My homemade gear hobber

Discussion in 'Machine Modifications' started by Ross Donelly, Jul 13, 2018 at 10:21 PM.

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  1. Jul 13, 2018 at 10:21 PM #1

    Ross Donelly

    Ross Donelly

    Ross Donelly

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    I thought some of you may be interested in this. I modified my milling machine to do gear hobbing. The goal is to turn the gear blank by exactly 1 tooth per revolution of the mill. To achieve this, I use a motor, shaft encoders on the mill motor and the gear blank motor, and an arduino running PID controller software to keep them exactly in sync. I also needed to tilt the mill head by the angle of the hob teeth, which I did with a thick shim.

    837.JPG

    Here's a picture of the hob cutting a plastic gear (just for a test). This was before I hardened the hob so I wanted to cut something soft.

    841.JPG

    And here are some gears I've made. Some spur gears and a worm gear (the worm was made on the lathe of course). I also used this to make the gears in my Webster engine.

    004.JPG

    Cheers,
    Ross.
     
    Cogsy and minh-thanh like this.
  2. Jul 13, 2018 at 11:27 PM #2

    natalefr

    natalefr

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    Good job !
     
  3. Jul 14, 2018 at 12:48 AM #3

    Herbiev

    Herbiev

    Herbiev

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    Great work. Thanks for sharing
     
  4. Jul 14, 2018 at 2:35 AM #4

    Engineville

    Engineville

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    This is a significant achievement. If you’re willing, please tell us more of how you made and configured this modified mini-mill. It appears that you have mounted an optical chopper/counter on the spindle of the mill, but is the spindle drive also a stepping motor? How do you synchronize the rotation of the mill spindle and the rotation of the a-axis holding the gear blank?
     
  5. Jul 14, 2018 at 8:28 AM #5

    Ross Donelly

    Ross Donelly

    Ross Donelly

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    Sure. There's a 2-bit opto-interruptor mounted on the mill motor frame and an interruptor disc with 18 notches mounted on the end of the motor shaft. The 2 bits feed the microcontroller, which is set up to count transitions. So the microcontroller knows how many notches have gone by and in which direction. Also knowing the gear ratio of the mill, it knows quite precisely the rotational position of the mill spindle. I have a similar setup on the gear blank motor (a-axis). Now given that we know both positions, we can measure the error in the a-axis position. The goal error is zero. A PID routine is set up to control the motor voltage and keep the motor moving at just the right speed that the error stays at zero (or as close as possible).

    The Arduino has PWM outputs that make motor control easy and accurate, and its inputs can trigger interrupts for counting. It's surprising how well this works. Once stabilized, it's accurate to within about 0.2 degrees. The one minor problem I've had is mill vibration occasionally getting into the opto interruptor, but I could fix that by mounting the LED/phototransistors more solidly.
     

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