Confused about cutting threads on a lathe

Bill Gruby said:
To be quite honest on this I think the 29.5 degree thing is quite over rated.

"Billy G"
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Agreed, I use the method mentioned above because I'm lazy and won't write down the last depth setting on the CS, answer the phone say, return to the machine and I can't remember the last pass, was it .06, .08 or .100?

The last manual threading job I did was 30 parts, 2 1/2"-4 thread X 7" long in 304 SS, the DD of the sharp V is .379", it took 25 passes on each part. 750 passes over all, it took nearly a week, also the total length of the part is 140" so a fair amount of time was spent loading and unloading the machine (-:
 
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I set it at 29.5 there abouts cause I was taught to. But I have watched so many video's and read so many threads that I dont think it matters. I understand the theory, but as Bill said, your taking so little that I dont think it makes a matter.

I will continue putting it at 29.5 cause it's how I was taught and it works. It aint broke so I aint fixing it
 
I've done it both ways and find that 29.5 gives me a slightly better finish, at least on larger threads. Perhaps it depends on how rigid your machine is.
 
I've found that cutting threads using the compound at 29 deg gives a slightly better finish than plunging straight in. My compound, however, provides the Z axis for my milling attachment so I do nearly all of my threading by plunging in with my cross slide. Both ways work.
 
I have plunged straight in using the cross slide only for about 40 years now and don’t regret it. Off the top of my head, it has one big advantage for me. The point of lead does not change/move. I can set-up my thread tool setting its distance to say a shoulder and that distance will not change. It would if I did the ~29.5° compound thing. Also, I can do a very sexy full radius stop at the end of a thread because I know the “V” will remain on center throughout its cutting. As far as finish, both ways should be fine. If you are having problems, try a different tool bit, different cutting oil, different depth of cuts, finer cuts at the end, or something?…Good Luck.
 
I've cut at both 90 and 29.5 on the CS with similar results, although I find the tool is less likely to chatter at 29.5. I like using the CS to feed the depth of thread as I use a stop on the cross feed to return to 0 every pass. At 29.5 the depth of thread is CS dial reading x 0.87. At 90 degrees it would be CS dial reading x 1.0.
You could use any angle you like but I know the COS of 90 and 30 pretty well by now. :)

I highly recommend using the stop on the CF. One less thing to worry about.
 
I do it like Bill. I just leave the tool post wherever it was to begin with and go from there. I usually take .002 deep cuts the first few passes, then go to final thickness from there. I just have to make sure I have the right gears on my 7X and the 9X20 levers are in the right position too.
 
my apprentices were required to cut a 3 lead treading project, internal and external. very few lathes can drop in at 3 separate but equal place. 2 ok 4 ok 8 ok but 3 no. You are required to advance your tool bit one third pitch for each thread. so you need to set your compound at 90 degrees( parallel ) to ways to cut the 3 lead thread. an example of not using 29.5 degrees. I have cut threads both ways many times. different method same results. bill
 
also when you cut acme threads 29 degrees included angle you set your compound at 14.5 degrees. another example of not using 29.5 degrees bill
 
bfd said:
also when you cut acme threads 29 degrees included angle you set your compound at 14.5 degrees. another example of not using 29.5 degrees bill
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14 degrees, for the same reason you use 29.5 degrees for 60 degree threads.
 
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