Beginner full profile threading question

[SRT Mike]

I posted earlier about a part I am making - I am playing around with the thread a bit and I want to be sure I am on the right track before I screw anything up

I am putting a custom thread on a part I am making. I am using a 24 pitch full profile laydown threading insert. The OD of the part I am turning an external thread on is 1.400. I am trying to figure out what to put in my G71 command to get a correct thread (Okuma OSP5000 control).

If you look at the following pictures, you can see my uncertainty. The full profile insert turns the flattened top of the thread (I marked this as 'a') as well as cutting right down to the V (marked as 'b'). When I turn the thread, I start by moving the cutting point (point 'b') in the X dimension to the major diameter of the thread (i.e. OD of the part), and in my G71 command, I have to enter the ending diameter as well as thread height and of course a few other things like thread length. The picture of the insert is somewhat inaccurate in that the tip of the insert has a radius - it's not perfectly pointed as it looks in the picture... but if you look at the 2nd picture (which I borrowed from Wikipedia), it shows the "theoretical" point of the V thread groove (the green line) as well as the actual bottom of the thread (the yellow line).

When I setup the X offset for my threading tool, I turned a piece of stock lengthwise to a nice smooth finish, lets say 1" OD finished diameter. Then I switched to the threading tool and brought it down until the point of the threading insert just grazed the stock and I called that 1" off X-zero. So basically my X0 position for the threading tool is point 'b' in the first picture. I also could have set the offset by touching off on the 'b' part of the insert, then using the "theoretical" length of the cutting V-part to set the X offset. The way I did the offset, the actual tip of the tool is X0, even though I know the tip of the tool has a slight radius. I could have touched off point 'b' and added on the theoretical length of the tip instead - what was the right way to set the offset?

With me so far?

So I *think* what I want to do in my G71 line is set the finsih diameter to 7/8*H (i.e. the green line in the diagram). That will mean that the tip ('b') of the threading insert will come down to the green line, and because it's a full profile insert, the part of the insert marked 'a' in the first pic will cut the tops of the threads flat (the blue line in the 2nd pic).

Am I right on that?

Or do I program to the red line (minor diameter) in my code? Or do I calculate the yellow line based on tool nose radius?

In the few threads I've cut so far, I have always set the end diameter as the minor diameter (i.e. I programmed G71 to end at the red line in the 2nd pic with the 4 colored lines on it) but the way I am reading the diagram I should have been programming to the green line instead.

It hasn't been a big deal until now because I'm always cutting both sides of the thread, and I would just start where it seemed right, then adjust the threads a few thou here and there until they mated nicely. But I'm hoping to learn from you guys the correcct way so I don't have to experiment as much and for when I need to mate with an off-the-shelf part like a bolt rather than cutting my own threads on both sides.

Bear in mind this is a 24tpi external thread on a 1.4" OD part. I know the normal thread would not be 24tpi for a 1.4" OD but I am doing this thread because I do not have a lot of length to screw in and I wanted to have a tight pitch so I will get plenty of threads to hold the part in.

I also have a 48tpi insert for when I am feeling real adventurous!

Thank for any pointers!


Cliffs notes:
1) How do you set the X-zero position of a laydown threading insert? How do you measure it and set it?

2) When you program a thread, do you set the end diameter to the minor diameter in pic 2 (red line), or put the threading insert tip right to the bottom of the "V" (the green line)???