Assembly drawings and bills of materials
Assembly drawings show how parts go together; the bill of materials lists what’s needed. Learn how they’re laid out and how they link to detail drawings.
In this lesson you will
- Lay out an assembly drawing
- Balloon parts and create a bill of materials
- Handle standard parts correctly
- Use exploded views and assembly notes
What assemblies show
An assembly drawing shows parts in their assembled positions — usually in section to reveal how they fit — with only the dimensions needed for assembly (overall sizes, fitting positions, clearances). Detail dimensions belong on the part drawings.
Balloons and the BOM
Each part gets an item balloon (a circle with a number) with a leader to the part. The bill of materials (parts list) — often above the title block — lists item number, part number/drawing number, description, material, quantity and supplier or standard. Balloons and BOM rows must match exactly.
| Item | Part no. | Description | Material | Qty |
|---|---|---|---|---|
| 1 | BRK-001 | Base bracket | S275 steel | 1 |
| 2 | SH-010 | Shaft ⌀20 | Stainless 304 | 1 |
| 3 | — | Hex bolt M8 × 25 (ISO 4017) | Steel 8.8 | 4 |
Standard parts
Bolts, nuts, washers, bearings and similar are standard parts: listed by their standard designation and size in the BOM, not drawn in detail and not given part drawings. Draw them simplified in the assembly.
Exploded views and notes
Exploded views (often isometric) show assembly order clearly for manuals and workshops. Assembly notes cover torques, adhesives, lubrication and inspection. Link the assembly and parts with drawing numbers so any part can be traced.
Common mistakes
Balloon numbers that don't match the BOM.
Generate the BOM from the model or check each balloon against its row.
Full detail dimensions on the assembly.
Keep detail dimensions on part drawings; show only assembly-critical ones.
Exercise
A simple assembly
- Model or draw a bracket, shaft and two bearings with fasteners.
- Draw the assembly in section.
- Balloon every item and create a matching BOM.
- Add an exploded isometric and two assembly notes.
You should end up with: An assembly drawing with a correct BOM.
Checklist
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Lesson quiz
Every balloon number must appear in the bill of materials.
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