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'nother question: Gear terms

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  • 'nother question: Gear terms

    Gear cutting fascinates me tho I haven't done any yet. Until recently all of my reading would reference the 'involute' gear form, which I seem to recall once seeing defined with an example using a string to illustrate the tooth profile. But lately I've started seeing the term 'Module' or 'module involute'. In his fine feature article of the HSM I recv'd yesterday, Terry Sexton touched on the issue and from that I deduced that the term 'module' describes a concept that's an alternative to Diametral Pitch. Can someone elaborate on the difference in these two gear forms?

  • #2
    usually module referrs to the nomenclature of the metric system, sometimes there is a use in the inch system.


    • #3
      An involute is a constantly changing curve meaning the tooth form is a constantly changing curve, not the 1/4 pitch dia as published in some how to books years ago.
      The module of a gear is equil to the pitch diameter divided by the number of teeth.
      Diameterical pitch is equil to the number of teeth divided by the pitch dia.
      Most gears today use an involute form and could be described in the module or diameterical terms, but aren't.
      Metrics are generally described in module terms, hun.
      Thank you,
      Machinerys Handbook page 2090 volume 26



      • #4
        Module is best described as the metric form of DP.
        Just as one inch equals 25.4mm then 1 Mod as it's called is equal to 25.4DP, 2 Mod is equal to 12.7 DP etc.
        In imperial measurement you get DP usually working in equal numbers 12,14,16 etc Mod is usually expressed as a decimal 1, 1.25, 1.5, 1.75, 2.0 etc.

        If you are fascinated with gears and a lot are then my advise is to buy the book called Gears and Gearcutting by Ivan Law. It's number 17 in the Nexus series. I believe Wise Owl and Linseys stock this in the US.
        Failing that TEE Publishing in the UK ship to the states. It's not an expensive book about 8 UKP but it covers most of the problems encountered in the home shop as regards the theory and practical cutting of gears.
        There are may other books but this one is written with the home shop guy in mind.

        John S.

        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.