Metrology and Measuring Instruments
Metrology and Measuring Instruments
The exam tests least count, correct reading of vernier and micrometer, limits and fits, and gauge selection.
Rules
- Accuracy is closeness to the true value; precision is repeatability.
- Least count: vernier LC = 1 MSD − 1 VSD (commonly 0.02 mm); micrometer LC = pitch / thimble divisions = 0.5/50 = 0.01 mm.
- Vernier reading = main scale value before the vernier zero + (coinciding vernier division × LC).
- Micrometer reading = barrel reading + (thimble division × LC), corrected for zero error.
- Fits: clearance (hole larger than shaft), interference (shaft larger), transition (either). The hole-basis system is standard, with H as the basic hole.
- Limit gauges: the GO gauge checks the maximum material condition, the NO-GO gauge the minimum; by Taylor's principle the GO gauge should check all features together.
- Angular measurement: sine bar with h = L sin θ; slip gauges are wrung together and used as end standards.
Worked example
Vernier calliper of LC 0.02 mm: the main scale reads 24 mm before the vernier zero and the 13th vernier division coincides. Reading = 24 + 13 × 0.02 = 24 + 0.26 = 24.26 mm. If the instrument had a zero error of −0.03 mm (vernier zero to the left of the main scale zero), the true size = 24.26 + 0.03 = 24.29 mm.
Traps
- Reading the main scale at the last vernier line instead of at the vernier zero.
- Adding a positive zero error instead of subtracting it.
- Confusing least count with accuracy.
- Using a sine bar without knowing its roller centre distance.
- Treating GO/NO-GO gauges as size-measuring instruments; they only accept or reject.