Making a threaded backplate for chuck?

[mu38&Bg#]

I've seen a few pages on making backplates. Is there anything to watch out for? It looks like the most critical part is sizing the thread correctly. This is going to mount a Sjorgen 5C chuck so I'm trying to make things true. If I could buy a blank for 1-3/4"-8 spindle threads I would just do that, but they are hard to find.

 

[Stan]

The secret to making threaded back plates is to make a replica of your spindle nose to use as a gauge. Make this piece to the highest level of accuracy that you can and over the years you will use it many times to make an adapter of some kind to fit your spindle.

You bore and thread the spindle from the back side so that you can turn the recess, face the mounting surface and single point the threads without taking it out of the chuck. If you have made your gauge carefully, you will be able to screw the new backplate onto the spindle and then fit the front of the backplate to the chuck. When you have the chuck mounted to the backplate, put adjacent punchmarks on the chuck and backplate so that it can be removed and replaced in the same position. HTH Stan

 

[joeby]

Stan has put it in a nutshell.

You need to make sure you do a good job on the counterbore on your backplate that fits over the register behind the threads on your spindle nose. The threads don't have to be all that tight because the register and front shoulder of the spindle nose locates the backplate, rather than the threads. If you keep the counterbore a nice slip fit on the register, all will be good.

Drill your chuck mounting holes before threading your new backplate on to finish the chuck side. That way you will be able to use bolts or pins in the holes and a bar to remove it if necessary.

Kevin