La Rioja PAU Technical Drawing Applied to Plastic Arts and Design II common mistakes
La Rioja PAU Technical Drawing Applied to Plastic Arts and Design II · Understand the traps and how to avoid them.
When reducing a figure by a rational scale factor, the student measures each segment with the ruler and multiplies by the decimal approximation of the fraction (e.g. 0.667 for 2/3), accumulating rounding drift so the reduced figure no longer closes on its last vertex.
They were taught similarity through numeric ratios rather than through the geometric construction of the fourth proportional or the homothety centre, so they trust the calculator instead of the compass. Decimal rounding of a non-terminating fraction is invisible until the polygon is closed.
What to do instead
Construct the reduction graphically: place the scale ratio on a ray from a vertex (Thales/fourth-proportional construction) and transfer every segment with the compass from that single graduated ray, never from a numeric product. Verify by checking that the closing side equals the same constructed ratio. If a numeric factor is unavoidable, keep the fraction in the compass opening rather than converting it.
On a standard sheet, the student draws the views of a solid without first laying out the axes of symmetry and the alignment lines between front view, plan and side view, so the plan ends up shifted horizontally relative to the elevation and the side view is not at the same height.
They treat multiview drawing as three independent sketches rather than as one projection system, and they start with the object instead of with the sheet layout. Without construction lines there is no reference to transfer heights and depths.
What to do instead
Before any object line, draw the horizontal ground line, the vertical fold lines, and 45° transfer line (or compass arcs) that carry depth from plan to side view. Project every feature with faint construction lines from view to view, and only then darken the outlines. Check alignment by running a straightedge along corresponding edges across all three views.
When dimensioning, the student places the dimension figure inside a narrow gap, lets extension lines cross the object's visible edges, and omits the overall dimensions, so the drawing is either ambiguous or over-dimensioned.
They dimension as an afterthought once the drawing is finished, choosing whatever space is left, and they do not distinguish functional dimensions from redundant ones. They also forget the standard rule that dimensions are placed above a horizontal dimension line and read from the bottom or right.
What to do instead
Plan dimensions before drawing: identify the overall length, width and height, then the functional sizes, then the location dimensions, and never repeat a dimension that can be deduced. Keep dimension lines outside the outline where possible, leave a clear gap between the object and the start of the extension line, and keep figures legible and unbroken by any line.
In isometric or perspective drawing, the student reduces all three axes by the same amount when drawing an isometric view, or draws the isometric axes at 45° instead of 30°, producing a distorted or non-standard axonometric.
They confuse the true isometric projection (where the axes are equally inclined to the picture plane and the natural foreshortening is 0.816) with the conventional isometric drawing (drawn full scale on 30° axes). Mixing the two produces inconsistent proportions.
What to do instead
State explicitly which convention is being used. For a standard isometric drawing, draw the three axes at 30° to the horizontal and measure all three at full scale; do not apply the 0.816 factor. For a true isometric projection, apply the foreshortening consistently to all three axes. Keep the vertical axis truly vertical.
When solving a tangency problem, the student finds the centre by eye or by trial and error with the compass, and the resulting arc touches the line or circle at an approximate point rather than a true point of tangency.
They have not internalised the locus theorems: the centre of a circle tangent to a line lies on a parallel at distance r, and tangent to a circle lies on a concentric circle of radius R±r. Without the locus, they guess.
What to do instead
Always reduce tangency to a locus intersection: offset the given line by the radius, offset the given circle by R+r (external) or R−r (internal), and intersect the two loci to fix the centre. Then drop the perpendicular to the line, or join the centres, to locate the exact tangency point before drawing the arc.
The student draws a perspective with a single vanishing point for all receding edges, even when the object is rotated relative to the picture plane, so two faces that should converge to two different points collapse to one.
They memorise 'one-point perspective' as the default and do not check the orientation of the object to the picture plane. They also place the horizon and station point arbitrarily without relating them to the observer's eye height and viewing distance.
What to do instead
First decide the object's orientation to the picture plane: if one face is parallel, use one point; if an edge faces the viewer, use two points. Set the horizon at the observer's eye height and the station point at a distance giving a comfortable cone of vision (roughly 60°). Construct each family of parallel edges to its own vanishing point, and check that verticals stay vertical in two-point perspective.
When asked to analyse a composition, the student lists the elements present ('there is a red circle, a diagonal, some texture') without explaining how contrast, balance, rhythm or focal emphasis are produced, and without linking colour choices to a stated colour system.
They confuse description with analysis and have not practised the vocabulary of composition and colour (value contrast, complementary or analogous relationships, weight distribution, directional lines). Under time pressure they default to inventory.
What to do instead
Structure the answer as claim, evidence, effect: name the compositional device, point to the specific area of the image that demonstrates it, and state what it achieves perceptually. For colour, identify the relationship explicitly (complementary, analogous, triadic) and the role of value and saturation in establishing hierarchy. Practise on past images with a fixed three-part template.
The student converts a scale incorrectly, for example treating 1:50 as 'divide by 50 and add centimetres', or mixing units so a 1:50 drawing of a 2 m object is drawn 4 cm instead of 4 cm... and then labels it 40 mm, producing a factor-of-ten error.
Scale is taught as a formula to memorise rather than as a ratio of like units, and students do not consistently convert to the same unit before dividing. Millimetres, centimetres and metres are mixed within one calculation.
What to do instead
Write the ratio as a fraction with both terms in the same unit before calculating: 2 m at 1:50 = 2000 mm / 50 = 40 mm. State the unit at every step and convert at the start, not the end. Sanity-check the result against the sheet size and the real object: a door drawn 4 mm wide is obviously wrong.
In the plan or side view, the student draws hidden edges as solid lines, or omits them entirely, so a hole or internal step visible only in one view is not represented.
They equate 'not visible from the front' with 'not part of the object', and they have not learned the convention that hidden outlines use dashed lines of consistent dash length.
What to do instead
For each view, systematically list every edge of the solid and decide whether it is visible from that direction; if not, draw it dashed. Keep dash length and gap uniform (roughly 3:1) and make dashes meet at corners. As a check, confirm that every feature appears in at least two views unless it is a simple through-hole normal to one face.
The student leaves construction lines, compass arcs and smudged graphite on the final sheet, or draws freehand where the exercise requires a ruler and compass construction, so the answer cannot be verified and marks are lost for presentation.
They work in one pass with a single pencil hardness, do not plan the layout, and treat construction marks as clutter rather than as evidence of method.
What to do instead
Use a 2H for construction and a HB for final outlines, keep the sheet clean with a soft eraser, and retain faint construction lines where the method is part of the answer while erasing irrelevant trial arcs. Rule every straight line and use the compass for every arc. Reserve a margin for the title block and check the sheet before submission.