Engineering components and assembly drawing
This domain tests standard representation of fasteners, threads, gears and springs, and the conventions of assembly drawings with parts lists.
Threads and fasteners
- ISO metric threads are designated M followed by the nominal diameter, with pitch only when coarse is not intended: M20 is coarse (2.5 mm pitch), M20 x 1.5 is fine.
- Representation: crests as thick continuous lines, roots as thin continuous lines on external threads, and internal threads as thin lines with the crest thick. Thread runout and the 120 degree drill point are shown on a tapped hole. The tapping drill diameter is roughly 0.85d.
- Hexagon bolts are specified by diameter and length; nuts by diameter; washers and spring washers by diameter.
Gears and springs
- Module m = pitch circle diameter / number of teeth. Addendum = m, dedendum = 1.25m, outside diameter = m(N + 2), centre distance = m(N1 + N2)/2.
- Compression springs: outside diameter, wire diameter, free length and number of active coils.
Assembly drawings
Show parts in their working positions, sectioned according to the same conventions, with a parts list giving item number, description, material and quantity, and balloons with leaders pointing to each part. Working drawings show one part fully dimensioned; assembly drawings omit manufacturing dimensions.
Worked example
Two spur gears mesh, 20 and 40 teeth, module 2 mm. Pitch diameters are 40 mm and 80 mm, outside diameters are m(N + 2) = 44 mm and 84 mm, and the centre distance is 2(20 + 40)/2 = 60 mm.
Traps
Using outside diameter instead of pitch diameter to find the module; hatching a solid shaft in an assembly section; drawing a thread root as a thick line; omitting material or quantity from the parts list.