Gauge block

A gauge block (also called a slip gauge or Jo block) is a precision-ground, hardened steel or ceramic block with two opposite faces ground and lapped to be perfectly flat and parallel. Gauge blocks are the fundamental standards of length in manufacturing — they define exactly what “one millimetre” or “one inch” means in a machine shop.

How it works

Gauge blocks are supplied in sets of graduated sizes. Individual blocks can be wrunged (stacked and twisted together) to build up any desired dimension within the set’s range. When two blocks are wrung together, the surfaces adhere so strongly that the combination is almost as accurate as a single block — the wringing force comes from molecular attraction between the ultra-flat surfaces.

Why they exist

Every measurement in a machine shop ultimately traces back to a gauge block. When you calibrate a micrometer, you check it against a gauge block of known dimension. When you set up a machine, you use gauge blocks to verify spindle positions. The gauge block is the link between the theoretical definition of a unit (the metre, the inch) and the physical tools that use it.

Grades

• Grade 0 (reference): The highest accuracy, used in calibration laboratories. Deviations from nominal size are measured in millionths of an inch.
• Grade 1 (inspection): For checking instruments and fine measurement.
• Grade 2 (workshop): For everyday use in manufacturing.
• Grade 3 (utility): For rough measurement and setup.

Materials

• Hardened steel: Traditional, durable, and relatively inexpensive. Prone to corrosion if not oiled.
• Ceramic: Harder, more wear-resistant, and corrosion-proof. More expensive but increasingly common.
• Carbide: Extremely hard and wear-resistant, used in the highest-precision applications.