How are separate components combined into a working CAD assembly?
Building assembly models from components using mating constraints and sub-assemblies, producing exploded views and a parts list (BOM).
An SQA Advanced Higher Graphic Communication answer on assembly modelling, covering how components are combined using mating constraints and sub-assemblies, and how exploded views and a parts list or bill of materials are produced.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this key area is asking
The SQA wants you to combine individual component models into an assembly by applying mating constraints, to organise larger products into sub-assemblies, and to produce exploded views and a parts list (bill of materials) that communicate how the product goes together. This is where separate parts become a working product.
Mating constraints and degrees of freedom
The skill is locating each part with the minimum sensible set of constraints, leaving free only the motion the product needs. A bolt is fully fixed; a hinge pin keeps one rotational freedom; a slider keeps one translational freedom. Over-constraining causes conflicts, while under-constraining lets parts drift, so the marker looks for parts that are located exactly as the mechanism requires.
Sub-assemblies
Sub-assemblies are an organisation and efficiency tool. A printer might have a paper-feed sub-assembly, a cartridge sub-assembly and a casing, each modelled and constrained on its own, then brought together. This makes large assemblies easier to navigate and edit, and it reflects real manufacturing, where modules are built before final assembly.
Exploded views and the parts list
These two outputs are how an assembly is documented for manufacture or instruction. The exploded view typically uses thin dashed trail lines along the axes to show how parts come apart. The parts list turns the visual into a countable, orderable schedule. The balloons tie the two together, so a reader can match item 3 in the list to the ballooned part in the view. Clear, consistent ballooning and a complete BOM are directly rewarded.
Examples in context
A pulley on a shaft uses a concentric constraint and a face mate, leaving rotation free. A cabinet is a top-level assembly of carcass, door and shelf sub-assemblies. A flat-pack instruction relies on an exploded view with trail lines and a numbered parts list. In every case, correct constraints make the assembly behave like the real product, and the exploded view and BOM make it buildable.
Try this
Q1. Name the constraint used to align the axes of a shaft and a bore. [1 mark]
- Cue. A concentric (axis-alignment) constraint.
Q2. State what an exploded view shows that a normal assembly view does not. [1 mark]
- Cue. The components separated along their fit axes, revealing the assembly order and relationships.
Q3. State three pieces of information a parts list gives for each component. [1 mark]
- Cue. Item number, description (name) and quantity.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksA shaft must sit centrally inside a bearing and be free to rotate but not slide out. Describe the assembly constraints that would locate the shaft correctly.Show worked answer →
Apply a concentric (axis-aligning) constraint between the cylindrical surface of the shaft and the bore of the bearing, so the two share a common axis and the shaft is centred.
Then apply a coincident or mate constraint between a shoulder face on the shaft and the face of the bearing to fix its position along the axis, stopping it sliding out.
Leaving the rotation about the shared axis unconstrained allows the shaft to rotate, as required.
Markers reward a concentric constraint to align the axes, a face mate to locate the shaft axially, and the recognition that the rotational degree of freedom is deliberately left free.
SQA AH style3 marksExplain the purpose of an exploded view and a parts list in an assembly drawing.Show worked answer →
An exploded view separates the components along their assembly axes so that each part, and the order and direction in which parts fit together, can be clearly seen; it communicates how the product is assembled.
A parts list (or bill of materials) tabulates every component with an item number, name or description and quantity, keyed to balloon labels on the drawing, so each part can be identified and counted.
Together they let someone assemble or order the parts without ambiguity.
Markers reward the exploded view showing assembly sequence and relationships, and the parts list identifying and quantifying each component with item numbers tied to balloons.
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