Making a threading dial

[Hopefuldave]

Mayhem Wrote:Do you just do that to get an initial mark and then plot the remaining lines from there? I noticed that at least one of the marks lines up and the half nuts won't engage.

Just to clarify, when I said the dial has no current marks, I meant to say numbers or lines. There are several centre punch marks, one of which I have marked with a dab of paint.

I'm glad I asked, as some of the stuff I have read makes it appear much more complicated.

I'm assuming your lathe is imperial not metric... Metric's a lot more fuss and needs a set of different gears for the indicator dial to do the different pitchesl... I'm also assuming your leadscrew's an even number of tpi - be very unusual if it wasn't!

1) You'll need to be certain of the leadscrew pitch, so measure over (e.g.) 8 turns with a vernier, to see how many tpi it is first.

2) Once you know how many tpi, count the teeth on the gear - the gear needs to be a whole, even number[1] of inches per turn so multiply teeth by tpi and see what you get - ideally it'll be 2" or 4"

3) If it *is* a nice number of inches in 2) above, divide the number of inches per *dial* turn by half-an-inch and divide it into that many segments (4 for 2", 8 for 4", 16 for 8" etc.) and mark the numbers at every inch -

The dial works by having the leadscrew and chuck in the same relative positions when you engage the half-nuts, this happens according to how often the thread pitch repeats - i.e. how far the carriage has to travel for the same positions to come around. For an 8 tpi leadscrew this happens with every 1/8" travel of the carriage and/or rotation of the leadscrew, so for threads divisible by 8 tpi you can engage anywhere - simples :) For divisible by 4 tpi, every 1/4" of travel or 2 rotations of the leadscrew - hence the thread dial!

I'll assume it works out at 2" per turn of the dial in 2) above, this way cutting "something and a half" tpi (obscure plumbing and gas fittings) uses the same numbered line each pass,
whole number tpi uses every "opposite" numbered line,
tpi divisible by 2 every numbered line etc,

When you get to tpi divisible by "number of inches per dial turn" you can use any mark (so for a 2" travel dial this would be 2, 4, 6, 8, 10, 12, 14, 16 tpi etc,).

If it works out at 4" per turn, the above get slightly modified (but your dial will work for something-an-a-quarter tpi!) so:
"quarter" threads - same numbered line;
"half" threads - opposite numbered
"tpi" threads - any numbered
"x2tpi" threads - any mark

I *think* I have this right, ideally I'd get Teacher to check....

Often the dial's attached to the gear's shaft with a friction fit, so for long, fine, odd-or-half-tpi threads you can rotate it to bring a number up approaching the index mark before you start threading - this can save a lot of time!
Another useful feature of the thread dial is that with the leadscrew stopped and half-nuts disengaged it approximates a travel indicator for the carriage... On mine it's 1/16" per mark, quite handy!

Dave H. (the other one)

[1] If the gear has a weird number of teeth, you may need to replace it... If you have a keyed leadscrew (for power feed) it can pretty much cut its own gear in Delrin / nylon if the edge of the keyway's sharp