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Please help me understand DQ542MA Drivers settings + CNC USB Controller settings

Discussion in 'CNC Mills/Routers' started by eamogensen, Feb 9, 2019.

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  1. eamogensen

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    Hello All,

    In my build I am following Schematix's build of his "Ox Metal" CNC. When I purchased my parts from Openbuilds the Pro board was not in stock so I went with the combo of 4 DQ542MA Stepper drivers and the CNC USB Controller.
    My questions is, What the heck should I be setting the drivers to (How many steps per revolution) and what speeds should I be using in the software to get the fastest and smoothest action out of my CNC? Finding any reference to this has eluded me (Maybe I'm not searching correctly).
    At the time of writing this I currently have the drivers set to 25600 steps and in the software my speed is set to a max speed of 690.
    If someone could point me in the right direction it would be greatly appreciated.

    Thanks,

    EAM
     

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  2. Rob Taylor

    Rob Taylor Master
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    The motors are natively 200 steps per rev. This is where they'll have the most torque and accuracy (ie. they'll stop where they say they're stopping). Microstepping is where the motor balances the current between two poles by a precise amount to try to hold the rotor at a certain angle. 2x microstepping gives you one extra step between each pole where it splits the current in half, doubling your perceived steps. 4x microstepping adds 3 steps with 1:3, half, and 3:1 current splitting (not exactly, I'm sure, but let's pretend). As you can see, the more you try to divide up the current to carefully juggle the position of the rotor, the less precise you're gonna get and the less force the poles are going to be able to exert.

    Therefore, doing any microstepping at all loses you a fair bit of torque, but obviously you gain some extra precision and smoothness by adding more steps. The tradeoff point is generally at 4x (800 steps) or 8x (1600 steps), depending on the motor you start with, your machine class, what voltage you're running, screws vs belts, and probably some other variables. After 4x the torque starts dropping fast. After 8x, it basically falls off a cliff. I wouldn't ever run any more than 1600 steps, personally, and only then if my system could sustain that loss of torque. I'd rather physically gear-reduce the motor and do 2x or 4x stepping. This also has other mechanical advantages that are well beyond the scope of this explanation.

    As for speed, it depends. On many machines, it's more the acceleration that defines the machine "speed" (cycle time) than absolute top speed. At least for hobbyist machines, it's generally tuned by hand, where you increase speeds and accelerations until things start stalling out, then you back them off about 20%. If you have to back off the acceleration, it'll stall out while it's still spooling up, or only just started moving. If the top speed is too high and the motor can't reach it or maintain it, you'll have problems at or near the end of the acceleration curve as the sound pitch becomes consistent. This is for rapids, of course, the machine will never come near them whilst cutting.

    There should be a lot of information out there on getting optimum machine performance out of your stepper tunings, as well as generally setting up drivers and how stepper motors work. Not quite sure why it wouldn't show up, but Googling some of the terms and phrases I've used here will probably be a good start. :)
     
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  3. Giarc

    Giarc Master
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    I routinely suggest people with questions like this to read the entire GRBL wiki. It will answer pretty much any question you have. But for your steps/mm, you need to know how far a nut (anti-backlash) moves on the screw per revolution. Then use the formula here gnea/grbl

    So if it move 8mm and you use 1/8 microstepping, (200 steps per rev X 8 microstepping)/8mm of travel per rev = 200 steps/mm. Do not use the 25600. Use the 1600 jumper setting which is 1/8 microstepping.
     
    #3 Giarc, Feb 9, 2019
    Last edited: Feb 9, 2019
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  4. eamogensen

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    Thanks Giarc,

    I'm playing with it now and much better
     
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