Worm wheel help

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  • Boostinjdm
    Senior Member
    • Mar 2011
    • 1160

    Worm wheel help

    I'm trying to cut a replacement worm wheel for my saw and am getting confusing results. I'm going to say right up front that I don't know what I'm doing and this is my first attempt at making a gear.
    I measured up the worm and made a hob to match. It is 1.182" OD and 4tpi. I don't know what that works out to for pitch.
    I cut one blank at 2.050" and ended up with 26 teeth that were ugly cuz I gashed it the wrong direction. So....round 2, I cut a blank at 2.000" and ended up with 27 teeth without gashing. The goal is 23 teeth.

    What gives?
  • Fasttrack
    Senior Member
    • Jul 2005
    • 6300

    #2
    What is the thread profile?

    As a close estimate: 23 teeth / 4 threads/inch = 5.75 inches. 5.75"/3.141 = ~1.83" so the diameter should be approximately 1.83"

    Comment

    • Boostinjdm
      Senior Member
      • Mar 2011
      • 1160

      #3
      So the pitch diameter should be about 1.83"? Or the OD of the blank?

      The old gear is at least 2" and worn pretty bad so I don't know where it started. That's what I was going off of when making the blank.

      Comment

      • winchman
        Senior Member
        • Aug 2003
        • 4030

        #4
        It sounds like the blank is being driven by the hob as it starts cutting the teeth. Since the OD is too large, it's cutting too many teeth. You need to drive the blank at the proper ratio to the rotation of the hob, or remove a little material from the blank (i.e. make roughing cuts between the tooth locations) so the hob won't have a chance to start cutting too many teeth.

        Keep in mind I've never tried to cut a gear, so this is all speculation on my part.
        Any products mentioned in my posts have been endorsed by their manufacturer.

        Comment

        • Fasttrack
          Senior Member
          • Jul 2005
          • 6300

          #5
          Originally posted by Boostinjdm View Post
          So the pitch diameter should be about 1.83"? Or the OD of the blank?

          The old gear is at least 2" and worn pretty bad so I don't know where it started. That's what I was going off of when making the blank.
          That's the pitch diameter.

          How are you gashing the blank? Can you use the old wheel to index the blank and gash it?

          Comment

          • Boostinjdm
            Senior Member
            • Mar 2011
            • 1160

            #6
            For the 3rd attempt, I gashed the blank on the rotary table in the mill. I also removed a bit of material down the center to try and get a smaller OD to engage the hob. I still ended up with too many teeth and a few partial teeth.

            Now the goal has changed. I don't care how many teeth it has as long as it's functional. I've had enough fail for one day. I'll try again tomorrow.

            Comment

            • John Stevenson
              Senior Member
              • Mar 2001
              • 16177

              #7
              Do the maths.

              4 tpi equates to 12.56 DP.
              Rule is for a worm wheel you add 3 teeth for an enveloping worm.
              So 23 + 3 = 26 / 12.56 = 2.070" OD.

              However offering this up to a hob will generate more than 23 teeth because the diameter is 3 teeth bigger than it should be.
              It will only cut 23 at start up if it's allowed to start on the pitch line.

              You need to pre gash the wheel first with 23 teeth then let the hob for the correct shape.
              .

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



              Comment

              • Boostinjdm
                Senior Member
                • Mar 2011
                • 1160

                #8
                If I cut my blank at 2.070" and feed it into the hob without gashing, will I get 26 even teeth?
                Or would it work to feed the blank in from the end of the hob so it can start on the pitch line without gashing?

                Comment

                • John Stevenson
                  Senior Member
                  • Mar 2001
                  • 16177

                  #9
                  The answer to that is a crap shoot.

                  It only take the first two 'threads' to set the pattern for what goes next.

                  Print a circle out 2 .07" diameter, draw 23 radial lines to the outside and glue this onto your blank.
                  Then with a hacksaw with 2 blades in it side by side make 23 shallow cuts across the face of the gear to correspond with the lines.
                  If you don't go deep you won't have to worry about the helix angle as the hob will clean up.
                  .

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



                  Comment

                  • Boostinjdm
                    Senior Member
                    • Mar 2011
                    • 1160

                    #10
                    This is what I meant by feeding it in from the end. It seems to work perfectly for this guy. http://www.youtube.com/watch?feature...&v=ie7dNOLgno0

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