Cantabria PAU Technical Drawing II (Dibujo Técnico II) common mistakes
Cantabria PAU Technical Drawing II (Dibujo Técnico II) · Understand the traps and how to avoid them.
When constructing a tangent line to a circle from an external point, the candidate draws the perpendicular bisector of the segment joining the point to the centre but then places the tangent points at the intersections of that bisector with the circle instead of with the circle having the segment as diameter.
They memorise the steps as a sequence of lines without understanding Thales' theorem: the tangent point lies on the circle with diameter OP, not on the perpendicular bisector of OP. The bisector is only used to find the midpoint of OP.
What to do instead
Before drawing, label the midpoint M of OP, draw the circle with centre M through O and P (Thales circle), and mark the intersections with the given circle as the tangent points. Then join the external point to those points. Check by verifying that the radius to each tangent point is perpendicular to the tangent line.
In an ellipse construction by concentric circles or by the rectangle method, the candidate connects the points in the wrong order or uses the major and minor axes as if they were conjugate diameters, producing a curve that is not an ellipse.
They confuse the definition of axes (perpendicular, through the centre) with conjugate diameters (not necessarily perpendicular). The concentric-circle method requires projecting points from the auxiliary circles onto verticals and horizontals, and the order of points along each quadrant is easily reversed.
What to do instead
For the concentric method, divide the major auxiliary circle into equal parts from 0° to 90°, project each point vertically to the minor auxiliary circle, then draw horizontals from the minor circle and verticals from the major circle; their intersections give the ellipse. Always number the points sequentially and join with a French curve, checking symmetry about both axes.
When finding the intersection of a line with a plane in dihedral projection, the candidate takes the point where the projections of the line cross the traces of the plane as the intersection point.
They treat the 2D drawing as if it were the 3D space, forgetting that the intersection point must lie on the line and in the plane simultaneously. The apparent crossing of a projection with a trace is not generally the projection of the intersection.
What to do instead
Use an auxiliary projecting plane that contains the line, find its intersection line with the given plane, and then intersect that line with the original line. In dihedral, this means: draw the auxiliary plane (e.g., a vertical plane through the line), find its intersection with the given plane (a line), and where that line meets the given line in projection is the intersection point. Verify by checking that both projections of the point lie on the corresponding projections of the line and on the plane's traces.
When determining the true shape of a section of a solid by an oblique plane, the candidate uses the section's projection directly as the true shape, without applying a change of plane or a rotation.
They do not distinguish between the projection of the section (which is foreshortened) and its true magnitude. In dihedral, an oblique plane does not appear in true size in either principal view.
What to do instead
Obtain the true shape by one of two standard methods: (a) auxiliary view - project the section points onto a new plane parallel to the cutting plane, using distances from a reference line; or (b) rotation - rotate the cutting plane about one of its traces until it is parallel to a principal plane, carrying the section points with it. Always check that the true shape has the correct number of vertices and that its area is larger than any of its projections.
When dimensioning a mechanical part, the candidate places dimensions inside the hatched area of a section view, or uses the same dimension line for two different features, or omits the diameter symbol on circular features.
They focus on covering every feature quickly and ignore the standard rules of UNE/ISO dimensioning: dimensions should be outside the figure where possible, not crossing hatching, and each feature should be dimensioned once. The diameter symbol is often forgotten because the circle is obvious in the view.
What to do instead
Before drawing dimension lines, list the features to be dimensioned and assign each a single dimension. Place dimension lines outside the outline, use extension lines from the feature, and never let a dimension line coincide with a centre line or an outline. For circles, always prefix the numerical value with Ø; for radii, use R. Check that no dimension is repeated and that all non-redundant features are covered.
When drawing the orthographic views of an object, the candidate aligns the front and top views incorrectly (e.g., places the top view below the front view in first-angle projection, or mixes first- and third-angle conventions).
They memorise 'top view above' or 'top view below' without linking it to the projection system. In first-angle projection (common in Spain), the top view is placed below the front view; in third-angle, it is above. Many candidates also forget that the side view is placed on the opposite side of the front view in first-angle.
What to do instead
Before starting, write '1st angle' or '3rd angle' on the sheet and apply the rule: in first-angle, the view is placed on the side opposite to the observer; in third-angle, on the same side. Use a 45° mitre line to transfer depths between top and side views, and check that all views share the same height and width. Confirm by mentally folding the views into a box.
In isometric drawing, the candidate draws the three axes at 120° but then measures true lengths along the receding axes instead of applying the isometric scale (or uses the wrong scale factor).
They confuse isometric projection (where axes are foreshortened by about 0.816) with isometric drawing (where true lengths are used along the axes). The distinction is often glossed over in class, and the candidate applies the 0.816 factor to a drawing that is meant to be a true-length isometric.
What to do instead
Decide at the start whether the problem asks for isometric projection (use the isometric scale, factor ≈0.816) or isometric drawing (use true lengths along the axes). In both cases, draw the three axes at 120° to each other, with the vertical axis upright. For circles, use the four-centre approximate ellipse method on the appropriate face, and check that the major axis of each ellipse is perpendicular to the third axis.
When constructing a plane perpendicular to a given line through a point, the candidate draws a line perpendicular to the given line in one view only and declares it the plane.
They do not apply the theorem that a plane is perpendicular to a line if it contains two intersecting lines perpendicular to that line. In dihedral, a single perpendicular line in one view does not define a plane; the plane must be represented by its traces or by two lines.
What to do instead
To define a plane perpendicular to a line L through point P, draw two lines through P that are each perpendicular to L. In dihedral, use the fact that if a plane is perpendicular to a line, its traces are perpendicular to the projections of the line (for a plane perpendicular to a line, the horizontal trace is perpendicular to the horizontal projection of the line, and the vertical trace to the vertical projection). Construct the traces accordingly, then verify by checking that the line's direction vector is normal to the plane.
When applying a surface finish symbol, the candidate places the symbol on an extension line or uses the wrong symbol for the required machining process (e.g., uses the basic symbol for a machined surface).
They treat the surface finish symbol as a generic tick and do not recall the ISO 1302 conventions: the basic symbol (a tick) means any method, the symbol with a bar means material removal required, and the symbol with a circle means no material removal. The symbol must point to the surface, not to a dimension line.
What to do instead
Memorise the three basic symbols: tick only (method not specified), tick with a horizontal bar (machining required), tick with a circle (no machining). Place the symbol on the surface or on an extension line of the surface, with the point touching the surface. Add the roughness value (Ra) above the tick, and the machining method or tolerance grade if required. Check that the symbol is not placed on a hidden line or a centre line.
When solving a tangency problem involving two circles and a given radius, the candidate uses the external tangent construction for an internal tangency (or vice versa), producing tangency points on the wrong side.
They do not distinguish between external tangency (circles outside each other) and internal tangency (one circle inside the other). The construction differs: for external tangency, the auxiliary circle has radius R - r; for internal, R + r.
What to do instead
Before constructing, identify the type of tangency required. For a circle of radius R tangent externally to a circle of radius r, draw an auxiliary circle centred at the given circle's centre with radius R - r (or R + r for internal). Then find the intersection of that auxiliary circle with the locus of centres, and from there determine the tangency points by joining the centres. Always check that the resulting tangent circle touches the given circle at exactly one point and does not cross it.