When producing internal threads, selecting the right thread mill diameter is key to operating efficiency. Greenfield along with Cutting Tools Chicago aka General Cutting Tools defines how to select the correct thread mill for your application. Thread mills are usually offered in several cutting diameters for a given threads per inch (TPI).
Generally, for coarser pitches (coarser than 14 TPI), select a cutting diameter no larger than 70% of the nominal thread size to be produced. 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 and produce more heat, resulting in excessive wear. This rubbing may also distort the thread form affecting the thread angle.
When choosing a thread mill, it may not be necessary to use the largest one that will fit the hole. The larger diameter thread mill will minimize deflection, particularly on coarse thread series, but is more prone to rubbing and chip congestion. A smaller mill will remove more cubic inches of metal than a larger one, resulting in greater efficiency and productivity. There will be more clearance for the tool and more space for coolant and chips. However, for optimum results, the tool may need to rotate much faster than the capability of the machine. Also, the thread length of the tool may be too short for the thread depth required.