Catalonia PAU Technical Drawing Applied to Plastic Arts and Design II common mistakes
Catalonia PAU Technical Drawing Applied to Plastic Arts and Design II · Understand the traps and how to avoid them.
In dihedral-system exercises, the student draws the second projection of a point or line using the distance measured on the paper of the first projection, instead of transferring the true distance from the reference line (LT) with the compass, so the two views are inconsistent and the depth is wrong.
Under exam time pressure the student treats the two projections as two independent drawings and eyeballs the distance, forgetting that in the dihedral system depth is conserved between the horizontal and profile planes and must be carried across with the compass or by parallelism to the folding line.
What to do instead
Before solving, mark the LT and label each projection with its plane (top, front, profile). For every point, apply the rule 'same depth, same height, same distance from LT' explicitly: transfer measurements with the compass from the LT, never from another point, and finish by checking that corresponding projections align on perpendiculars to the LT.
When drawing the development (unfolding) of a cylinder, cone or prism, the student unfolds only the lateral surface or only the bases, or places the bases attached to the wrong edge, producing a net that cannot be folded back into the solid.
The student memorises 'development = rectangle plus two circles' without reasoning about which edges are shared, and loses track of the seam line and the tangency points between lateral surface and bases.
What to do instead
Always draw the seam line first, then attach each base to the edge that actually corresponds to its perimeter, marking shared vertices with the same letter/number on the solid and on the net. Verify by mentally folding: every shared edge must appear exactly twice in the net.
In isometric or cavalier/cabinet axonometric drawings, the student applies the same scale on all three axes, or measures true lengths along the receding axis in isometric, so the object looks stretched and proportions do not match the multiview.
The student confuses the reduction coefficients of the different axonometric systems and does not distinguish isometric (equal reduction on all axes, drawn at 30°) from cavalier (true scale on the receding axis) and cabinet (half scale on the receding axis).
What to do instead
Write the coefficient rule at the top of the sheet before starting: isometric uses the same unit on the three axes at 30°/30°; cavalier uses full scale on the receding axis; cabinet uses half scale. Take all dimensions from the given multiview, never from the axonometric itself, and check the overall bounding box at the end.
In conic perspective, the student places the horizon line and the measuring points arbitrarily, or draws parallel receding lines that converge to different vanishing points, so the perspective is geometrically impossible and the object appears distorted.
The student works by visual intuition ('it looks deep') instead of applying the method: all lines of one direction must converge at a single vanishing point on the horizon, and heights must be scaled according to distance.
What to do instead
Fix the horizon and the ground line first, then locate each vanishing point by drawing the direction of the real edges from the station point. Use the measuring-point method for depths and check that all parallel edges of the same family meet at exactly one point; verify heights with the height line at the picture plane.
When dimensioning, the student writes dimensions inside the drawing over the object lines, omits the overall dimensions, duplicates dimensions, or uses arrows and witness lines that cross each other, making the drawing unreadable or ambiguous.
The student treats dimensioning as an afterthought added at the end and has not internalised the priority order (overall dimensions first, then partial) or the rule that dimensions go outside the contour on parallel dimension lines.
What to do instead
Dimension in a fixed order: overall length, width and height first, then features, then radii and diameters with their symbols (R, Ø). Place dimension lines outside the view, keep a consistent gap from the object, avoid crossing witness lines, and never repeat a dimension that can be deduced.
The student mixes line types and weights: uses a thick continuous line for hidden edges, a dashed line for axes, or draws all lines at the same thickness, so the multiview cannot be read hierarchically.
The student considers line type a cosmetic detail and does not connect each line type to its meaning (visible edge, hidden edge, axis of symmetry, cutting plane, auxiliary line).
What to do instead
Keep a small legend on the sheet: thick continuous for visible edges, thin dashed for hidden edges, thin chain line for axes and symmetry, thin continuous for dimension and auxiliary lines. Before handing in, check each line against the legend and erase construction lines that are no longer needed.
In plane-geometry constructions (tangencies, bisectors, regular polygons, arcs of circumference), the student solves by trial and error with the ruler, or leaves construction arcs and does not mark the exact tangency points, so the result is approximate and loses points.
The student knows the final shape but not the construction sequence, and skips the auxiliary steps because they seem like extra work under time pressure.
What to do instead
Memorise and write the sequence before drawing: for a tangent line to a circle from an external point, draw the auxiliary circle with diameter equal to the segment between the point and the centre, and mark the tangency point where it cuts the given circle. Always leave the construction arcs visible and label the tangency points with T.
When sketching or making a CAD drawing for a product, the student draws the object in an arbitrary orientation without choosing the principal view, so the multiview does not show the characteristic shape and requires unnecessary hidden lines.
The student starts drawing the first face that comes to mind instead of deciding which view best describes the object, a step that is critical both in freehand sketching and in CAD.
What to do instead
Before drawing, choose the front view as the one showing the most characteristic contour and the fewest hidden edges, then derive top and side views from it. In CAD, set the work plane and the coordinate origin at the base of the object and build from the principal view outward.
In colour and texture work, the student uses colour as decoration without a stated criterion (no defined palette, no value contrast plan, no relation to the product's function), so the proposal cannot be justified in the presentation.
The student separates the plastic-expressive part from the technical project and does not link colour decisions to legibility, hierarchy or material, which is exactly what the exam asks to justify.
What to do instead
Define a limited palette (three to five colours) with stated roles (dominant, secondary, accent), justify each choice by function and contrast, and show the value scale in grey to check legibility. Present the palette next to the product and explain the criterion in two or three lines.
In the project presentation, the student hands in loose sheets without a defined order, without a title block or scale indication, and without explaining the process, so the examiner cannot follow the solution from the statement to the result.
The student concentrates all effort on the final drawing and treats layout and memory as bureaucracy, running out of time to organise the deliverable.
What to do instead
Reserve the last minutes to assemble the deliverable in a fixed order: statement and analysis, sketches and process, final drawings with title block (name, scale, date, sheet number), and a short justification of decisions. Number the sheets and check that every requested item is present before handing in.