What's wrong with my threading technique?

[Erich]

I don't agree. Start a thread by cutting the OD to the nominal major diameter.
Now cut the thread to nominal pitch diameter. Measure it over wires and confirm the PD is correct.
Now turn the OD down some amount. Pitch diameter did not change. Remeasure over wires if you are not sure.
What did change is the width of the flats on top of the thread.
On thread with a flat on top, PD and MD are independent features. On a sharp V thread they are linked.

[754]

I forgot to mention
As far as a few thou reduction from nominal size goes...
Measure diameter of a few commercial bolts, you may be surprised..

[J Tiers]

They sort of do. Consider an extreme example. If you require the MD to be exactly 0.3114 and the minor to be exactly 0.2618 and the cutter is a perfect 60 degree edge, then what is the tolerance on the PD? How can it be anything other than 0.2843?

Now take that same example and move the MD to 0.3104. That means that you will cut the thread .001 deeper. Now measure the PD. Remember, the PD is that point where the thread and groove are of equal width. That's the definition. Is your PD still 0.2843? Nope. It's moved down by .0005

What does this tell us? If you allow a range of MD to be acceptable, you have to allow a corresponding range of PD too.

Dan

You are starting at the wrong end.... The PRIMARY PARAMETER is the pitch diameter. That PD is shared by sharp V, and UN threads. A thread of the correct PD will fit with the proper clearance for the thread class, as long as the thread form is correct within the tolerances allowed

The MD is something that FOLLOWS FROM having the correct PD and thread form.

I suppose there might be a few cases in which the MD may be a factor that is wanted within certain limits. As long as those special tighter limits are within the standard limits established for the thread size (1/4-20 or whatever) then the PD should be accepted anywhere in the tolerance for that size and fit class.

[danlb]

I don't agree. Start a thread by cutting the OD to the nominal major diameter.
Now cut the thread to nominal pitch diameter. Measure it over wires and confirm the PD is correct.
Now turn the OD down some amount. Pitch diameter did not change. Remeasure over wires if you are not sure.
What did change is the width of the flats on top of the thread.
On thread with a flat on top, PD and MD are independent features. On a sharp V thread they are linked.

You are confusing the major diameter with the outside diameter. They are often two different things.

[tom_d]

Somebody's going to call me a purist, but I'll say it anyway. Using a triangular file on the groove of the threads negates the value of using wires (or thread mic) to check the pitch diameter.
Dan

I think we're both purists. I guess I just didn't make myself clear enough earlier about de-burring the thread. If I was better at drawing I would have included an illustration.

When you de-burr the thread crests use a small three sided file held so that the length of the file is in line with the helix of the thread, and tipped so it does not contact the thread flanks. I usually make two passes, tipping the file so it favors one side of the thread then the other, with the cutting face about 15 degrees off the thread axis........

Of course the file should not run against the thread form. Holding the cutting face of the file at a slight angle puts a slight (about 15 degrees) chamfer on the crest of the thread, without touching the flank. I use the three sided file out of convenience. They tend to come in sizes that are long enough for me to grip safely, as opposed to the smaller needle files.

[J Tiers]

You are confusing the major diameter with the outside diameter. They are often two different things.

You may want to check the diagrams you yourself posted...... see what the MD is shown as.....

[Erich]

You are confusing the major diameter with the outside diameter. They are often two different things.

What is it (major diameter) when they are different?

[danlb]

The major diameter of a screw is the maximum size that the screw crest should be. The minor diameter will be 5/8 of the double thread height less than than that. The OD can be smaller than the major diameter. That does not impact the PD since the cutting tool will still be cutting the same path to the same depth.

Dan

[danlb]

You may want to check the diagrams you yourself posted...... see what the MD is shown as.....

Please, do explain.

[J Tiers]

Please, do explain.

With pleasure....

Please observe the diagram you posted in post 77.

The "major diameter" is clearly labeled as being from the TRUNCATED CREST, i.e. THE FLATTED SURFACE, and NOT from the crest of the sharp V thread.

It is the real OD, with a tolerance... the "as-made" OD, which must be within limits.

The major diameter of a screw is the maximum size that the screw crest should be. The minor diameter will be 5/8 of the double thread height less than than that. The OD can be smaller than the major diameter. That does not impact the PD since the cutting tool will still be cutting the same path to the same depth.

Dan

With the MD shown as the actual OD, then the actual OD might be OUT OF SPEC, but the actual major diameter and the actual OD of the thread are the same thing.

You seem to continue to base many things on the major diameter. This is the incorrect way to view threads.

The primary measurement is the pitch diameter.

