Tapping drill size rule: “The hole drilled for tapping should be smaller than the tap size by twice the thread depth.” Evaluate this statement for standard metric/Unified 60° threads.

Introduction / Context:
Selecting the correct tapping drill size is critical. If the pilot hole is too small, taps break; if too large, thread engagement is insufficient. A common shop approximation uses the pitch, not “twice the thread depth,” particularly for 60° metric and Unified threads.

Given Data / Assumptions:

• Standard 60° V threads (metric/Unified).
• Conventional percentage of thread engagement (typically 60–75%).
• Through or blind holes with suitable chamfering and lubrication.

Concept / Approach:
For 60° threads, an accepted quick rule is: tapping drill diameter ≈ major diameter − pitch. This ties directly to thread geometry where the fundamental triangle height is related to pitch, and practical engagement (not the theoretical full depth) is targeted to reduce torque. Saying “smaller by twice the depth of thread” overestimates removal and leads to an undersized drill, excessive torque, and potential tap failure.

Step-by-Step Solution:

Verification / Alternative check:
Handbook tables corroborate the approximation and provide exact recommended drill sizes for specific engagement percentages. Practical experience shows dramatic torque rise if the hole is made too small relative to pitch-based recommendations.

Why Other Options Are Wrong:
Limiting agreement to coarse/fine/trapezoidal threads is misleading; these thread forms have specific geometry, but none uses “twice thread depth” as the general drilling rule for tapping standard 60° forms.

Common Pitfalls:
Confusing thread “height” with usable depth; assuming 100% engagement is desirable; ignoring material, lubrication, and tap type (spiral point/spiral flute) when selecting the drill.

Final Answer:
Disagree