# Adapting A Back Plate



## miro (Apr 13, 2015)

I recently got a nice  4 jaw chuck for my 1929 South Bend 13 in lathe.
I've been looking for a while.

The 4 jaw chuck has a back plate but the bore and the thread are not good for the SB lathe.
So as an exercise in machine work, I though that I could machine a filler plug that threads into the old back plate.
Then tack weld the plug onto the plate.
Then bore and thread the plug so that it fits the SB lathe spindle.
I will be using the outer diameter of the back plate which is used as the registering reference for the chuck.

The old bore is big enough that there will be some material into which I can cut the new thread .
The pictures show the back plate of the 4 jaw  and a back plate of a chuck that fits the SB lathe
I think I'm OK up until the last step so here's my question:

How do I know that the new threaded bore is OK to fit the spindle ? 

I'm using the SB lathe to do the machining so I cannot take the chuck off to see if it fits properly.
I think I can allow myself some leeway, because it is a 4 jaw and I will be using a dial indicator when I mount anything in the chuck.

miro


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## Mark_f (Apr 13, 2015)

I made a plug gauge to match my spindle thread and then it is available any time I need to thread 1 1/2-8 for any tooling. I used my extra chuck to check the fit as I made it.


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## aametalmaster (Apr 13, 2015)

Either make a plug or do a reverse fit where you remove the chuck and spin it 180 degrees and see if it fits. Use wood shims under the chuck so it just goes straight on..Bob


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## mzayd3 (Apr 13, 2015)

So you will essentially be making a bushing?  Be aware that the threads just hold the chuck on.  It is the straight, unthreaded section near the spindle bearing that registers the chuck for concentricity.  Don't ask how I know!


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## kd4gij (Apr 13, 2015)

That back plate should be cast iron.


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## Thoro (Apr 14, 2015)

That is with certainty a cast iron dog drive face plate adapted into a backplate.  This is actually common.  I'd even venture to guess that youur spindle is 1 7/8" x 8 tpi?  this one looks like a 2 1/4" x8 pi backplate.  A bit of creativity may yeild great results!  Perhaps welding might not be the best.....I'm thinking loc-tite


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## miro (Apr 15, 2015)

I thought more about the tack weld and I agree that's not a good idea.
So I figured two 8-32 set screws tapped and Loctite 'd in, at the rim of the plug and the back plate should keep it secure.


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