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A machine is designed as parts, but it is built, bought and serviced as an assembly. In CAD, the assembly model is where parts meet: it shows whether they fit, whether they move as intended and what has to be made or bought. The bill of materials (BOM) is the list that comes out of it, and it drives purchasing, stock, production and cost.
Parts, sub-assemblies and assemblies
- A part is a single piece with one material: a shaft, a bracket, a bolt.
- A sub-assembly is a group of parts that is built and handled as one unit: a gearbox, a wheel hub, a motor with its mounting plate.
- An assembly (or top-level assembly) brings parts and sub-assemblies together into the finished product.
Splitting a design into sensible sub-assemblies keeps large models manageable and matches how the product is actually built: sub-assemblies are put together on the bench and then installed as a unit.
Mates and constraints
Parts are placed in an assembly with mates (also called constraints or joints, depending on the software). Each mate removes degrees of freedom, just as a real joint does.
| Mate | What it does | Real-world equivalent |
|---|---|---|
| Coincident | Makes two faces, edges or points touch or lie in the same plane | Two faces bolted together |
| Concentric | Lines up two cylindrical axes | A pin in a hole, a shaft in a bearing |
| Parallel / perpendicular | Sets the orientation of faces or axes | Guide rails, square mounting |
| Distance / angle | Sets an exact offset or angle | Spacers, adjustable stops |
| Gear, rack and cam | Couples motion between parts | Gear trains, cam followers |
| Limit | Sets a range of motion | Hinge stops, slide end stops |
The first part (usually the base or frame) is fixed; everything else is mated to it directly or through other parts. A good habit is to mate parts the way they are located in reality: a gear concentric to its shaft and coincident to the shoulder. Then the model moves like the real machine, and a design change carries through correctly.
Top-down and bottom-up design
In bottom-up design, parts are modelled separately and then assembled. In top-down design, parts are created inside the assembly, referencing each other: a cover drawn from the edges of the box it fits. Most real projects mix the two: standard and bought parts are inserted bottom-up, while parts that must fit each other are designed in context.
Checks the assembly model makes possible
- Interference detection finds parts that overlap.
- Clearance checks confirm gaps for tools, fingers, cables and thermal expansion.
- Motion studies move mechanisms through their range to check for collisions.
- Mass properties give the total weight and centre of gravity.
The assembly drawing
An assembly drawing tells the workshop how the parts go together, not how each is made (that is on each part’s own drawing). It usually shows:
- General views of the complete assembly, often with an exploded view that separates the parts along their assembly direction.
- Balloons: numbered circles with leaders, one per item, matching the item numbers in the BOM.
- The bill of materials table, normally above the title block.
- Overall and interface dimensions only: the size of the assembly and the positions of mounting holes and connections.
- Assembly notes: bolt tightening torques, adhesives and thread lockers, lubrication, adjustments and tests.
The bill of materials
A BOM lists everything needed to build one assembly. Typical columns are:
| Column | Contents |
|---|---|
| Item | The balloon number on the drawing |
| Part number | A unique identifier, or the standard for bought parts (for example ISO 4017) |
| Description | Name, size, material or grade |
| Quantity | Number per assembly |
| Material, finish | For made parts |
| Supplier, catalogue number | For bought parts |
| Mass | For weight roll-ups |
Single-level and indented BOMs
A single-level BOM lists only the direct children of one assembly. An indented (multi-level) BOM shows the full structure, with each sub-assembly’s parts indented under it, and is what production planning and purchasing need.
There are also two views of the same product. The engineering BOM (EBOM) follows the design structure in CAD. The manufacturing BOM (MBOM) follows the way the product is built, and adds things the CAD model doesn’t show, such as adhesive, packaging and grease.
Worked example: rolling up quantities from an indented BOM
A machine frame (top level) contains 2 identical motor mount sub-assemblies and 6 M10 bolts of its own. Each motor mount contains 4 M10 bolts. How many M10 bolts are needed to build 50 machines?
Step 1: bolts per machine. Multiply each sub-assembly’s quantity by the number of sub-assemblies, then add the top-level bolts.
Step 2: for the batch.
A single-level BOM of the top assembly shows only “6 bolts”, which is how parts shortages happen. Always order from the rolled-up quantities of the indented BOM.
Good practice
- One part number per unique part. If two parts differ in any way (length, finish, material), they need different numbers.
- Name and number parts consistently in CAD so the BOM fills itself from file properties.
- Use the CAD library for standard parts with correct standards and grades in their properties.
- Control revisions. When a part changes in a way that affects fit or function, it gets a new revision, and the BOM records which revision is used.
- Keep the BOM linked to the model so it updates when the design changes, instead of typing it by hand.
Key points
- Parts make sub-assemblies, which make assemblies; structure them the way the product is built.
- Mates remove degrees of freedom like real joints; fix the base part first.
- Assembly drawings use exploded views, balloons, a BOM, interface dimensions and assembly notes.
- Indented BOMs roll up the true quantities for purchasing and production.