Hobbing a gear with a tap help
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  1. #1
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    Default Hobbing a gear with a tap help

    I've been reading all morning and now my head hurts, no really, it hurts. I need to make three worm gears and have a worn sample but was able to take measurements from the unused edges but can't figure out what pitch of tap I would need. I realize the math may not work out to where this "hobby" method may even work but I can't even come up with a number.
    Here's what I have :
    OD 1.560"
    Circumference 4.898"
    Pitch Diameter 1.466 (maybe?)
    Root diameter : 1.372
    30 teeth

    Any help is greatly appreciated.

  2. #2
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    You say WORM GEAR, so I am not figuring WORM

    I will assume SINGLE START worm

    Using your maybe, the pitch is 1.466 X "pi" / 30 = .15352

    The reciprocal of that is the TPI

    .15352 / 1 = 6.5138 threads per inch - not that the profile of such a tap matches in the least with your worm profile - unless you make it so

    Quote Originally Posted by esaresky View Post
    I've been reading all morning and now my head hurts, no really, it hurts. I need to make three worm gears and have a worn sample but was able to take measurements from the unused edges but can't figure out what pitch of tap I would need. I realize the math may not work out to where this "hobby" method may even work but I can't even come up with a number.
    Here's what I have :
    OD 1.560"
    Circumference 4.898"
    Pitch Diameter 1.466 (maybe?)
    Root diameter : 1.372
    30 teeth

    Any help is greatly appreciated.

  3. #3
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    Correct, not the worm. Haha, I had that number, .1535 and it didn't make sense because I didn't know I needed the reciprocal. Thanks.

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    Now all you need to do is to figure out what the dimension translates into as far as either a diametrical pitch (inches) or module (metric) tooth size, and see if you lathe can be set up to cut a worm that size to use as a cutter, because you are not going to find a tap that size.

    My best guess is that it’s cheaper and a lot faster, to pick a likely suspect out of the gear catalogs.

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    Here is an online article explaining the process of using a tap to hob a worm gear. Hope this helps.


    Making Wormgears


    Sent from my iPhone using Tapatalk

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    The OP may not really NEED a worm wheel of that size and thread pitch but is intrigued by the process and wants to play with it.

    If that's so, he needs to choose a tap to work with and get a little more guidance as John has done to wind up with something that looks more like a part than an industrial accident.

    The relationship of TPI to pitch, and then to circular pitch will lead him to something with a whole number of teeth and a blank size. Plus the hint that gashing the correct wheel tooth count to start with will encourage the development along the right lines might get him on his way.

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    If you have some leeway with the depthing, like if the distance from worm to gear is adjustable, you might get away with a metric tap with a 4mm pitch (slightly larger gear) or a 6tpi tap (slightly smaller). If this is a low speed low confidence item, it may work out and you'll be ok. That is if like John says the profile is even close. Boston gear has worm wheels with 1.25" PD in 24 diametral pitch, or 1.875" PD in 16 DP in stock, I wonder if one of those might work with some effort.

    I remember when I was a late teenager I had a reciprocating fan from perhaps the 1940s that stripped its phenolic gear for the reciprocation. Not having any machinery, I epoxy laminated a few guitar picks together (thought nylon might be OK), drilled them, mounted to arbor in drill to turn the blank, and cut the teeth with a file. That was about '93, it ran until I gave it away when I moved here in '01- and I lived in GA, AZ, and muggy Cincinnati in those years so it ran a lot! Sometimes a hack works out. But if this is for a paying customer or a high confidence item forget the tap hack. It does work for making rope knurls though...

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    couple of tips
    make a hob, single point a cutter out of tool steel , make it slightly bigger then the worm, turn it and mill it.
    look it up, on you tube or world wide web, then heat treat it.
    the worm and worm gear have to be a match set. it's checked by center distance, and pattern check.
    the worm gear can not be check by MOW,
    be back later for more

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    The worm gear is for a Bodine gearmotor 720:1 ratio used to drive a conveyor used in making chocolate-covered confectionary products. A similar motor is about $500 and would have to be modified (or the conveyer) to work. The manufacture of the conveyor has been out of business for 15-20 years but because it was proprietary, Bodine won't sell replacement parts. I've tried twice (in case another customer agent was more friendly, soon as I give them the CAT number, they say I have to order parts from the equipment mfg. Yes, I've explained they are closed permanently. The chocolate company has three of these lines and had extra motors from when they were available but they are now on borrowed time. The rest of the motor is fine, just the brass worm gear has worn so thin the teeth fall off. I figured if I could make three gears vs 3 new motors @ $500 each plus the labor and agavation of mounting them.

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  12. #10
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    A commercial venture?

    Sounds to me like someone ought just bite the bullet and buy a new supportable motor with a worm drive on it. a 30:1 reduction is pretty easy to find. As is a 24:1, Those together get you 720:1, if my math is right.
    Or you could cobble together a reduction system using jackshafts and pulleys or chain sprockets to keep things rolling along in the meantime.

    Or, find a guy that makes gears for a living, and have him make you a couple spares. Could be a bog-standard gear set, bought off the shelf, or it could be some gawdawful bastard of a modified tooth form made just for the purpose. Going to take some careful and accurate measuring to get sorted, so the gear maker may wish to have hands on the actual gear case.

    It seems to me that you are well in over your head, and it's going to be cheaper in the long term, to hire someone that actually has the needed knowledge.

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    I can't see the tap method working for this because the tooth profile is all wrong. Your worm probably doesn't have a 60° triangular thread.

    I've used the tap-as-a-hob method to make rope knurls and I've seen projects where the resulting worm gear I'd used with a bit of threaded rod as a light duty worm gear, but it's never going to mate properly with anything but the thread that it was cut with.

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    I have run into this proprietary part from a de-funk company before and know it can be frustrating. The motor manufacturer is probably up against a legal problem. They could be sued by the owners of that "de-funk company" if they still exist. And the motor manufacturer may have no easy way to determine if those owner(s) still exist. It's a problem, but there seems to be no easy way around it.

    Have you considered becoming a new OEM for a functional equivalent of this motor. Instead of asking the motor manufacturer for a proprietary part for that now de-funk company, just develop your own motor with the appropriate specs. You would need to make drawings or determine a standard configuration of the motor's mount and shaft positions. Specify the external dimensions or an envelope that it must fit within. Then add the final RPM needed. Do not specify the actual motor RPM or the gear ratio, let them do that. Let them do the actual design for everything inside that envelop. Ask for a quote for a quantity, say ten, of them. It would be your choice to specify if this new design would be protected by the same proprietary designation or if it would be open for third parties to purchase.

    This way, you become the owner of the new propriety motor and can then order them whenever your client needs one.

    You could even send the specs package to several motor manufacturers and get competitive bids.


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