1. What is Tap Drill Diameter?

The tap drill diameter 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 diameter formula:

Where:

• Major Diameter — Outer diameter of the thread (inches)
• Percentage — Desired thread percentage (typically 60-85%)
• TPI — Threads per inch of the tap

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

3. Importance of Correct Drill Size

Details: Using the correct tap drill diameter is crucial for creating threads with proper strength and engagement. Too small and the tap may break; too large and the threads will be weak.

4. Using the Calculator

Tips: Enter major diameter in inches, percentage as a whole number (e.g., 75 for 75%), and TPI from the tap specifications. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What percentage should I use for general purpose threading?
A: For most applications, 75% thread engagement provides a good balance between strength and ease of tapping.

Q2: Where can I find the major diameter of a thread?
A: Thread specifications typically list major diameter. For standard threads, consult engineering references or tap manufacturer charts.

Q3: Does material affect the calculation?
A: The formula is material-independent, but harder materials may require slightly larger drill sizes to reduce tap breakage.

Q4: What about metric threads?
A: This calculator is for imperial units. For metric threads, use pitch instead of TPI in a different formula.

Q5: How precise should the drill size be?
A: For best results, drill to within ±0.001 inches of the calculated size when possible.