Selecting the correct tap drill size is one of the most common calculations in machining. Get it wrong, and you'll either break taps or produce weak threads. This guide covers everything you need to know about calculating drill sizes for tapping operations.
Understanding Thread Percentage
Thread percentage refers to how much of the theoretical thread depth you're actually cutting. A 100% thread would require a hole exactly at the minor diameter—which would break your tap. Most applications call for 65-75% thread engagement.
Here's the key insight: a 75% thread is only about 5% weaker than a 100% thread, but requires significantly less torque to tap. This is why 75% is the sweet spot for most applications.
When to Use Different Thread Percentages
- 50-60%: Soft materials like aluminum, brass, or plastics where tap breakage is a concern
- 65-75%: General purpose machining in steel and most metals (recommended)
- 80-85%: High-strength applications in tough materials where maximum holding power is critical
The Tap Drill Formula
For unified threads (UNC, UNF), the formula is:
Tap Drill = Major Diameter - (1 ÷ TPI × Thread%)
For metric threads, use:
Tap Drill = Major Diameter - (Pitch × Thread%)
Let's work through an example. For a 1/4-20 UNC thread at 75%:
- Major Diameter: 0.250"
- TPI: 20
- Calculation: 0.250 - (1 ÷ 20 × 0.75) = 0.250 - 0.0375 = 0.2125"
- Nearest drill: #7 (0.201") for ~75% thread
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Common Tap Drill Sizes Quick Reference
Here are the most frequently used tap drill combinations in the shop:
| Thread | Tap Drill | Decimal |
|---|---|---|
| 1/4-20 UNC | #7 | 0.201" |
| 5/16-18 UNC | F | 0.257" |
| 3/8-16 UNC | 5/16" | 0.3125" |
| 1/2-13 UNC | 27/64" | 0.4219" |
| M6×1.0 | 5.0mm | 0.197" |
| M8×1.25 | 6.8mm | 0.268" |
| M10×1.5 | 8.5mm | 0.335" |
Avoiding Tap Breakage
Broken taps are expensive—not just the tap itself, but the time and potential part damage from extraction. Follow these guidelines:
- Use the right drill size: Too small means excessive torque; too large means weak threads
- Maintain proper depth: Drill deeper than the tap length to allow chip clearance
- Apply cutting fluid: Essential for steel and stainless; helps with chip evacuation
- Consider spiral flute taps: Better chip evacuation in blind holes
- Check for chip buildup: Back out periodically in deep holes
Clearance Holes vs. Tap Holes
Don't confuse tap drills with clearance holes. A clearance hole allows a bolt to pass through without engaging threads. For a 1/4-20 bolt:
- Tap drill: #7 (0.201") - creates threads in the part
- Close fit clearance: 17/64" (0.266") - minimal play
- Free fit clearance: 9/32" (0.281") - standard clearance
Tips for Better Threads
Beyond drill size, these practices improve thread quality:
- Chamfer the hole entrance to help the tap start straight
- Use a tap guide or CNC rigid tapping for perpendicularity
- Match tap style to material (spiral point for through holes, spiral flute for blind)
- Consider roll-form taps for production—no chips and stronger threads
Getting tap drill sizes right is fundamental to quality machining. While charts and calculators speed up the process, understanding the underlying principles helps you make better decisions when standard sizes don't apply.
