drill and tap

There's a rule of thumb for calculating the diameter of the hole to be drilled for tapping:

Major Diameter - Pitch = Drill Size

The Major Diameter is the nominal "outside diameter" of the male thread, and the Pitch is the "lengthwise distance" between two successive thread crests.

This rule of thumb works for the three most common thread forms found in the US, the pre-WWII US Standard (aka American National), the post-WWII Unified, and the ISO Metric.

Let's work an example or two:

1. For a US Standard or Unified 7/16-14, the Major Diameter is 7/16 inch, which is 0.4375 inch, and the Pitch is 1/14 inch, which is 0.0714 inch.

Major Diameter - Pitch = Drill Size
0.4375 inch - 0.0714 inch = 0.3661 inch

2. For a M8 x 1.25 ISO Metric thread it's even simpler. The Major Diameter is 8 millimeters and the Pitch is 1.25 millimeters.

Major Diameter - Pitch = Drill Size
8 millimeter - 1.25 millimeter = 6.75 millimeter.

Only a couple things complicate this:

1. There probably won't be a standard drill size that exactly matches the calculated size. In almost all cases, though, using the nearest standard-size drill will be fine.

2. We 'Murkins don't always describe drill sizes by their diameters, so you might need to find a drill size chart to know what to call the nearest standard-size drill. In the case of the 7/16-14's 0.3661 inch calculated drill size, the nearest US standard drill size is "Letter U" at 0.368 inch diameter.

John

I'd go a hair larger than the largest drill that will fit through a nut because the tap extrudes threads besides cutting them and the resulting hole ends up smaller than the tap drill and if you get the hole small, the tap WILL bind and may break. In any case for every fraction of a turn forward you need to back the tap out a quarter turn to break off the chips or it will bind anyway.

A lubricant is good too, I keep pipe threading oil around for steel, kerosene is good for aluminmum and there are magic elixors for many metals.

Gerald J.