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Y axis on a OX style machine

Discussion in 'General Talk' started by KE6MTO, Aug 5, 2019.

  1. KE6MTO

    KE6MTO New
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    I've been slowly building up an OX style machine. I've been testing it and have been noticing Y-axis issues. Running into an issue when I run a "hello world" file doing air cutting, one Y motor starts jerking around off and on. What I've noticed is right before it happens the motor is not moving with the other Y motor. Inputting light pressure on the motor there is no resistance, basically as if the power is off. These are NEMA23 motors and the controller is a Protoneer Raspberry Pi CNC controller running 4988 motor controllers. Since this is my first machine build would like some input to troubleshoot this. Is this indicative of overheating and the motor controller is temporarily going offline or maybe not enough drive current?

    -Chris
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    A4988 drivers weren't really designed for use with NEMA 23 steppers. At a minimum you should be using 8825 drivers. You might also consider giving the TB67S109 drivers a try. It's what is used in the BlackBox and offers sufficient amperage to fully cover the NEMA 23 steppers. I don't recall anyone on the forum having tried them yet with the Protoneer board but I don't see any reason they shouldn't be worth considering.

    Another thing you might look for is loose wires. This has been noted to be the cause of intermittent operation more than a few times.
     
  3. KE6MTO

    KE6MTO New
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    Thank you for the input. I will look at getting a set of 8825 or other higher amperage drivers.

    Chris
     
  4. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    8825s on polulu sized breakouts overheat at around 0.8amp even with a fan blowing on them. Texas Instruments has whitepapers on proper PCB design for their thermal pad chips: if you checkout section 11.3.2 of http://www.ti.com/lit/gpn/DRV8825 and follow the links to the various other documents you will see
    A) that the chip heatsinks through the bottom. Those breakouts with the heatsinks on top of the chip is a joke
    B) it needs substantial PCB surface as it dissipates heat via copper pours on the PCB. Pololu sized PCB is way too small. Datasheet recommends 20cm^2 of surface area below the chip for a GND pour. Pololu footprint is about 3cm2

    The design of that small driver size limits them to under 1A, still too little to be useful. They will still just go into thermal shutdown :)

    Size of the pcb is the problem, not the choice of chip as much. A4988 can do 2A if on a proper sized PCB too

    Even TB67S109s on a pcb that small will result in disappointment. Thats why we didnt go for little breakouts and instead engineered the Thermals on BlackBox 4x properly. It has a massively engineered PCB stackup to pull heat from the chips, dump them into 24cm2 of aluminum heatsink, and then blows a fan too. A lot more thermal management going on that is immediately visible. Worth spending on a BlackBox unless your time is free and you want to spend more time making pololu sized parts work than using the machine :)

    So, if you want to do yourself a favour: BlackBox Motion Control System
     
    Mark Carew likes this.
  5. KE6MTO

    KE6MTO New
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    Yeah, was going to pull the trigger on that but they are out of stock :(

    Thank you for the info. I initially thought to have the rPI and controller as one would work out well, not so sure now... Been kicking myself on why I was getting jumping/jerking and was thinking it was a mechanical issue. Looks like the whole time it's been an underpowered driver issue.
     
  6. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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  7. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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  8. KE6MTO

    KE6MTO New
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    Just an update, since the BlackBox was not in stock, I got a set of TMC2208 drivers. Boy was that a good move in the short term. Problem is gone, movement is dead silent and was able to make my first cut this weekend.

    -Chris
     
    David the swarfer likes this.

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