Percentage of thread and tap selection
Many variables affect the performance of threads and the faucet used to generate them. A very important factor is the size of the drill used to generate the hole that will be mined. Most of the taps are only about the size of a drill bit, which will produce about 75 percent of the threads. Generally speaking, using a lower thread can significantly improve the service life of the tap tool, and we recommend using 60% to 70% of the value in most applications. The percentage of thread strength is not directly proportional to the percentage of threads. For example, 100% thread specification is 5% better than 75% thread specification, but requires three times the torque to be produced.
The metric thread of ISO is the same as the 60 degree line of the UN thread, but the diameter and pitch are not interchangeable. The gauge drill table has the same drill size problem because they use the same formula as the discarded threads. In the ISO and UN thread series, there are a series of useful drills that allow designers and mechanics to adjust the effective thread depth for long holes, various materials and thread strength requirements. It also allows the combination of inches and metric taps and drills to operate easily.