My homemade gear hobber

Discussion in 'Machine Modifications' started by Ross Donelly, Jul 13, 2018.

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  1. Jul 13, 2018 #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.
     
  2. Jul 13, 2018 #2

    natalefr

    natalefr

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    Good job !
     
  3. Jul 14, 2018 #3

    Herbiev

    Herbiev

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    Great work. Thanks for sharing
     
  4. Jul 14, 2018 #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 #5

    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.
     
  6. Jul 21, 2018 #6

    Wizard69

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    Thanks for the post! This cries out for a video though, a thousand words and all.
     
  7. Jul 22, 2018 #7

    graywd

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    great post but I'm missing something .... if the hob is spiral, why do you need to tilt the mill ---- or is the hob straight?
     
  8. Jul 22, 2018 #8

    Mechanicboy

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    The hobbing gear cutter has spiral hence it has a angle, when you are creating the gear wheel, then the hob must be tilted to a right angle then the gear tooth is straight. Each hobbing gear cutter has angle marked on then one can set the correct angle on hobbing gear cutter to make straight tooth. If one want the helical gear, then angle + helical angle or angle - helical angle.
     
  9. Jul 22, 2018 #9

    graywd

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    Thank you for the quick response. I had assumed that the spur blank rotated upward to match the helical rise of the hob so that the cutter always engaged the same spur. Part of my assumption was that if the head was tilted so that the hob cutter is flat, then the blank would be incremented only when the next spur is to be cut. I'll need to re-think this.
     
  10. Jul 23, 2018 #10

    kstrauss

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    Did you use a standard Arduino PID module or did you roll your own?
     
  11. Jul 24, 2018 #11

    jsimpson

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    Love it...
     
  12. Jul 24, 2018 #12

    Ross Donelly

    Ross Donelly

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    I rolled my own. I'd post the program but I'm on vacation and don't have access to it. There's not much to it, mainly tuning the 3 coefficients.
     
  13. Jul 24, 2018 #13

    kstrauss

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    Absolutely no rush so enjoy your vacation. I just like to see how others do things.
     
  14. Aug 1, 2018 #14

    necchiom

    necchiom

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    Wow! Awesome job.
     

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