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Work holding in a rotary table.

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  • Work holding in a rotary table.

    Spent a couple days thinking about a project I could use my new rotary table with and came up with a flywheel to use with an old crank I had. Tried to fix an old leaf blower last summer but the cylinder was scored. I can't stand throwing anything out (gotta be a sickness) so in a box it sat. Has a beautiful little crankshaft in it with ball and needle bearings. Figured I could hack away most of the case and use it for a model engine. Found a set of plans for a Factory engine by Elmer but am scaling up and want to change a few things.

    I have finished the lathe work on the flywheel but need to mount it on the rotary table so I can cut the spokes out. Problem is the table is 6" and the flywheel is 5-3/8", any ideas? I can't cut a morse taper yet, my attachment needs some work.

    Also want to use a Walschaert style valve remote from the cylinder instead of the slide type. Does anyone have a design for the valve they could share or where I could find it.

    Cheers, Bob

  • #2
    Have had this problem myself before, " getting a quart out of a pint pot", where the rotary table is concerned, frequently it is not big enough, What i do, is get a piece of steel plate, greater in dia, than the rotary table, set it up on the drilling machine, & drill a hole in the centre, to suit a centring plug on the table centre hole, drill 4 holes to suit alan screws to hold table to tee slot nuts, next drill holes near periphery of plate to accommodate clamping bolts & clamps, used to anchor component


    • #3
      Discs/plates on a rotab

      There are several viable options.

      I had that same problem with my 6" rotary table (rotab). I also have an 8" rotab as well but for smaller jobs it is a PITA to lift up and down from the mill table so I save it for when needed.

      For my 6" rotab, I did 2 things:
      I bought a 5" 3-jaw chuck with front-mounting which has three 3/8mm (5/16") hex socket screws which are recessed into the face of the chuck and then pass through it to 3 "T" nuts in my 6" rotab table. The ("inside") jaws of the chuck extend to about 5" (130mm) or so.
      (second from top in following link).

      Soft jaws can be fitted to extend the opening range if required.

      This chuck will bolt directly onto the rotab table.

      I was given an old 190mm (7 1/2") face-plate that was screwed to fit a lathe spindle.

      I made an adaptor for it as a permanent fitting. It will bolt directly via three 8mm (5/16") bolts to my lathe flange, rotab or the mill table.

      There are any amount of variations of these principles.


      • #4
        If your lathe has a compound, you can cut a morse taper..... I can and I don't own a taper attachment.

        Also, you can bolt a larger plate to a R/T, and then clamp your work to that...... centering your plate via a spigot plugged into your morse taper center hole (I assume that is why you mentioned the morse taper)........

        The chuck idea is good too, as it allows transferring between machines and keeping centers.

        Keep eye on ball.
        Hashim Khan


        • #5
          I made an 8.5" diameter faceplate out of 1" thick aluminum for my 6" rotab. Instead of making slots in it, I drilled and tapped a bunch of holes for 3/8" strap-clamps. The reason for aluminum (besides being in the cut-off bin at the metal supplier) was that it can be sacraficial. If I need to mill or drill completely thru something, I can. Once the faceplate gets too messy, I just toss it in the lathe and face it off.

          I use 4 socket-head cap-screws and t-nuts to attach it to the rotab. I also have an MT#2 pin to align the plate with the rotab incase I need to mount it accurately.


          • #6
            Plate to table

            I'd suggest forgetting about the morse taper pin and spigot to locate the plate on the rotab table.

            There is actually quite an allowable tolerance on concentricity between the table bearings centre and the morse taper. So you have an unknown error to begin with.

            There has to be some - even if quite small - clearance between the spigot and the plate. This can be additive or subtractive with regard to the morse taper.

            The best, cheapest (zero),most accurate fastest method to align a hole or circle/cylinder (any size) with the rotab table bearing centre without any spigot or morse taper use is to locate the plate roughly centred and just "nip" it with the socket (or any other suitable) screws to the rotab table. Put a dial indicator on the bore/hole/cylinder to be aligned to the table centre, disengage the worm from the wheel and rotate the table by hand. Tap the plate to align it by getting the total indicated run-out (TIR) as close as you need to zero, tighten the securing bolts/screws and re-check the TIR.

