Can you identify this spindle style please. 1930 Reed Prentice Lathe as near as I can tell. I want to put a new chuck on it. .

Its a typical sort of threaded spindle nose, lots of mfr's used that method but not a lot of standardization. Unless you find a donor backplate or get lucky you'll have to machine a backplate for the new chuck. Its not a difficult job but you need a functioning lathe to do it.

I've done it cut-and-try, which works but is a pain unscrewing the chuck-with-backplate workpiece from the spindle, flipping it around to test fit on the spindle then flipping it back to machine some more.

The 2nd time I did one I made a spindle gage, essentially as close a duplicate of the nose as you can make. Then no flipping things around to test, you use the gage to test the backplate as you machine it.

The goal is something like a snug slip fit on the smooth portion, the threaded portion doesn't need to be super close but a good fit is always nice. The radius between the spindle and register needn't be an exact duplicate but of course the register face should be machined flat.

Once the new backplate fits the spindle, then the rest of it is machined to accept the new chuck.

Unmachined backplates can be found on ebay et al- or maybe you can find one already machined but with enough material left so it can be remachined for your lathe.

Seems like an awfully short threaded section.

Catalog just says 2 3/4-4 - but you need actual dimensions of what you see there - like measurement over three "wires" of the appropriate diameter.

In the process you will need to make a tread plug gauge to those dimensions in order to check the progress of machining a new back plate. One of the great all time books on the subject is Van Keuren Precision Measuring Tools

If you have a chuck that's just worn out, chances are the back plate is fine. Granted they're not always removable but it would be worth looking at as even if you have to doctor the diameter or hole pattern to fit a new chuck, you'll have to do that anyway (and more) making a whole new plate. Once you have one reliable chuck, then making more plates is easy. Often you can buy cast blanks to bore and thread, and keep an eye on the classifieds for used ones. IMO chucks wear out eventually, but the threaded back plates SHOULD be part of that individual lathes kit for its whole life.

Looks like a square or acme thread. Even a ruler should get you close enough to identify the thread, then work from there. 2-3/4” 4tpi doesn’t seem to be a standard acme thread, so you may be looking at making a new back plate.

fciron said:

Looks like a square or acme thread. Even a ruler should get you close enough to identify the thread, then work from there. 2-3/4” 4tpi doesn’t seem to be a standard acme thread, so you may be looking at making a new back plate.

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Maybe - but I suppose it would have something to say other than 2 3/4 - 4

"Seems like an awfully short threaded section."

Hmm. My wife often says that about me....

2 3/4 4TPI acme sounds right. You will absolutely need to make a gage to represent the spindle threads.

If old backing plate is removable, that's an easy solution. If new chuck hasn't same pattern, it's a simple job to add.
Making new or re-machining an old one, you can get away with adequate threads of correct pitch, but the pilot/ register and shoulder are key to running properly. I wouldn't proceed without a sample test gauge, not fit alone, verifying instruments agree.

Use the compound at 30° (or even less, 15°) for the last passes dusting off those tenths for best possible fit. I know one instance where a bad chuck with integral back, was further disassembled,then all the chuck material milled and turned away. Inexpensive, a rather unpleasant job; but again, inexpensive.

Does look like square threads - the slam-dunk way to machine a chuck backplate for something like this is to remove the spindle from the machine and use that as a gage when cutting the backplate. This pre-supposes you have a buddy with a lathe to do the job on. Which is not a bad thing to have, overall.

jim rozen said:

Does look like square threads - the slam-dunk way to machine a chuck backplate for something like this is to remove the spindle from the machine and use that as a gage when cutting the backplate. This pre-supposes you have a buddy with a lathe to do the job on. Which is not a bad thing to have, overall.

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Nope, it's definitely Acme thread. I have the same machine and cut the threads for both the gage & backplate. I made two gages. The first was made from some mild steel mystery metal and I invented new ways to gaul steel. For the second, i bought an appropriately sized piece of 12L14 and the result was much better. I checked the pitch diameter using thread wires. I made the backing plate from dura-bar.

Best wire size for measuring 29 degree Acme 4 pitch is .12911 in above mentioned Van Keuren book above. "flush" wires (.1258") are made to stand .010" above the Acme thread OD - and I suppose this applies to both "top" and "bottom" wires

OK, acme. Tough to see from the photo angle - but the point stands: sounds like the long way around but using the spindle itself as the gage works wonders.

One trick that does work, if you have *any* well-fitting tooling for the machine's spindle, is to set up the threading job on that, get it close, then unscrew the work from the chuck and reverse it to test. When you flip it back it will be close, not perfect, but close enough that the next cut will clear up the missed areas. Repeat until perfect.