South Bend 9 Skinner Chuck Fitment
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  1. #1
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    Default South Bend 9 Skinner Chuck Fitment

    Hello,

    I have a four jaw skinner chuck #4006-56 that came with the lathe I purchased. (South Bend 9a.) The chuck wasn't installed when I bought it. My issue is that the chuck won't thread onto the spindle. I've measured the threads and it appears to be 1 1/2 8 like the spindle, but for some reason will only thread on for about 3/4 of a turn. The threads are clean and clear of any chips or burs. I'm trying to figure out why it won't fit. I've removed the back plate from the chuck and have tried screwing it onto the spindle from both sides. Same issue. 3/4 turn and stops. The threads on the chuck have a somewhat flat spot on the crest. Could the chuck threads possibly be acme? Did any of these chucks come that way? I'm fairly confident they are not metric. Couldn't get my metric thread gauges to match up. I'm attaching a few pictures of the chuck. Any thoughts on what I'm dealing with here?






  2. #2
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    67 views and no comments. Interesting. No-one must know why this chuck will not fit. I can only assume it's an oddball issue. Might try and find another backplate for it. If anyone knows where I could find one, please let me know!

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    Post pictures of you spindle.

    What’s the EXACT dimensions of the back plate threads (diameter/TPI)?

    What YEAR is your 9a

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    I have three other chucks that fit fine. The spindle is 1 1/2 inch diameter and 8 tpi for sure. The chuck backplate is the same (as best I can tell.) Just the threads look like they may be acme. (Was hoping someone could confirm or deny that chucks from this era could have acme threads.) It starts fine, just binds after 3/4 turn as stated in the original post. The lathe is from the mid to late 40's as best I can tell. Don't have the serial number handy at the moment.

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    There are other manufacturers whose 1-1/2" x 8 backplates do not fit a South Bend spindle. My guess would be that your threads simply aren't deep enough in the backplate. You might want to try to mount the backplate somehow and see if you can cut the threads a little deeper. Or, if you have a 1-1/2" x 8 tap, running the tap thru would probably be simpler.

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    now that slk001 says that I remember I actually did cut a thread plug gauge for my SB16 and didn’t get the threads deep enough and sure enough this is exactly what it felt like. 3/4 turn into my backplate and dead lock up.

    I shelved that part and still haven’t tried to correct it, the anxiety got the best of me

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    Thanks. Maybe I'll try and pick up a 1 1/2-8 tap and see what I can do with it. I have a feeling the cost of the tap may be almost as much as the chuck however.

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    I have that tap. send me the back plate and I'll run the tap through.
    PM me

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    Thanks for the generous offer. I bought a used tap off ebay. Not sure how good it will be. If it's junk, I'll definitely take you up on it!

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    Years ago I bought a check and a backplate from Grizzly for my 9A, and I had the same problem. I wish I could remember the technical description for why the threads would not fit, but I am not a machinist so the technical details elude me. But it had something to do with thread tolerances or something like that. The spindle on the lathe needed either more, or less contact. I contacted Grizzly and they asked me to send it back and they would tap it for me (at THEIR expense). When I got it back, it did fit, however when registered it worked well for me.

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    F.Y.I. I have had good results with backplates purchased from The Little Machine Shop for my 9" SBL in the past. That back plate looks awful ruff on the threads. The tap should clean out any high metal. But even after that, I'm not too sure if I would use it again. Just my opinion. Ken

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    Thanks. The threads are a little pitted, but I didn't feel it was enough to prevent me from using it. The picture might make it appear worse than it is. I'm no machinist. Just a hobbiest. Guess I'll see what it looks like (and if it fits) after running a tap through it.

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    It looks like cast iron, so the threads will look crappy close up. Just run the tap thru it, then mount the BP on the lathe and take a skim cut off the mating surface (full coverage cut, but as little as possible). The chuck will then work like a charm for you.


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