# Confused about cutting threads on a lathe



## Al Slitter (Oct 10, 2014)

Greetings all;
I am hoping that someone might be able to clear up the confusion in my mind about cutting threads on a lathe ?

I had always thought that one sets the compound to 29.5 degrees when cutting threads, however I have seen a couple of videos where they have to compound set at 90 degrees to the work when cutting 40 TPI.
I had thought that by using the compound set to the 29.5 degrees allows the left hand cutting edge of the tool to lead in to the threads being cut.

When and why use the cutter set and 90 degrees to the piece being cut?


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## Ulma Doctor (Oct 10, 2014)

there is going to be arguments both ways from many people. for some is personal preference, for some is mechanical theory, for some some is real world experience
both methods will make threads.
the 29.5* will take less energy to make the thread.
 as you said the left edge is doing the cutting instead of both edges being plowed into the work when feeding with the compound.

my advise would be to try both methods on scrap in different materials and in different TPI so you can see for yourself which you prefer.
try different tooling(carbide inserts and HSS) as well to get a real feel.


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## Rbeckett (Oct 10, 2014)

One advantage of HSS is you can shape the tool with a bend in the end and grind it to 60 degrees and get up to shoulders and  do inside cuts a little deeper too.

Bob


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## Terrywerm (Oct 11, 2014)

One other point to consider is that when cutting 40 TPI, the threads are not very deep, so plunge cutting them is not too much of a problem. The compound could be set to any angle you desire because you would not need to use it.


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## Al Slitter (Oct 11, 2014)

Thank you all for your replies.
I was not aware that both types of set up will work.

Thanks again!


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## chuckorlando (Oct 11, 2014)

Yea they all seem to work and anyone who is a die hard to one concept or the other swears the others dont work well ahahaha


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## Terrywerm (Oct 11, 2014)

Like the old saying goes: There is more than one way to skin a cat!


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## NightWing (Oct 11, 2014)

Years back, my Mazak QT-8's had a threading menu that allowed zig-zag threading.  The tool cut first on the front edge and then the back edge over and over until the thread was to size.  It gave a nice clean thread.


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## Wreck™Wreck (Oct 12, 2014)

If I have many parts to do on a manual I use the compound at 30 Deg.

Zero the cross slide and the compound at the major dia.(assuming a male thread), advance the compound for the desired depth of cut for the pass. Thread.
Retract the cross slide and return to the start position, advance the CS back to zero then advance the compound for the next pass.

You will always know how deep the last pass was by looking at the compound dial. Return both to zero and begin the next part.


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## Bill Gruby (Oct 12, 2014)

To be quite honest on this I think the 29.5 degree thing is quite over rated. No matter what angle you put the compound at the tool still has to be 90 degrees  to the work. It's a matter of how hard you wish to push the tool. Remember, when you are cutting threads as we do you are only taking a few thou. at a time, not .020. I cut threads from where ever the compound is at the time. It's just the way I do it.

 "Billy G"


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## Wreck™Wreck (Oct 12, 2014)

Bill Gruby said:


> To be quite honest on this I think the 29.5 degree thing is quite over rated.
> 
> "Billy G"



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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## chuckorlando (Oct 12, 2014)

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


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## John Hasler (Oct 12, 2014)

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.


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## epanzella (Oct 15, 2014)

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.


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## chips&more (Oct 15, 2014)

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.


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## mws (Oct 15, 2014)

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.


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## Rbeckett (Oct 16, 2014)

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.


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## bfd (Sep 18, 2016)

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


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## bfd (Sep 18, 2016)

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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## John Hasler (Sep 18, 2016)

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


14 degrees, for the same reason you use 29.5 degrees for 60 degree threads.


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## Firestopper (Sep 18, 2016)

I've done it both in 29.9º and 90º with excellent results. I use a robust insert and take .005 per pass. I will add, when teaching someone for the first time, I demonstrate with the compound set to 29.5º.


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## T Bredehoft (Sep 18, 2016)

I remember (its been 16 years since I retired) that there was positive stop on the cross feed, (you screwed in a knob to set it) that would obviate advancing the crossfeed. Is this no longer available?  It was great,  you got to the end of the thread, backed out the cross feed, returned to the start, ran the cross feed in to the stop, advanced the compound .002 or whatever, and engaged the leadscrew.  Repeat until done.
Do lathes no longer have this feature?


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## epanzella (Sep 18, 2016)

I use the xslide to thread and keep track of where I am by always retracting the tool .100" which is one full turn on my handwheel and also on the DI connected to my slide. If I make a cut at .030 I retract the tool out to the next .030. For the next cut I go back in to .030 plus my next incremental DOC which would bring me to .035 for a typical .005 cut until I get close. My last cut is always right there under my nose.


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