A technician recently reached out to ask how deep the pilot hole should be drilled for tapped holes.
As shown in the diagram: A = actual thread engagement length, C = effective thread depth, B = pilot hole depth. His core question was whether the dimension B-C must strictly comply with values listed in design manuals, and if it could be reduced.
Different engineering references offer conflicting design recommendations:
Some state blind pilot hole depth = effective thread depth + 3~5 pitches
Others advise adding an extra 2~6 mm to the effective thread depth
A third standard suggests effective thread depth + 0.5D (D = thread major diameter)
Many engineers get confused by these varying guidelines. To clarify the logic, we first need to understand the purpose of this extra clearance and the tapping process.
Tapping tools (taps) come in multiple variants, but every tap features a tapered cutting chamfer at the front, followed by a full-thread guiding section. Longer cutting chamfers deliver smoother cutting and extend tool service life.
Type A (taper tap): Designed for manual tapping
Type B/D (plug tap): Widely used for CNC machining with long service life
Type C (modified bottoming tap) & Type E (short bottoming tap): For blind holes, with short chamfers to create threads close to the hole bottom
The length of this cutting chamfer directly dictates the required extra depth of the pilot hole.
Plug taps (Type B/D) are the most commonly used style, which explains the widespread rule of thumb: blind pilot hole depth = effective thread depth + 3~5 pitches — these 3 to 5 pitches match the standard chamfer length of plug taps.
The recommendation of adding 0.5× major diameter (0.5D) aligns with the 3–5 pitch rule. For standard metric threads, the major diameter is roughly 8 times the pitch. Taking the midpoint of 4 pitches equals half the major diameter (0.5D).
Do not get misled by inconsistent surface-level rules; always focus on the underlying mechanical logic.
Back to the original question: Can we shrink the B-C clearance dimension? Yes, it is feasible. For example, Type E bottoming taps only require an extra 1.5–2 pitches of clearance. However, as noted earlier, this drastically shortens tap lifespan, so extreme minimal clearance should only be adopted when absolutely necessary.
Hope this technical breakdown helps you.
Many seemingly insignificant design details require our attention, such as Easily Overlooked Threaded Hole Chamfer Dimensions.
