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Pitch-diameter micrometers (aka thread mics) have a matching set of thread-shaped tips for measuring the pitch diameter of screw threads. Limit mics have two anvils and two spindles, and are used like a snap gauge. The part being checked must pass through the first gap and must stop at the second gap in order to be within specification.
A dedicated type of micrometer, called a thread mic or pitch mic, which has a V-anvil and a conical spindle tip, contacts the thread flanks for a direct reading. A general-purpose micrometer (flat anvil and spindle) is used over a set of three wires that rest on the thread flanks, and a known constant is subtracted from the reading.
Many differential screw configurations are possible. The micrometer adjuster pictured uses a nut sleeve with different inner and outer thread pitches to connect a screw on the adjusting rod end with threads inside the main barrel; as the thimble rotates the nut sleeve, the rod and barrel move relative to each other based on the differential between the threads.
The 3-wire method is also used when high precision is needed to inspect a specific diameter, commonly the pitch diameter, or on specialty threads such as multi-start or when the thread angle is not 60°. Ball-shaped micrometer anvils can be used in similar fashion (same trigonometric relationship, less cumbersome to use).
As the knob is rotated it moves the anvils in or out with respect to the measurements. The knob usually has a slipping mechanism to take the feel out of the device and increase reliability between measurements. The measurement given is the mean diameter of the three anvils, and is usually good to 0.001 mm (3.9 × 10 −5 in). [1]
A thread gauge, [1] also known as a screw gauge [2] or pitch gauge, [3] is used to measure the pitch or lead of a screw thread. Thread pitch gauges are used as a reference tool in determining the pitch of a thread that is on a screw or in a tapped hole. This tool is not used as a precision measuring instrument, rather it allows the user to ...