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  • Rack cutter profile

    Hi all,

    I've been doing a lot of research on making gears with my shaper and I've not been able to find this piece of information. I know the cutter has a rack profile, which is just like a truncated threading tool with the same angle as the pressure angle of the gears, but what I can't find is how much should be taken off the end of the tool for a specific module...

    I'll be using the process where the shaper traverses and cuts 1 tooth of the gear at a time while rotating the gear blank to generate the involute profile, and I'm using the module system.

    Does anyone have a diagram of a rack cutter?

    Cheers,
    Andrew
    Andrew

  • #2
    Download and install GearSpec from here: http://www.wmberg.com/tools/

    I think it will tell you what you need to know.
    ----------
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.
    Don't own anything you have to feed or paint. - Hood River Blackie

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    • #3
      "Gear Cutting Practice" by Colvin and Stanley has dimensions for the various "modified" profiles.

      Of course the "basic" profile is simply a trapezoid with the pressure angle as the angle of sides. However, many modifications have been made for practical reasons.

      Lindsay books carries it (they reprinted it).
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        Thanks very much, but gearspec didn't really tell me anything about the profile of the cutter.

        Of course the "basic" profile is simply a trapezoid with the pressure angle as the angle of sides
        That's what I'm looking for, but how do I find the dimensions of the trapezoid? I understand it has 20 degree sides for a 20 degree pressure angle, but I need another dimension to create the cutter.

        Cheers,
        Andrew
        Andrew

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        • #5
          Well you know the DP of the gear, so you know the pitch diameter, and the OD, so you know the "addendum". The "dedendum", or distance below the pitch circle on the gear to bottom of toothspace is the pitch circle less the addendum, with a bit more off as an allowance for clearance.

          On teh rack, the pitch circle is up from teh bottom of the toothspace the distance of the addendum, and the distance to end of tooth is that again, plus the allowance, as a fist approximation.

          To do it right there are all sorts of formulas.

          The Colvin and Stanley book is pretty good. Details cutting and measuring of spur, helical, and bevel gears. Discusses the various ways of cutting.

          I don't have the Ivan Law book, I understand it is less theory and more direct how-to. If so you may want both.
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            Cutter point width.

            Originally posted by jones
            Hi all,

            I'll be using the process where the shaper traverses and cuts 1 tooth of the gear at a time while rotating the gear blank to generate the involute profile, and I'm using the module system.

            Does anyone have a diagram of a rack cutter?

            Cheers,
            Andrew
            No but:

            Point width of cutter = Tooth thickness of cutter-(2*Addendum of cutter*Tanfunc of Pressure angle).



            The addendum of the cutter is the Dedendum of the gear, also the fillet radius is about 0.30*Diametral pitch.

            So: 1 Metric Module = 0.0383" or 0.9728 mm for 14.5° PA.

            And 0.0287" or 0.7290 mm for 20° PA. OK?



            Cheers,
            Les H.
            The Impossible Takes Just A Little Bit Longer!

            Comment


            • #7
              Search google for Sunderland gear cutting process

              Picture of the process here http://www.slv.vic.gov.au/miscpics/0...mp021020.shtml
              Precision takes time.

              Comment


              • #8
                Plain ol' racks don't have to get very sophisticated if they're used for slow moving applications like in arbor presses and such. An accurately ground shaper tool is all you need.

                Powered motion racks like for driving planer tables need to have the rack tooth profile reccommended for that service. The rack tooth has to have tip and root relief so the load tapers into the mesh and back out. This little detail makes for smoother operation and greater durability. The standard rack profile used for gear hobs etc and gear shaper cutters have these reliefs ground into them so you just use them and the tip and toor relief come automatically. Single point tools to be used in powered racks may need to be machine ground unless an optical comparator is handy and a die diler equipped with a slip stone is used to refine the needed features into the tooth space profile.

                You can see the effect of tip and root relief in gear pairs that have seen enough service that wear polish is apparent. At the tips and roots the wear polish blends away from the as-finished flank. These are very subtle features detectable in racks only with instrument and in the case of very large pitches by a scale held against a rack tooth flank and sighted to the light.

                The standard rack shown in Machinery's Handbook illustration shows this relief feature and it's discussed in the text if you hunt for it. You might see it described as "cycloidal relief".
                Last edited by Forrest Addy; 11-18-2009, 05:06 AM.

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                • #9
                  Pitching and rolling

                  A rack while straight should be seen as a small part of a very large gear. It is easier to visualise that way.

                  Here is a graphical representation. It is one of a series that I have on photobucket.


                  The distances between teeth is the same as the circular pitch on the gear you are matching it with.

                  The metal strips that make the gear blank rotate have to work on a cylinder the same diameter as your finished gear pitch diameter so that the gear "rolls" on the pitch circle - like this:


                  Basic gear data is here:

                  .

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                  • #10
                    Wow, thanks very much everyone. That's exactly what I was looking for. Now to get on to making a jig for the shaper which rotates the gear blank...

                    Cheers,
                    Andrew
                    Andrew

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