When producing internal threads, selecting the right thread mill diameter insures it will operate efficiently.
Thread mills are usually offered in several cutting diameters for a given threads per inch. Smaller diameters are used for small thread sizes, such as 3/8-16 NC. A larger tool diameter could be used for producing a 3/4-16 NF. However, the smaller thread mill could be used to produce the larger 3/4-16 as well. Generally, for coarser pitches (coarser than 14 TPI), selecting a cutting diameter no larger than 70% of the nominal thread size to be produced is recommended. For finer pitches, the thread mill can be as large as 75% of the nominal diameter. Although the tool has radial clearances similar to end mills, if the tool diameter is too close to the thread diameter, the tool may rub, producing more heat that could result in excessive wear. This rubbing may also distort the thread form affecting the thread angle.
The question is, should the largest thread mill that will fit the hole be used? The answer is, not necessarily! For the greatest efficiency, smaller mills will remove more cubic inch of metal than a larger one, resulting in greater productivity. There will be more clearance for the tool and more space for coolant and chips. However, to optimize the tool, it will be rotating much faster, which may exceed the capability of the machine. Also, the thread length on the tool may be too short for the thread depth required. On the other hand, the larger diameter thread mill will minimize deflection, particularly on coarse thread series, but is more prone to rubbing and chip congestion.