Fitting a new backplate bolted from the back

[vic neweyParticipant @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?

[Grindstone CowboyParticipant @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

[DC31kParticipant @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.

[Chris CrewParticipant @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.

[JasonBModerator @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.

[Adrian R2Participant @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.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, basically the same as jasonB has said.

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.

[vic neweyParticipant @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.

[Howard LewisParticipant @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

[Frank GorseParticipant @frankgorse]

There is a method that I saw in a ME mag years ago that needs no DRO,trig or transfer screws and is pretty much foolproof,even if the holes in the chuck are not precisely located.

First,find or make a bit of rod that is a close fit in the tapped holes. The shank of a drill will do at a pinch. Put this in the chuck of the drilling machine.

Now open the jaws of the chuck to max or remove them altogether and place the chuck face down on the drill table with one of the holes carefully located under the drill. Clamp in place without obstructing the upper surface. Now put the backplate in place and drill through in the usual way.

Repeat x2 , drilling with backplate screwed in place.

 

 

 

 

[vic neweyParticipant @vicnewey60017]

An excellent idea Frank! I have now completed my backplate using a piece of clear acetate as a template which worked o.k but wish l had known your method then

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