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Adapting a 4 jaw chuck for SB9C

bking

Plastic
Joined
Jun 21, 2019
Mostly by chance I acquired a 1938 SB 9C (manual apron) and after doing the ILION renovation and repainting, I've been slowly adding more tooling for it.

I've just bought a 4 jaw chuck and adapter plate from Little Machine Shop. The adapter plate needs to be machined to fit the chuck.

So my first two questions are:

1) The boss on the adapter plate needs to be 0.0001" - 0.0008" smaller than the chuck. Since I don't have a 3-4" micrometer, is it even remotely possible do this with a digital caliper? My inexpensive digital caliper goes to the nearest .0005".

The machinists I knew in my small town all up and moved so I don't have anyone at the moment to borrow from and I doubt I'll ever have another use for a 3-4" micrometer.

2) The chuck adapter is already threaded but the threads look kinda ugly. I don't have any experience machining cast iron, so maybe this is normal? see photo

My background: back in the early '80s I worked as a machinist/technician building cyclotrons and experimental apparatus for the physicists. Oddly enough, they didn't tend to use cast iron for cyclotrons.
 

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I don't know why the chuck adapter needs to be .0001"-.0008" smaller than the chuck counterbore. I doubt it's possible to do and it's not practical.

Every chuck adapter I have ever fitted I have skimmed the face to clean up the axial runout and then turn the OD to .025" smaller than the ID of the chuck counterbore. Then oversize the bolt holes. Then chuck a known round piece of stock in the chuck then indicate and bump it to center. For a 4 jaw I would indicate the OD of the chuck body. Remember it's a 4 jaw you will have to indicate the part.
 
Yeah and really crappy locking threads ! The back plates I've got from Shars-CME etc were always clean and sharp on the threads. Personally I would send it back.
 
If the threads on this, and also the register diameter, fit you machine, it will work fine.

The boss on the chuck can be 0.001 or 0.002 inch smaller than the chuck backside bore. As mentioned just be
sure to take a facing cut once the backplate is installed. Also as mentioned, if the chuck is threaded make the
holes in the backplate generous clearance.

After taking the facing cut and turning the OD to fit things, the backplate will lock onto the spindle pretty well. You won't
have an easy time unthreading it. C-clamp a piece of flat stock to it, so you have a bit of a handle to work with.
 








 
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