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Boring Gears

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  • Boring Gears

    Can someone tell me the best way to hold a gear on a lathe for boring and the best tool(carbide,Hss) to use since the gear hole has a keyway? I've thought about pressing an aluminum key in the keyway to avoid the whack-a-whack cut.

  • #2
    Some additional information about the gear would be helpful. How big is the gear, what is it make of and how much do you have to take out of the bore? Is the gear flat or is there a boss on it?

    Carbide does not like interupted cut as it will chip. You probably need to use HSS.



    • #3
      Depending on the size & number of teeth, they often center perfectly in a 3-jaw. Try that first & indicate the I.D., you may be surprised.

      4-jaw would be my second choice (although you'll have just as many people tell you it's their first choice!)

      If neither of those work a faceplate should do the trick. On really odd-shaped parts I make an adapter plate that bolts to the faceplate to give more clamping options.

      The keyway may not be a problem if it's small. On larger keyways I mill a piece of keystock to the depth of the keyway and superglue it in (don't laugh til you try it).

      All my boring bars use carbide inserts. With the correct insert grade & proper chipbreaker selection they can work well even on interrupted cuts. I'll email a copy of Valenite's catalog if you want (it's 96.7MB). Great tech section on grades & chipbreakers.

      Barry Milton
      Barry Milton


      • #4
        it will depend a little on the application. For my sheet metal roller (low rpm, low duty cycle and the meshing of the gears varies with the sheet's thickness), as Barry suggested the three jaw sufficed.

        For a more demanding app, I'd stay away from the three jaw as it will not centre it perfectly . Depending on your three jaw it will be a couple to many thou out - that will impact meshing.

        I’ll be first to make Barry's prediction come true - hold it in the four jaw and indicate the bore.


        • #5
          I'd probably do it on a faceplate. If your lathe allows, put a pin of appropriate diameter in a collet and slide the gear onto that, push it up against the faceplate, and clamp it in place. Then remove pin+collet and bore the gear.

          A filler piece in the keyway wouldn't hurt. I'd use a HSS toolbit.
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          There are lots of people who mistake their imagination for their memory. - Josh Billings
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          • #6
            I've bored many a gear.

            If you have a keyway HSS really is the best for withstanding the interrupted cut. Modern carbides are tough but the last place you want the edge to break down is when you're finishing a bore with the remnant of a keyway in it.

            Grip the gear in the 4 jaw using 0.062 annealed copper or soft aluminum to pad the jaws and protect the teeth from bruising. Dial into the gear OD and the face. If the job is real persinckety use a pin in the tooth spaces to dial into the gear's pitchline. Rock the pin in the space to ensure a min reading on the dial indicator.

            If you prefer to use a 3 jaw or you have many gears to bore use soft jaws bored to suit the gear. Be sure to dial in the face of the gear as well as the OD. The last thing you want in a gear is a drunken tooth bearing.


            • #7
              From a hobby machinist having done this a few times- I usually mount a round disc of mdf to a faceplate, turn out a recess to press the gear into, then fasten the gear using bolts through the gear, the wooden disc, and the faceplate.
              I figure the recess is turned with no runout, so the gear will also be turned with no runout or wobble. I like this method when there are multiple discs of any kind to do.
              I just did this with two steel discs that came close to being too large in diameter to turn in my lathe, and had no problems with the boring operation.
              I keep a dedicated vacuum cleaner close to the lathe to clean up after the woodturning operation.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-