Which 3D CAD modelling techniques build a solid model, and how is a 2D sketch turned into 3D geometry?
3D CAD modelling techniques: sketch-based modelling with constraints, the feature commands (extrude, revolve, sweep, loft), and editing features (shell, fillet/chamfer, array and boolean) to build and modify a solid model.
An SQA Higher Graphic Communication answer on 3D CAD modelling techniques, covering sketch-based modelling and constraints, the feature commands extrude, revolve, sweep and loft, and editing features such as shell, fillet, array and boolean operations.
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What this key area is asking
The SQA wants you to build and edit a 3D CAD solid model: sketch-based modelling with constraints, the feature commands (extrude, revolve, sweep, loft), and the editing features (shell, fillet/chamfer, array and boolean) that modify a model. At Higher you are expected to know which command builds which kind of geometry and why parametric, constrained models are easy to change.
Sketch-based modelling and constraints
Constraining captures design intent: a fully defined (parametric) sketch can be resized by changing a value, and the relationships hold, so the rest of the sketch and the features built on it rebuild automatically instead of breaking.
The feature commands
Choosing the right command is the skill: a uniform shape is extruded, anything turned is revolved, anything that follows a route is swept, and anything that morphs between shapes is lofted.
Editing features
Worked example
Examples in context
Every product you own was likely modelled this way: bottles and wheels are revolves, casings are extrudes that are shelled and filleted, pipes and cables are sweeps, and streamlined or transitioning shapes are lofts. The parametric, constrained model is also what drives downstream processes, feeding the assembly, the rendered presentation and the production drawings from one master model.
Try this
Q1. State the command used to make a part that is circular about an axis (such as a wheel). [1 mark]
- Cue. Revolve.
Q2. State what the shell command does. [1 mark]
- Cue. Hollows a solid to a uniform wall thickness.
Q3. State the difference between a fillet and a chamfer. [2 marks]
- Cue. A fillet rounds an edge (a radius); a chamfer bevels an edge (a flat angle).
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 Higher (style)4 marksDescribe the extrude, revolve, sweep and loft commands, and give an example of an object best made by each.Show worked answer →
Extrude takes a 2D profile and pushes it straight along a path perpendicular to the sketch, giving a prism of constant cross-section. Example: a rectangular bracket or a hexagonal nut blank.
Revolve rotates a 2D profile around an axis (up to 360 degrees) to make a part that is circular about that axis. Example: a bottle, a wheel, a shaft or a turned handle.
Sweep takes a 2D profile and drives it along a chosen 2D or 3D path, so the cross-section follows the path. Example: a length of pipe that bends, a handle that curves, or a skirting/moulding following a route.
Loft blends between two or more different profiles on separate planes, creating a smooth transition between them. Example: a boat hull, a transition duct (round to square), or a bottle that changes shape along its height.
Markers reward: extrude = push a profile straight (constant section), revolve = rotate a profile about an axis (circular parts), sweep = profile along a path, loft = blend between different profiles, each with a sensible example.
SQA Higher (style)3 marksExplain how constraints (dimensional and geometric) are used in a CAD sketch and why they make a model easier to edit.Show worked answer →
A CAD model usually starts from a 2D sketch. Constraints fix the sketch so it is fully defined. Dimensional constraints set sizes (lengths, diameters, angles) as values that can be changed. Geometric constraints set relationships, such as two lines being parallel, perpendicular, equal, horizontal, vertical, tangent or concentric, without giving a number.
They make the model easier to edit because the design intent is captured: when one dimension is changed, the constrained relationships are kept, so the rest of the sketch updates correctly instead of falling apart. A fully constrained, parametric sketch can be re-sized by editing a value, and features built on it rebuild automatically.
Markers reward: dimensional constraints set changeable sizes, geometric constraints set relationships (parallel, perpendicular, tangent, etc), and constraining captures design intent so editing one value updates the model predictably (parametric editing).
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