PAU Technical Drawing Applied to Plastic Arts II (Valencian Community) common mistakes
PAU Technical Drawing Applied to Plastic Arts II (Valencian Community) · Understand the traps and how to avoid them.
Leaving construction arcs, centres and auxiliary lines as a faint tangle instead of distinguishing them from the final solution, so the examiner cannot tell which line is the answer and which is scaffolding.
Candidates draw everything with the same 2H pencil and the same pressure, then run out of time to go back and reinforce the solution; they also believe 'showing the construction' means leaving every mark equally visible.
What to do instead
Work in two passes: first all auxiliary construction in 2H with light pressure, then overdraw the final result (the polygon side, the tangent arc, the ellipse) in HB/2B with a clear weight difference. Erase nothing that proves the method, but never let an auxiliary line be heavier than the solution. Practise by re-drawing one tangency exercise three times with a deliberate weight hierarchy.
Applying Thales' theorem or a proportion but measuring the segments with the ruler and rounding, instead of transporting them with the compass, so the fourth proportional or the divided segment drifts by a millimetre or two.
Candidates trust the ruler because it is faster, and the error is invisible at the drawing stage; it only shows when the resulting polygon closes badly or a tangent misses.
What to do instead
Force the compass-only habit: mark each transferred length as an arc on the auxiliary ray, never a ruler reading. After finishing, verify by checking that the constructed figure closes exactly and that the ratio between measured segments matches the requested ratio within the drawing tolerance.
Constructing a regular polygon by trial with the protractor (dividing 360° by n and marking angles) rather than by the general compass method, which loses accuracy and, in exams that demand a specific procedure, loses the method marks.
The protractor is taught early and feels reliable; candidates do not realise that cumulative angular error on a 9- or 11-sided polygon produces a visible gap at the closing vertex.
What to do instead
Learn the specific constructions: general method on a circumscribed circle for any n, the particular shortcuts for triangle, square, pentagon, hexagon, octagon, and the side-given methods. Use the protractor only as a final check, never as the construction. Time yourself doing a 7-gon by the general method in under four minutes.
Solving tangencies without locating the exact tangency points, drawing an arc that 'looks' tangent and joining it to the line or circle by eye.
The eye accepts a near-tangency at small scale; the candidate stops when the curve appears to touch, and does not realise the tangency point must lie on the line joining the two centres (or on the perpendicular to the line).
What to do instead
Always construct the tangency point explicitly: for line-circle, drop the perpendicular from the centre; for circle-circle, draw the line of centres and mark where it cuts the circles (external tangency: distance between centres equals sum of radii; internal: difference). Then draw the arc between the two marked points only. Check that the arc meets each element with a common tangent, not a corner.
Mixing up homology, homothety, affinity and inversion - using the centre of homology as if it were a homothety centre, or treating an affinity as a homothety - so the transformed figure has the wrong shape or the wrong size.
All four are taught in the same block, all use a centre or an axis plus a coefficient, and candidates memorise the procedures without the invariant that distinguishes them (homothety preserves direction and shape; affinity preserves parallelism but not ratios along the direction; homology relates plane figures through a centre and an axis; inversion maps lines and circles to each other).
What to do instead
Before starting, write one line identifying the transformation and its invariants: does it keep parallelism? does it keep the shape? does it have an axis? Then apply the matching procedure. Build a one-page comparison table of the four with a worked mini-example each, and redraw it from memory before the exam.
Drawing an ellipse or parabola by freehand or by plotting too few points, so the curve has flat spots and does not meet the axes or directrix correctly.
Candidates underestimate how many points are needed and skip the construction of the tangent at each point, which is what makes the curve smooth.
What to do instead
Use the standard constructions: for the ellipse, the two focal circles / the affine method from the auxiliary circle, with at least 8–12 points per quadrant and the tangent at each point obtained from the focal property (the tangent bisects the angle between the focal radii). For the parabola, use the directrix-focus definition with points taken on the axis of the ordinate. Draw the curve with a French curve through the constructed points, never freehand.
In dihedral projection, forgetting to project the depth consistently - measuring a distance from the ground line in one view and from the wrong reference in the other - so the plan and elevation do not correspond and the third view cannot be built.
The two views are drawn at different times and the candidate switches reference lines; the error is small enough to pass unnoticed until the auxiliary view is requested.
What to do instead
Draw the ground line and the vertical reference first, and always transfer depths with the compass from the ground line, marking each transfer with a small tick. Before finishing, check three corresponding points (a vertex, an edge end, a centre) across both views and confirm the depths match exactly.
Dimensioning a drawing with redundant or contradictory dimensions, dimensions that do not start from a real reference edge, or dimensions placed inside the figure crossing its lines, and omitting the scale or the units.
Candidates treat dimensioning as labelling and copy the numbers they measured rather than the numbers that define the geometry; they also forget that every dimension must be readable from one direction and must not be repeated.
What to do instead
Dimension the defining magnitudes only: one dimension per feature, chained from a datum edge, never closing a chain with a redundant total. Place dimensions outside the outline, on parallel lines, with arrowheads touching the extension lines. Always state the scale (e.g. 1:2) in the title block and give units once. Do a final read-through: can every dimension be read without turning the sheet?
Treating the applied-art question as a free commentary on the artwork's meaning and style, without identifying the geometric structure (proportional grid, symmetry axes, perspective vanishing points, golden or modular ratios) that the question is actually asking about.
Candidates revise art history and forget that the subject is technical drawing applied to plastic arts; they write about iconography when the marks are for geometric analysis.
What to do instead
Structure the answer in three moves: (1) identify the representational system and the geometric armature (axes, grid, vanishing points, module); (2) measure or estimate the proportions and name the ratio or construction; (3) explain how that structure produces the composition's effect. Use the technical vocabulary (focal distance, horizon line, module, symmetry) explicitly.
Running out of time because the sheet is laid out badly: the figure is drawn too large or too small for the format, or placed off-centre, so it either overflows the border or leaves no room for the auxiliary views and the dimensioning.
Candidates start drawing at the first free spot without planning the layout, and only discover the space problem when the auxiliary view or the dimensions have to be added.
What to do instead
Before drawing, sketch the layout in the corner: bounding box of the main figure, positions of the auxiliary views, space for dimensions and title block, all inside the margin. Choose the scale from that box, not from the paper size. Keep a fixed margin (e.g. 10 mm) and align views on construction lines drawn first.