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Reboring a step pulley

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  • Reboring a step pulley

    Hi folks,

    I've just removed a 4-step aluminium step pulley from the motor on a lathe I'm rebuilding.

    It goes from about 3 inches up to about 6 inches and has a 5/8ths bore.

    I'm upgrading the size of the motor on the lathe and don't think I can get a new motor of the HP I want with a 5/8ths shaft.

    SO... I think I need to rebore the step pulley. I have no suitable lathe so will need to use the mill.

    Couple of questions I was hoping someone could help with-

    1) Can I just clamp this thing down like a pyramid on my mill table? I presume the bottom face is machined perpendicular to the axis?

    2) I have an RF45 clone mill. I can tram the head accurately. Am I in danger of getting too much runout by using the mill?

    3) I don't think I have a means of re-slotting the keyway to the larger size that will undoubtedly be required. I don't have a broach set or a slotting head. I'm not sure what my options are here- I could machine a special 'stepped' key, use some kind of setscrew arrangement or shrink fit the pulley on and hope I don't need a key?

    I considered turning the shaft of the motor down on my other lathe (which is too small to host the pulley itself), but that wouldn't solve the fact that the new motor would have a keyway bigger than the old one.

    When I list all the problems above I wonder if I wouldn't be better just paying for a new step pulley that's already bored to the right size, but I know from experience the cheap ones can have a lot of runout and I'd like to reuse this one - it's British made and is a big lump of aluminium to throw away!


  • #2
    1) Maybe. Best bet would be indicate the sides of one of the pully grooves and center over that, then check the other side of the groove for parallel
    Consider everything has inaccuracys, your best bet is dialing into the part that matters, the pully grooves.
    Easyer might be indicate into the existing bore, or a long straight shaft inserted into the bore.
    2) Not really, Its a pully, very low accuracy needed as belts have slop and strech, Worse case you end up with a bad vibration from being out of balance.

    3) Files work to make a keyway, with some time.

    4) What else do you have to do with your machine time today? Is it going to be more fun and/or productive then making a new pully?
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


    • #3
      Two thoughts:

      You have a shaper in your mill. Grind a bit to cut the keyway, mount it in the (locked) spindle, and crank the spindle down into the bore to cut the pulley.
      Use the X-feed to deepen the cut.

      If you decide to reduce the diameter of the motor shaft, you can bolt the motor to the mill table, turn on the motor, then use the mill (with a cutter) to act as a spinning lathe tool against the (also spinning) shaft.
      I have used this method to remove awkwardly mounted pulleys, works well. Take it easy until you see how it cuts.

      You would still have a groove to cut, but you could probably come up with a variation of the first suggestion, above.
      Or you could just secure the shaft from turning and run the appropriate width endmill along the shaft to cut the groove.


      • #4
        yes, mount the pulley to the mill bed on parallels, tram the head to suit the existing bore or the pulley grooves, check a few of the V belt grooves to get concentricity.

        you could drill and tap a grub screw through the bottom of the V in the largest pully into a dimple or flat on the shaft for drive, the belt won't be bothered about a 1/4" or even bigger thread for a grub screw. if you go for multiple grub screws place them at 90 or 120 degrees to each other so they don't fight (i've seen them at 180 degrees which doesn't work terribly well). put a drop of locktite on the shaft/pully and grub screws during final assembly if you want to be sure it doesn't slip

        or as above grind some tool steel and mount it in the spindle to slot the bore if desired


        • #5
          If you want to post it to me, NG10 area, I'll bore it and keyway it and send it back.
          You just need to pay carriage both ways. I'm guessing you want 19mmط x 6mm key ?

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


          • #6
            Originally posted by John Stevenson View Post
            If you want to post it to me, NG10 area, I'll bore it and keyway it and send it back.
            You just need to pay carriage both ways. I'm guessing you want 19mmط x 6mm key ?
            Now were else would you find such a nice bunch of guys

            please visit my webpage:


            • #7
              A pulley problem may arise again. You need to have a backup. I would machine a temporary replacement in the mill, a non stepped pulley, with a set screw, even just a piece of ally rod with a hole in it, which would get the lathe going so you can do a proper job on the stepped pulley.

              Then I'd make a mandrel, bore out the stepped pulley, mount it on the mandrel and skim it to run perfectly true.

              You're investing in a new motor - do it the honour of a good pulley.


              • #8
                Hi folks,

                Thanks for all the replies so far.

                John - Thanks for the kind offer of help here, I've sent you a PM.

                Now that I think about it making a slotting tool for the mill is probably quite easy. Only trouble is there is no way to lock the spindle on my mill... I will have to devise something.

                Will let you know what the outcome is once I've solved it!



                • #9
                  Read through this thread. A few posts down a fella tells how to make a simple broach. I've done this and it works quite well.


                  Good luck


                  • #10
                    Hi folks,

                    Just a quick update - I put the pulley on the mill tonight, checked the mill was in tram (was 0.02mm out over about 200mm), spent about 45 minutes trying to dial the part in though. Turns out that tightening the Z axis on my mill throws everything off about a mill, and much the same for the other two! Made it a bit of a nightmare as I was having to tighten things incrementally and each one threw another one off a bit.

                    Anyway, got it indicated in eventually and set about it with a 12mm x 150mm indexable boring bar from the lathe.

                    Managed, through fluke, to get it absolutely bang on. Came out about 24.03mm. Squeezes onto the motor with a few taps of a hammer, and is a bugger to get back off.

                    Next step is making and using a keyway cutting/slotting tool for the mill, which will also require a spindle lock of some kind. Will keep you posted.

                    Cheers for the advice.


                    • #11
                      Good on yer Rich.

                      11 out of 10 for doing it.
                      -1 for the posed photo

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


                      • #12
                        Hi folks,

                        As promised here's an update on this.

                        I tried to make a shank to insert into a collet which took a small HSS tool held with a grub screw, but I cocked it up in various ways.

                        After that I thought about keeping things simple so I ordered a 100mm x 8mm HSS blank and an 8mm square R8 collet, and set about the pulley as shown.

                        It worked perfectly, I got good results taking about 0.05mm - 0.07mm per pass and the resulting keyway was very neat and straight.

                        I put the pulley on the motor and to my dismay noticed a bit more runout than I thought I had. I think the key may be pushing the pulley out a bit. But I ran the motor on the bench at 3000rpm and it wasn't vibrating a tad, so I can live with the runout.

                        Thanks for the advice.


                        • #13
                          Did you make the keyway deeper than the key, as keys are supposed to be in tight contact only to the sides of the keyway?

                          Otherwise congrats, nice looking job and great to see that folks still take the effort of making their own tools even for those oneoffs
                          Amount of experience is in direct proportion to the value of broken equipment.


                          • #14
                            Well done. Just shows what a fellow can do when he has time and has a brain he can put in gear. Congrats, Peter
                            The difficult done right away. the impossible takes a little time.