PAU Technical Drawing II (Castilla y León) common mistakes
PAU Technical Drawing II (Castilla y León) · Understand the traps and how to avoid them.
When constructing a triangle from three given sides or from two sides and an angle, the candidate draws both possible solutions without discussion, or draws only one and never checks whether the data admit 0, 1 or 2 triangles (ambiguous case with given angle not between the given sides).
Constructions are practised as a single recipe rather than as a case analysis; the candidate does not test the triangle inequality or compare the given angle with the altitude, so the existence and number of solutions are never examined.
What to do instead
Before drawing, run the existence check explicitly: for three sides verify a+b>c for all pairs; for two sides and a non-included angle compare the given angle with the right angle and the opposite side with the altitude. Write the number of solutions in the answer and justify it in one line, then draw each valid solution with its arcs left visible.
On tangent problems (line tangent to a circumference, common tangents to two circles, tangent circles of given radius), the candidate guesses the centre position by eye and draws the tangency point without locating it as the foot of the perpendicular from the centre.
The candidate remembers the final picture of textbook tangencies but not the locus reasoning behind them (centre lies on a parallel line at distance r, or on a circle/hyperbola of centres), so the construction collapses into approximate drawing.
What to do instead
Always reduce a tangency to a locus intersection: centre at distance r from the line, or at distance r1±r2 from the other centre. Mark the tangency point as the intersection of the centre line with the circumference and verify perpendicularity with a set square. Practise the four standard cases (external, internal, line-circle, circle-circle) until the locus step is automatic.
When asked for the tangent to an ellipse, parabola or hyperbola at a point or from an external point, the candidate uses the focal property incorrectly, e.g. treats the tangent as the bisector of the angle between the focal radii instead of the bisector of the angle between one focal radius and the extension of the other.
The three conics have similar-sounding focal tangent properties and the candidate memorises them as one blurred rule; the parabola case (bisector of the angle between the focal radius and the line parallel to the axis) is confused with the ellipse/hyperbola case.
What to do instead
Keep a one-line card per conic: ellipse - tangent bisects the exterior angle of the focal radii; hyperbola - tangent bisects the angle between the focal radii; parabola - tangent bisects the angle between the focal radius and the parallel to the axis through the point. Construct each once from scratch and check the result by the perpendicular-from-focus property (the foot of the perpendicular from the focus to the tangent lies on the tangent at the vertex).
In dihedral-system problems the candidate determines the intersection point of a line with a plane, or the intersection line of two planes, only in one projection and leaves the other projection undetermined or drawn through an arbitrary point.
The candidate works in a single view because it looks complete there, forgetting that a point in space needs both projections and that the second projection must be obtained by projecting onto the other plane, not by eye.
What to do instead
Use the auxiliary-plane method systematically: take a projecting plane containing the line, find its intersection with the given plane, and mark where that intersection meets the line; then carry the point to the other view with the reference line perpendicular to the fold. For two planes, find two common points, each fully determined in both views, and join their projections.
When a solid is cut by an oblique plane, the candidate draws the section as a polygon whose vertices are guessed on the visible edges only, missing vertices on hidden edges and on the back face, so the section is incomplete.
The candidate reads the section directly off the drawing instead of intersecting the cutting plane with every face; hidden edges are mentally suppressed, and the auxiliary view that would reveal the true section is skipped.
What to do instead
Number every face of the solid, intersect the cutting plane with each one in turn, and label each section vertex in both views before joining them. Then obtain the true shape by a change of plane parallel to the cutting plane (or by rotation), and check that the number of section sides equals the number of faces actually crossed.
In isometric and cavalier/cabinet drawings the candidate applies the same scale along all three axes, and in cabinet projection uses a reduction factor other than one half for the receding axis; ellipses in isometric are drawn as true circles or as arbitrary ovals rather than as the correct isometric ellipse with its major axis perpendicular to the receding axis.
The candidate transfers the orthographic outline directly and does not distinguish between true lengths, axonometric scales and the conventional coefficients of the particular system.
What to do instead
State the coefficients before drawing: isometric uses equal reduction on all axes (or full scale with the drawing enlarged), cavalier uses full scale on the receding axis, cabinet uses one half. For circles, construct the isometric ellipse from the circumscribed square (four-centre method) and keep the major axis perpendicular to the axis normal to the circle's plane; check the ratio of major to minor axis (about 1.22 in true isometric).
Dimensioning errors: dimensions repeated in two views, a dimension chain closed on the same element (over-dimensioning), radii and diameters confused, and dimensions placed inside the hatched area of a section or on a hidden line.
The candidate dimensions what is easy to reach on the sheet rather than following the rule that each dimension appears once and that functional dimensions are given from datums; the difference between R and Ø and the ban on dimensioning hidden edges are not applied.
What to do instead
Before dimensioning, list the features and assign each exactly one dimension, choosing datums for functional sizes and chains only for non-critical ones. Check the drawing for closed chains and duplicate sizes. Use Ø before the value for full circles, R for arcs, and move any dimension off hatched or hidden geometry to a visible edge or an outside extension line.
Thread and fastener representation: the candidate draws the minor diameter as a continuous thick line in the end view, omits the 120° chamfer lines, does not leave the thread run-out undefined, and shows a bolt head with three visible faces in a view where only two should appear.
The conventional simplified representation of threads (thick line for crest, thin line for root, thin line at 3/4 circle in the end view) is memorised loosely, and hexagonal heads are drawn from a generic picture rather than from the standard proportions (across-flats versus across-corners).
What to do instead
Learn the standard convention pair by pair: crest thick, root thin, root arc open over one quarter in the end view; thread line stops at the run-out. For hex heads, derive the view from the across-flats distance and remember that in the view along the axis only three faces are seen while the side view shows two. Redraw one M10 bolt, nut and washer to the standard proportions until the conventions are exact.
Tolerances and surface finish: the candidate writes tolerance values without the letter symbol or writes the symbol without the grade, places the surface-finish symbol on a hidden line or on a non-machined face, and omits the machining allowance or the general tolerance note.
ISO limits-and-fits notation (e.g. 40 H7/g6) is treated as decoration rather than as a paired shaft/hole specification, and the surface texture symbol is placed where it is visually convenient instead of on the surface concerned.
What to do instead
Write fits as nominal size plus hole tolerance plus shaft tolerance, and be able to look up or state the meaning of the letter (position of the tolerance zone) and the grade (width). Place the surface-finish symbol on the visible outline or extension line of the machined surface, add the general tolerance note in the title block area, and check that every toleranced feature has both a size and a finish specified.
In freehand sketching and CAD-based representation tasks the candidate produces a sketch with no construction lines and wrong proportions, or a CAD drawing with layers, line weights and scale not set, so the printed result has uniform line thickness and a title block that does not match the scale of the geometry.
Sketching is treated as a quick doodle rather than as a scaled freehand construction, and CAD is used as a digital pencil without setting up the drawing environment (layers, line types, dimension style, scale factor).
What to do instead
For sketches, block in the bounding box and centre lines first, keep proportions by comparing lengths against a chosen unit, and darken only the final outlines. For CAD, set layers for outlines, hidden, centre and dimensions with the correct line types and weights, set the dimension style to the drawing scale, and verify the title block scale against a measured dimension before submitting.