Confused about cutting threads on a lathe

[Al Slitter]

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?

 

[Ulma Doctor]

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.

 

[Rbeckett]

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

 

[Terrywerm]

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.

 

[Al Slitter]

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

Thanks again!

 

[chuckorlando]

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

 

[Terrywerm]

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

 

[NightWing]

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.

 

[Wreck™Wreck]

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.

 

[Bill Gruby]

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"