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calibrate spindle speed

Discussion in 'CNC Mills/Routers' started by Dale Y, Mar 11, 2017.

  1. Dale Y

    Dale Y Journeyman

    Jun 20, 2016
    Likes Received:
    Getting my machine together, just need to finish the wiring, and get the belts in. Trying I have a 400 w variable spindle. How do I figure out the correct speed commands in file files?
  2. Jonathon Duerig


    Nov 25, 2015
    Likes Received:
    Something between trial and error and doing some calculation. Typically, the 'speed' setting you specify is a PWM value that acts as a % of full power. So find he maximum speed, then choose the PWM value that gives you the speed you want. I think with GRBL, it is a value between 0 and 1000 with 1000 meaning 100%. Also keep in mind that when the spindle encounters resistance, its actual speed will decrease.

    Determining what speed you want the spindle and the feed rate of the device (feeds and speeds) is a complicated question that depends on the material and the bit. Each bit has chip load data indicating how much material it can remove per revolution. Thinner bits are not as robust so you need to run them more slowly and at shallower depths or you might break your bit. The materials matter a lot where harder materials require you to move more slowly and some materials (like plastic or aluminum) can melt if things are moving too fast. I cut a lot of plastic and find that even with my settings dialed in, my bit will eventually dull enough to start melting plastic and that is how I know I need to change bits.

    The Shapeoko community has a bunch of people trying out feeds/speeds and reporting the results here:

    Materials - ShapeOko

    That might give you a good sense of a start place. Expect to break bits while you dial in the settings. That is one reason why it is a good idea to stick to a few materials that you know well, record your settings and the results, and slowly branch out. If every new piece is a different material, you will always be on the edge of a failed job.

  3. Dale Y

    Dale Y Journeyman

    Jun 20, 2016
    Likes Received:
    Been doing some research and I found a chart about that gives different feed speeds depending on materials, bit size /# flutes. As far as controlling the RPM, I borrowed a tachometer and checked spindle RPMs at different "S" commands. At no load, in my tests, the settings are not a % of full. On my spindle, running 48 volt supply, it is rated at 12000 rpm max. Here is an example of my tests:
    S100 - 2060 rpm
    S200 - 4315
    S300 - 6300
    S400 - 7710
    S500 - 8850
    S600 - 9820
    S700 - 10730
    S800 - 11480
    S900 - 11900
    S1000 - 11980

    So, on these tests, each step up gives diminishing increases. But at least I know what I have going in.
    hairyMug likes this.
  4. David the swarfer

    David the swarfer OpenBuilds Team
    Staff Member Moderator Builder

    Aug 6, 2013
    Likes Received:
    If you are running GRBL v1.1 then you can set the variable
    $30 - Max spindle speed, RPM
    to your max spindle speed. Then you don't have to map your S values (-:
    so, set
    and then your Gcode and use actual RPM like 'M3 S12000' for fullspeed.

    Will it be spot on? Probably not.

    There are also some options in GRBL's config.h that can affect this.
    Grbl v1.1 Configuration · gnea/grbl Wiki · GitHub
    snokid likes this.

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