z axis.



  • Thanks for the information. I'll be able to work through that when trying to validate the machine's performance. For work's machine I'm being careful not to fall victim to upgraderitus! If something is broke, or a bottle neck I'll tackle it. If not then it stays as is.



  • What effect is a finer pitched lead screw offer to the issue.



  • @greecher:

    What effect is a finer pitched lead screw offer to the issue.

    I did a bit of post on my blog about this https://somei3deas.wordpress.com/2017/01/31/z-axis-lead-screws/.



  • Hi Ian,

    Thanks for the link to your blog, and good chatting with you briefly early on at TCT a few weeks back.

    Edit: Who supplied your trapezoidal screws?



  • Hi DocTrucker,

    Sorry I don't recall the chat but then I got to chat to a lot of people at that show. 🙂 I suspect you maybe didn't introduce yourself as DocTrucker?

    Ref the screws - Chinese finest via Amazon https://www.amazon.co.uk/Length-1000mm-Copper-THSL-1000-8D-printer/dp/B019ML9576/ref=sr_1_5?ie=UTF8&qid=1510923171&sr=8-5&keywords=8mm+lead+screw+1mm+pitch

    HTH



  • DocTrucker, thanks for the info, I'll be trying some 2mm ones first, but am saving that link back for even finer.

    dave


 

Looks like your connection to Duet3D was lost, please wait while we try to reconnect.