The 10:1 rule
A good rule of thumb: your instrument should be about ten times more precise than the tolerance you are checking. Checking a ±0.05 mm tolerance needs an instrument good to around 0.005 mm.
Calipers
Vernier, dial and digital calipers measure outside, inside, depth and step dimensions with one tool. Typical resolution is 0.01–0.02 mm. They are versatile and fast, ideal for general machining and incoming checks, but less accurate than micrometers.
Micrometers
Micrometers measure a single dimension with resolution to 0.001 mm and a controlled measuring force via the ratchet. Use them for tight-tolerance diameters and thicknesses. Each micrometer covers a 25 mm range, so you may need a set.
Gauges and indicators
Plug, ring and snap gauges give fast go/no-go answers in production. Dial and test indicators measure run-out, flatness and position. Height gauges on a surface plate, and gauge blocks as references, complete the toolkit. For complex geometry, CMMs measure in 3D.
Keep them trustworthy
- Calibrate on a fixed schedule
- Measure at stable temperature
- Clean faces before use
- Store in cases, away from coolant and chips