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: Fine (UNF), Course (UNC), 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 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 table below also lists the standard sizes for both close and free clearance holes.

Tap styles - (bottoming, plug, taper)

Common Tap and Clearance Drill Sizes

thread sizetap drillclearance (close)clearance (free)
0-80 UNF3/64 (0.047")52 (0.064")50 (0.070")
2-56 UNC50 (0.070")43 (0.089")41 (0.096")
4-40 UNC43 (0.089")32 (0.116")30 (0.129")
6-32 UNC36 (0.107")27 (0.144")25 (0.150")
8-32 UNC29 (0.136")18 (0.170")16 (0.177")
10-24 UNC25 (0.150")9 (0.196")7 (0.201")
10-32 UNF21 (0.159")9 (0.196")7 (0.201")
12-24 UNC16 (0.177")2 (0.221")I (0.228")
1/4-20 UNC7 (0.201")F (0.257")H (0.266")
1/4-28 UNF3 (0.213")F (0.257")H (0.266")
5/16-18 UNCF (0.257")P (0.323")Q (0.332")
3/8-16 UNC5/16 (0.313")W (0.386")X (0.397")
1/2-13 UNC27/64 (0.422")33/64 (0.516")17/32 (0.531")