Myford 4-Jaw Backplate

[HillclimberParticipant @hillclimber]

I have this little Burnerd 4-jaw chuck warming in my AGA before finalising the register on a new backplate for my S7, and I am suddenly struck with doubt!

It had a little, unidentified backplate on it for some other lathe (last pic). The outer diameter of that backplate corresponds with the diameter of the register of the chuck. Thus it is located by the chuck register and on the machined internal face of the chuck.

I have been machining the new, larger backplate assuming that the chuck would instead be located by the register, but on the outer rim of the chuck rather than that internal, turned space. This obviously means there will be a small gap between the backplate and the internal face of the register, where the locating bolts are.

Question is, am I getting this wrong? Should it be mounted as before, and I can simply reduce the overall diameter of the backplate to achieve that? Or am I barking up the better tree with this larger backplate and using the outer edge of the chuck?

Cheers, Colin

[HillclimberParticipant @hillclimber]

[Clive FosterParticipant @clivefoster55965]

Backplate only needs to be large enough to reach the register and accommodate the fixing bolts.

The layout of a four jaw chuck lets the PCD of the fixing bolts be smaller because they can go between the jaws instead of outside the scroll. In the sizes used on "our" lathes at least the register is generally made smaller and a smaller backplate used.

Clive

[HillclimberParticipant @hillclimber]

Clive, I hope 'we' got that one right?

The alarm bell was that the bolts would not have passed through the clamping faces with my initially-considered approach. This makes more sense.

Wielded the hacksaw to reduce the cast backplate somewhat before finish machining. Caught a good fit on the register, sticking the chuck in the AGA earlier helped. Now ready for those smaller parts that warrant higher speeds. This chuck has been in a drawer the past year, and pleased to deploy it now.

Cheers, Colin

[not done it yetParticipant @notdoneityet]

Chuck should not be a shrink fit on the register – that could possibly prevent the mating surfaces clamping together properly. Also you might have trouble removing the backplate, to service the chuck.

It is generally the independent jawed chucks that /bolt from the face (space between the adjusters), but there are some scroll chucks that can be secured from the front. The latter will mate to the surface machined adjacent to the outer circumference, of course.

[HillclimberParticipant @hillclimber]

No shrink fit. When the chuck and backplate were finally assembled at common ambient temperature, they were a beautiful smooth fit.

I just wanted to ensure that the chuck was warmer than the workpiece when machining. The biggest pain with my larger 'for Myford' 4-jaw is that the fit is so tight it becomes awkward to swap it on when it emerges from the cabinet and the machine mandrel is already warm through use….

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