How Do I Calculate the Drill Size for a Forming Tap?
This is the industry-standard formula used by CNC machinists and tooling manufacturers worldwide:
Forming Tap Drill Size = Tap Major Diameter – (0.5 × Thread Pitch)
This formula applies to all standard metric forming taps and delivers a 60%–70% standard thread height, which is the ideal thread percentage for maximum strength and tap stability.
Practical Calculation Example (Metric)
Example: Calculate drill size for M6×1.0 forming tap
Major diameter of M6 = 6.0 mm
Pitch = 1.0 mm
Calculation: 6.0 – (0.5 × 1.0) = 5.5 mm
Result: Use a 5.5mm drill bit
Another Example: M8×1.25 Forming Tap
8.0 – (0.5 × 1.25) = 7.375 mm
Closest standard drill size:7.4 mm
3. Imperial (UNC / UNF) Forming Tap Drill Formula
For imperial unified threads, use the same logic with inch units:
Drill Size = Major Diameter – (0.5 × Pitch)
Example: 1/4-20 UNC Forming Tap
Major diameter = 0.250 inch
Pitch = 1/20 = 0.050 inch
0.250 – (0.5 × 0.050) = 0.225 inch
Closest standard drill: #1 drill (0.228 inch)
4. Quick Reference Chart: Most Popular Forming Tap Drill Sizes
Below are the most commonly used sizes for quick shop floor reference:
Tap Size | Pitch | Calculated Drill Size | Standard Drill Bit |
|---|---|---|---|
M3 | 0.5 | 2.75 mm | 2.8 mm |
M4 | 0.7 | 3.65 mm | 3.7 mm |
M5 | 0.8 | 4.60 mm | 4.6 mm |
M6 | 1.0 | 5.50 mm | 5.5 mm |
M8 | 1.25 | 7.375 mm | 7.4 mm |
M10 | 1.5 | 9.25 mm | 9.3 mm |
M12 | 1.75 | 11.125 mm | 11.1 mm |
5. Adjust Drill Size Based on Workpiece Material
The standard formula works for general use, but different metals require minor adjustments due to ductility difference:
Soft Ductile Materials (Aluminum, Copper, Brass)
These flow easily during cold forming.Use the exact calculated size for full, clean thread crests. No adjustment needed.
Mild Steel / Low Carbon Steel
Stick to the standard drill size for optimal 65%–70% thread height, balancing tap life and thread strength.
Stainless Steel (304/316)
Stainless steel is tough and creates high friction. Drill 0.05–0.1 mm larger to prevent tap binding and breakage, with negligible impact on thread strength.
Hard or Brittle Materials
Do not use forming taps on cast iron, hardened steel, or powder metal. These materials cannot displace and will damage the tap.
6. Common Mistakes to Avoid
Mistake 1: Using cutting tap drill sizes
This is the #1 mistake. Cutting tap holes are too small for forming taps. It will cause extreme torque, overheating, and tap fracture.
Mistake 2: Pursuing 100% thread height
Full 100% thread height offers almost no extra strength but doubles tap wear. 60%–70% thread height is the industrial sweet spot for forming taps.
Mistake 3: Ignoring material ductility
Running the same drill size for aluminum and stainless steel will lead to inconsistent yields and frequent tool failures in mass production.
Mistake 4: No tolerance control
Even 0.1mm deviation can cause fit issues. Always use precision drill bits for threading operations.