Inch and Metric Tap Drills

Inch Tap Drill Sizes. Screw sizes covered here are No. 0 to 5/8.
- Overview. Since the internal UNJ threads are not required to have a root radius, a standard UN tap with the proper class of fit may be used, which requires a larger tap drill diameter. (Reference "Standards and Dimensions for Ground Thread Taps" published by the United States Cutting Tool Institute, USCTI, year 2000).

Cut Threads. UNC,UNF Least expensive, reference Unified Inch Screw Threads - UN and UNR Thread Forms per FED-STD-H28/2 (replaces MIL-S-7742) and companion spec ASME B1.1-2003.
Minor Diameters. See the appropriate Military Specification, listed above for male class 2A and 3A minor diameter ranges or for internal threads check this combined list of UN and UNJ tap drill sizes.
Precentage of thread depth. Machinery’s Handbook 27^th Edition states that not more than 55 to 60 percent of the standard thread depth is necessary, since test have shown that anything over 60 percent does not sicnificantly increase the thread strength, however; 75 percent threads are commonly used to provide an extra measure of safety. It goes on to say if the screws enter more than 1 1/2 times the diameter, 50 to 55 percent thread is sufficient. FED-Std-H28/2 states that the maximum precentage of thread permissible is 83.3 %. From this information one might conclude that for economical tapping you could use drill sizes that will yield 55 to 75 percent of thread depth, but not more than 83 & 1/3.

The formula for calculating percentage of thread depth as defined in USCTI's ."Standards and Dimensions for Ground Thread Taps" is as follows: [percentage of full thread = no. of threads per inch x ( basic major dia of thread - drill hole size / .013)]

Rolled Threads. UNJC,UNJF Most expensive, high strength, reference Unified Inch Screw Threads - UNJ and UNJR Thread Forms per MIL-S-8879 (SUPSEDED by SAE AS8879), Fed-STD-H28/4 and companion spec ASME B1.15-1995.
Simplified rule of thumb for inch tap drill diameters. Subtract from the outside diameter of the tap an amount equal to 1 divided by the number of threads per inch and select the nearest commercial drill size (Machinery’s Handbook 17th Edition) example: #0(.060)-80=.060-1/80.
Thread Binding. Some rolled male threads are not compatible with female cut threads, for example, a UNJC male thread will bind in a UNC tapped hole.
Thread Engagement. For most conditions minimum thread engagement should be 1 1/2 x body diameter.
Thread Designation A standard method for designating inch screw threads is shown here. Reference Screw Thread Representation per ASME Y14.6-2001.

Metric Tap Drill Sizes Tap drill sizes are listed here ranging from M1,4 (.0551) to M22 (.8661).
- M Profile Screw Threads. The M profile is intended for general purpose metric screw thread applications. With the standard tolerance class 6H/6g it is to be used for metric applications where inch class 2A/2B has been used. Reference Screw-Thread Standards For Federal Services, Metric Screw-Threads per ANSI/ASME B1.13 and Screw-Thread Standards For Federal Services, Metric Screw-Threads per FED-STD-H28/21.
- MJ Profile Advantages The MJ external thread has an increased minor diameter with a controlled rounded root. This greatly reduces the concentration of stress, hence increasing the fatique life of the part. This type of thread more than doubles part fatique life and improves the static tensile strength.
- MJ Profile Screw Threads. The Mj profile is to be used for metric applications where inch class 3A/3B and UNJ threads have been used. It is recommended for screw threads requiring the most efficient use of material (maximum loads with minimum material), fatique life and stress levels commensurate with the physical size and weight of the product. Reference Screw-Thread Standards For Federal Services, Metric Screw-Threads per ANSI/ASME B1.21 and Screw-Thread Standards For Federal Services, Metric Screw-Threads per FED-STD-H28/21.
- Simplified rule of thumb for metric tap drill diameters. Subtract the pitch from the outside diameter of the tap and select the nearest commercial drill size. example: mj3x0,5=3-0,5.
- Thread Designation. A standard method for designating metric screw threads is shown here. Reference Screw Thread Representation per ASME Y14.6-2001.