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Stepper driver question

Discussion in 'CNC Mills/Routers' started by Simon Tavis, Jan 2, 2016.

  1. Simon Tavis

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    Hi All

    I'm currently in the middle of building the large OX CNC and whilst waiting on a few more parts I decided to look into the electronics area of of the project.

    I am using 4 Nema 23s and have the Arduino Uno + the CNC Shield (Pololu A4988 drivers). What I am confused about is the 5 different microstep resolutions and which one I should be setting for my particular setup. The resolutions are full/half/quarter/eighth/sixteenth.

    I will mainly be using the CNC for wood machining of MDF/Plywood/softwood/hardwood.

    Any help and explanations would be greatly appreciated.

    TIA ... Simon
     
  2. Steve Fox

    Steve Fox Veteran
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    The more steps, the less torque. You will want to use the least number of steps that give you the resolution you want.
    In addition, on my controllers, half and quarter run quieter than full. Depending on the type of cutting you plan on doing, full or half steps will probably be adequate.

    Everything is a compromise. If I were you, I would start with full steps and see if it does what you want. If it doesn't, up it a level until it does.
     
  3. David the swarfer

    David the swarfer OpenBuilds Team
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    1/8 steps is probably what you want on the belts. gives a nice balance between resolution and power IMHO.
    however, if you can live with lower, like 1/4, then do it. you can always change it later.

    note that your belt drive microstep settings can and probably should be different to the Z axis leadscrew setting, calculate that separately.

    for example I have an 8x8mm leadscrew, ie 1 turn = 8mm.
    so full stepping (200step per rev) would give 0.4mm per step. a bit high for me, and more resonance issues.
    half stepping gives 8/400 = 0.02mm per step. lovely! 2/100's of a mm is plenty small enough.
     
  4. Christian Knüll

    Christian Knüll Journeyman
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    Hi,

    this is actually a common misconception:
    • Motor shaft torque is roughly the same for any step resolution setting. In fact motor torque is - slightly - higher and more uniform for microstepping because it reduces motor resonances.
    • What decreases drastically is the "incremental torque" between microsteps. This is the external force required to rotate the motor shaft back or forth one microstep but will not result in a permanent step loss.
    • In other words: Microstep positions are "soft" and external forces can deflect the motor shaft rather easily to its next full step position which is much harder to overcome (and will result in a step loss).
    • This "softness" is also the reason why microstepping will in fact not improve resolution as much as expected: The cutting forces will just deflect the motor shaft towards the next or previous full step causing a heavily load dependent non-linearity for microsteps.

    The main benefit of microstepping is to reduce resonances and vibrations. The higher the microstepping setting the more quiet and smooth your motors will run - especially at low speeds.
    This is however limited by your controllers ability to generate steps fast enough. If your controller is able to create steps @ 30kHz you get 9600rpm for full step but just 600 for 1/16 microstepping.
    Also: the differences between full, half and 1/4 steps are very significant. 1/8 still adds some improvement but 1/16, 1/32 and so on will only add marginal improvements.
     
  5. Joe Santarsiero

    Joe Santarsiero OB addict
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