Lathe backplates

Discussion in 'Workbenches, including workshop techniques.' started by Jon Nazareth, 15 November 2019.

  1. Jon Nazareth

    Jon Nazareth Western Thunderer

    Today I bought a second hand TOS 10mm 4 jaw self centring chuck for my Myford but, it came without a backplate. Myfords don't have any blanks for about three weeks as they are waiting for a delivery. There are three holes already in the rear of the chuck but, I don't know how to transfer these to the new backplate. Any ideas, anyone? It may be easier to make new holes but I'd be interested to hear what others think. I am a little worried about having to make something that needs to be so accurate but I have no choice.

  2. phileakins

    phileakins Western Thunderer

    Turn up some short transfer punches to fit in the holes in the chuck?

    Last edited: 15 November 2019
  3. Jon Nazareth

    Jon Nazareth Western Thunderer

    Yes, I looked on Youtube and there was a chap who used some transfer punches. He used an allen key and hand ground cones on the ends on his bench grinder, it seemed to work. However, I'm not so sure that I could grind perfect cones by hand. I'll have to give it a try.

  4. neaston

    neaston Active Member

    There should be a recess in the back of the chuck. Turn the same diameter on the surface of the back plate so that it just fits the recess on the chuck. It is then a simple job to drill and tap the holes.
    It is still worth checking with a dial gauge that the check is centered properly : a little adjustment may still be required.

  5. Brian McKenzie

    Brian McKenzie Western Thunderer

    Before securing your 4-jaw SC chuck to a new backplate, I'd want to check for runout by having the chuck jaws clamp on a freshly turned spigot in the lathe - something suitably solid, at least 15mm but preferably more in diameter. In the absence of a dial-test-indicator, set up a boring tool in the lathe to use initially as a visual indicator for any runout.

    Should runout be excessive, the bore diameter could be recut, but you may wish to have someone else view this first.

    The three bolt mounting holes are likely to have been drilled using a nice round number PCD (pitch circle diameter). If this diameter can be determined, then scribing or drawing out a similar circle on the backplate, then using dividers around the circumference of this circle will position the holes with sufficient accuracy.

    Last edited: 15 November 2019
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  6. Jon Nazareth

    Jon Nazareth Western Thunderer

    Needless to say, I've been thinking about how to get this work done as best as I can. One thing that came to me was that it wouldn't matter if the clearance holes were a little oversize as the recess would be the thing holding the plate true. Am I right? The PCD is 90mm and my lathe is imperial but I'm sure that I can sort something out when the time comes.


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  7. Brian McKenzie

    Brian McKenzie Western Thunderer

    Yes, the holes can be a little larger, but better that you drill to no more than screw diameter size initially - leaving opportunity to tweak the hole positions marginally if needed.

    You could also test this out first by drilling a sheetmetal trial piece, or use a card template pressed over neat fitting pins in the screw holes.

    phileakins likes this.
  8. Phil O

    Phil O Western Thunderer

    You could use the good old fashioned way of making a gasket to make a template, get a piece of cornflake packet, other cerals are available and a small ball pien hammer and using the ball end tap around the outside diameter of the chuck to eventually cut out a circle of card and then repeat the process for each of the holes. You can now use the template too fairly accuratly mark the holes in the backing plate.
  9. Les Golledge

    Les Golledge Active Member

    If making a gasket template to locate the holes a method I used at when making gaskets may prove of use, when you have cut the first screw hole fit a screw in it, cut the second hole and fit a screw before cutting the third hole as the gasket is firmly located. Perhaps add some tape to secure the template to the chuck.
    Hope this helps.
    Best wishes,