That's an interesting thought on form taps. And this is perhaps the first time I have seen it expressed. I have not used form taps, yet.
Is this due to formed threads not having a fully formed crest? If so, what is the amount of the crest that is typically missing? I assume that the crest on a formed thread would be a radius, not a flat.
And could that be changed by using a smaller tap drill?
I may have a bit more experience with this than many - simply b/c I have set up many thread roll heads on multi-spindles. I Have rolled many Dryseal pipe (NPTF) threads back-in-the-day, and to make that 6-step profile gauge fit, it's got be be dead nuts! When dooing that, you will be looking at your form a lot with a magnifier while dialing in your timing. You can also see it when setting up your Fette head, but since timing is not an issue on a Fette, you typically are not studying your thread form much. Mostly just measuring PD. But you will see the problem easily on form tapped holes with too large of a start hole as well.
But essentially, when you are forming, you are pushing down the root, and displacing the material to fill in the form on your tap into the crests. But seldom doo you ever get to 100% on an internal thread. I used to run a 3/8-16 hole in 6061 in an oil slinger that way, and I loved the look of those threads, but that would likely not be attainable in steel, nor in coolant. (never tried, so not sure)
So, your formed thread is a line of W's.
Looking at that W, envision that the bottom of the W is the bore of your hole, and the W represents only one full thread pitch.
The outside edges of that W are where the fwd and aft thread on your tap is forming.
The V (upside down for this arguement) in the middle is the zone that is getting squeezed and pushed up into the form of the tap.
Now the W shown here is an over-dramitization as it is all that I have handy, and I say this b/c the V in this W goes all the way to the root diameter, when in real life we are starting at a little under pitch D.
But the catch is that until you get to full crests (almost never on an internal thread) you are essentially running a double thread. Picture your 3/8-16, with 5/16-16 running inside of it. If you have a large start hole, it is not uncommon for the customer to actually have troubles getting their fastener started - as it wants to try to start in that in-between thread 1/2 the time. This is the noticeable issue that folks are aware of.
But what most either don't understand, or prefer to disregard simply b/c they want to run this style tap, is that even when you have your 70% thread that so many are OK with, the 25% of the crest that has been formed up past the 45% start hole, is unsupported. Go back and look at that W aggin.
So, lets say that if you look at the cross section of your thread, and at the diameter of the root of your unformed crest*
(see daffynation below) of your W shows 1/3 formed V, then 1/3 unfinished crest, and then finishing with another 1/3 formed V. We started at 45% full thread, and it has been formed up to - what? If you are measuring your Minor D now, and you show 70%, the root of that open area between is going to be more like 50-55% prolly. So, your formed thread has a good solid 50% thread, and another 20% that has very little strength since there is not much backing it up.
So, you drop to a smaller drill size, and now you are up to 75% on your Minor D. Now the middle gap of your thread is pushing up quickly as you have more material pushing up into an ever smaller void, and maybe now your gap is at 65%?
Now you drop to another letter size and your Minor D is now up to 80-85%. Now the space that your V occupies is getting much smaller. You have much less V sticking up above your gap, and your bolt doesn't even want to think about cross threading anymore. I would say that by this point your 80-85% formed thread is likely as strong as your cut tapped 70-75% thread, b/c the gap between your threads on that cut thread is flat, and fully supported at your measured Minor D.
There will ALWAYS be a W profile until you get to 100%.
* Meaning, look at your W thread. I am talking about the depth of what is left of that upside down V in the middle of your W.
You can prolly find some pics online that would show this better than my text.
Think Snow Eh!