Hand-tapping

[mklotz]

A formula is always more compact and easier to remember than a chart. This from one of my shop papers, hope it helps...

Tap drill calculation...

TD = tap drill diameter
MD = major diameter of thread
DOT = depth of thread expressed as percentage
TPI = thread pitch expressed as threads-per-inch

TD = MD - DOT / (75*TPI)

Example:

MD =0.5
DOT = 65 %
TPI = 12

TD = 0.5 - 65 /(75* 12)
TD = 0.428 (closest drill = 27/64 = 10.87 mm)

The more simplistic formula...

TD = MD - 1/TPI

makes the assumption that:

DOT / 75= 1

or:

DOT = 75 %

which is actually more than needed in many situations.

Suggested DOTs:

MILD AND UNTREATED STEELS 60-65
HIGH CARBON STEEL 50
HIGH SPEED STEEL 55
STAINLESS STEEL 50
FREE CUTTING STAINLESS STEEL 60
CAST IRON 70-75
WROUGHT ALUMINUM 65
CAST ALUMINUM 75
WROUGHT COPPER 60
FREE CUTTING YELLOW BRASS 70
DRAWN BRASS 65
MANGANESE BRONZE 55
MONEL METAL 55-60
NICKEL SILVER (GERMAN SILVER) 50-60

ISTR that the Whitworth is a 55º thread. My formulae are for a 60º thread but, the difference doesn't seem to matter much. The formula result agrees well with BJ's information.