An assembly drawing is needed for all products or inventions that have more than one part. These drawings list all parts and sub-assemblies that make the final product. A BOM (Bill of Materials) on the drawing lists each part number, part name, and part quantity.
The dimension line is a thin line, broken in the middle to allow the placement of the dimension value, with arrowheads at each end (see diagram below). An arrowhead is approximately 3 mm long and 1mm wide. That is, the length is roughly three times the width. An extension line extends a line on the object to the dimension line.
For a technical or engineering drawing, however, different strategies have to be used. This is where an isometric drawing becomes useful. An isometric drawing allows the designer to draw an object in three dimensions. Isometric drawings are also called isometric projections.
Isometric projection is a method for visually representing three-dimensional objects in two dimensions in technical and engineering drawings. It is an axonometric projection in which the three coordinate axes appear equally foreshortened and the angles between any two of them are 120 degrees.