Measurement, Metrology, Inspection and Quality Control
What the exam tests
Least counts, correct reading of instruments, limits and fits, and choosing the right inspection tool for a feature.
Core rules
- Vernier least count = 1 main scale division − 1 vernier scale division. A 49 mm/50 division vernier on a 1 mm main scale gives LC = 0.02 mm.
- Micrometer least count = screw pitch / thimble divisions = 0.5/50 = 0.01 mm.
- Slip gauges are wrung to build a size, using the fewest blocks and starting from the last decimal place.
- Sine bar: sin θ = h/L. A 100 mm sine bar with 50 mm of slips sets 30°, and needs a surface plate and dial indicator for checking.
- Limits and fits per IS 919 use the hole basis (H hole) wherever possible; clearance, transition or interference is decided by the tolerance zones, not by nominal size.
- Taylor's principle: the GO gauge is full-form and checks all features together, the NO-GO gauge checks size only.
- The standard reference temperature is 20 °C; parallax and temperature are the largest shop-floor errors.
- Ra is arithmetic mean roughness in micrometres; Rt is peak-to-valley height.
Worked example
A vernier shows main scale 23 mm with the tenth vernier line coinciding: 23 + 10 × 0.02 = 23.20 mm. If the instrument has a zero error of −0.03 mm, the actual size is 23.20 + 0.03 = 23.23 mm.
Traps
- Forgetting to add a negative zero error instead of subtracting it.
- Using outside jaws for a bore and ignoring jaw thickness.
- Treating a GO–NO-GO pair as a measuring instrument; it only accepts or rejects.
- Reading the micrometer thimble from the wrong datum line.