The pitch diameter is what fits a nut or threaded hole. THAT is what sets the fit class, the slop between the nut and bolt. You need to start with the pitch diameter, and then realize that the other features then have a relation to the PD, they are dimensioned FROM the PD.

So, for a 5/16-24 thread, of class 2A

"BASIC" diameter is 0.3125"
Major diameter max 0.3114
Major diameter min 0.3042
Pitch diameter max 0.2843
pitch diameter min 0.2806
minor diameter max 0.2603

And, for a 5/16-24 thread, of class 3A

"BASIC" diameter is 0.3125"
Major diameter max 0.3125
Major diameter min 0.3053
Pitch diameter max 0.2854
pitch diameter min 0.2827
minor diameter max 0.2613

There is a range of 7 thou or so on the major diameter in class 2A, but half that for the pitch diameter.

In class 3A, the maximum major diameter is larger, and so are the others. But the pitch diameter tolerance is tighter at 0.0026, yet the MD tolerance is the same as for class 2A. That clearly shows which is the important measurement, the closer fit class has a smaller allowed range of PD, but no change to the range of MD.

a theoretical ideal threadform would follow the PD as it changed. but since the allowed variation of MD is larger than the allowed variation of PD, and has no fixed relation to it, obviously there has to be a range of MD for any PD. If they were tightly linked, the ranges would obviously be the same, they would all vary together.

That is essentially why the PD is a basic specification of the thread. The MD, etc are considerations of thread FORM, the PD is a consideration of thread FIT.

Of course thread form is important, but it comes from, is based on, the pitch diameter.

Reference for sizes:

https://www.engineersedge.com/screw_threads_chart.htm

[danlb]

OK Jerry. Since you can type so much about how the MD does not matter, here's a test. Put up or shut up?

I'll give you a the PD of a real life part as measured with the wires over thread technique. I'll also give you the pitch. According to to you, that's all you need in order to duplicate the thread and have a perfect fit of whatever class you desire.

Your challenge is to write down the steps needed to single point a male thread from start to finish. Show all work and formulas. You say over and over that that's how you single point. In this thread you even claim that you single point quite often, so it should be really easy for you.

I'm betting that you can't do it.

After you do your's I'll list the steps to duplicate that thread using just the pitch and MD.

[Your Old Dog]

Don't know your experiance level as I am new to the board myself. Keep in mind you have to back up a bit before you send the show forward into the thread to eliminate you slop or run out of the lathe.

Also, remember you back in and out of the cut with the cross slide BUT you advance the depth of cut with the compound slide.

When threading I wouldn't bother with the optimal cutting speed of the material !! Things happen way to fast for that LOL

Good luck
.

[danlb]

Just for grins I measured the PD of the 5/16-24 thread that I cut in post 102. I'd forgotten how much I don't like thread measuring wires.

I used the "PeeDee" thread measuring set that I've had for years. 24 tpi calls for a .029 wire** with a constant of .05092. I measured 0.3357 with a mic. Doing the math, that comes out to a measured pitch diameter of .28478 inches. The range for the pitch diameter on a class 3A (per J tier's post above) is max 0.2854 min 0.2827.

Nice to see that using the old fashioned technique gives me a PD that's right in the middle of the proper range. The old fashioned technique is to infeed a calculated distance from the proper starting point with a 60 degree tool.

Dan
** Like most thread wire sets, they use a limited number of thread diameters to cover all threads from very fine to very coarse. This set uses the same wire for 24 through 20 TPI. Obviously the wire will be hitting the sides of the thread at different heights for each of those. This means it does not really measure AT the pitch diameter; it measures how deep the wire fits into the groove and gives you the math to figure the PD based on the assumption that it's a clean 60 degree groove.

[txfireguy2003]

Wow, sorry guys, I didn't realize I was going to start such a debate. Clearly, I need to go back and study, as I'm just a dumb fireman trying to learn I new skill to save me some money on gunsmith labor, and maybe make a few bucks to help support my rifle addiction at some point in the distant future.

Guess I need to buy some training videos so I can learn this stuff. I've pretty much taught myself to do just about everything I ever wanted to do, but machining has so many variables and moving parts and 15 different ways to skin the same cat, I'm clearly going to need some professional education.

Sent from my SAMSUNG-SM-G891A using Tapatalk

[tom_d]

Absolutely no need to apologize for asking legitimate questions about a procedure you're not yet familiar with. The way to learn is by asking questions. Yes, there have been some spirited conversations here, but that comes with the passion that many have, and their willingness to share with you and others. So keep asking, and sharing your experiences. Participation is key to this forum.