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How to Calculate the Minor Diameter of Thread

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A screw is made of a head, where the screwdriver or drill bit is inserted, and a body, which is composed of threads that bore into a material. In construction and engineering settings, you might need to identify the size of the threads being used in the screw to properly find replacements. One measurement used to identify the size of a screw is minor diameter, which is the distance between the roots of the thread.

Place the tip of the depth micrometer, also called the "thimble," into one of the threads on the screw.

Turn the dial at the opposite end of the micrometer until the flat part is flush against the screw body.

Turn the locking lever to hold the micrometer in place.

Read the micrometer from left to right to determine the thread depth. As an example, the micrometer reads 0.25 inch.

Multiply the thread depth by 2. In the example, 0.25 times 2 equals 0.5.

Subtract the result from the screw's major diameter. In the example, the screw has a major diameter of 0.75. Subtracting 0.5 from 0.75 equals 0.25. The screw has minor thread diameter of 0.25 inch.

Tip

Familiarize yourself with the parts of a micrometer before working. The dial, for instance, is above the handle in the middle of the tool and the anvil locking mechanism is at the top.

Digital micrometers are more expensive, but are also more accurate and easier to read.