Making a threading dial

[Mayhem]

I had originally been told that as my leadscrew is 2tpi, I can cut any threads divisible by 2 without using the threading dial, as it will always be in sync. I have cut 12tpi threads this way and it worked but recently I had some issues with 6tpi threads. To test this, I took two consecutive 'scratch' passes and the two did not line up. So I must have been lucky on the 12tpi thread!

So, I decided to use the threading dial but soon discovered a problem: With the half nuts disengaged the dial turns normally. Once the half nuts are engaged the dial stops turning. On closer inspection, the gear on the dial loses contact with the leadscrew once the half nuts engage. I'm unsure why this is????

I have been planning to make a new dial for a while but have no idea where to begin. All the information I have seen is based around 8tpi leadscrews and transposing this to a 2tpi leadscrew doesn't work. The current gear is very roughly cut from some brass plate and some teeth are wider than others. There are no numbers on the dial - only markings are a series of centre punch marks. As such, I have been using only one mark which makes the process rather time consuming.

Any suggestions on how to go about making a new dial? I'm especially interested in being able to correctly stamp numbers on the dial. Of course, I will need to figure out why the dial has stopped rotating.

[f350ca]

I'm quite certain someone or everyone will correct me if I'm wrong but pretty sure the dial should stop when you engage the half nuts. While the carriage is stopped the dial turns as the tread moves past the teeth. Once you engage the half nuts the carriage is moving at the same speed as the thread so there is no relative movement to turn the gear.

[Mayhem]

That makes sense Greg - I cannot recall having seen it stop before though. Mind you, I haven't used it very often.

Either way, I'd still like to make a new face, so I don't have to wait for the one mark to come around.

[TOM REED]

the thread dial is supposed to stop when the 1/2 nuts are engaged. You are supposed to engade the 1/2 nuts on the mark as the thread dial comes around. You should be able to use any quadrant on even # threads but I believe you will have to use the same quadrant each time on odd # threads, metric is another question especially on a change gear lathe. Your thread dial is definitely not broken. tom

[Mayhem]

Well the reason I want to modify/make a new dial is that there are no markings on the current one. Is it as simple as marking out 8 lines (every 45*) and numbering every second one or is there more to it than that?

[Tony Wells]

That's how they should be marked. You might want to engage the half-nuts to identify the correct position for the index line. Once they are engaged (spindle stopped btw), use a marker or a center punch or something to make a mark on the dial face.

[Mayhem]

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 wont 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.

[big job]

Lets see, while threading with half nut the dial stops and half nut disengauged it
revolves. If it revolves there is nothing wrong with it. This has been asked before
simply in short say if you are in a bus in front and bus travels walking speed and
you walk to the rear exactly the speed of the bus look out window and you are stopped
you have not moved at all so each other canceled movement. The bus-carriage-stops
you-leadscrew-are still walking and around she goes. best I can do on that one. My
dial I made out of an exhaust valve I marked the lines and numbers but I turned a
ring with a witness mark >then engauge 1/2 nut cut power then turn the ring to what
ever no. or line I want then try it under power suppose a big washer will do.

[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

[Mayhem]

Thanks Dave - I have some counting to do!

I wont be back in the workshop until Friday, so I'll work through this then. I'll need the time to read this again a few more times. Your assumptions are correct in that it is an imperial lathe, with an imperial leadscrew (metric wasn't invented when this old girl was made). The leadscrew is 2tpi but I will double check to make sure.