What is an assembly drawing?
An assembly drawing shows a complete product or sub-unit made of several parts. It does not give every size of every part (that is the job of the detail drawings). It shows how the parts fit and work together.
- General assembly: the whole machine.
- Sub-assembly: one group of parts, like a wheel and its bearing.
- Exploded view: parts pulled apart along their lines of assembly, easy for people building it.
It usually gives only overall sizes and fitting sizes (like Ø20 H7/g6), not every dimension.
Balloons and the bill of materials
Each different part gets a balloon: a circle with a number, joined to the part by a thin leader line ending in a dot. Same parts share one number.
The bill of materials (BOM) or parts list is a table, usually just above the title block. Each row: item number, part name, quantity, material, and often the standard (for example ISO 4014 bolt) or drawing number of the part. Bought parts like bolts, nuts and bearings are listed by their standard name, not drawn in detail.
Sections in assemblies
Section views show how inner parts sit inside each other. Rules:
- Each part has one hatching style in every view.
- Neighbouring parts use opposite directions or different spacing.
- Bolts, nuts, shafts, pins, keys, rivets and balls are not hatched when cut along their length.
- Very thin parts (gaskets, sheet metal) may be filled solid black.
Joints on assembly drawings
Detachable (can be undone)
Bolts and nuts, screws, studs, keys and pins. Threads are drawn in a simple way: a thin line for the thread root and a thick line for the outside. Washers spread the load; lock nuts and spring washers stop loosening.
Permanent (cannot be undone without damage)
Rivets (lap and butt joints), welds (shown by an ISO welding symbol: an arrow, a reference line and a shape such as a triangle for a fillet weld), adhesive bonding and soldering.
How to read an assembly drawing: title block → parts list → find each balloon → follow how each part touches the others → identify joints → imagine the order of assembly.
Key formulas and definitions
- Balloon number = item number in the bill of materials
- Neighbouring parts → opposite hatching
- Not hatched along length: shafts, bolts, nuts, pins, keys, rivets
- Detachable joints: bolt, screw, stud, key, pin
- Permanent joints: rivet, weld, adhesive, solder
- Fillet weld symbol: triangle on the reference line
Worked examples
1. A pulley assembly has a shaft, a pulley, a key, 2 bearings and 4 screws. How many rows does the BOM need?
One row per different part: shaft, pulley, key, bearing, screw = 5 rows. Quantities: 1, 1, 1, 2, 4.
2. Three plates touch in a section view. How should they be hatched?
Plate 1 at 45° one way, plate 2 at 45° the other way, plate 3 with different spacing or angle (for example 30°) so no two neighbours look the same.
3. Which joint would you choose for a car wheel, and why?
Bolts (wheel nuts), because the wheel must be removed to change a tyre. A weld or rivet would be permanent.
Common mistakes
- Giving two identical parts two different balloon numbers. Same parts share one number; the quantity goes in the BOM.
- Hatching neighbouring parts in the same direction, so they look like one part.
- Hatching a bolt or shaft cut along its length.
- Putting every dimension of every part on the assembly. Only overall and fitting sizes go there.