Fitting and Measurement
Core Concepts
Fitting covers limits and fits, tolerance, allowance, clearance and interference, and assembly methods. Measurement covers steel rule, vernier caliper, micrometer, dial indicator, height gauge, bore gauge, and slip gauges. You must understand least count (resolution), zero error, and how to read scales correctly.
Limits, Fits and Traps
Know the hole-basis system: H is the basic hole, and shaft letters (h, g, f, k, p, etc.) give clearance, transition, or interference fits. Learn the terms: nominal size, basic size, upper and lower deviation, tolerance zone, and allowance (difference between hole and shaft at maximum material condition). Traps include confusing tolerance with allowance, mixing up clearance and interference, or misreading a vernier where the zero is offset. Also know surface roughness (Ra) and how it relates to fit and function, and the use of feeler gauges and dial indicators for alignment.
Worked Example / Rule of Thumb
Vernier least count = 1 main scale division − 1 vernier division. For a 0.02 mm vernier: main scale 1 mm, vernier 50 divisions = 0.98 mm, so LC = 0.02 mm. Reading = main scale value before the vernier zero + (coinciding vernier line × LC). For a micrometer: reading = barrel reading + (thimble division × 0.01 mm). Always check zero error and apply the correction.