Measurement, Marking Out and Tolerances
Measurement, Marking Out and Tolerances tests correct instrument choice, scale reading, marking from datums, and the meaning of tolerance and fits.
Instruments and precision
A steel rule is graduated in 0.5 mm. A vernier caliper has a least count of 0.02 mm: read the main scale before the vernier zero, then find the vernier line that aligns. A micrometer reads to 0.01 mm: read the barrel and add the thimble reading. A try square checks 90 degrees, dividers scribe arcs, odd-leg calipers find centres, and a surface gauge marks from a surface plate.
Marking out
Work from a datum, a true edge or face. Mark all dimensions from the same datum to avoid accumulated error. Use a scriber on metal, a centre punch for drill positions, and witness marks for matching parts. Remove burrs before measuring.
Tolerance and fits
Tolerance is the permitted variation in size; allowance is the intentional difference between mating parts. A clearance fit allows movement, an interference fit requires force, and a transition fit may be either. Limits are the maximum and minimum permitted sizes.
Worked example: vernier reading
The main scale reads 12 mm. The vernier zero is past 12 mm, and the 7th vernier line aligns with a main-scale line. With a least count of 0.02 mm, the reading is 12 + (7 x 0.02) = 12.14 mm. Reasoning: each vernier division represents 0.02 mm, so seven divisions add 0.14 mm.
Traps
Parallax error from viewing a scale at an angle, not zeroing a caliper, reading the wrong vernier line, confusing tolerance with allowance, and marking each dimension from the last one instead of a datum all cost marks. State the unit and the number of decimal places the instrument justifies.