Canary Islands PAU Technical Drawing Applied to Plastic Arts & Design II common mistakes
Canary Islands PAU Technical Drawing Applied to Plastic Arts & Design II · Understand the traps and how to avoid them.
When drawing the ellipse of a cone section, the candidate marks the vertices on the major axis but never checks that the sum of distances to the two foci is constant for intermediate points, so the curve is drawn as a freehand oval that does not close symmetrically.
In plane-geometry practice the ellipse is often reproduced from memory as a 'flattened circle'; the focal definition is memorised as a statement but not used as a construction check, so the candidate has no verification step and trusts the eye.
What to do instead
Before inking, construct at least four points by the focal method (arcs of radii r and 2a−r from each focus) and confirm they lie on the already-drawn curve; then draw the curve through the constructed points with a French curve, not freehand. Keep the focal construction visible in the answer as evidence.
For a tangent line to a conic from an external point, the candidate draws a line that merely touches the curve visually and does not construct the auxiliary circle or the polar, so the point of tangency is off by several millimetres and the line actually cuts the curve.
Tangent problems are treated as drawing exercises rather than constructions; the candidate skips the auxiliary-circle/thales-circle step because it looks like extra work, and there is no habit of marking the exact tangency point.
What to do instead
Always build the auxiliary circle of the conic centred on the major axis, draw the circle with diameter joining the external point to the relevant focus, intersect to get the tangency points, then join. Mark each tangency point with a small cross and label it T1, T2 so the construction is auditable.
In dihedral projection the candidate finds the intersection of a plane with a solid by locating only the points where the cutting plane meets the visible edges in one view, ignoring edges hidden in that view, so the section polygon is incomplete (a triangle where a pentagon is correct).
Attention is anchored on the view where the section is easiest to read; the candidate forgets that every edge of the solid must be tested against the plane, including those whose projection is hidden or coincides with another line.
What to do instead
Work edge by edge from a numbered list of the solid's edges, record the intersection of each with the plane in both views, then order the points around the section. Cross-check the number of section vertices against the number of edges actually cut before drawing the polygon.
When asked for the true shape of a section, the candidate measures lengths directly on the tilted projection instead of applying a change of plane or a rotation, so the resulting figure is foreshortened and its angles are wrong.
The candidate conflates the projected outline with the true magnitude; the rule that true length and true shape require the element to be parallel to a projection plane is known in words but not applied as a procedure.
What to do instead
Introduce an auxiliary projection plane parallel to the cutting plane (or rotate the section about a line of the section that is parallel to a projection plane), project all section points onto it, and only then measure. State in one line which change of plane was used and why it makes the plane parallel.
In the development of a truncated cylinder or cone, the candidate divides the base circle into equal parts but transfers the corresponding heights with dividers taken from the wrong view, producing a development whose seam does not close.
The heights exist at true length only in the view where the axis is parallel to the projection plane; the candidate measures them in the view where they are foreshortened, or uses a different number of divisions on the base and on the development.
What to do instead
Fix the number of divisions first (e.g. 12), number the generators on the solid, take each generator's true length from the view where the axis is parallel to the projection plane, and lay them out in the same numbered order. Verify closure by checking that the first and last ordinates are equal.
In axonometric projection the candidate draws the axes at arbitrary angles and applies a single foreshortening factor to all three, so a cube looks like a distorted box and circles in the coordinate planes become incorrect ellipses.
The candidate remembers 'isometric' as a look rather than as a set of axis angles and coefficients; the relationship between the axonometric axes, the coefficients and the ellipse axes of the inscribed circles is not recalled.
What to do instead
For isometric, use axes at 120° with equal coefficients and draw circles in each coordinate plane as ellipses with major axis 1.22d and minor axis 0.71d, major axis perpendicular to the third axis. For dimetric or trimetric, derive the coefficients from the chosen angles before drawing, and mark the axes and coefficients on the sheet.
In conical perspective the candidate places the measuring points and the horizon by eye, so parallel edges of the object converge to different vanishing points and the height of a repeated element changes arbitrarily along the depth.
Perspective is treated as an intuitive sketch; the candidate does not establish the station point, picture plane and horizon as a constructed framework before drawing the object, and does not use measuring points for depths.
What to do instead
Set up the framework first: horizon, ground line, station point at a stated distance, and vanishing points for both horizontal directions. Obtain depths only through the measuring points on the ground line, and check every set of parallel edges against its single vanishing point before adding detail.
On a dimensioned drawing the candidate omits the overall dimension, repeats the same dimension in two views, and places dimensions inside the hatched area of a section, so the drawing is ambiguous or unreadable.
Dimensions are added view by view as the drawing progresses rather than planned as a set; the conventions about not duplicating a dimension and about keeping dimensions clear of hatching are known but not applied systematically.
What to do instead
Dimension after the views are complete: first the overall length, width and height, then feature dimensions, each exactly once on the view that shows the feature most clearly. Keep dimensions outside hatching, use a consistent arrow and text height, and run a final check that no dimension is repeated and none is missing.
When drawing a section view the candidate hatches the whole cut area uniformly across two adjacent parts, and omits the cutting-plane line with its arrows and letters on the parent view, so the section cannot be related to the cut.
Hatching is seen as shading rather than as a code for material; the candidate also treats the cutting-plane indication as optional decoration and does not check the direction of the arrows against the resulting view.
What to do instead
Use a different hatch angle or spacing for each adjacent part, never hatch a rib or a shaft cut longitudinally, and always draw the cutting-plane line with thick ends, arrows showing the viewing direction and the same capital letters used to title the section. Verify the arrows point toward the side actually shown.
In a layout or signage composition the candidate sets the type at a size chosen by eye, ignores the baseline grid and the modular margin, and lets the optical centre sit at the geometric centre, so the page looks unbalanced and the hierarchy is unclear.
The candidate designs on screen at an arbitrary zoom and judges by feel; the relationship between type size, line length, leading and the modular grid is not used as a decision tool, and optical centring is not distinguished from geometric centring.
What to do instead
Define the format, margin and baseline grid before placing any element; set type sizes from a modular scale, keep line length within a stated measure, and align all text to the baseline grid. Place key elements on the optical centre (slightly above the geometric centre) and check the hierarchy by squinting at the layout so only the intended first, second and third levels remain visible.
In the design project the candidate presents only a final rendered view, with no dimensioned sketches, no orthographic views and no specification of materials or dimensions, so the object cannot be built from the documentation.
The project is treated as an illustration task; the candidate works directly in a rendering tool and never produces the ideation sketches and technical drawings that the brief requires as evidence of process.
What to do instead
Structure the deliverable as a documented process: ideation sketches with annotations, a chosen solution with overall and feature dimensions, at least front, plan and side views at a stated scale, an exploded or section view where relevant, and a short specification of materials and finishes. Check that a reader could build the object from the drawings alone.