PAU Technical Drawing Applied to Plastic Arts and Design II common mistakes
PAU Technical Drawing Applied to Plastic Arts and Design II · Understand the traps and how to avoid them.
When a problem asks for the true shape of a plane defined by three points, the candidate constructs the auxiliary view directly from the plan without first establishing the change of plane perpendicular to a line contained in the plane, so the resulting figure is a distorted projection rather than the true magnitude.
The candidate memorized 'auxiliary view gives true shape' without internalizing the two-step logic (edge-on first, then true size), and under time pressure collapses the sequence into one projection.
What to do instead
Before drawing anything, identify a line in the plane whose true length is known or constructible; project the plane edge-on onto a plane perpendicular to that line, then take a second change of plane parallel to the edge-on trace. Practise five three-point plane problems checking each intermediate edge-on view for collinearity of the three points.
In tangent constructions (line tangent to a circle from an external point, or common tangents between two circles), the candidate guesses the tangent point by eye instead of constructing it via the Thales circle on the segment joining the external point to the centre.
The candidate treats tangency as a visual property rather than a locus condition (the tangent point lies on the circle with diameter equal to the distance from centre to external point), so freehand placement feels adequate.
What to do instead
Always construct the auxiliary circle of diameter OP (or O1O2 for common tangents) and intersect it with the given circle; mark tangent points with a small tick and verify the radius to that point is perpendicular to the tangent line using a set square before inking.
When drawing the multiview representation of a solid with inclined faces, the candidate aligns the top view and side view correctly but omits the 45-degree mitre line or misplaces it, so depths in the side view do not correspond to depths in the top view.
The candidate transfers dimensions by counting squares or by eye across the sheet rather than through a formal projection mechanism, and does not understand that the mitre line is the geometric guarantee of depth correspondence.
What to do instead
Establish the mitre line at 45 degrees at the intersection of the top-view and side-view ground lines before drawing any feature; transfer every depth by horizontal projection to the mitre line and then vertical projection into the side view. Check three arbitrary points per view for consistent depth.
The candidate draws hidden edges as solid lines, or omits them entirely on the grounds that they are 'not visible', producing views that are ambiguous or that contradict the visible outline.
Line convention is treated as cosmetic rather than semantic; the candidate does not realize that a missing hidden line changes the reading of the solid's topology.
What to do instead
Adopt a rule: every edge of the solid must appear in every view where it projects as a line, either solid (visible) or dashed (hidden). After finishing a view, count edges in the isometric sketch and reconcile the count against solid plus dashed lines in each orthographic view.
In isometric drawing, the candidate measures true lengths along the 30-degree axes but also measures true lengths along non-isometric directions, or uses an isometric scale inconsistently, so the drawing is neither true isometric nor a correct isometric projection.
The candidate conflates isometric drawing (full scale along axes) with isometric projection (0.816 reduction), and applies foreshortening selectively rather than uniformly.
What to do instead
Decide at the start whether the task requires isometric drawing or isometric projection and state it on the sheet. For drawing, use full scale only along the three axes and construct all other lengths by coordinate offsets parallel to the axes, never by direct measurement along inclined edges.
When constructing a two-point perspective of a box or interior, the candidate places both vanishing points without relating them to the station point, so the horizon, station point, and vanishing points form an inconsistent set and the resulting perspective looks distorted or the measuring points fall off the sheet.
The candidate draws perspective as a set of converging lines rather than as a projection from a station point onto a picture plane, and does not check that the angle at the station point between the lines to the two vanishing points is 90 degrees for mutually perpendicular directions.
What to do instead
Fix the picture plane, horizon, and station point first; construct each vanishing point by drawing through the station point a line parallel to the corresponding real-world direction and dropping it perpendicular to the horizon. Verify the 90-degree condition at the station point before drawing any object edge.
On a dimensioned drawing, the candidate dimensions the same feature twice (once in each view), places dimensions inside the outline where they cross other lines, or omits the overall dimensions while giving only chain dimensions.
The candidate thinks of dimensioning as labelling rather than as a unique, non-redundant specification, and works view by view instead of planning the dimension scheme for the whole part.
What to do instead
Plan dimensions before drawing: assign each feature to exactly one view, reserve overall length, width, and height for the view that shows them most clearly, and keep dimensions outside the outline with extension lines that do not touch the object. Do a final audit crossing out any duplicated value.
The candidate applies line-thickness conventions uniformly, drawing visible outlines, hidden lines, centre lines, and dimension lines all at the same weight, so the drawing has no visual hierarchy.
The candidate treats line weight as an aesthetic choice rather than a coded convention that tells the reader which lines carry geometry and which are auxiliary.
What to do instead
Set three weights on the sheet: thick for visible outlines, medium for hidden lines and dimension/extension lines, thin for centre lines, axes, and hatching. Use a 0.5/0.25/0.13 mm or equivalent hierarchy and check the drawing at arm's length to confirm the hierarchy reads.
When asked to analyse the proportion or compositional structure of a given form, the candidate describes what the object is and lists its parts instead of identifying the underlying geometric scheme (regulating lines, root rectangles, golden-section divisions, symmetry axes, or modular grids).
The candidate confuses description with analysis and lacks a vocabulary of proportional systems, so the answer stays at the level of appearance rather than structure.
What to do instead
Overlay candidate regulating lines on the form: diagonals of the bounding rectangle, halves and thirds, root-2 and root-3 constructions, and the golden section. State which scheme the significant edges actually align with, give measured ratios as evidence, and only then discuss effect.
In a design-project deliverable, the candidate produces technically correct orthographic views but presents them at inconsistent scales across the sheet, or mixes scales within a single view, so the documentation cannot be read as a coherent set.
The candidate treats each view as an isolated drawing task and does not plan the sheet layout, scale selection, and title-block information as a single communication problem.
What to do instead
Choose one principal scale from the standard series before layout, derive all views from it, and reserve any enlarged detail for a clearly labelled separate scale. Lay out the sheet with a grid plan, place the title block and scale note first, and verify by measuring a known dimension in every view against the stated scale.