Mechanical Part and Assembly Drawing
Detail and assembly drawings
A detail drawing defines one part: shape, size, tolerances, surface finish, material, heat treatment and general notes. An assembly drawing shows how parts fit together: overall dimensions, functional dimensions, item balloons, a bill of materials and the fits at mating features. Do not dimension individual part manufacture on the assembly drawing; use the BOM and detail drawings for that.
Assembly conventions
Balloons carry item numbers that match the BOM and point to the part with a leader ending in a dot or arrow. Sectioned assemblies hatch adjacent parts differently. Show only the parts needed to explain function; simplify fasteners and standard components. Include overall envelope dimensions, centre distances, and the fit between shaft and housing.
Worked example
A gear is mounted on a Ø30 shaft in a housing. The shaft is Ø30 k6 (+0.015/+0.002) and the gear bore is Ø30 H7 (+0.021/0). The maximum clearance is 30.021 − 30.002 = 0.019 mm and the maximum interference is 30.015 − 30.000 = 0.015 mm, so the fit is transitional and needs a press or controlled assembly force. On the assembly drawing, show the fit as Ø30 H7/k6 and balloon the gear, shaft, bearing and cover; put the actual limits on the detail drawings. The reasoning is that the assembly states the fit, while the details state the manufacturing limits.
Traps
Omitting a BOM item, ballooning a part that is not in the BOM, dimensioning the same feature on both assembly and detail drawings, forgetting the fit designation, and using a leader that touches two parts.