What is the difference between external and internal micrometers?
Micrometers are precision measuring tools used in mechanical engineering, machining, and metrology to measure small distances or thicknesses with high accuracy—typically to the nearest 0.01 mm or 0.001 inch. They come in various types, with the two main categories being internal micrometers and external micrometers, also known by alternate names that reflect their specific functions.
External Micrometers (Outside Micrometers)
Also known as: outside micrometers, external screw gauges
Function: Designed to measure the outside dimensions of an object—such as the outer diameter of a shaft, rod, or bolt, or the thickness of a flat part.
Structure: External micrometers feature a C-shaped frame, a stationary anvil, and a movable spindle operated by a calibrated screw and thimble. The object to be measured is placed between the anvil and spindle, and the thimble is rotated until it gently contacts the object. A ratchet or friction stop ensures consistent measuring pressure.
Typical Applications:
Measuring the diameter of turned parts
Thickness of metal sheets or machined plates
Quality control in manufacturing processes
Internal Micrometers (Inside Micrometers)
Also known as: inside micrometers, internal bore micrometers
Function: Designed to measure internal dimensions, such as the diameter of a hole, bore, or cylinder.
Structure: Internal micrometers usually consist of a head with a calibrated screw and interchangeable rods or jaws that expand outward. Unlike external micrometers, they apply outward pressure against internal surfaces. Some variants resemble telescoping gauges or use a three-point contact system for self-centering inside bores.
Typical Applications:
Measuring cylinder bores in engines
Inspecting internal grooves or tube diameters
Checking precision holes in manufacturing
Key Differences
| Feature | External Micrometer | Internal Micrometer |
|---|---|---|
| Measurement type | Outside dimensions | Inside dimensions |
| Contact orientation | Object placed between contacts | Contacts expand outward |
| Common measurement | Shaft diameter, thickness | Bore diameter, hole width |
| Accuracy and use | High, often used for go/no-go | High, but alignment-sensitive |
| Calibration reference | Reference surface (external) | Often needs setting rings |
Summary
While both internal and external micrometers operate on the same principle—a precision screw translating rotation into linear displacement—their construction and applications diverge based on whether the goal is to measure the inside or outside of a feature. External micrometers dominate in general-purpose precision work, while internal micrometers are specialized tools often used in conjunction with other instruments for calibration.