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Lathe work, How do we find center?

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  • Lathe work, How do we find center?

    Hello Group,

    A buddy of mine is working on a steam tractor and the engine drive gear system needs some repair. This original gear is a cast gear that was faced when new, but needs to be bushed now as the shaft has worn a larger out of round hole. The plan is to bore to 1.25" and then press fit brass into the opening, then machine the hole for a 1" shaft with final size boring and reaming.

    The Question is, how do we find center so the gear, which is a planetary gear that has to work with 2 others on a larger gear for the engine drive system. Old stuff so close probably will work but even finding center has us both stumped.

    Will try and answer any questions or get additional info if needed.

    Click image for larger version

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    mr fixit for the family

  • #2
    You appear to have a raised boss just a touch inboard from the root of the teeth. I'd indicate off of that using an indicol or other holder for the indicator.

    It looks like the hole is still pretty much in the center. You might start by centering over that and then check it against the boss.

    At the end of the project, there is a profound difference between spare parts and extra parts.

    Location: SF East Bay.


    • #3
      Get a piece of pipe or tubing with an OD larger than the gear teeth. Bore the ID to be a close fit to the gear teeth OD. Cut a slot through one side of the tube. Put the gear in the tube and clamp the tube in the lathe chuck. The hole you bore in the middle of the gear will be concentric to the OD of the gear.


      • #4
        I'd say toolguy is not a good answer. My first thought is that it's a tapered tooth gear so never mind centering it. Just holding it to start would be a bit of a trick.

        I'd go even a bit further too. I'd step bore the "holder" so that the smaller end of the teeth fits into a reduced ID to center the small end and the larger side of the holder is the same ID as the big end of the teeth. In fact so it looks like below.

        Click image for larger version  Name:	gear holder.jpg Views:	0 Size:	34.6 KB ID:	1852284
        Chilliwack BC, Canada


        • #5
          Quick and dirty? I'd say chuck it up in a 3 jaw, with the jaws between the teeth (you get lucky with this tooth count) and flat against the face of the chuck. Then check the bore with an indicator. who knows, you might get lucky and its centered up good enough to bore and sleeve. If not, then go for the more complex solutions of special fixturing. It would literally take about 5 minutes to do a quick check. Not sure what size your lathe chuck is but you might have to use same size pins in between the gear teeth.
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          Ya, I know it's a bevel gear, but the bevel angle doesn't look that steep. It would be a light machining setup for sure.

          Another option would be on a mill, with a boring head. Not every lathe problem has a lathe solution.....


          • #6
            What I would try is put a piece of square plate bigger than the gear in a four jaw. Drill a hole roughly in the center of the plate and bolt the gear through its center to the plate using a large washer. Indicate the tips of the teeth and center the gear with the four jaw by its moving the square plate. When the gear is centered, drill between 3 of the gear teeth and bolt the gear to the plate using a small rectangular piece of steel that spans 2 teeth each. Remove the center bolt and washer, drill and bore.


            • #7
              Errrr... Cast iron gear perhaps? I clicked on and expanded the pictures and it sure looks like it's a cast iron gear by the casting marks on the ends of the teeth. A little too much pressure? Not sure I like that idea.... It would be a lot safer in a little turned bore "pot collet".
              Chilliwack BC, Canada


              • #8
                Cast bosses are not generally usable for reference. I agree that the best you can do is use the outer tooth edges and in this case that is going to be a complete swag. If you have a milling machine with a DRO, you can locate center off the three teeth that look the best to you. Because of the odd shape, I'd use a mill anyway to avoid difficult holding issues mentioned by BC.
                Southwest Utah


                • #9
                  +1 on Dan's setup in a three jaw chuck. I've done that a number of times.


                  • #10
                    Hrrm, 9 teeth, so I would chuck it in the 3 jaw and indicate off the bore. Sufficiently light cuts on cast iron won't hurt it. Once it is bored and bushed, chuck it by the bore on a mandrel and clean up the OD a few thou with a skim cut. A few thou shorter teeth won't hurt a cast gear, it'll get lost in the original loose tolerances.

                    Another way to hold odd shapes so to pour molten lead around them in a casting. Hold onto that and center up and machine however you want, machining the lead away with the part as needed, melt it off when finished. They make low melting alloys for just this purpose and I've seen that done before, but most people don't have that.
                    Last edited by nickel-city-fab; 01-30-2020, 06:26 PM.


                    • #11
                      Indicate the crests or the root of the teeth, whichever is less worn.


                      • #12
                        What are we seeing in the pics? Is that the two ends of the same gear?

                        Anyway, it looks a bit worn as to the teeth as well as the hole, and there does appear to be a heck of a taper on it, which is a bit unusual. That would eliminate the idea of an outside sleeve to hold it.

                        If it is cast, can the OD be relied upon at all? Might have casting defects, and if it is worn, there may be a burr from the wear, plus the OD is not an essential factor in a gear anyway. The right idea is something along the lines of what Dan Dubeau showed, hold it by pins that hit the pitch line, except for that pesky taper.

                        Assuming the gear needs to be pretty accurate, you may want to put in the bush, one a fair bit longer than needed. Measure center relative to the pitch line (from pins) on each side, then mark and center the too-long bush at each side. Turn the stuck-out part true with it on the centers, finally chucking it by the stick-out part and boring the hole in the bush. The excess bush can be cut off, and the part faced with it on an arbor.

                        That may be more fuss than needed, but it would re-center the bush nicely.

                        Keep eye on ball.
                        Hashim Khan


                        • #13
                          Make a new gear


                          • #14
                            It is probably powdered metal.


                            • #15
                              Wow Guys'

                              What a great site this is, so many responses, and so quickly, Thanks. Yes the pictures are of the same gear, just wanted to show both sides. The hole is warn out of shape, the shaft that it fits on is a straight 1" shaft.
                              We tried indicating the crest and root tooth edges and your correct, they do very a bit. We also tried the large flat surface edge and it seemed better but still out of round a bit. We are probably being to critical for what it is, but we both like to see things done right.

                              J Tiers, Could you help those of us (ME) that are less knowledgeable reexplaining the setup your describing.

                              As always the help here is fantastic.

                              Mr fixit for the family