Somethings been bugging me. If you're turning threads on the lathe, you set the compound for an angle, and use that to feed. When I first started doing it, I saw and read that the proper angle was 30 degrees. Which makes perfect sense to me. Half of the 60 degree thread angle. But since then, I've also read that the correct angle to set the compound to is 29 degrees. I've even read 29 and a half degrees. So what gives? Why the variation. And how much difference can one degree or one half degree make? That's not very much of an angle change on the compound. And, as for 29.5 degrees, the marking on my compound at thicker than a half degree. So I can aim for 29.5 but I'm just as likely to be set to 29.8 or 29.2. So is there one correct answer to this, or is it a personal preference thing? Does the 1.5 degree variation make much difference?
I was taught to use 29 1/2 degrees when I went to the Navy's machinist school. However, I've always used 30 degrees since then.
the 29.5 or 29 or whatever is basically saying as close to 30 without being thirty....it does not determine the thread profile and is only done so that the cutting tool cuts on half of the profile (reducing cutting force)
the last cut is done with a .001 in feed on the crossfeed so the final thread form will be exactly that of the thread tool (and its alignment)
Just aim for 291/2 deg infeed. The 1/2 deg is for cleaning up one side of the thread by just scraping along and not making a chip. Your thread will be 60 deg because your tool bit is 60 deg. . Note: 29 1/2 deg from the straight in 0 DEG infeed. Some lathes are marked 90 deg. off.
Your going to get a lot of opinions on this subject. I prefer using 30 deg because the thread is a 30 deg per side thread. The idea of the 29 or 29,5 deg angle is to chase the back side of the cut. The truth is it will still chase the back side of the cut at 29 or 30 deg but just not as much at 30 deg.
The thing you don't want to do is go over 30 deg so anything between 29 and 30 is just fine.
Do it the way it suits you best.
It's only ink and paper
Since the cutter is what creates the actual angle and a half-degree won't mess up your trig, the under-30-degree compound setting is simply to insure that the back flank of the thread is smooth. The slight under-angle makes it so you slightly shave the back face of the thread but the bulk of cutting pressure pushes back on the front face. Imagine if you set it to 31° rather than 29° - the profile of the back face of the thread would be stair-stepped instead of smooth, thus the 29° setting. It's just a little safety margin.
The last cross-slide infeed of .001" is commonly done I guess because I hear it all the time, but it's an error to do that. This would introduce a neutral cutting pressure into your method and increase the chance for a less accurate thread right at the end after you've been doing so well and possibly muck things up when you were almost done.
Truth is that the shallow cut is almost a spring pass and it shouldn't hurt much which is why folks get by with it. You can do whatever you want, but technically it's not proper practice.
What Tyrone says about the 29 or 29.5 cut is what I have always read, but I suspect the real reason is just a bit deeper.
Consider your comments about the marks on your compound being more than 1/2 degree wide and your doubts about being able to set it that accurately. I suspect that this is true for most lathes to one degree or another. There is no Vernier scale on the compound and I would bet that +/- 1/2 degree is the best any lathe manufacturer would be likely to claim if they were to actually write a spec on it. Oh, and then there is the tolerance on the angle of the lathe bit used to cut it. Just how sure are you that it is exactly 60 degrees? Perhaps it is 59 or 61. How many shops have the ability to accurately measure an angle with such short sides?
So, with at least a half degree of doubt in the actual angle the compound is set to, if you set it to the 30 degree mark, you are just as likely to be at 30.5 as you are to be at 30 or 29.5. If you cut a 60 degree thread at 29.5, theoretically one flank of thread will show a set of steps from each successive pass and this is not desirable. So, by setting the compound to a slightly smaller angle, you are much more confident that both flanks of the thread you are cutting are smooth and not stepped.
I don't think it really makes any difference weather you are cutting at exactly the angle of the flank or at a slightly smaller angle which will shave a bit more off on each pass. But if you are at a slightly greater angle, you will leave a set of steps on one flank. So it is better to err in the direction of a smaller angle instead of a greater one.
Make it fit.
My lathe scale like many is "inverted". I have to set mine at 60 degrees on the scale.
Given it's inverted, if was to bother with the "1/2 degree", do I set it for 60.5 or 59.5
I just set mine to 60 and forget it. Works great with the laydown inserts.
Here is a site that explains it all with pictures and words.
Anywhere between 29 and 30 deg will produce good threads. I used a precision angle finder to make a witness mark at 29.5 as the quadrant on the lathe is not accurate.