Creating Threaded Holes
Screw Threads - To ensure that parts will fit together as designed, we often rely on pre-defined standards. In the context of American (non-metric) screw threads, the common standard is "Unified National", and comes in four flavors: Course (UNC), Fine (UNF), Extra Fine (UNEF), and Special (UNS). In general, this is redundant with the pitch (threads per inch), and could be omitted from an engineering drawing, but is often there to provide some redundancy in specifying the desired thread.
Tapping - This is the process of cutting threads on the inside of a part, and is typically done with a fluted tap. The process generally includes using a spot drill or center drill to create a divot, drilling a pilot hole (see the chart below for the proper size "tap drill"), then using the tap to produce the threads. The tables below also list the standard sizes for both close and free clearance holes.
Tap styles - There are three different kinds of hand taps: taper, plug, and bottoming. Each refers to the amount of tapered threads at the start of the tap. Taper taps have around 8-10 tapered threads, providing a relatively gradual cutting action. They are useful for hard and difficult-to-work materials. Plug taps are the most commonly used taps, having around 3-5 tapered threads. They are useful as a general purpose tap for ferrous and nonferrous materials. Bottoming taps only have 1-1.5 tapered threads and are most useful when tapping blind holes that need threads to the bottom of the hole. Note that a plug or taper tap needs to be used to cut threads first and that a bottoming tap should not be used to start a thread.
Drill and Tap Chart (Inch)
| Thread Size | Tap Drill (75% Thread) | Tap Drill (50% Thread) | Clearance (Close) | Clearance (Free) |
| 0-80 UNF | 3/64 (0.0469") | 55 (0.0520") | 52 (0.0635") | 50 (0.0700") |
| 2-56 UNC | 50 (0.0700") | 49 (0.0730") | 43 (0.0890") | 41 (0.0960") |
| 2-64 UNF | 50 (0.0700") | 48 (0.0760") | 43 (0.0890") | 41 (0.0960") |
| 4-40 UNC | 43 (0.0890") | 41 (0.0960") | 32 (0.1160") | 30 (0.1285") |
| 4-48 UNF | 42 (0.0935") | 40 (0.0980") | 32 (0.1160") | 30 (0.1285") |
| 6-32 UNC | 36 (0.1065") | 32 (0.1160") | 27 (0.1440") | 25 (0.1495") |
| 6-40 UNF | 33 (0.1130") | 31 (0.1200") | 27 (0.1440") | 25 (0.1495") |
| 8-32 UNC | 29 (0.1360") | 27 (0.1440") | 18 (0.1695") | 16 (0.1770") |
| 8-36 UNF | 29 (0.1360") | 26 (0.1470") | 18 (0.1695") | 16 (0.1770") |
| 10-24 UNC | 25 (0.1495") | 20 (0.1610") | 9 (0.1960") | 7 (0.2010") |
| 10-32 UNF | 21 (0.1590") | 18 (0.1695") | 9 (0.1960") | 7 (0.2010") |
| 12-24 UNC | 16 (0.1770") | 12 (0.1890") | 2 (0.2210") | I (0.2280") |
| 12-28 UNF | 14 (0.1820") | 10 (0.1935") | 2 (0.2210") | I (0.2280") |
| 1/4-20 UNC | 7 (0.2010") | 7/32 (0.2188") | F (0.2570") | H (0.2660") |
| 1/4-28 UNF | 3 (0.2130") | 1 (0.2280") | F (0.2570") | H (0.2660") |
| 5/16-18 UNC | F (0.2570") | J (0.2770") | P (0.3230") | Q (0.3320") |
| 5/16-24 UNF | I (0.2720") | 9/32 (0.2812") | P (0.3230") | Q (0.3320") |
| 3/8-16 UNC | 5/16 (0.3125") | Q (0.3320") | W (0.3860") | X (0.3970") |
| 3/8-24 UNF | Q (0.3320") | S (0.3480") | W (0.3860") | X (0.3970") |
| 1/2-13 UNC | 27/64 (0.4219") | 29/64 (0.4531") | 33/64 (0.5156") | 17/32 (0.5312") |
| 1/2-20 UNF | 29/64 (0.4531") | 15/32 (0.4688") | 33/64 (0.5156") | 17/32 (0.5312") |
Drill and Tap Chart (Metric)
| Thread Size | Tap Drill (75% Thread) | Tap Drill (50% Thread) | Clearance (Close) | Clearance (Free) |
| M1.6x0.35 | 1.25 (55) | 1.35 (54) | 1.70 (51) | 1.75 (50) |
| M2x0.40 | 1.60 (52) | 1.75 (50) | 2.10 (45) | 2.20 (44) |
| M2.5x0.45 | 2.05 (46) | 2.20 (44) | 2.65 (37) | 2.75 (7/64) |
| M3x0.50 | 2.50 (39) | 2.70 (36) | 3.15 (1/8) | 3.30 (30) |
| M4x0.70 | 3.30 (30) | 3.50 (28) | 4.20 (19) | 4.40 (17) |
| M5x0.80 | 4.20 (19) | 4.50 (16) | 5.25 (5) | 5.50 (7/32) |
| M6x1.00 | 5.00 (8) | 5.40 (4) | 6.30 (E) | 6.60 (G) |
| M6x0.75 | 5.25 (4) | 5.50 (7/32) | 6.30 (E) | 6.60 (G) |
| M7x1.00 | 6.00 (B) | 6.40 (E) | 7.40 (L) | 7.70 (N) |
| M7x0.75 | 6.25 (D) | 6.50 (F) | 7.40 (L) | 7.70 (N) |
| M8x1.25 | 6.80 (H) | 7.20 (J) | 8.40 (Q) | 8.80 (S) |
| M8x1.00 | 7.00 (J) | 7.40 (L) | 8.40 (Q) | 8.80 (S) |
| M10x1.50 | 8.50 (R) | 9.00 (T) | 10.50 (Z) | 11.00 (7/16) |
| M10x1.25 | 8.80 (11/32) | 9.20 (23/64) | 10.50 (Z) | 11.00 (7/16) |
| M12x1.75 | 10.30 (13/32) | 10.90 (27/64) | 12.60 (0.5156") | 13.20 (33/64) |
| M12x1.50 | 10.50 (Z) | 11.00 (7/16) | 12.60 (1/2) | 13.20 (33/64) |
| M14x2.00 | 12.10 (15/32) | 12.70 (1/2) | 14.75 (37/64) | 15.50 (39/64) |
| M14x1.50 | 12.50 (1/2) | 13.00 (33/64) | 14.75 (37/64) | 15.50 (39/64) |