Threading Question

Lenard · Nov 29, 2019

I have been thinking about an answer to this question on threading.
On a external threading, you set up as 29.5 degrees for this type of thread.
My question is "do you set up this degree for a internal threading or leave at zero degree? (straight in/out)
This is keeping me up at night. Have never read anything about the setup.
Thanks for any information on this question in advance.
Lenard

awander · Nov 29, 2019

Just like for external threading, you can either feed in along the flank of the thread(29.5 degrees or so for a UN or Metric thread), or you can feed straight in.

Internal thread is no different. Just be aware that the compound will need to be set differently(opposite) than for an external thread, assuming you are threading on the inside front wall.

Lenard · Nov 29, 2019

Thank you for this answer. I thought that I was thinking right about the straight in on the internal threads. You know how something pops into your mind and just can't get it out. I have been doing it right, which is good. I do understand about the compound, also. Thanks again.

Cole2534 · Nov 29, 2019

awander said:
Just like for external threading, you can either feed in along the flank of the thread(29.5 degrees or so for a UN or Metric thread), or you can feed straight in.

Internal thread is no different. Just be aware that the compound will need to be set differently(opposite) than for an external thread, assuming you are threading on the inside front wall.

I don't see why youd move the compound, I don't think the tool cares which flank it advances toward? Obviously this only holds true for a symmetric profile, buttress would probably care.

FWIW- I'm a proponent of feeding straight in and leaving the compound at 45 for easy chamfers.

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eKretz · Nov 29, 2019

If a lathe has a very light or free moving carriage it can be advantageous to feed along the flank at 29.5° when threading relatively coarse threads, especially if the half nut is a bit worn. When the thread starts the tool is loaded on one side against any slop/backlash in the system that way. Helps prevent "drunken" thread where the lead gets hopped around a bit at the start of the thread then settles in correctly after a thread or two. This isn't usually a problem on fine threads or lathes with a heavy carriage.

Gordon Heaton · Nov 29, 2019

Cole2534 said:
I don't see why youd move the compound . . .

I use the exact same compound angle for both (using a wedge-type QCTP) and will often run male and female threads in one setup. I use a start position at least one leadscrew thread ahead of the work and have never had 'drunken thread' syndrome either, although that could happen if threading from an inside shoulder outbound.

rons · Nov 29, 2019

Lenard said:
On a external threading, you set up as 29.5 degrees for this type of thread.
My question is "do you set up this degree for a internal threading or leave at zero degree? (straight in/out)
This is keeping me up at night. Have never read anything about the setup.

I have a couple popular books in two volumes. One book says to do exactly as your set up for 29.5 degrees.

I had known a German machinist that had been in the business since WW II doing work for the army in machine shops as a boy and then did clock making after the war.
His way is to go straight in for cutting threads.

I have tried both and they both work for partial profile tool bits. I invest in partial profile bits and make whatever dimensions I want for the thread.
Another choice is full profile. I think it makes sense to go with full profile if you are doing the same thread over and over again.

Two books, volumes 1 & 2. Volume 1 has a threading section that uses my machine as an example with pictures.

Machine Shop Practice, Vol. 1: Karl Moltrecht: 9780831111267: Amazon.com: Books

eKretz · Nov 29, 2019

I've always done straight in unless there was a problem. That scenario is pretty rare.

hawkfan9 · Nov 29, 2019

Tool pressure vs. lathe hp. On a light duty lathe, cutting on once side of the tool is easier, especially on coarser threads. On a heavy duty lathe, like my Monarch Series 61, it doesn't matter worth a can of beans. It mattered a lot on my old 1 hp Logan. Either way works just fine, as the tooth geometry is the same either way. If you have a light duty lathe, and don't mind taking 40 passes on a coarse thread, then you can go straight in if you want.

hanermo · Nov 29, 2019

On a thread in "Practical Machinist -general", non-cnc, lathes will have limited speed, for good sfpm in steel.

So steel parts smaller than about 3/4"-1/2" D tend to have threads that are "torn".
When using the direct-approach system, via x axis, and 0 degrees approach angle.

Even heavy industrial turning centers will have poor looking threads, when using the 0 degree method, mostly.

The threads may have the right od, id, and pd, mostly, but the surface finish is about 4-6/10, aka crap.
As per a Haas STY30 demo, I dealt with (excellent results overall btw).

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