Quality · Released
Measurement Planning
Plan which instrument to use, which datum to reference, and what acceptance criteria apply. Good measurement planning prevents bad parts from shipping and good parts from being rejected.
Instrument selection guide
| Feature | Tolerance range | Recommended instrument | Resolution needed |
|---|---|---|---|
| External dimensions | ±0.05 mm or looser | Vernier caliper | 0.02 mm |
| External dimensions | ±0.02 mm | Outside micrometer | 0.001 mm |
| Hole diameter | ±0.05 mm | Inside micrometer / telescoping gauge | 0.01 mm |
| Hole diameter (precision) | ±0.01 mm | Bore gauge / CMM | 0.001 mm |
| Depth / step | ±0.02 mm | Depth micrometer / dial depth gauge | 0.01 mm |
| Surface finish | Ra 0.8–6.3 µm | Surface roughness tester | 0.01 µm |
| Angles | ±0.5° | Bevel protractor | 0.1° |
| Complex geometry | Any precision | CMM | 0.001 mm |
| Threads | Go/No-Go | Thread plug/ring gauge | — |
Measurement principles
10:1 rule: instrument resolution should be at least 10× better than the tolerance being measured. If tolerance is ±0.05 mm, use an instrument with 0.005 mm resolution or better.
Same datum: always measure from the same datum that the machine used to cut the feature. Measuring from a different datum introduces alignment error.
Calibration: check calipers and micrometers against gauge blocks before each shift. A zeroed micrometer is more reliable than a CMM that hasn't been calibrated in 6 months.