Member postings for Lloyd Bowers

Lloyd,

First, to cut threads less than ¼” in diameter, it is probably easier to use a die than to cut them on your lathe, though the lathe can be a great help in ensuring a die-cut thread is true, if the stock is held in the chuck and the die in a tailstock dieholder. Annual rotation is probably in order, unless your slowest speed under power is very slow – 60 RPM or so is OK.

Looking at threads between ¼” and 1” in diameter, all Whitworth, BSF, Unified Coarse (UNC) and Unified Fine (UNC) threads will be covered if you can cater for 28, 26, 24, 22, 20, 18, 16, 14, 13, 12, 11, 10, 9 and 8 threads per inch ("TPI". Note that the first, second … seventh threads in that list have double the TPI of the eighth, ninth …. fourteenth.

There is a formula which works out the gears required to cut threads of the required TPI on a lathe with a given leadscrew TPI. I believe your leadscrew is 8 TPI. If your driver gear (the one on the spindle) is 24T, that formula can be simplified. Just multiply the TPI you want to cut by 3 to get the number of teeth needed on your driven gear, which is the one you need to put on the leadscrew.

So, looking at the second seven TPIs listed in the second paragraph above, the driven gear to go on your leadscrew is 24T for 8TPI, 27T for 9TPI, 30T for 10TPI, 33T for 11TPI, 36T for 12TPI, 39T for 13TPI and 42T for 14TPI (none of which you have; your 24T is already on your spindle). You will need to bridge the gap between the 24T on the spindle and whatever is on the leadscrew with one or more idler gears of any convenient size. As long as there are no compound gears in the train, the idlers (which will include those on a tumbler reverse, if you have one) have no effect on the overall gear ratio, which is determined only by the first and last gears in the train. A compound gear comprises two gears on the same shaft which are fastened together so they revolve as one. One of the two is driven by the gear above it in the train, and the other drives the one below it.

But a compound gear will be useful to get at the first seven TPIs in the list. If in place of the idler (or one of the idlers) you compound two gears together, one of which has twice the tooth count of the other, and use the larger one as a driven gear and the smaller as a driver. Then, you will get twice the TPI out of whatever gear is on your leadscrew. That will give you the first seven in the list.

I have assumed that the bore of the 24T on your spindle is such that you can’t change it for any of the other gears; if you can, then further, more complicated calculations might produce the right ratios from fewer gears.

As to metric threads, the only way to get these exact is to use a compounded pair and a 127T gear somewhere in the train. But given the rather odd tooth counts of your present set, you may be able to produce “near enough” metric threads for practical purposes.

Finally (at last!) : before investing in gears meant for a Boxford or South Bend, check that their bores match your spindles, or be prepared to make new spindles to fit them.

Andy