Assembly Drawings

≈ 40 min

Assembly Drawings

A detail drawing describes one component so it can be made. An assembly drawing shows how components fit together so the product can be built.

The two answer different questions and carry different information:

A detail drawing is fully dimensioned — every size needed to manufacture that part.
An assembly drawing carries only functional dimensions: overall sizes, centre distances between mating features, and any dimension needed to assemble correctly. It does not repeat the manufacturing dimensions, which live on the detail drawings.
Each component carries an item number in a circular balloon with a leader line, keyed to a parts list.

Worked example. A bearing housing assembly: the detail drawing of the bracket dimensions every hole, boss and fillet. The assembly drawing shows the bracket, bearing, shaft and bolts in position, and dimensions only the shaft centre height and the bolt-hole centres — the numbers the fitter needs to build it.

Core checkpoint: Dimension an assembly for assembly, not for manufacture. Repeating manufacturing dimensions creates two sources of truth.

Which dimensions belong on an assembly drawing?

Functional ones — overall sizes, centre distances between mating features and anything needed to assemble correctly.

Every dimension of every component.

No dimensions at all.

Only the dimensions of the largest component.

What is the name of the circled number with a leader line that identifies each component on an assembly drawing?

Why is fully dimensioning every component on the assembly drawing a problem?

It duplicates the detail drawings, so a later revision must be corrected in two places and eventually will not be.

It uses too much ink.

Assembly drawings are not allowed to carry any dimensions.

It makes the drawing take longer to produce, but is otherwise fine.

A fitter needs the height of a shaft centre above the base. Which drawing carries it?

The assembly drawing — it is a functional dimension needed to build the product correctly.

The detail drawing of the shaft.

Neither; it is measured on site.

Both, so that it can be cross-checked.