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Acceleration

Discussion in 'CAM' started by DavidCNC2017, Apr 3, 2021.

  1. DavidCNC2017

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    Hello,

    How to disable acceleration during cutting , but leave it when jogging or move spindle to preprogramed position?

    Thank You.
     
  2. David the swarfer

    David the swarfer OpenBuilds Team
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    You cannot, the stepper motors will stall and lose position. (unless you are cutting extremely slowly, like 50mm/minute slowly).
    You would also have EEPROM write delays that will make your movement very jerky (and your EEPROM will wear out too).
     
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  3. DavidCNC2017

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    May be I did not explain the problem exactly.
    The problem is that during cutting process each time spindle change cutting direction it start to accelerate, move at constant speed, and decelerate.

    For example, spindle start to move along X axis. It start with acceleration, then constant speed, then deceleration. After that , for example, if direction changes to move along Y axis it make the same: accelerates, goes constant speed and decelerates.

    So , each time the moving direction is changed, this 3 processes repeated.

    What I want is: movement to start with acceleration, to reach the constant speed and move all the path with this speed until the end, and make deceleration just before the end point of cutting.

    Hope now I am more clear.
     
  4. David the swarfer

    David the swarfer OpenBuilds Team
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    It can only do that if the corners are less than about 20 degrees. Otherwise the motors will stall due to the abrupt direction change.
    upload_2021-4-3_12-8-54.png

    so the left shape will have to decelerate into the corner and accelerate out of it.
    The right shape will likely be able to maintain constant speed, heavy machines will still need to slow down due to kinetic energy.
    Physics will always rule!

    You have not said if this is a laser or a router?

    If it is a laser and is running GRBL, then there is a laser mode to solve overburn in corners. Read the wiki.

    Just to be clear, physics dictates that 'instant' speed or direction changes are not possible, all machines always have some value of acceleration, even a Datron Cube, even though the acceleration is very high, it still exists and can still limit surface finish in certain circumstances.
     
  5. DavidCNC2017

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    Thank You so much for You detailed explanation. Here Some more info about what is going on.

    Some more info:
    1. CNC router running GRBL, DM 542 stepper drivers, Machine is not heavy, kind of Openbuilds Sphinx 1000 x 1000. with 1.5 kW spindle
    2. The point is, that on my old C beam router , running on Arduino+gShield I never had such "issue". I even did not saw that movement accelerates or decelerates. Visually it was just cutting with constant speed, even in case of sharp corners. And now I have this.... and it is really very nervous, especially in small parts, when all the time the spindle just accelerates and decelerates, even may be sometimes without reaching its regular speed.
     
  6. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    It happened. Just faster. The stepper motors has to accelerate/decelerate to speed, they cannot start like a DC motor. Tune your acceleration appropriately on any new machine. Fast as you can, but not so fast it stalls. Same for Max Rate. Refer the Grbl wiki it explains the process
     
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  7. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Wait spindle or carriage acceleration? If its the spindle, do you maybe have Laser Mode enabled? Spindle should stay constant.

    Otherwise your CAM may be introducing spindle speed changes during cornering. Post a sample gcode file?
     
  8. DavidCNC2017

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    Sorry, I mean carriage of course. The spindle speed stay constant.
     
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  9. DavidCNC2017

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    I agree that the stepper motor has to accelerate/decelerate to speed. What I can not understand is why , accelerating once it have to change speed at every single corner (sharp or not so sharp).
    Assuming that feed rate is low, not 50 miles/h :)
     
  10. Alex Chambers

    Alex Chambers Openbuilds team
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    If it's a sharp corner one axis (therefore one stepper motor) has to come to a halt. The other axis has to accelerate to its set speed. No motor can stop instantaneously, or start instantly at a (relatively) high speed. You can probably tweak your acceleration settings to improve things but, as Scottie would say in Startreck, you can't change the laws of physics. The rotor in the stepper motor has mass and it will take time to get it up to speed.
    Alex.
     
  11. Peter Van Der Walt

    Peter Van Der Walt OpenBuilds Team
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    Post your Grbl settings just for a sanity check to see if you changed any of the cardinal defaults that shouldn't be changed.

    The basic theory and operation stands.

    However, one can make things worse than it should be, by messing with Junction Deviation etc, so do paste the settings just so we are sure you are experiencing the settings as they are supposed to be

    Grbl Settings tab > Backup Settings > open the resulting text file and copy/paste content in a reply
     
  12. DavidCNC2017

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    Ok Peter, no need to send GRBL settings. I already figured it out. My mistake was that I put very small acceleration value in settings, that is why the acceleration was very noticeable visually, and not only.
    Now I change the setting making acceleration values bigger, so acceleration/deceleration is taking place within several mm in few seconds and it is not boring. Visually it looks like end mill just stop.

    Thank You and everybody for response !
     
  13. rscamp

    rscamp Well-Known
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    Acceleration can be set surprisingly high before losing steps. FWIW, commercial machines are set with acceleration as high as practical. Low accelerations make the machine sluggish (as you found out) thus increasing job run time. In the case of a router, they also cause the cutter to travel farther on path with suboptimal (inadequate) chip load. Commercial machines also have settings for "S-curve" or "Jerk" (rate of change of acceleration) to allow even higher average acceleration with softer (less jerky) acceleration force changes.
     

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