LC DCG common mistakes
LC DCG · Understand the traps and how to avoid them.
When producing an auxiliary view, the candidate projects the new view perpendicular to the auxiliary plane instead of perpendicular to the true-length edge, so the auxiliary view comes out foreshortened or distorted rather than showing the face in true shape.
The candidate has memorised 'project at 90°' without attaching it to the correct reference: the lines of sight must be perpendicular to the auxiliary plane, which means the projection lines are drawn perpendicular to the reference/fold line, not perpendicular to the object edges. Confusion between the fold line and the object geometry causes the error.
What to do instead
Before drawing anything, state in writing which edge or face is being made true length/true shape and which fold line it relates to. Then draw all projection lines perpendicular to that fold line, transfer heights from the adjacent principal view with a compass or dividers, and verify by checking that the known true-length edge appears at its stated dimension in the finished auxiliary view.
On orthographic layouts, the candidate mixes up first-angle and third-angle projection, placing the plan above the elevation (or the end view on the wrong side) and producing a drawing whose views are in the wrong relative positions.
Both systems are taught and the symbol is often memorised as a picture rather than understood as a projection relationship. Under exam pressure the candidate defaults to whichever layout they last practised, without checking the projection symbol on the sheet.
What to do instead
Adopt a fixed rule to recite at the start of every layout: in first angle the object sits between observer and plane, so the view from above goes below the elevation; in third angle the plane sits between observer and object, so the view from above goes above. Check the given projection symbol first and mirror the layout to it before committing to construction lines.
The candidate draws a line or curve as tangent to a circle or arc by eye, without constructing the point of tangency, so the join is visibly kinked and the tangency condition fails when marked.
In freehand sketching and rushed CAD work, tangency looks acceptable at small scale, and candidates do not realise that a tangent point must lie on the line joining the centres (for two circles) or on the perpendicular from the centre (for a line to a circle).
What to do instead
Always construct tangency: for a line tangent to a circle, drop a perpendicular from the centre to locate the tangent point; for two arcs, mark the tangent point on the line of centres before drawing either arc. In CAD, apply the tangent constraint and then check the geometry with a dimension or by zooming to the join rather than trusting appearance.
When developing a surface (radial-line or parallel-line development), the candidate measures true lengths from a view in which the relevant edges are inclined and therefore foreshortened, giving an undersized or misshapen development.
The candidate treats any visible edge as measurable. They have not internalised that only lines parallel to a projection plane appear true length, so inclined edges in the front or plan view are shortened and cannot be transferred directly.
What to do instead
Before developing, audit each edge: identify which are parallel to a principal plane and which are not. For inclined edges, first obtain true length by auxiliary projection or by the difference-in-height construction, then transfer those true lengths to the development. Label the true-length source of every dimension on the development so the marker can follow the method.
The candidate builds a CAD model as a single complex sketch with many overlapping profiles and then extrudes or revolves it in one operation, leaving unresolved sketch constraints and a model that cannot be edited or that fails when a dimension is changed.
This comes from sketching the way one draws on paper, adding lines wherever needed, rather than modelling features sequentially. The candidate also under-uses constraints, dragging geometry into place instead of dimensioning it, so the sketch is under-defined and unstable.
What to do instead
Model in discrete features: one simple, fully constrained sketch per feature, using geometric constraints (coincident, tangent, equal, symmetric) plus dimensions until the sketch reports fully constrained. Add fillets, chamfers and holes as separate features after the base solid, and rebuild the model after changing a driving dimension to confirm it updates cleanly.
In an assembly or part drawing, the candidate applies dimensions to the drawing view rather than to the model, or dimensions the same feature twice from different references, producing conflicting or non-driving dimensions that do not match the geometry.
The candidate is thinking of the drawing sheet as the primary artefact and annotates it to look complete, rather than treating the model as the source of truth. Duplicate dimensions arise from adding references 'for clarity' without checking what is already defined.
What to do instead
Dimension the sketch or feature that creates the geometry, then bring those driving dimensions into the drawing view so they are automatically consistent. Before finishing, run a duplicate/over-defined check and remove any dimension that repeats information already given by another dimension or by a datum, keeping one clear dimensioning scheme from datums.
The candidate jumps straight into a final solution drawing without recording the brief, constraints, or alternatives, so the design cannot be shown to meet the stated specification and loses the analysis and evaluation marks.
Time pressure and the habit of treating design as drawing lead candidates to skip the problem-definition and idea-generation stages. They believe a neat final drawing is what is rewarded, not realising the assessment rewards evidence of a design process.
What to do instead
Structure the answer to the brief: a short written statement of the problem, constraints and user needs; at least two or three sketched concepts with brief pros and cons; a justified choice; then the resolved solution with dimensions and materials. Keep each stage short but explicit, and annotate the final drawing to show how each constraint from the brief is satisfied.
The candidate's solution ignores a stated constraint (for example, overall size, material, manufacturing method, or a required mechanism), or adds features that conflict with the brief, and does not notice the conflict before finishing.
Candidates focus on the visual appeal of the artefact and treat the constraint list as introductory text. They also fail to carry out a checking step, so a solution that exceeds a maximum dimension or uses an unavailable process is submitted unchecked.
What to do instead
Convert every constraint in the brief into a checklist item with a measurable test, then verify the final drawing against each item and annotate where it is met. If a constraint genuinely cannot be met, state the trade-off explicitly and justify the compromise rather than silently ignoring it.
In the geometry and trigonometry option, the candidate applies a trigonometric ratio to a triangle that is not right-angled, or uses the sine rule where the ambiguous case applies without considering the second possible solution.
Formulae are recalled as a list without their conditions of use. The candidate sees a triangle with a known angle and side and reaches for sine, cosine or tangent without checking for a right angle, and forgets that the sine rule can yield two valid triangles.
What to do instead
Before calculating, annotate the triangle: mark any right angle, or drop a perpendicular to create one if using SOHCAHTOA. When using the sine rule for an angle, check whether the given data is SSA and evaluate both the acute and obtuse possibilities, then test each against the figure and the stated context. State the condition being used alongside the working.
The candidate computes an angle or length in a three-dimensional context by treating a sloping face or a skew line as if it lay in the plane of the paper, measuring the angle in the projected view rather than finding its true magnitude.
Angles and lengths in orthographic views are foreshortened, but the candidate reads them directly off the drawing. They do not connect the trigonometry option to the descriptive geometry method of finding true length and true angle by auxiliary projection.
What to do instead
For any 3D angle or length, first identify the true-length lines involved and construct an auxiliary view or a right triangle whose hypotenuse is the true length of the line. Measure the angle only in a view where both lines appear true length, or calculate it from the true lengths using trigonometry, and show the construction that establishes true length.