3D CAD modelling

Modelling features

• Extrude: a 2D profile pulled out to a set distance to make a solid.
• Revolve: a profile rotated around an axis. State the axis and the angle: 360° for a full solid, or e.g. 90° for a quarter.
• Loft (Higher): joins two or more different profiles on offset work planes, e.g. a circle to a smaller circle 80 mm above it.

Higher modelling features HIGHER

• Extrude (sweep) along a path: a profile on a work plane perpendicular to the start of a path, swept along it. Give the path with all its dimensions and the profile size.
• Helix: a profile offset from an axis (the offset is the helix radius) swept round in a spiral. Give any two of pitch (distance per turn), revolutions and height: e.g. 11 revolutions at pitch 20 gives a height of 220. The 2025 report noted many candidates mixed up pitch and revolutions.
• Loft at Higher often combines with shell and offset work planes.

Modelling edits

• Subtract (extrude subtract / cut): removes material, e.g. a hole or slot.
• Add (union): joins solids.
• Shell: hollows a solid to a wall thickness, removing chosen faces. State the thickness and the faces removed.
• Fillet: rounds edges to a radius. Chamfer: bevels edges.
• Higher adds intersect (keeps only the overlapping volume), mirror and array (linear, box, radial), and regular or irregular (variable) fillets and chamfers.

How to answer a "describe how to model" question

These are worth 4–7 marks and are where most of the marks are awarded. For each stage, write:

1. the 2D profile (sketch) with its key sizes, and where it is drawn (which face or work plane);
2. the feature command (extrude, revolve, loft…) with the distance, angle or axis;
3. any edit (subtract, shell, fillet, chamfer) with the size and which edges or faces.

Example: "Sketch a Ø50 circle and extrude 50. On the top face sketch a Ø35 circle and extrude-subtract 40. Fillet the bottom edge R2 and chamfer the top edge 2.5 × 45°."

• Use the correct terms from the course specification: "cut out" or "pull up" are not awarded marks.
• Include every relevant dimension; read them carefully off the drawing.
• Get the order right: shell before adding internal features (in 2025 N5, shelling at the end was not awarded marks); shell before a radial array of holes.
• At Higher you don't need to describe the 2D commands used to draw a profile; the profile with all its sizes is enough.
• For a revolve, name the centre axis and the angle: both carry marks.

Modelling terms and concepts HIGHER

• Sketch / profile: the 2D shape a feature is made from. Work plane: a flat plane to sketch on, which can be offset from a face. Axis: a line to revolve around. Face, edge, vertex: the surfaces, lines and corners of a solid.
• Feature: each operation in the model. Datum: a reference plane, axis or point. Suppress: temporarily switch a feature off.
• Modelling tree (history): the ordered list of features. To change a model, edit the sketch or feature in the tree; the model updates. In 2024 a candidate who remodelled from scratch instead of editing the tree scored 0.
• Modelling plan: the planned order of features before you start.

Key terms

Quick check