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  • Concentric bore

    I need to make a small pulley which has a bore concentric with the outside of the hub. How would you go about doing this:
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  • #2
    For something that small, even in steel, you could chuck the slug, make the bore, wedge it on a tapered mandrel and turn the outside. You'd need to support the outside end of the mandrel as well.
    Southwest Utah

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    • #3
      have it stick out from the chuck and do all the turning and boring at one setting. Or swap it around, ideally with a collet. What aspect is concerning you?
      in Toronto Ontario - where are you?

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      • #4
        Originally posted by Mcgyver View Post
        have it stick out from the chuck and do all the turning and boring at one setting. Or swap it around, ideally with a collet. What aspect is concerning you?
        Simply drilling the bore may or may not be concentric as drill bits are prone to wander. The walls of the hub are fairly thin, so not a lot of force can be used to hold it between dead centers.

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        • #5
          Originally posted by elf View Post

          Simply drilling the bore may or may not be concentric as drill bits are prone to wander. The walls of the hub are fairly thin, so not a lot of force can be used to hold it between dead centers.
          Gun drilling would be safe bet but even ordinary twist drills work often suprisingly good when you make a good starting hole. Dont use center drills, single-point turn the starting hole instead.
          https://www.practicalmachinist.com/v...0/#post3044390
          Location: Helsinki, Finland, Europe

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          • #6
            What is the concentricity tolerance?
            It will often look like so on the drawing.

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            • #7
              Originally posted by elf View Post

              Simply drilling the bore may or may not be concentric as drill bits are prone to wander.
              good point.....I've posted this so many times I'm bored of doing so lol, but its guaranteed to work. i put a 1/8 hole through 3 inches and it was within a tho out....apparently it was 11 years ago...post 18 here

              https://bbs.homeshopmachinist.net/fo...-hole-drilling

              Last edited by Mcgyver; 02-25-2021, 08:25 PM.
              in Toronto Ontario - where are you?

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              • #8
                YES! I would do the bore first. If you are worried about a drill bit going sideways, use a gun drill of a slightly smaller diameter and then a 1/8" reamer. Add a generous, 60 degree chamfer at the outer end of the bore for a tail stock center for supporting the outer end.

                Using that center in the tail stock, machine the pulley except for the rear face, working from the outer end towards the chuck holding the stock. Then part off.

                If your parting operation did not leave a nice enough face on the end of the pulley, turn it around in the chuck, face it, and chamfer the edges. That facing cut should be quite accurate even if your part is not perfectly centered. The chamfers may be a bit off center, but that will probably not matter.



                Originally posted by Mcgyver View Post
                have it stick out from the chuck and do all the turning and boring at one setting. Or swap it around, ideally with a collet. What aspect is concerning you?
                Paul A.
                SE Texas

                And if you look REAL close at an analog signal,
                You will find that it has discrete steps.

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                • #9
                  Yes, I would avoid using a center drill for starting the bore. I would use a short spotting bit and you can use one that is larger then the bore (3/16" or 1/4") and go just deep enough to make a crater that is 1/8" in diameter or LESS. I keep a small collection of spotting bits just for things like this.



                  Originally posted by MattiJ View Post
                  Gun drilling would be safe bet but even ordinary twist drills work often suprisingly good when you make a good starting hole. Dont use center drills, single-point turn the starting hole instead.
                  https://www.practicalmachinist.com/v...0/#post3044390
                  Paul A.
                  SE Texas

                  And if you look REAL close at an analog signal,
                  You will find that it has discrete steps.

                  Comment


                  • #10
                    Any reason you couldnt single-point the bore then do the rest of the turning on a mandrel? Sure, need a small boring bar for that, but mirco100 bars arent prohibitively expensive

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                    • #11
                      I would chuck up the stock in a four jaw and rough in the outside shape along with the groove. Then drill undersize and ream the finish hole. Then fit the hole with a snug fitting pin and re-indicate the finished bore. Then finish the outside profile and it's a good job.
                      Sarge41
                      Last edited by sarge41; 02-25-2021, 09:48 PM. Reason: thick fingers

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                      • #12
                        The bore needs to be conc with the outside of the HUB, no mention of the pulley part? In any case, do all in one set-up. Rough turn all OD--- drill and finish bore the ID ( yes a small bar)--- then come back and do a very light clean up on the critical OD surfaces -- cut off. ALSO , no mention of the material or quantity? these will affect how one processes the part

                        Joe B

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                        • #13
                          Does everything need to be exactly concentric, or just the ID and the 162* V profile? Rough in one side and drill a little over half way through leaving a few thou under size. Flip the part and rough the profile, then drill and finish the ID to size with a reamer. Put a piece of shafting in the ID and check how the hole runs. If it is concentric enough for your specs then finish that side along with the V profile, flip the part and finish the non critical diameters. If the ID isn't running how you would like then hold the shaft concentric and slide the piece on. Provide some tail stock support. The part can be gripped on the ID with some epoxy. Finish the diameters concentric to the ID and shaft, then heat the epoxy to remove the shaft. You may need to clean up the ID with the reamer by hand to remove any residual adhesive.

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                          • #14
                            The material will be steel. Bearings will ride on the hub. Ideally, there should be 0 runout at .75". I don't know at this point what the max acceptable runout will be. A round poly belt will be used and it should not touch the bottom of the groove, so its concentricity won't be as critical. I only need one good one, hopefully it won't take too many tries

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                            • #15
                              Drill and ream the part. Then chuck a piece of say .75 diameter stock and turn a 1/8 diameter + stub on it. Press your part onto the stub and face it. Remove, turn the stub down to 1/8 exactly and push the part onto it with the faced side facing the chuck. Apply pressure from the tailstock to hold the part against the .75 diameter shoulder, and machine the pulley.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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