3D Modelling and CAD
3D Modelling and CAD
This domain tests whether you can build and document a model with design intent, not just draw something that looks right. It covers sketch constraints, feature-based solid modelling, assemblies, drawing extraction, rendering and file exchange.
Core rules
- A sketch should be fully constrained: geometric constraints control shape, dimensional constraints control size, and the sketch should not move when dragged.
- Model with features in a logical history: base sketch and extrusion, then cuts, fillets, shells and patterns. Later features reference earlier geometry, so order matters.
- Use symmetry, equal constraints and datum planes to capture design intent so a change to one driving dimension updates the model predictably.
- Assemblies use mating constraints such as coincident, concentric, parallel and distance; parts should not be moved by dragging unconstrained components.
- Generate orthographic drawings from the model, then add dimensions, sections and a title block; do not redraw views manually.
Worked example
Model a 60 × 40 × 8 mm plate with four Ø6 mm holes, each 6 mm from two edges, and 10 mm corner fillets. Sketch a centred rectangle on the top plane, add horizontal and vertical constraints, dimension 60 and 40, then constrain hole centres symmetrically and dimension 6 mm offsets from the edges. Extrude 8 mm. Add the corner fillets before cutting holes so the fillet tool does not select hole edges. Cut the four holes through all. If the plate length changes to 70 mm, the symmetry and offset constraints should keep the holes correctly placed.
Traps that cost marks
Under-constrained sketches, deleting a parent feature and breaking children, mixing part and assembly constraints, applying a fillet larger than the available edge, and failing to update the drawing after a model change.