Machine Drawing: Fasteners, Joints and Bearings
What the exam tests
Machine drawing tests conventional representation of threads, bolts, nuts, rivets, keys, cotter joints, welded joints and bearings. The exam expects BIS conventions, empirical proportions and correct sectional treatment of standard parts.
Core rules
- A screw thread is shown by a thick continuous line for the major diameter and a thin continuous line for the minor diameter, with the thread runout and end view convention.
- Standard parts such as bolts, nuts, rivets, keys, shafts and pins are not sectioned longitudinally; section them only when the question specifically requires a transverse section.
- Use BIS tables for actual sizes. Where empirical proportions are allowed, the nominal diameter d controls head and nut sizes; do not invent sizes that contradict the table.
- In riveted joints, use rivet diameter, pitch, edge distance and row spacing consistently; show the lap or butt strap and the correct number of rivet heads.
- Bearings are drawn by their housing, bore, shaft and cap; hatching on the housing must change direction at a joint.
Worked example
Represent an M20 bolt in elevation. The nominal diameter is 20 mm, so draw the major diameter as a thick line 20 mm apart. The minor diameter is about 0.85d = 17 mm, shown as a thin line inside the major diameter. Draw the thread runout near the end and the bolt head with a nominal across-flats size taken from the BIS table, not guessed. In the end view, show the thread as a broken circle: thick major circle and thin minor circle.
Traps
Drawing thread crests and roots as two equally thick lines is a convention error. Sectioning a bolt along its axis through the threaded portion is wrong. Do not mix rivet and bolt proportions. Hatching should stop at the shaft; a shaft is not hatched in longitudinal section.