4 mm pitch, you want a 25-tooth to give a nice neat 100mm per turn, surely?
A 25-tooth 20DP will be a touch bigger than 20mm diameter though, closer to 35mm
Dave H. (the other one)
I confess to being somewhat confused about the whole shebang.
First let me explain that my lathe came with threading instructions that tell me to never disengage the half-nut lever during threading. They say to cut to the end of the thread, then without disengaging, back the tool out and reverse the direction of the lathe to get back to the beginning for the next cut.
Using this method I can indeed cut all the threads the lathe is capable of cutting. The problem is that it's a real pain to have to do it this way. There is no brake on the machine so it coasts a long way after being shut off. Because of that you need a lot of room between the end of the thread and when the lathe will finally come to a stop. And secondly it takes time to reverse it and wait for it to return to the beginning, especially if you are cutting a long thread.
I can't understand why they didn't include a thread dial so you can disengage the half-nut lever. That would have been such a minor option to add for something like a lathe. In any case, it is what it is. So making a thread dial sounds like a good project for this lathe.
My lathe came with gears to cut the following Metric threads:
The Roman numeral I and II at the top of the chart indicate two lead screw speeds the lathe has.
On the right is a diagram of the threading gears. Gear A is driven from the spindle, gear B is an idler, and D drives the lead screw. B is set to 120 teeth for all metric threads.
Gear D comes in sizes from 24 teeth to 60 teeth, and gear A comes in sizes from 24 to 75 teeth.
As I say, I can cut all these metric threads as long as I don't disengage the half-nut lever and just reverse the lathe.
However after some research I discovered that there are 6 Metric thread pitches that I can cut by disengaging the half-nut because in the case of these threads it doesn't matter where the half-nut is reengaged. Therefore no threading dial is required for the following threads highlighted in orange, red, and green.
This is certainly nice to know. At least I have six threads I can cut normally without the need for a threading dial.
Unfortunately I need to cut a 1.5 mm pitch thread, and clearly I'll need a threading dial for that if I want to disengage the half-nut between cuts.
My lathe will also cut the following Imperial threads:
Unfortunately I don't think I can cut any of those without a threading dial. Unless I settle for just reversing the lathe, etc. But like I say, that can be a real pain.
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As a somewhat crazy caveat:
I did watch a video where a fellow explained how to cut any thread without a thread dial and still be able to disengage the half-nut lever between cuts. It's a slightly more complicated process, but it basically requires three step.
1. Mark the lathe chuck or spindle with a scribe line and pointer.
2. Do the same for the lead screw.
3. Finally place a table stop or indicator on the ways to mark the precise starting point of the table.
Supposedly when you reach the end of the cut and disengage the half-nut you can bring the table back to the stop (or indicated zero position), and turn the lathe over until both the chuck and lead screw are lined up with their respective marks. Engage the half-nut and make the next cut.
That might sound like a lot of stuff, but it's actually not too bad and allows disengaging the half-nut rather than depending on the lathe to coast to a stop and have to run it in reverse back to the beginning.
Obviously having a threading dial would be the easiest way to go.
I think different gears may be required on the threading dial for cutting metric and imperial threads?
Currently I'd just like to cut a 1.5 mm pitch thread and be able to do it by releasing the half-but between cuts.
I've been thinking about using either a 1 mm or 2 mm pitch thread instead. The problem is that I would then need to make matching nuts as it would probably be difficult to find commercial nuts the right diameter with those thread pitches.
For example the thread that's called for is M10 X 1.5. I could make my own M10 x 1 or M10 x 2, which I could do without a thread dial, but then where would I get nuts to fit? I'd have to make those too.
