Diameter Pitch

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Pitch Diameter vs Thread Diameters



Thread Pitch Diameter Display[edit]

A display with 2 studs of the same thread pattern and pitch diameter but with a varying minimum and maximum diameter of the thread.


Purpose of Project[edit]

Two threads with different diameters but having the same pitch diameter might visually appear to be of different sizes. However, both are essentially the same and will fit in the same tapped hole (provided the hole is tapped according to the respective size). This can cause confusion as two threads with different pitch diameters but the same thread diameter will appear to be of the same size but will not fit in the same tapped hole. Therefore, when working with threads it is important to fully understand this concept. We aim to achieve this visually and logically in this page.

Thread Diameter vs Pitch Diameter[edit]

Pitch Diameter vs Thread Diameters

Thread Diameter[edit]

There are two different thread diameters. The outside and inside thread diameters.

Major Diameter:The largest diameter of the thread of the screw or nut. When working with thread on screws the common term used is "outside diameter" and on threads of a nut the common term is "full diameter".

Minor Diameter:The smallest diameter of the thread of the screw or nut. When working with thread on screws the common term used is "core diameter" and on threads of a nut the common term is "inside diameter".



Pitch[edit]

The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis. In inches for example, the pitch is calculated by 1 divided by the number of threads per inch.

Pitch Diameter vs Thread Diameters

How to Calculate Pitch[edit]

Thread Measuring Wires Kit
Thread Measuring Wire Chart
Procedure for measuring the pitch of a thread using the "Three Wire Method":
Example Description
Measuring Outer Thread Diameter
1. Calculate the outer thread diameter.


Use either a micrometer gauge, ruler, or some calipers. However keep in mind the more precise of an instrument, the better.

Measuring threads per inch
2. Calculate the threads per inch.


To do this simply use a ruler and count how many threads are located within a 1 inch distance.

Calculating Wire Diameter
3. Calculate the wire diameter.


To do this, take a thread measuring wire and put it between the threads. You will have found the correct one once its center point is parallel to the endpoints of the threads on either side of the wire. Next align the other two thread measuring wires in the threads as demonstrated by the graph (Three Wire Method). Make sure they too are parallel. Record the wire diameter.

Equation to calculate M.
4. Calculate M.

Use Equation to calculate M.

PEE DEE Thread Charts
5. Final Step.


Use the PEE DEE Thread chart and threads per inch to find CONST, subtract this from M value. The resulting value is the pitch diameter.