1. What is the Drill Size Calculation?

The drill size calculation determines the appropriate drill bit size needed before tapping threads. The formula accounts for the major diameter of the tap, the desired thread percentage, and the threads per inch (TPI).

2. How Does the Calculator Work?

The calculator uses the following equation:

Where:

• Major Diameter — Diameter of the tap (inches)
• Percentage — Desired thread percentage (unitless)
• TPI — Threads per inch
• 0.013 — Constant factor

Explanation: The equation calculates the proper hole size needed to achieve the specified thread percentage when tapping.

3. Importance of Correct Drill Size

Details: Using the correct drill size is crucial for achieving proper thread engagement and strength. Too small a hole can cause tap breakage, while too large a hole results in weak threads.

4. Using the Calculator

Tips: Enter the major diameter of the tap in inches, the desired thread percentage (typically 60-75%), and the threads per inch (TPI). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What percentage should I use for general applications?
A: For most applications, 75% is recommended as it provides a good balance between strength and ease of tapping.

Q2: How do I find the major diameter of a tap?
A: The major diameter is typically marked on the tap or can be found in tap drill charts for standard sizes.

Q3: What's the difference between TPI and pitch?
A: TPI (threads per inch) is the reciprocal of pitch (distance between threads). For metric threads, use pitch in mm.

Q4: Can I use this for metric threads?
A: This formula is designed for imperial threads. For metric threads, different calculations are needed.

Q5: What if my calculated drill size isn't a standard size?
A: Choose the next smallest standard drill bit size to ensure proper thread engagement.