The tap drill depth for tapped holes should be deep enough to allow for the minimum thread depth, the taps chamfer, any chips that may accumulate in the bottom of the hole and any over-travel of the spindle as it slows to a stop before reversing. So what minimum drill depth would be ideal?
To answer that, we start with this basic rule: At a minimum, the tap drill depth should be at least of one thread or pitch beyond the chamfer of the tap. In other words, for a 1/4-20 tapped hole requiring a 3/8” (.375) thread depth, if a two thread bottoming chamfer is used, having a length of .100”, and adding a minimum of one additional thread or .050”, the minimum drill depth would be .525” (.375 + .100 + .050 = .525”).
However, many times using the “one-pitch rule” does not prevent the tap from hitting the bottom of the hole. In this case three things need to be taken into consideration.
The first is the machines capability to stop the spindle rotation at the bottom. Many of the newer machines are able to stop and reverse in one-half turn or less. If this is the case, the “one pitch rule” may be sufficient.
However, depending on the age of the machine, many spindles over-travel one, two, or even three pitches. In this case, the additional drill depth should be at least one pitch beyond the over-travel amount.
The second consideration is the chips produced. Spiral taps extract chips, preventing them from accumulating in the bottom of the hole.
However, chips from straight flute hand taps are likely to be flushed to the bottom by the coolant. The front of the tap may bottom on these chips, chipping or breaking the tap. In this case, one diameter of additional depth is recommended.
The third consideration is part of the tap design. If a plug or taper chamfer is used, tap sizes through 3/8” diameter are manufactured with a 90° external centers that could hit the bottom.
Therefore, when using a plug chamfer a rough rule is to add one-half the diameter to the length for sufficient clearance.