Thread inspection using thread rolls make a lot of sense when you have a better understanding of how it all works.
Both the PG-6000 and RG-7000 gages use 2 thread rolls that are precision ground to seat into the thread form and contact the thread at the pitch line.The gages are preset and zeroed to a calculated dimension for the particular thread being inspected. The indicator on the gage reveals how the thread deviates from nominal size. This inspection is also known as a functional size inspection. The thread rolls see variations in form, lead, taper, and height all at the same time just as a ring or plug gage does.
This is just another NOK (Nugget of Knowledge) from Jim Douglas.