Analog micrometer screw gauge

An analog micrometer screw gauge, often simply called a micrometer, is a precision measuring instrument used to measure small linear dimensions, such as the thickness of a sheet, the diameter of a wire, or the outside diameter of a shaft, with high accuracy. It’s more precise than a vernier caliper or a ruler.

How it Works

The micrometer’s operation is based on the principle of a screw and nut. A precisely machined screw (the spindle) moves axially as a rotating thimble is turned. The amount of axial movement is directly proportional to the rotation of the thimble, allowing for very fine measurements to be read from calibrated scales.

Key Components

An analog micrometer typically consists of the following parts:

1. Frame (C-Frame): The rigid, C-shaped body that holds all the other parts in alignment. It’s usually made of a sturdy material like hardened steel or cast iron.
2. Anvil: A small, stationary cylindrical part located at one end of the frame. The object to be measured rests against it.
3. Spindle: A movable cylindrical rod that moves towards or away from the anvil. It has a precisely ground screw thread that engages with a nut inside the barrel.
4. Sleeve (Barrel): A stationary cylindrical part attached to the frame. It has a linear scale (main scale) marked in millimeters (or inches) and often half-millimeter (or 0.025 inch) divisions.
5. Thimble: A rotating part that slides over the sleeve. It has a circular scale (thimble scale) divided into typically 50 or 100 divisions. One full rotation of the thimble corresponds to a specific axial movement of the spindle (e.g., 0.5 mm or 0.025 inches).
6. Ratchet Stop (or Friction Thimble): Located at the end of the thimble, this device ensures consistent measuring pressure. When the spindle touches the object, the ratchet slips, preventing excessive force that could damage the object or the micrometer.
7. Lock Nut (or Lock Clamp): A mechanism that locks the spindle in place once a measurement has been taken, preventing accidental movement while reading the scale.
8. Zero Adjusting Screw: Some micrometers have a small screw or mechanism to adjust the zero reading if there’s a zero error.