Lab tests have shown that 50 to 65% of a full thread holds as much as 75 to 100% of the load.

To determine a tap drill size, a formula or a table is used. Tables derived from mathematical calculations using the 75% factor are readily available and are widely used. Tap drill size other than 75 percent are also published. An example is printed in the table above. However threads other than the standard UNC and UNF series are rarely listed in a table.  For these threads reference to the Machinery’s Handbook should provide sufficient information from which tap drill sizes can be derived. 

Please note that blueprints will often specify a countersink or a bevel at the top of the hole to guide the tap and eliminate the possibility of getting a thread burr.

When formulas must be used, the simplified versions that follow will serve for most threads.  The formulas required are:

  1 divided by N equals P, where 1 is one inch, N is the number of threads per inch, and P is the thread pitch.  The thread pitch is the distance from one thread to the next.

And:

D subtract P equals TD, where D is the nominal thread diameter, and P is pitch as calculated in the previous formula.  The resulting TD is the decimal diameter of the theoretical Tap Drill.  In some cases the calculated tap drill size does not coincide with an actual drill size.  When that happens, use the next nearest drill size that is listed.  Sometimes this may be a few thousandths of an inch larger or smaller.  In most cases, use the nearest drill size to the actual calculation.

Using the thread in the example chart: 1/20=.05 (1 divided by 20 equals .050)  followed by: .250-.050=.200 (.250 minus .050 equals .200).  The result is a tap drill size of .200, this falls between the #7 and #8 drills in the chart. Dietrich Kanzler