Harold V --
I don't know if I should agree with you or argue with you.
Agree is best, and for many reasons. That's assuming that most folks don't get too caught up on the proper tool tip modifications so the threading tool complies to thread form for each pitch, which they don't. Hell, I don't do it all the time, and I know better! [img]/ubb/images/graemlins/blush.gif"%20alt="[/img]
My point is that if you cut by the formula everyone seems to be so caught up with, the pitch diameter may or may not be right. I'd suggest that anyone choosing to use the formula use it as a guide at best, never to gage thread sizes. If the minor diameter of a thread is too small, thanks to a pointed tool, the thread will still function. Whether it would pass inspection or not would be another issue. For the home shop guy, though, it's clearly not an issue, but a thread that won't fit a nut, or a mating part, certainly would be. That's the reason I suggest that threads be cut to pitch diameters, without using the major or minor diameters as reference points, and that's exactly what the formula suggests. If you use the major diameter as a starting point, and it is in error, the pitch diameter will have a corresponding error unless one compensates. That doesn't take into account the fact that the tip flat or radius is likely to not be correct. The number of mistakes I've made by using a surface as a reference point (not threading, but other operations) without further measuring tells me I'm right. The point of using the tool tip as compared to the major diameter of an external thread as the basis for cutting depth is further complicated by the angle one chooses to use on the compound. There's just too much to keep track of, only to be measuring a feature that won't tell you where the pitch diameter is anyway. Don't lose site of the fact that no go gages are truncated at the pitch line and tell no lies.
I've mentioned on more than one occasion that I cut threads for an extended period of time in a missile facility, and received my baptism by fire as a result. To insure thread form, all threading tools (this goes way back, long before inserts were available) had to be inspected by the QC department before you could make a first article, and then had to be re-inspected when they were removed from the setup for sharpening. QC inspected them with an optical comparator for angle and the appropriate flat at the point (that's all we were concerned with then, no radius) for the pitch required for the specific part being threaded. While we had both A and B gages, we often measured the threads with wires, depending on the situation at hand. The point is, if the threading tool is proper, the major diameter doesn't influence the pitch diameter, but the pitch diameter is distinctly influenced by the variations in the major diameter and/or the flat on the tool..
The bottom line on threads is that all features should be right, but when's the last time you saw anyone in the home shop worrying about the pitch diameter, the most crucial dimension of all?
Yes, John, I realize you know and understand what I'm talking about, because you said the same thing:
Your point about cutting a screwthread based on Pitch Diameter is excellent, and you are absolutely right about the effect of the toolbit-tip flat on the infeed. But Major Diameter and Pitch Diameter control is not the whole of threading; that flat (or optional radius) needs to be correctly sized and the flank angle correct in addition to the Pitch Diameter correct for the thread to be acceptable.
I maintain that a machinist or engineer who understands the fundamental geometry of screwthreads has a big advantage over the machinist or engineer whose knowledge of screwthread dimensioning is restricted to the limits and formulae in a table. If anything, the advantage of understanding fundamental screwthread geometry is greater in the home shop and in remote locations where the threading toolbit is a product of the machinist's knowledge and skill with a bench grinder.
No argument from me. Screw threads are amongst the more complex and tight tolerance of things generated in the shop, yet are treated as simple objects. The bottom line is that when machining them, measuring them by the most critical dimension, not the one easiest to measure, is the best way to go. Without checking the charts to see if I'm right, I'm pretty sure that the tolerance on pitch diameter is much more restrictive than the minor diameter, and for sure the major diameter. On the smaller threads it's not uncommon to have only a couple thou from one end to the other. That's the dimension that should be considered as critical (without disregarding thread form, for sure).
As an exercise, ask the next half-dozen machinists and mechanical engineers you encounter to describe the theoretical geometry of the Unified threadform, or if you're pinched for time, just ask them how wide/long the flat at the end of the toolbit should be theoretically.
In my 27 years of asking the latter question, I would guess that fewer than ten percent of the engineers and machinists I've asked -- all of them working in North America -- know the right answer. The correct answer is either another question: "For cutting an internal or external thread?" or is a conditional statement "For cutting an external thread the flat should be one-forth the Pitch, for an internal thread one-eighth the Pitch." (Incidentally, many current machine-shop textbooks don't answer this question correctly.)
I agree, not many know, nor understand, that of which you speak. Considering my lack of formal education, you could include me amongst them had I not had the real life experiences of which I've spoken. To make sure I know and understood proper threads when I was actively machining, I used my H28 manuals, which lack for nothing if you're willing to dig out the information.
You can thank modernization (CNC) for the decline in skills, which are no longer really needed. Why would a "machinist" need to know about thread form when all he does is apply the tool that he takes out of a package? Rarely does the modern day "machinist" have to sharpen a tool. Many home shop types, in an attempt to avoid the learning curve (sharpening proper cutting tools in general), are now running carbide, many on machines that clearly don't benefit by its use.
Incidentally (Mk II version), I don't think I've ever seen a 60-degree center gage with Double Depth of Thread tables containing the correct Double Depth values for the Unified threadform (developed in 1949 and incorporated into the US Federal and Military standards shortly thereafter); every one that I've noticed has listed values for the obsolescent-for-fifty-years U S Standard threadform.
That's an interesting observation. To be VERY frank, the only thing I've ever used the double depth for is in determining the length of the threading tool such that it will thread to a shoulder and provide a maximum amount of thread on the object. It's very useful in that regard. As a result, I don't really care how precise they are, for that wouldn't influence my thread. As I said, those of us that know and understand threading from the old school don't gage threads by the depth. We rely on gages or wires and measure the pitch line. That the thread tables, or the markings on a threading gage, are in minor error is of little consequence to me, and I assume for some others.
You're a rare cat, John. Hardly anyone I encounter discusses that which you just have.
Harold