Fitting a new backplate bolted from the back

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Fitting a new backplate bolted from the back

Home Forums Workshop Tools and Tooling Fitting a new backplate bolted from the back

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  • #789046
    vic newey
    Participant
      @vicnewey60017

      I’ve made a new backplate for a 5″ chuck,  the 3 fixing holes on the chuck don’t go right through so it’s bolted from the back. I have to find a way to drill the holes in the new backplate to line up with those the chuck and I wondered if there any tricks to easily do this accurately?

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      #789051
      Grindstone Cowboy
      Participant
        @grindstonecowboy

        Transfer screws are what you need, available here and probably lots of other places too.

        Rob

        Links to https://www.arceurotrade.co.uk/Catalogue/Measurement/Punches-Scribes/Transfer-Screws

        #789053
        DC31k
        Participant
          @dc31k

          In principle, all three holes in the chiuck should be tapped on the same pitch circle diameter.

          For peace of mind, just in case that is not so, mark the chuck and backplate (marker pen) when you do the transfer so at least you know it will fit in that position.

          If, after drilling, it fits in any of the three positions, then you can erase the marks.

          #789133
          Chris Crew
          Participant
            @chriscrew66644

            This is the way I do it. The chuck will be held concentric to the backplate by the register you turn on it so make this as accurate as possible. You will need to measure the diameter of the fixing screws and the diameter of the backplate you have made. This may be exactly the same as the chuck already or you may have made it slightly larger for turning off later when the chuck has been fitted so take this into account. With a caliper measure the distance from the side of the chuck to the outside of a fixing hole and add to it half the diameter of the fixing screw and then double the result. Subtract this result from the diameter of the chuck and you have the PCD of the fixing holes. You can then measure from the edge of the backplate, make a mark and from this mark scribe a circle with a sharp pointed tool the PCD of the fixing holes by pulling the lathe round by hand. You can then scribe a horizontal line through this PCD, index the lathe round one third and again one third if it is convenient on your machine. It is on a Super7 or 7R with the 60t bull wheel, or you will have to remove the backplate to a rotary table or other indexing device for marking. You have a little leeway with the size of the fixing holes when drilling because the backplate register will hold the chuck concentric to the lathe axis.

            #789144
            JasonB
            Moderator
              @jasonb

              I’d stick 2 bolts into the chuck, measure the distance across them and deduct 1 diameter. Put that number into an online PCD calculator and then drill the backplate to the PCD. That can be done in a number of ways depending what you have to hand. Clearance holes are usually quite generous so you could be 0.1mm off and it would still fit OK as the register is doing the accuracy bit.

              #789148
              Adrian R2
              Participant
                @adrianr2

                Assuming ODs are similar enough, make a paper template for the holes from the chuck as you would for a gasket, then transfer to the backplate and mark out? For belt and braces, check measurements as above.

                #789161
                Nicholas Farr
                Participant
                  @nicholasfarr14254

                  Hi, basically the same as jasonB has said.

                  Trig solution0001

                  Than after turning the register, stick a dead centre into your headstock, mark a PCD with a suitable tool in the tool-post, and set it to the radius from the dead centre. Drill the holes with a suitable clearance size.

                  Regards Nick.

                  #789207
                  vic newey
                  Participant
                    @vicnewey60017

                    Thank you all for the advice, the thread is 5/16 whitworth so not much chance of getting transfer screws but other ideas seem doable so l will see how l get on.

                    #789302
                    Howard Lewis
                    Participant
                      @howardlewis46836

                      You could always make your own transfer screws.

                      Three, or four 5/16 BSW bolts, or studs with the end turned to a 60 degree taper can act to mark the backplate.  Painting the backplate with spirit blue will make life a little easier.

                      They don’t need to be hardened. You are not likely to use them very often!

                      Howard

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