First of all, you have to be cutting
VERY small threads to be worrying about single digit microns. As in,
way below the application range of laydown threading inserts.
For a typical thread that you'd cut with even a size 11 laydown insert, if you hold crest diameter close to mean sizes, then even a 10µm variation in insert geometry is unlikely to push you out of tolerance on PD.
I have never done any kind of statistical study to determine the limits of variation, beyond stating that, in my experience, cutting the crest diameter to mean with a Sandvik insert has always produced an in-tolerance pitch diameter, and I have done this a
lot, for
years.
Marcus, yes I do have the confidence to measure a thread this way in the machine, and prove it offline. I don't tend to do that as a matter of course, but I have done it plenty of times and, touch wood, it has not bit me yet. Like I said before, it
has bit me when using other manufacturers inserts, and consequently I will only ever use non-Sandvik threading inserts if I absolutely have no other option (which is almost never).
For actual pitch diameter gauging, balls on the inside, wires on the outside, although I much prefer
Marlco parallels over wires as they are less susceptible to false measurements due to form error, and the PD can be read directly without calculation. Gagemaker rolls are a functional equivalent for internal threads, although you need deep pockets for those, and they are not explicitly measuring pitch diameter.
In many cases measuring the internal PD can be done with a single ball and a micrometer from a known concentric OD. Where that is not possible, three balls, a small grooved parallel from my gauge block accessory kit, and a stick mic. I have only ever once cut a very small internal thread where I absolutely had to measure the PD directly in order to satisfy the customer, and I used about a dozen balls, some grease to hold in them in the thread, and gauge pins. That was a huge pain in the arse, impossible to do in machine so it took two or three attempts. Thankfully most customers are satisfied with go/nogo gauges.