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Thread: 21 Tooth Worm Gear

  1. #1
    Join Date
    Mar 2004
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    170

    Question 21 Tooth Worm Gear

    Need a replacement for a 21 tooth bronze worm gear. About 2.75" dia pitch diameter, 3/4" thick, 1" keyed bore. Boston gear only shows a 20 tooth. Any ideas? Thanx.

  2. #2
    Join Date
    May 2003
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    52N 122W Western Kanuckistan
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    Do you need the worm or the worm wheel?
    Free software for calculating bolt circles and similar: Origin now settable to bottom left! All values positive. Click Here

  3. #3
    Join Date
    Mar 2004
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    Post

    Worm wheel not the worm.

  4. #4
    Join Date
    Jun 2002
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    205

    Post

    I use my rotary table to cut odd tooth gears. 360/21 = 17.142857 degrees per tooth.
    Most rotary tables are accurate to +/- 45 seconds or so. On a 2.75" diameter gear,
    a one minute variation would be:
    3.14159 * 2.75/360 * 60 = .0004.

    Probably having a variance of +/- 1/2 mil
    on a gear will work pretty well.

    Of course the RT is marked in minutes, no decimal fractions of a degree, so you need to convert. I always make up a chart of the reading in degrees and minutes for each tooth, and check it off as I cut.

    Regards,

    Jeff E.

  5. #5
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    You can hob one.

    Make a hob using an Acme type profile with the correct pressure angle. Grind a few slots in the threads to create the teeth and case harden or make it from drill rod.

    You need to make a fixture to hold the gear blank so it can rotate and offer it to the hob in the lathe at the right height. It will generate the correct number of teeth if the diameter of the blank is correct. It helps to gash the blank first to the pitch circle depth.




    Free software for calculating bolt circles and similar: Origin now settable to bottom left! All values positive. Click Here

  6. #6
    Join Date
    Mar 2005
    Location
    Toronto
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    Evan, obviously the results speak for themselves, but how did you get away with not backing off behind the teeth?
    .

  7. #7
    Join Date
    Sep 2003
    Location
    BC Can.
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    You so called amateurs never cease to amaze me. After 40 years in the business, I would really like to run as an apprentice with one of you.

  8. #8
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    It cuts slowly without relief but it cuts. It wouldn't be hard to relieve the teeth but it isn't really necessary for aluminum or brass.
    Free software for calculating bolt circles and similar: Origin now settable to bottom left! All values positive. Click Here

  9. #9
    Join Date
    Mar 2005
    Location
    Nashville, TN
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    393

    Post

    Here's a nice site with lots of pics on hobbing some gears on a small lathe-
    http://bedair.org/Worm/Worm.html
    You never learn anything by doing it right.

  10. #10
    Join Date
    May 2002
    Location
    Beaumont, TX
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    <font face="Verdana, Arial" size="2">Originally posted by Evan:
    It cuts slowly without relief but it cuts. It wouldn't be hard to relieve the teeth but it isn't really necessary for aluminum or brass.</font>
    So Evan, did you feed the cutter radially or axially into the blank? That's a detail that is often not clear in the descriptions of hobbing.

    Paul A.


    Paul A.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

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