Determining the Minimum Tap Drill Depth
The tap drill depth for tapped holes should be deep enough to allow for the minimum thread depth, the tap’s 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.
To determine the ideal minimum drill depth, start with the basic rule: the tap drill depth should be at least one thread or one pitch beyond the chamfer of the tap. In other words, for a 1/4-20 tapped hole requiring a 3/8″ (.375″) thread depth, using a two-thread bottoming chamfer with 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″).
It is important to note that using the “one-pitch rule” does not always prevent the tap from hitting the bottom of the hole. In this case, consider the following three factors:
- Understand the machine’s capability to stop the spindle rotation at the bottom. Many 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, an older machine may allow the spindle to over-travel one, two, or even three pitches. Here, the additional drill depth should be at least one pitch beyond the over-travel amount.
- Consider how the chips are produced. Spiral taps extract chips, preventing them from accumulating in the bottom of the hole. Chips produced 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.
- Understand the tap design. If using a plug or taper chamfer, note the tap sizes through 3/8″ diameter are manufactured with 900 external centers that could hit the bottom. A general rule is to add one-half the diameter to the length for sufficient clearance.