Where on the threads would I need to measure to find the nominal size of a bolt?
If I measure the major diameter of an external thread (bolt), the size is always considerably smaller than the nominal size. I.e. if I measure an M12 bolt, it will measure around 11.8 mm or so. Similarly, if I measure a 1/2-inch bolt, it will measure around 12.5 or 12.6 mm, where 12.700 would be exactly half an inch.
The problem occurs when I want to make an external thread on the lathe. So far, I have always turned the stock to the nominal size (12 mm for an M12 thread), and then cut the treads. This consistently produces terrible results, where the nut will not fit until the thread is cut way too deep.
So, I suspected that maybe the nominal thread diameter would be referenced from the theoretical point where the flanks of the treads would meet to a sharp point. To check this, I drew a quick thread in Fusion 360 (it has built-in threads), and did some measurements there. Alas, it still does not work out. In this case, the size comes out too large, by about the same amount it was small before.