Here's a closer view of the gear set. Each of these gears will mesh with every other gear in the set, which will allow me to assemble trains with a wide range of ratios, both reduction and increase. That's important, because it's the rotation ratio between the spindle and the lead screw, along with the pitch of the lead screw itself, that determines what thread pitches can be cut on the lathe. I plan to make interchangeable English and metric lead screws, rather than mess with trying to come up with gear ratios that will let me cut metric threads on an English lead screw, or vice versa -- but the English screw will come first, both because I grew up with English units and because it's easier to get the lead screw in English threads.
The lower image is a direct scan of the gears on a flatbed scanner -- it's been reduced to 50%, effectively 150 dpi. If you look closely at the larger two gears, you can see some lettering molded into the web; "CANADA" presumably indicating the country of manufacture, and "T50" or "T40" which is the number of teeth on the gears. The five gears, in fact, are 50, 40, 30, 20, and 10 teeth, all in compatible 24DP tooth. The gears have a bore of 1/8", but it looks as if I can ream them out to 1/4" without difficulty. There might be some interesting times trying to key the gears, however, as there isn't much meat in the center hub, and I might find I have to leave them at 1/8" bore in order for the keyways in the gears to hold.
The combination of the small center bore and the plastic construction might, in fact, mean these gears won't stand up for use as change and tumbler gears; 1/8" diameter bobbins seem awfully light, given that I can bend 1/8" music wire or drill rod with my fingers (given a length of a few inches to work on). I plan to perform some tests on these gears before I build the complete banjo setup and install them on the lathe; I'd rather find out they're too weak before spending a lot of time mounting them than when I'm trying to make my first screw cuts or power feeds. At a minimum I'm looking at broaching a keyway in each gear, keying a 1/8" or slightly larger bobbin shaft for up to six wheels in the change train, and installing the T20 gear on the spindle and a T20 and T10 for the tumbler gears along with a T10 or T20 idler gear. That's a lot of work -- which would have to be redone practically from ground up if I strip the gears on the first cut, or find them deforming after a few cuts. With the four sets of these gears only $12 plus shipping, I don't regret getting them -- but four sets doesn't leave many spares on the T20 size, which I'll need several of. I might well wind up buying new gears
The first set of parts
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