Basic Engineering Practice: Units, Tolerances, and Engineering Judgment
Scope and mindset
Basic Engineering Practice is the judgment domain: units, statistics, economics, tolerances, standards, and ethics. Questions rarely demand deep theory; they test whether you convert units correctly, apply the right design factor, and avoid rounding errors.
Concepts to master. Consistent units and the use of g_c; significant figures; mean, standard deviation, and normal-distribution reliability; worst-case versus statistical (root-sum-square) tolerance stack-up; present worth and annual-cost comparison; design factor versus safety factor; the code hierarchy (ASME BPVC, B31.1, Y14.5, AGMA, ISO); documentation and professional ethics.
Common traps. Mixing lbf and lbm; converting ksi to psi (x1000) but forgetting psi to MPa (1 psi = 6.895 kPa, so 1 ksi = 6.895 MPa); applying a design factor to ultimate strength when the governing code specifies yield; adding tolerances arithmetically when the question implies independent random dimensions; rounding intermediate results.
Worked rule of thumb. Four dimensions each +/-0.005 in: the worst-case stack is +/-0.020 in; the statistical stack is sqrt(4 x 0.005^2) = +/-0.010 in. If a question asks for the clearance guaranteed for every part, use worst case. If it asks about typical rejection rate or states that dimensions are independent, use RSS. Keep units explicit and convert to N, m, and Pa before calculating.