Professional / Trade Practice
Professional / Trade Practice covers how a supervisor converts a drawing, work order and standard into a correct job: measurement, fits, tools, process sequencing, inspection and documentation. It is the practical bridge between engineering theory and execution on a BHEL shop floor or site.
Core rules and concepts
- Read drawings in this order: title block, views, dimensions, tolerances, surface finish, notes, revision status.
- Tolerance is permissible variation; allowance is an intentional difference between mating parts. Clearance = hole size − shaft size.
- Fits: clearance fit always has positive clearance; interference fit always has negative clearance; transition fit may be either.
- Use calibrated instruments within their range; record calibration status and least count.
- Follow the work order and approved drawing revision; never machine to a superseded dimension.
- Material traceability: match grade, heat number and test certificate before cutting or welding.
- Inspection: first-piece inspection, in-process checks and final documentation.
Worked example
A hole is specified 25.000–25.021 mm and a shaft 24.980–24.997 mm. Minimum clearance = minimum hole − maximum shaft = 25.000 − 24.997 = 0.003 mm. Maximum clearance = maximum hole − minimum shaft = 25.021 − 24.980 = 0.041 mm. Since both values are positive, it is a clearance fit. The reasoning is to pair the worst-case limits, not the nominal sizes.
Traps
Confusing tolerance with allowance; using a worn micrometer without checking zero; mixing mm and inch; ignoring drawing revision; and accepting a part because the nominal size matches while the limits are out.