Editing Tap and die (section)

===Holes===

Whether manual or automatic, the processing of tapping begins with forming (usually by drilling) and slightly [[countersink]]ing a hole to a diameter somewhat smaller than the tap's major diameter.  The correct hole diameter is listed on a [[drill and tap size chart]], a standard reference in many [[machine shop]]s. The proper diameter for the drill is called the '''tap drill''' size. Without a tap drill chart, you can compute the correct tap drill diameter with:
:<math>TD = MD - \frac {1}{N}</math>
where <math>TD</math> is the tap drill size, <math>MD</math> is the major diameter of the tap (e.g., {{frac|3|8}} in for a {{frac|3|8}}-16 tap), and <math>1/N</math> is the [[thread pitch]] ({{frac|16}}&nbsp;inch in the case of a {{frac|3|8}}-16 tap).  For a {{frac|3|8}}-16 tap, the above formula would produce {{frac|5|16}}, which is the correct tap drill diameter.  The above formula ultimately results in an approximate 75% thread.

Since metric threads specify the pitch directly, the correct tap drill diameter for metric-sized taps is computed with:
:<math>TD = MD - \text{pitch}</math>
where <math>TD</math> is the tap drill size, <math>MD</math> is the major diameter of the tap (e.g., 10&nbsp;mm for a M10×1.5 tap), and pitch is the pitch of the thread (1.5&nbsp;mm in the case of a standard M10 tap) and so the correct drill size is 8.5&nbsp;mm. This works for both fine and coarse pitches, and also produces an approximate 75% thread.