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Bolt Pitch Diameter Calculator

Pitch Diameter Formula:

\[ PD = D - \frac{0.6495}{TPI} \]

inches
threads/inch

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1. What is Pitch Diameter?

The pitch diameter (PD) is the diameter of an imaginary cylinder that passes through the threads where the width of the thread and the space between threads are equal. It's a fundamental measurement in thread design and manufacturing.

2. How Does the Calculator Work?

The calculator uses the pitch diameter formula:

\[ PD = D - \frac{0.6495}{TPI} \]

Where:

Explanation: The formula accounts for the relationship between major diameter, thread pitch, and the theoretical perfect fit between mating threads.

3. Importance of Pitch Diameter

Details: Pitch diameter is crucial for determining thread fit class, calculating thread strength, and ensuring proper thread engagement in bolted joints.

4. Using the Calculator

Tips: Enter major diameter in inches and threads per inch. Both values must be positive numbers. For standard thread sizes, consult engineering references for exact dimensions.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between major and pitch diameter?
A: Major diameter is the largest diameter of the thread, while pitch diameter is the theoretical diameter where thread width equals space width.

Q2: How accurate is this calculation?
A: This provides a theoretical value. Actual manufactured threads may vary slightly due to tolerances and manufacturing processes.

Q3: Does this work for metric threads?
A: No, this formula is for imperial threads. Metric threads use pitch (distance between threads) rather than TPI.

Q4: What is the 0.6495 constant in the formula?
A: It's derived from the geometry of the 60° thread form (UN/ISO standard) where the height of the fundamental triangle is 0.866025p and pitch diameter occurs at 0.75 times this height.

Q5: When would I need to calculate pitch diameter?
A: When designing custom threads, checking thread gauges, or calculating thread strength and fit classes.

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