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In need of, an at least 8 " diameter 4 jaw chuck with backing plate (1 1/2 X 8 tpi)

T

Tom Fitzpatrick

Active Member
In need of, an at least 8 " diameter 4 jaw chuck with backing plate (1 1/2 X 8 tpi) for my 12" Craftsman metal turning lathe. Bought a used one from My Machine Shop, but every thread hole for each jaw has a number of broken or missing threads so the action of moving the independent jaws is jerky and stiff in some spots. My Machine Shop wants me to return it so he can scrap it, but doesn't have any others in this size (new or used). Any of you members have an idea on other places to purchase this size chuck. Wondering if it can be salvaged.

In addition to this, does anybody have a method for smoothing this action up (i.e. cleaning up/chasing the threads, some kind of lubricant, new thread inserts, etc.). Thanks so much.
 
Can you post some pics of the offending parts in the current chuck?
 
1619460651547.webp

was able to post one pic - will send another in another post. Not sure how to do multiples.
 
Jeepers, I don't see fixing that. I'd return it at their cost.
 
Just took it all apart & soaking it in solvent to wash away accumulated crud. My neighbour suggested cleaning threads really well with a battery brush. After that going to blow in clean with compressor then reassemble with a really light coating of machine oil. Learned that it was media blasted before I received it and not 100% sure what media was used. Neighbour was wondering if it was sand & might have damaged the tips of the threads. It’s such a great size (8”) I’m going to try & salvage it. Will let you know what comes of the Rx. Thanks for your initial response.
 
You could try and make yourself a thread chaser. Copy the thread that is on the pinions and make them just a bit larger on your chaser tap. Probably won‘t take much to clean up the ridges off the existing ones. This will not help you with the broken off pieces, but at least the jaws will move freely once more.
 
First step I'd use is a rotary wire brush in a die grinder or dremel tool. Lots of varsol. After a thorough wipe, you can evaluate next steps. The most likely is to chase the threads with a 60 degree extra hard die stone in about 120 or 160 grit. If you do it by hand you can feel every bit of damage, and detect any raised spots. This will also smooth over any raised spots that were caused by chips caught in the threads. If the female thread is this bad, then the male thread will probably need the same treatment.

Frankly there are 8" chucks out there. This one might not be worth the effort.
 
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