PAU Technical Drawing II (Aragón) common mistakes
PAU Technical Drawing II (Aragón) · Understand the traps and how to avoid them.
In a tangency problem (line/line or circle/line with a given radius), the candidate draws the bisector or parallel offset lines freehand 'by eye' and then places the tangent point wherever the compass happens to land, instead of constructing the tangency point as the foot of the perpendicular from the found centre to the line (or the point on the line joining the centres for circle-to-circle tangency). The resulting arc looks right but does not actually touch.
Textbook figures show the finished arc, so candidates copy the visual outcome and skip the two-step logic: (1) locus of centres (offset parallel or concentric arc), (2) perpendicular foot gives the tangency point. Under time pressure they collapse both steps into one drawing action.
What to do instead
Adopt a fixed written routine for every tangency: list the loci that generate the centre, intersect them, then always drop a perpendicular (line tangency) or draw the centre line (circle tangency) to mark the exact tangency point before striking the arc. Verify by checking that the radius segment meets the given element at exactly 90 degrees.
On an ellipse/hyperbola/parabola construction, the candidate marks points on the directrix or on the auxiliary circle but then joins them with a French curve that misses several of the constructed points, or draws the curve through the axis endpoints without respecting the symmetry of the conjugate diameters.
Candidates treat conics as 'sketch a smooth oval' rather than as a point-by-point locus, and they run out of time to plot the minimum number of points needed to control the curve.
What to do instead
Plot at least 8-12 symmetric points per branch using the defined construction (foci/directrix for parabola, foci and transverse axis for hyperbola, auxiliary circles for ellipse), mark them lightly, then fit the curve so it passes through every plotted point and is tangent to the vertex tangents; check symmetry about both axes before inking.
When obtaining the true shape of a section or a face, the candidate measures lengths directly on the projection (e.g., an edge seen foreshortened in the plan) instead of performing an auxiliary view or a rotation/change-of-plane, so the true-shape polygon comes out too small or distorted.
The plan and elevation look 'complete', and candidates forget that only lines parallel to a projection plane keep true length; they confuse a projected section outline with its true magnitude.
What to do instead
Before measuring anything, identify which edges are parallel to which projection plane. For any edge not parallel, construct a change of plane (auxiliary elevation perpendicular to the edge, or rotate the plane about a trace) and take all true dimensions from that view; label the new projection plane explicitly on the sheet.
When finding the intersection of a plane with a polyhedron or of two solids, the candidate draws the intersection polygon by joining visible corner points guessed from the drawing, omitting the points where edges cross the cutting plane behind the visible faces, so the section line is incomplete or self-crossing.
Candidates work from the picture rather than from the geometric rule, and they do not extend edges and cutting-plane traces far enough to find the hidden piercing points.
What to do instead
Use the systematic method: extend the cutting plane's trace to meet the base plane, find where each edge of the solid pierces the plane (using the trace and auxiliary lines), number those points, then join them in the correct order around each face. Check that the number of section vertices equals the number of faces actually cut.
In dimensioning, the candidate places dimensions inside the view, crosses dimension lines, omits the overall (total) dimensions, and mixes arrowhead styles or writes values without the correct number of decimals or without the diameter/radius symbol.
Candidates dimension as they draw, adding numbers wherever there is space, and they have not internalized the priority order (overall dimensions first, then position, then detail) or the symbol conventions.
What to do instead
Dimension in a deliberate order: overall length/width/height first, then feature positions, then sizes; keep dimension lines outside the outline where possible, never cross extension lines, use Ø and R consistently, and apply one decimal convention throughout. Do a final pass checking that no dimension is repeated and none is missing for manufacture.
The candidate uses the wrong scale factor direction on a reduced/enlarged drawing (e.g., divides instead of multiplies when reading a 1:50 plan) or applies a graphical scale incorrectly, producing measurements off by the scale factor.
Scale is treated as a label rather than an operation, and candidates do not distinguish 'drawing measurement x scale = real measurement' from the inverse when converting real dimensions to paper.
What to do instead
Write the scale as an equation before starting: real = drawing x denominator (for 1:n reductions). Build a graphical scale on the sheet and use it to check at least two dimensions; convert every given real dimension to paper length explicitly in a small table before drawing.
In isometric or other axonometric views, the candidate draws the axes at arbitrary angles, reduces only some of the three directions, and then draws ellipses for circles with their major axis horizontal instead of perpendicular to the axis of the circle's plane.
The isometric axes (30 degrees to the horizontal, vertical Z) and the 0.816/1.0 reduction are memorized vaguely, and the rule linking the ellipse major axis to the normal of the circle's plane is easily forgotten.
What to do instead
Set the axes with a 30-60 set square from a vertical reference, apply the chosen reduction uniformly (or use full-scale isometric drawing consistently), and for each circular face locate the ellipse's major axis along the projection of the circle's normal and its minor axis along the projected diameter; construct the ellipse from its conjugate diameters rather than guessing.
In a homothety/similarity/inversion construction the candidate picks the centre or the ratio arbitrarily, or in inversion draws the inverse of a point without using the power of inversion (OP x OP' = k squared), so the inverse figure is not actually similar or inverse to the original.
Transformations are learned as vocabulary rather than as constructions with a fixed invariant; candidates do not check the defining relation after drawing.
What to do instead
State the invariant before drawing: for homothety all lines through the centre are fixed and ratios of corresponding segments equal k; for inversion OP x OP' = k squared with O, P, P' collinear. After constructing, verify two or three corresponding points against the invariant numerically or by measurement.
In CAD, the candidate models or edits using absolute coordinates typed by hand and then cannot reproduce the drawing after a change, or plots without checking lineweights, scale, and layer/pen assignment, so the printed sheet has hairlines, wrong paper size, or missing title block.
Candidates treat CAD as digital drafting-by-eye, relying on snaps and manual coordinates rather than parametric constraints and object snaps, and they leave plotting to the last minute without a page-setup check.
What to do instead
Use object snaps and geometric constraints (coincident, tangent, perpendicular) so geometry stays correct when dimensions change; keep a disciplined layer scheme (outline, hidden, centre, dimensions, text). Before plotting, verify page setup: paper size, plot scale 1:1 or the required scale, monochrome/pen table, lineweight display on, and a preview showing the full title block.
In freehand sketching of industrial or architectural elements, the candidate starts with detail (a bolt thread, a window mullion) before establishing overall proportions and axes, so the sketch runs off the sheet or has wrong proportions, and construction/centre lines are omitted.
Sketches are treated as quick pictures rather than as proportion-first drawings, and candidates are reluctant to spend time on light construction lines they will later erase.
What to do instead
Block in overall bounding box and principal axes lightly first, subdivide by proportion (halving, thirds), then add detail; keep construction and centre lines visible until the sketch is confirmed, and check overall proportions by measuring the bounding box against the reference before committing to dark lines.