Tap drill sizing rule For internal threads produced by tapping, the diameter of the drilled hole (tap drill) is generally how related to the outside (major) diameter of the tap/thread?

Introduction / Context:
Before tapping a nut or a threaded hole, an appropriately sized tap drill is used so that the cutting action forms the thread profile without excessive force or inadequate engagement.

Given Data / Assumptions:

• Standard metric or unified coarse threads.
• Through or blind holes with adequate allowance for chip evacuation.

Concept / Approach:
The tap drill diameter must be less than the major diameter to leave sufficient material for the thread flanks. A common quick rule for metric coarse threads is: tap drill ≈ major diameter − pitch. For unified threads, charts give specific values close to major diameter minus thread height allowance.

Step-by-Step Solution:
Define need: leave material for full thread height.Choose drill < major diameter to achieve the desired percentage of thread (often 60–75%).Confirm from standard tap drill charts for the thread size.

Verification / Alternative check:
Reference tables universally list tap drills smaller than the major diameter for both metric and inch series threads.

Why Other Options Are Wrong:

• Equal or greater than major diameter would leave no flank engagement.
• “Independent of size” is false; each thread size has a specific recommended drill.
• Pitch diameter is not used as the drill size; it is a different geometric parameter.

Common Pitfalls:
Using too small a drill leading to high torque/broken taps; using too large a drill yielding weak threads.

Final Answer:
Smaller than the outside (major) diameter