Engineering Drawing and Reading of Machine Assemblies
What this domain tests
Reading orthographic views, interpreting limits and fits, geometric tolerances, surface finish symbols, section views and assembly drawings.
Core rules
- First-angle projection: the view is placed on the far side of the object, so the top view sits below the front view and the left-side view sits on the right. Third-angle is opposite. Indian drawing practice uses first angle.
- Lines: continuous thick for visible outlines, dashed for hidden detail, chain dotted for centre lines and pitch circles, and thin continuous for dimension and projection lines.
- Limits and fits: a size written as 25 H7 means basic size 25 mm with an H7 hole tolerance. H7 for 25 mm is +0.021/0, and g6 is −0.007/−0.020. Maximum clearance = 0.021 − (−0.020) = 0.041 mm; minimum clearance = 0 − (−0.007) = 0.007 mm.
- GD&T: form (flatness, straightness, circularity), orientation (parallelism, perpendicularity), location (position, concentricity) and run-out. The datum is the reference from which the tolerance is measured.
- Surface roughness Ra is in micrometres; a tick with a number indicates the required value, and the tick with a circle means no machining is needed.
- Assembly drawings show part numbers, a bill of materials, fits and key dimensions; threads are shown by their major and minor diameter lines, with the thread noted as M10 × 1.5.
Worked example
A drawing specifies a 25 mm hole and shaft with H7/g6. Hole limits 25.000 to 25.021 mm; shaft limits 24.980 to 24.993 mm. Every permitted combination gives a clearance between 0.007 and 0.041 mm, so the fit is a sliding clearance fit suitable for a locating spigot.
Traps that cost marks
- Placing views in third-angle when the drawing is first-angle.
- Reading a bilateral tolerance as unilateral.
- Confusing concentricity with circularity.
- Ignoring the surface-finish symbol, which can override the general tolerance note.