1. What is Tap Drill Size?

The tap drill size is the diameter of the hole that should be drilled before tapping threads. It's calculated based on the major diameter of the thread, the desired thread percentage, and the threads per inch (TPI).

2. How Does the Calculator Work?

The calculator uses the tap drill size formula:

Where:

• Major Diameter — Outer diameter of the thread (inches)
• Percentage — Desired thread engagement percentage
• TPI — Threads per inch
• 0.013 — Constant factor for imperial threads

Explanation: The formula accounts for the relationship between thread dimensions and the required hole size for proper thread engagement.

3. Importance of Correct Drill Size

Details: Using the correct tap drill size ensures proper thread engagement strength while preventing excessive torque during tapping that could break the tap.

4. Using the Calculator

Tips: Enter major diameter in inches, percentage (typically 60-75% for most applications), and TPI. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What percentage should I use?
A: For most applications, 75% is standard. Use lower percentages (60-65%) for harder materials or when higher strength isn't required.

Q2: How accurate is this formula?
A: This provides a good estimate for most imperial threads. For critical applications, consult specific tap drill charts.

Q3: Does this work for metric threads?
A: No, metric threads use a different calculation based on thread pitch rather than TPI.

Q4: What if I get a non-standard drill size?
A: Round to the nearest standard drill size. The calculator shows the theoretical size for reference.

Q5: Why is the constant 0.013?
A: This empirical constant accounts for the relationship between thread height and pitch in imperial threads.