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Chuck for Pratt Whitney lathe

Joined
Sep 15, 2011
Location
North Dakota USA
I have a pratt whitney lathe I have restored to new but have no chucks for it. The threaded spindle measures just under 1 3/4 in diameter. Does anyone know where I can find a couple chucks for this lathe. Since is measures a 1/16 under 1 3/4 is it called a 1 3/4 spindle or some guys tell me it is a 1 7/8 spindle with that measurement. Thanks for your time!
 
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This is probably one of those cases where you have to "make yer own." This lathe is likely from the age and time where there was no established standard and each spindle was made individually and each backing plate/faceplate/chuck made to fit.

You are probably stuck with getting one of those "chinese" chucks and backing plate. Actually, these chucks are reasonably accurate and work well in a "basic" sort of way. They're at least as accurate as the chucks such a lathe might have been supplied with originally. Keep in mind that on the back of each of these chucks has a machined "step" or recess which is the "index" for the chuck: machining a spindle mounted backing plate to this diameter assures that the chuck will be tolerably concentric when you get everything mounted together. NOT to say that concentricity can't be improved, however. Many machinists are quite good at using a ball pein hammer and center punch to "finagle" a chuck/backing plate into a bit more accuracy.

In a nutshell, given an appropriate sized backing plate and a faceplate or chuck able to take it, starting with a hole drilled or bored to the approximate root diameter of the spindle thread, you somehow use the lathe to make the internal threads.

How to do this is the eternal question, especially if you don't have a faceplate or a chuck already that fits.

Lacking a proper faceplate/chuck for your lathe, you may be able to use the thru-hole (if there is one) and a piece of plate and allthread to hold the plate to the spindle by friction alone. This can then be used as a sort of ersatz faceplate to hold your backing plate for internal threading. Admittedly, this is bordering on the "ingenious" or possibly "lucky" if it works and the plate doesn't move around on the spindle and ruin your threading.

On a lathe with a 1"-8 spindle (standard UNC thread) I welded a nut onto a round plate and used that as a faceplate for a good long time. I used that home rigged affair to generate at least one backing plate for that lathe. I think I still have that "faceplate" around here somewhere. (one never throws away something that might still be useful.)

You may not want this sort of challenge. Instead it may be possible to bring a backing plate to a friendly neighborhood machinist along with the spindle out of the lathe you're trying to match. The machinist will use another lathe and chuck and cut threads "turn by turn" using the loose spindle you wish to match as a go-no go gauge. Mostly people who are into this sort of thing are sympathetic to those endeavoring to improve their own machine capability, and will do it for token or no payment.

Do this machinist a favor and bring only the spindle. If he has to horse around the entire headstock, or even the spindle with bull gear, pulleys, etc. attached, he might turn you down on "too much work" aspect. And horsing a spindle around while still attached to the accoutrements grows old and gets tiring REAL fast when you have to check fit, take a cut, check fit, take a cut - you get the idea.

Your machinist buddy may be able to use the "three wires" method to measure the thread up directly. But generally, machinists who can do this and do it reliably/consistantly, can't afford to spend the time to help out just anyone with a machining problem that walks in. Their services/talents are likely in too much demand from paying customers. Not to say that you won't find your fairy godmother in this person either.

Anyway, I hope I've given you a fair appreciation of the issues involved. It's hard to create something where nothing had existed before. You'll appreciate well the man who had to make the first lathe.

By the time you're done messing around with mounting a chuck to a lathe spindle, you'll be ready to take on surface plates (in groups of threes.) None of it is really all that hard, once you understand the principles involved.

Good luck,
Joe
 








 
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