PAU Technical Drawing II (Andalusia) common mistakes
PAU Technical Drawing II (Andalusia) · Understand the traps and how to avoid them.
When constructing a triangle from given side lengths, the candidate draws arcs with the compass set to the wrong radius (e.g., using the difference of two sides instead of one side), so the intersection does not match the required triangle.
The candidate confuses the radii for the arcs: to locate the third vertex, the compass must be opened to the length of each side from the respective endpoints. A common error is to use the sum or difference of sides, or to forget to reset the compass after drawing a previous arc.
What to do instead
Before drawing, label the given sides a, b, c. Place segment a as the base. From one endpoint, draw an arc of radius b; from the other endpoint, draw an arc of radius c. The intersection gives the third vertex. Verify by measuring all three sides with the ruler after construction.
In dihedral projection, the candidate draws the traces of a plane without checking that the horizontal and vertical traces meet at a single point on the ground line, or draws them parallel when they should intersect, leading to an inconsistent plane.
The candidate forgets that the traces of a plane (except when the plane is parallel to a projection plane) must intersect on the ground line. This happens because the candidate treats each trace independently and does not apply the condition that the ground line is the intersection of the two projection planes.
What to do instead
Always locate the point where the horizontal and vertical traces cross the ground line; they must coincide. If one trace is parallel to the ground line, the other must also be parallel (plane parallel to the ground line) or the plane is a projecting plane. After drawing, extend both traces to the ground line and verify they meet at the same point.
When determining the intersection of a line and a plane in dihedral system, the candidate uses an auxiliary plane that is not perpendicular to a projection plane, or fails to find the correct piercing point, resulting in an incorrect intersection point.
The candidate does not systematically choose an auxiliary plane that contains the line and is perpendicular to one of the projection planes (e.g., a vertical projecting plane). Without this, the intersection of the auxiliary plane with the given plane is not easily found, and the candidate guesses.
What to do instead
Use the standard method: draw an auxiliary plane that contains the line and is perpendicular to the horizontal (or vertical) projection plane. Find the intersection line of this auxiliary plane with the given plane (using traces). The point where this intersection line meets the given line is the piercing point. Check that the point lies on both projections of the line.
In tangent constructions to conics (e.g., tangent from an external point to an ellipse), the candidate uses the directrix or focal property incorrectly, often drawing a circle with radius equal to the major axis but centering at the wrong point, so the tangent line does not touch the ellipse.
The candidate memorizes a procedure without understanding the underlying property: for an ellipse, the tangent from an external point can be found by drawing a circle centered at the external point with radius equal to the distance to one focus, then intersecting with the circle centered at the other focus with radius equal to the major axis. Mixing up which focus gets which radius leads to failure.
What to do instead
For an ellipse with foci F1 and F2 and major axis 2a, to draw tangents from point P: draw a circle centered at P with radius PF1. Draw a circle centered at F2 with radius 2a. Their intersections Q give the points where the tangent lines from P touch the ellipse: the tangent is the perpendicular bisector of F1Q. Practice with a specific numerical example and verify by measuring that the point of tangency lies on the ellipse.
When drawing the development of a polyhedron (e.g., a regular tetrahedron or a prism), the candidate lays out the faces without ensuring that adjacent faces share the correct edge lengths and angles, so the net does not fold into the solid.
The candidate draws each face independently using given dimensions but does not check that the shared edges have identical lengths and that the angles at the vertices are consistent. This often happens with oblique or truncated solids where the candidate assumes all edges are equal.
What to do instead
Before drawing the net, calculate or measure the true lengths of all edges and the true angles between faces. Construct the net face by face, using the shared edge as a hinge and transferring the exact length. After drawing, mentally fold or use a paper model to verify that the net closes. For truncated solids, include the true shape of the cut face.
In axonometric perspective, the candidate applies the reduction coefficients incorrectly, using the same scale for all three axes when the projection is isometric, dimetric, or trimetric, or forgetting to apply the isometric scale to the axes.
The candidate does not distinguish between the real lengths and the axonometric lengths. In isometric projection, all three axes are equally foreshortened by a factor of about 0.816, but in isometric drawing (the common practice) the axes are drawn at full scale, which is a different convention. Confusion between these leads to distorted drawings.
What to do instead
Clarify the convention: in isometric projection, use the isometric scale (multiply true lengths by 0.816) along the axes; in isometric drawing, use full lengths along the axes but draw the axes at 120° to each other. For dimetric or trimetric, use the specific coefficients given. Always state the convention used and check by comparing the drawing with the true dimensions.
When dimensioning a technical drawing, the candidate places dimensions inside the figure, uses the same dimension line for multiple features, or omits necessary dimensions, making the drawing ambiguous or non-standard.
The candidate focuses on the geometry and overlooks the rules of standardized dimensioning (e.g., UNE or ISO standards). Common errors include dimensioning to hidden lines, using chain dimensioning without a datum, or forgetting to indicate the diameter symbol for circles.
What to do instead
Follow the basic rules: place dimensions outside the figure where possible, use a single dimension line per feature, avoid duplicating dimensions, and use symbols (Ø for diameter, R for radius, □ for square). Dimension from a common datum to avoid tolerance stack-up. Check that every feature has exactly one dimension and that no dimension is missing or redundant.
In similarity and scale problems, the candidate sets up the proportion incorrectly, for example by placing corresponding sides in different orders, or by using the scale factor inversely when converting from drawing to reality.
The candidate does not clearly identify corresponding parts or confuses the direction of the scale (e.g., 1:50 means 1 unit on paper represents 50 units in reality). This leads to a wrong ratio and thus an incorrect result.
What to do instead
Always write the proportion with corresponding quantities in the same order: (drawing length)/(real length) = scale factor. For a scale 1:50, if the drawing length is 4 cm, the real length is 4 cm × 50 = 200 cm. Use a table to list drawing and real values side by side and verify that all ratios are equal. For similar figures, explicitly mark corresponding vertices and sides before writing ratios.
When constructing a regular polygon (e.g., pentagon or hexagon) given the circumscribed circle, the candidate uses a general method that is approximate for some polygons, or misplaces the compass steps, resulting in unequal sides or vertices not on the circle.
The candidate relies on a single general method (such as the general method for any polygon) without realizing that it is only exact for certain polygons or requires precise division. For example, using the approximate method for a pentagon when an exact construction exists. Also, cumulative compass errors occur if the candidate does not verify the last step.
What to do instead
Use the exact specific construction for each polygon: for a hexagon, step the radius around the circle; for a pentagon, use the golden ratio construction (or the standard method with perpendicular bisectors). After construction, verify that all sides are equal by measuring, and that all vertices lie on the circle. For general polygons, use the exact division of the circle into n parts via the method of central angles (360°/n) with a protractor if allowed, or the exact compass construction for that n.
In CAD fundamentals, the candidate draws without setting up layers, units, and object snaps, leading to inaccurate geometry (e.g., lines not meeting exactly) and non-standard output.
The candidate treats CAD like manual drawing and ignores the setup phase. Without proper units and snaps, points are placed approximately, and without layers, line weights and types are incorrect, affecting the final plot.
What to do instead
Before drawing, set the units (e.g., millimeters), create layers for different line types (continuous, hidden, center) with appropriate colors and line weights, and enable object snaps (endpoint, midpoint, intersection, tangent). Use precise coordinate input and modify commands (trim, extend, fillet) rather than drawing approximate lines. After drawing, check with the distance and angle tools that geometry is exact.