Metrology Basics: Parallax Error, Calipers and Micrometers Explained
Metrology and Measurement Fundamentals
Metrology is a theoretical science and practical discipline covering the context of measurement.
Parallax Error
Parallax error is avoided when reading the scale by positioning your view perpendicular to the vernier scale. Reading at an angle introduces considerable errors.
Caliper
A caliper is an instrument used to measure linear dimensions such as internal, external and depth dimensions of a workpiece. It consists of a graduated ruler with a fixed jaw, on which a sliding jaw moves.
- Fixed ear
- Movable ear
- Vernier scale (inch)
- Locking screw
- Slider
- Fixed inch scale
- Fixed jaw
- Fixed stop
- Movable stop
- Movable jaw
- Vernier scale (mm)
- Depth probe
- Fixed millimeter scale
Types of Calipers
The quadrimensional caliper is used for internal, external and step measurements, as well as depth. It is the most commonly used type.
- Depth caliper: used to measure the depth of blind holes, slots, recesses and similar features. It may have a simple rod or a hook.
- Digital caliper: used for quick readings, free of parallax error, and ideal for statistical process control.
Note: The vernier caliper is the part whose purpose is to provide a resolution to a lesser extent (more precise) than that achieved with the fixed scale alone.
Micrometer
A micrometer is a linear measuring instrument used when high precision is required.
Micrometer Components
- Frame: made of special cast steel, heat treated to remove internal stresses. It is fitted with a thermal insulator that prevents expansion caused by heat transmitted from the hands or the instrument.
- Micrometric spindle or moving rod: constructed of hardened special steel and ground to guarantee the accuracy of the thread pitch.
- Measuring faces of the rod or fixed anvil: these contact the workpiece and are parallel. In rigorous measuring instruments, the contacts are carbide, offering high wear resistance.
- Adjustment nut: allows adjustment to remove play in the micrometer spindle when necessary.
- Thimble (drum): located on the proximal scale. It rotates with the micrometer spindle. Each revolution equals the spindle's pitch of displacement.
- Ratchet or friction stop: ensures a constant measuring force.
- Lock: allows the spindle to be immobilized at a predetermined position.
