Threading logic check

[GlennW]

OK, now I understand a bit more what you are looking for, and why.

This is everything that you would ever need to do what you are asking.

The internal thread minor diameter is within your size range.

[heavyg603]

That is awesome thank you . What tool is that ?

[GlennW]

ME ThreadPal

[John Hasler]

heavyg603 writes:
> I follow over to the 20 tpi line and see a single depth of thread at .0379. So by using the correct compound depth I feed to .043.
> When I finish I indeed have a good thread that has a depth of thread at .0379.
> So thinking this through I now have a Minor diameter of 1.320 and a major at 1.3579 ?

You have a minor at 1.320 and a major at 1.320 + 2 * .0379 = 1.3958. But as others have said, this is the wrong approach anyway.

[dbstoo]

I've been successfully cutting threads the way Heavyg603 did for years. It is no problem to cut both Internal and external threads that way and have them fit well without resorting to making a "test male thread" to use as a go/nogo gage.

The trick is that the thread form that he's created has a flat crest and a V root. He's using the National thread (not unified) chart, so he has to refer to page 99 of his book to see if the male and female threads are the same. With Unified Threads a) the female thread has a crest that is 1/4 of the thread height wide and b) the male thread has a crest that is 1/8 height wide.

For Unified thread compatible using a sharp V tool, the thread matches this drawing. Note that the green sections are cut away with the V tool.


Dan

[Harold_V]

I've been successfully cutting threads the way Heavyg603 did for years. It is no problem to cut both Internal and external threads that way and have them fit well without resorting to making a "test male thread" to use as a go/nogo gage.

I don't have an argument with you in regards to how you create threads, but I DO have one in regards to who you're trying to satisfy, and that they "fit well". I say this because I was trained and worked in a facility where every feature of a thread was inspected (major, minor, pitch diameters as well as thread form). If you create threads that work to your satisfaction, they may, or may not, meet the stringent requirements of rigid specifications. That's the point I've tried to drive home. If you cut threads, any threads, by dial settings, alone, pretty good chance you're going to miss the pitch diameter. But then I'm assuming that that matters. Some times it doesn't, at least for the person in charge (you).

To be clear, I trust dials. I also cut to marks on dials. However, I verify the marks by constant measuring---which is the way a thread is best created, assuming features matter. To some, they don't. Only you know if they matter to you, assuming you're not doing machining for gain. If you are, and you have to satisfy QC, you may get a rude awakening.

One thing for everyone to consider. The width of the required flat demands a large portion of a tool to be eliminated in order to be of proper size. That alters the depth required for the thread to hit the required pitch diameter. If the width is wrong, yet still in tolerance, the resulting pitch diameter may not be in tolerance. That's without even considering the error in the diameter upon which the setting was determined.

You learn these things when threads you swore were perfectly good are rejected (parts scrapped) because you felt the system was adequate. That I learned the hard way.

Harold

[dbstoo]

The Unified thread is defined by the geometric relationships of it's features. Because humans are involved, there are multiple tolerance classes, and they are also defined. In a commercial endeavor you have to meet tolerances. In a home shop you usually don't. Since the OP has already violated the Unified standard by using a sharp V, then there is no need to expect a class 4 thread that will pass inspection.

The fact that the geometry of the thread form is based on an isosceles triangle makes several things true. 1) If a 60 degree sharp tool penetrates to the proper depth, it will create a viable thread form and 2) it should be well within tolerances for pitch diameter on a class 1 fit and 3) even using a sharp V tool will create a thread that is interchangeable with a UN thread as long as the differences between male and female threads are taken into account.

Lastly, using dials and geometry, you can get good threads virtually every time on the first try. Yes, you need to check the major, minor and pitch diameter to be 100% sure if you need a specific class of fit, but most uses don't.

Dan

[Harold_V]

Everything you've stated is true, of that there is no doubt, assuming one works with perfect tool geometry, but most don't. If nothing else, the flat on the tool is wrong, which leads to erroneous conclusions. It is difficult, at best, to grind a proper threading tool without the use of a cutter grinder, so while the theory holds true, in practice some folks aren't going to enjoy success. If a guy is using insert tooling, and selects the proper pitch insert, yeah, he has a fighting chance.

It is not my intention to determine what others find acceptable, but I would be remiss in my duties as a moderator if I was to allow a post to stand that creates a false illusion that would give others the idea that working to the pitch diameter isn't necessary, as it is. It is the ruling tolerance of threads, and can be easily missed if ignored (by working to other features). The width of the flat, alone, can vary enough to consume all the tolerance of the pitch diameter, and yet remain perfectly within tolerance itself. For the home shop guy who isn't concerned with such things, yeah, it doesn't matter, but a guy who's trying to satisfy stringent QC is going to get one hell of a rude wakeup call.

Plain and simple---one does not work to major and minor diameters when creating threads. Observe proper tool geometry and pitch diameter and the other features will be correct, as they have greater tolerance. The critical feature of threads is the pitch diameter.

Harold

[heavyg603]

I very much appreciate everyone's input. It's been very educational for me and discovered some new approaches in the process. For clarity, figured I'd post a pic of one of my threading tools. I do use an insert one (don't have the number off the top of my head). I've always thought it was a sharp v as I couldn't detect a flat, but like everything else, could be wrong.

[heavyg603]

I very much appreciate everyone's input. It's been very educational for me and discovered some new approaches in the process. For clarity, figured I'd post a pic of one of my threading tools. I do use an insert one (don't have the number off the top of my head). I've always thought it was a sharp v as I couldn't detect a flat, but like everything else, could be wrong.

Update, this is a SECO 16NRAG60-A1 insert, pic is crappy I know. It has a 0.08 MM or 0.003149606" radius at the nose so it isn't a complete sharp point I guess but smaller than I could see.

[dbstoo]

. For the home shop guy who isn't concerned with such things, yeah, it doesn't matter, but a guy who's trying to satisfy stringent QC is going to get one hell of a rude wakeup call.

Plain and simple---one does not work to major and minor diameters when creating threads. Observe proper tool geometry and pitch diameter and the other features will be correct, as they have greater tolerance. The critical feature of threads is the pitch diameter.

Harold

Don't mean to beat a dead horse, but given the assertion "Observe proper tool geometry and pitch diameter and the other features will be correct" , does it not follow that you will reach the calculated depth if you observe proper tool geometry and pitch diameter? And given that, a tool with the proper geometry that cuts to the proper depth would also result in "the other features will be correct"?

[John Hasler]

I find this site useful: http://www.boltscience.com/

Relevant to this thread: http://www.boltscience.com/pages/screw1.htm