Extremadura PAU Technical Drawing Applied to Plastic Arts and Design II common mistakes
Extremadura PAU Technical Drawing Applied to Plastic Arts and Design II · Understand the traps and how to avoid them.
When solving a conic section problem (e.g., finding the focus of an ellipse given its major axis and a point), the candidate draws the major axis horizontally by default and then forces the given point to fit, often leading to an incorrect solution or an inconsistent construction.
The candidate assumes a standard orientation because textbook examples often show conics aligned with the coordinate axes, ignoring that the problem may require a rotated or translated conic. This leads to misapplying the definition of the conic (e.g., sum of distances to foci for an ellipse) and misplacing the foci.
What to do instead
Always analyze the given data first: if the major axis is given as a segment, use it as a reference but do not assume it is horizontal. Use the definition directly: for an ellipse, the sum of distances from any point on the curve to the two foci is constant and equal to the major axis length. Construct the foci by intersecting circles centered at the given point with radii that sum to the major axis length, or use the auxiliary circle method. Verify by checking that the constructed point satisfies the definition.
In a tangency problem (e.g., drawing a circle tangent to two lines and passing through a point), the candidate draws a trial circle that appears tangent but does not use the angle bisector or the locus of centers, resulting in a circle that is not exactly tangent.
The candidate relies on visual approximation rather than geometric construction. They may not recall that the center of a circle tangent to two intersecting lines lies on the angle bisector, and that the distance from the center to each line equals the radius. Without this, the construction lacks precision.
What to do instead
Systematically apply the locus method: for tangency to two lines, draw the angle bisectors; for tangency to a line and passing through a point, use the parabola definition (distance to point equals distance to line) or reflect the point across the line. For a circle tangent to two lines and through a point, find the center as the intersection of the angle bisector with the perpendicular bisector of the segment joining the point to its reflection across one line. Always verify by checking equal distances.
When determining the intersection of a line and a plane in descriptive geometry, the candidate draws the intersection point where the line crosses the plane's trace in one view, without projecting to the other view to confirm that the point lies on the plane and on the line.
The candidate confuses the intersection with the plane's trace (the line where the plane meets a projection plane) and forgets that the intersection point must satisfy both the line's and the plane's equations in 3D. They may also neglect to use an auxiliary plane to find the true intersection.
What to do instead
Use the auxiliary plane method: pass a projecting plane through the given line, find its intersection with the given plane (a line), then intersect that line with the original line. Always project the resulting point to both views and verify it lies on the plane's traces and on the line's projections. Practice with planes given by traces and by two intersecting lines.
In problems requiring the true shape of a planar section of a polyhedron, the candidate constructs the section in one view and then measures lengths directly from that view, assuming it is true shape, without applying a change of projection plane or a rotation.
The candidate does not distinguish between the projection of a section (which may be foreshortened) and its true shape. They may not recognize that the section plane is oblique to all principal planes, so its true shape requires an auxiliary view perpendicular to the section plane.
What to do instead
First determine the section polygon by finding the intersection of the cutting plane with each face. Then establish a new projection plane parallel to the cutting plane (using change of projection planes) and project the section vertices onto it. The resulting polygon is the true shape. Verify by checking that the new projection plane is perpendicular to the cutting plane and that distances from the folding line are preserved.
When dimensioning a technical drawing, the candidate places dimensions inside the figure, uses chain dimensioning from multiple references, and omits the overall dimensions, leading to cluttered and ambiguous drawings.
The candidate is not familiar with ISO/UNE dimensioning standards (e.g., UNE 1-039-94) and copies informal sketches. They may not understand that dimensions should be placed outside the views, aligned with the dimension lines, and that a single datum should be used for baseline dimensioning to avoid tolerance stack-up.
What to do instead
Follow standard rules: place dimensions outside the outline, use baseline dimensioning from a common datum where possible, include overall length, width, and height. Use arrowheads and extension lines correctly, avoid crossing dimension lines, and ensure text is readable from the bottom or right. Practice by redrawing given figures with proper dimensioning and comparing with standard examples.
In a descriptive geometry problem asking for the distance from a point to a plane, the candidate draws a perpendicular from the point to the plane in one view and measures the segment length in that view, without finding the true length.
The candidate forgets that the perpendicular segment is generally oblique to the projection planes, so its projection is foreshortened. They may not know that the true distance requires the perpendicular to be parallel to a projection plane, which can be achieved by a change of projection plane or by the right-triangle method.
What to do instead
To find the true distance: first draw the perpendicular from the point to the plane (using the theorem of three perpendiculars or by constructing a line perpendicular to the plane's traces). Then determine the true length of that perpendicular segment using the right-triangle method (difference in coordinates) or by a change of projection plane that makes the perpendicular parallel to the new plane. Verify by checking that the perpendicular is indeed perpendicular to two intersecting lines in the plane.
When developing the surface of a polyhedron (e.g., a truncated prism), the candidate unfolds the faces by measuring edge lengths directly from the projection, without correcting for foreshortening, resulting in a development that does not fold back into the correct shape.
The candidate assumes that the lengths in the projection are true lengths, but in orthographic projection, edges not parallel to the projection plane are foreshortened. They may not know that true lengths must be obtained from auxiliary views or by revolution.
What to do instead
For each edge of the polyhedron, determine its true length by constructing an auxiliary view where the edge is parallel to the projection plane (e.g., using change of projection planes or rotation). Then lay out the development face by face, using the true lengths and the true angles between edges (which may also require auxiliary views). Check by ensuring that the developed net closes correctly.
In a graphic design layout task, the candidate uses default software settings for margins, grids, and typography, resulting in a composition that lacks visual hierarchy and fails to communicate the intended message.
The candidate may not have training in design principles (balance, contrast, alignment, proximity) and relies on software templates. They may also ignore the brief's requirements for format, target audience, and reproduction constraints.
What to do instead
Before starting, define a clear visual hierarchy: establish a grid system (e.g., modular or column grid) and use it consistently. Choose type sizes and weights to create contrast, align elements to the grid, and group related items. Test the layout by squinting or viewing from a distance to check readability. Always refer back to the brief and consider the final output medium (print or screen).
When sketching a solution for a design problem, the candidate produces a detailed, shaded drawing that looks attractive but lacks accurate proportions and construction lines, making it impossible to derive precise measurements or verify geometric relationships.
The candidate confuses presentation drawing with technical sketching. They may focus on aesthetics rather than communication of geometry, and they may not use construction lines, axes, or proportion grids to ensure accuracy.
What to do instead
Adopt a disciplined sketching approach: start with light construction lines (axes, bounding boxes, proportion guidelines) using a hard pencil. Build the sketch from general to specific, checking proportions with a pencil or ruler. Keep the sketch clean and annotated with dimensions or notes as needed. Remember that in technical drawing, clarity and accuracy are more important than shading.
In an applied project, the candidate submits a set of drawings that are individually correct but inconsistent in scale, line thickness, or labeling, and lacks a cover sheet or index, making the documentation difficult to interpret.
The candidate treats each drawing as an isolated exercise and does not consider the project as a whole. They may not have been taught document control practices, such as using a consistent title block, layer naming, and sheet numbering.
What to do instead
Before starting the project, establish a documentation standard: define line weights for different element types (e.g., thick for outlines, thin for dimensions), use a consistent scale (e.g., 1:50 for plans, 1:20 for details), and create a title block with project name, date, scale, and sheet number. Assemble the drawings in a logical order with a cover sheet and index. Review all sheets together to ensure consistency.