            If/when the TIR is OK - no further effort is required as the hole/cylinder and the table axis are aligned. Re-engage the worm and wheel and you are set to go.

            Note that TIR is actually twice/double the actual eccentricity (out of alignment) so it can be very accurate.

            I can align my face-plate, 3-jaw chucks and my ER-32 collet adaptor (which also fits my lathe spindle flange) in a matter of minutes just with a dial indicator and common fastening bolts. "T"-nuts etc.

            A big advantage too of not (having to) use a location pin is that I have full use of the distance from the face of the plate/chuck etc. front face to face of the mill table (ie right through the rotab) for holding longer stock/jobs which would only other-wise be limited or obstructed by the spigot in the rotab table.

            Give it a go - I think you might be pleasantly surprised.

            This "tap-true? method is the alternate to but identical in principle to the much more expensive "set-true" devices on lathes etc.


            • #7
              Thanks folks, great ideas. Collected this piece of scrap cast a while back and will build an adapter plate as suggested with several tapped holes that will fit my studs, but. Have never turned anything this large, actually the flywheel above is the biggest so far.

              Need some advice for the smallest dimension DIA. A in the drawing below could be while still maintaining an accurate face cut. The overall on this rough piece is 11.25" right now. Their is no hole through at this point, just a shallow blind taper.

              This is the only way I can think of to mount it in my 3 jaw chuck. Not sure what the finished diameter will be yet but thinking as large as possible while still clearing the index plates on the R/T. Not quite sure on the finished thickness at this point as well.

              Cheers, Bob


              • #8
                I think I'd make it 3/4" and turn with caution.


                • #9
                  You need a faceplate! Most have slots cut in them and you could just pre-drill the holes smaller than final diameter that you would use anyway to bolt the big plate to the RT. Tap those holes and then just run bolts through the lathe faceplate slots into the holes and face away.....You have a completely clear tool path all the way across the front of the work that way.

                  If you don't have a faceplate, put that high on your priority list. You can often find them used -- a quick search on ebay turned up quite a few.

                  Paul Carpenter
                  Mapleton, IL


                  • #10
                    old brake discs make good disposable faceplates
                    Just got my head together
                    now my body's falling apart


                    • #11
                      Originally posted by Swarf&Sparks
                      old brake discs make good disposable faceplates
                      I thought so, too. Unfortunately, the 4 discs I had from replacing warped rotors on my Mercury Mistake were too big for my little SB9. Sorry, guys - I should have asked here if anyone wanted them.

                      The curse of having precise measuring tools is being able to actually see how imperfect everything is.


                      • #12
                        Originally posted by oldtiffie
                        I'd suggest forgetting about the morse taper pin and spigot to locate the plate on the rotab table.
                        There is actually quite an allowable tolerance on concentricity between the table bearings centre and the morse taper. So you have an unknown error to begin with.
                        There has to be some - even if quite small - clearance between the spigot and the plate. This can be additive or subtractive with regard to the morse taper.
                        Of course, the cheaper the table, the worse the alinement........

                        A good table should be as accurate as a lathe, a bad one might wobble a lot.

                        If you have to have half-tenth accuracy, a spigot etc isn't your best approach.

                        But for regular "toss it on there and go" work, it is fine. Probably far more accurate than a 3jaw chuck.... and, you CAN always check it.............Then you have YOUR word for it, and won't depend on mine, or Tiffie's.

                        Keep eye on ball.
                        Hashim Khan


                        • #13
                          You dont need any tooling.
                          If you are going to be cutting out a portion of the flywheel as I understand it,
                          Layout some of the wheel by hand and then drill 2 holes at 180 degree that fall into the "waste" area.
                          Now clamp down to the T nuts, and mill the other voids( say 4 of a 6 spoke pattern). Then use the open areas to refasten the flywheel so you can remove the first set of clamping screws.
                          If they are in the way, you may have to re-indicate when remounting.
                          To keep position (and use the original rotab degree marks) BEFORE removing the first screws, clamp a machinist clamp on the rim up against a rigid stop, like one of the rotab keyways. Then when you remount, the orientation will be repeated exactly if you use the same keyway.I mark with magic markers.