Terminology and Definitions
Terminology and Definitions tests precise language: the exam expects you to separate terms that sound alike but carry different technical meaning, and to read a specification or drawing note literally. Marks are lost here through approximation rather than hard calculation, so slow down on the defining sentence.
Size, tolerance, and fit
- Nominal size is the label identifying a part; actual size is what measurement shows.
- Tolerance is the total permitted variation (upper limit minus lower limit). It is not the same as clearance.
- Clearance is the gap between mating parts; allowance is the intentional minimum clearance designed in.
- Clearance fit: the hole is always larger than the shaft. Interference fit: the shaft is always larger, so assembly needs force, heat, or cold.
Measured quantities: look-alike pairs
- Mass (kg) is quantity of matter; weight (N) is the gravitational force on it. A 10 kg object weighs about 98 N at standard gravity.
- Force is in newtons; torque is in newton-metres (force × lever arm). They are not interchangeable.
- Gauge pressure is measured relative to atmosphere; absolute pressure = gauge + atmospheric.
- Resolution is the smallest display increment; accuracy is closeness to truth. A fine resolution does not guarantee accuracy.
Specification language
"Shall" is mandatory, "should" is a recommendation, "may" is permission. Calibration establishes the relationship to a reference; verification only confirms that an item meets a stated requirement. Traceability is an unbroken chain of calibrations back to a reference.
Worked example
A hole is specified 25.00–25.05 mm and a shaft 24.96–24.98 mm.
- Minimum clearance = smallest hole − largest shaft = 25.00 − 24.98 = 0.02 mm.
- Maximum clearance = largest hole − smallest shaft = 25.05 − 24.96 = 0.09 mm.
- Both values are positive, so the hole is always larger: this is a clearance fit with a clearance range of 0.02–0.09 mm. Had the result been negative, the fit would be interference.
Traps that cost marks
- Calling the ± value the clearance.
- Confusing tolerance with the natural spread of a process.
- Treating mass and weight as interchangeable in force calculations.
- Using gauge pressure where an absolute value belongs.
- Reading "should" as a requirement in a compliance question.