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  1. #1
    Engineer clough42's Avatar
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    May 2014
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    Fixture for CNC Milling Hobbed Bolts

    I just posted a YouTube video showing my fixture for milling hobbed bolts. I thought folks here might be interested.

    I was trying to come up with a way to make high-quality hobbed bolts without investing in lots of tooling. I only had a benchtop manual mill and no 4th axis or rotary table. I tried hobbing with a tap, but never liked the results. In the end, I made a fixture to hold and rotate the bolts for milling.

    This is the fixture. It starts with a NEMA17 stepper motor mounted to an aluminum bracket. The motor drives a 36-tooth GT2 pulley that has been drilled out and threaded to take the bolt and rotate it. There's a bearing in the fixture to support the head of the bolt, and another recessed into the pulley to support the thread end. The bolt is just screwed in by hand, locked in place using the set screws and milled.







    The fixture is then mounted in the mill vise and the mill is used to remove material around the bolt and then cut the teeth.





    Since I didn't have a CNC mill, I originally used an Arduino to control the stepper motor. One button rotated the bolt a full revolution to make the waist cut, and a second button rotated 1/20 of a turn at a time to cut the teeth. I just turned the Y-axis handwheel of the mill to feed in and then back out for each tooth. I used a dial indicator set to the correct zero point to control the tooth depth. A third button on the controller rotated the bolt back to the home position so I could reach the set screws to take out the bolt and put in another.

    Today, the mill has been converted to CNC, so I can automate the cuts and drive the stepper motor off the A axis of the controller. The process is exactly the same, but now I don't scrap parts when my mind wanders.

    Here are the resulting teeth under a microscope:



    The full video is here:

    https://youtu.be/AK0NvaKbRqc

  2. #2
    Super Moderator curious aardvark's Avatar
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    very neat.

    For the sake of about 1.50 - is it worth making your own ?

  3. #3
    Staff Engineer Davo's Avatar
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    That's similar to how we hob our motor shafts.

  4. #4
    Engineer clough42's Avatar
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    Quote Originally Posted by curious aardvark View Post
    very neat.

    For the sake of about 1.50 - is it worth making your own ?
    For me, it isn't as much about the price as it is about being able to control the quality. I wanted them to be crisp, clean and consistent so I could stand behind them, and for me that meant making my own.

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