MATSEC SEC Graphical Communication common mistakes
MATSEC SEC Graphical Communication · Understand the traps and how to avoid them.
Internal bores, slots and back edges are drawn with the same thick continuous line as visible edges, or left out of a view altogether because the candidate only draws what is obvious from the given pictorial.
The candidate treats each view as an independent picture of the object rather than a projection of the same solid from a different direction, so features hidden in that direction are never reasoned about.
What to do instead
Before starting, list every feature of the solid and, for each of the three viewing directions, decide visible or hidden. Adopt a fixed line hierarchy on every sheet: thick continuous for visible edges (about 0.7 mm), thin short-dashed for hidden edges (dashes about 3 mm with 1 mm gaps), thin chain-dotted for centre lines. A hidden line must break wherever it coincides with a visible line.
The plan and end elevation do not line up with the front elevation: widths differ by a few millimetres, the side view is placed by eye, and centre lines and the projection symbol are missing or mirrored.
Views are drawn one after another from the pictorial instead of being projected from a common reference, and first-angle layout has been memorised as 'plan underneath' without being linked to the truncated-cone symbol.
What to do instead
Set out the three view boxes and a 45-degree mitre line first, then project every point across with construction lines that stay on the sheet. In first angle the object sits between observer and plane, so the plan goes below the elevation; in third angle the plan goes above. Draw the truncated-cone symbol before any view so the chosen convention is fixed on the sheet.
The candidate draws receding edges at 45 degrees for an isometric, or draws at 30 degrees but then measures depth at half scale as if it were a cabinet oblique.
Isometric, cavalier and cabinet oblique are conflated into one 'slanted drawing' idea, and the 0.816 foreshortening factor of true isometric projection is applied (or misapplied) to an isometric drawing.
What to do instead
Fix the rules: isometric has both receding axes at 30 degrees to the horizontal with all three axes at full true length, because isometric drawing is not foreshortened. Cabinet oblique uses 45 degrees with depth at half scale; cavalier oblique uses 45 degrees with full depth. Write the chosen convention in the title block and check every measurement is taken along an axis, never across one.
Circles in isometric are drawn with a compass as true circles or as freehand ovals, and diagonals on a face are measured directly along the slanted line rather than being located from the axes.
The candidate does not accept that a circle projects as an ellipse and that only lines parallel to the three axes are measurable in a pictorial view.
What to do instead
For each circle, draw the enclosing isometric square, locate its midpoints and use the four-centre construction; the major axis of the ellipse lies perpendicular to the axis of the cylinder and measures 1.22 times the true diameter, the minor axis 0.71 times. For any non-isometric line, mark its two endpoints by axis measurements and join them; never scale along the line itself.
All construction arcs are rubbed out, or are left so heavy that they are indistinguishable from the required answer, so the bisectors, perpendiculars and tangent points that earn the method marks cannot be seen.
The candidate believes a clean drawing scores better and treats construction as private working rather than as assessable evidence of method.
What to do instead
Keep construction thin (about 0.3 mm, hard pencil) and leave it on the sheet; darken only the final answer. Mark tangent points with a small tick or letter, and where a line is tangent to an arc, show the radius to the point of contact so the tangency is proved, not assumed.
Loci such as cycloids, involutes or paths of a point on a rolling shape are drawn as a freehand curve through five or six roughly estimated positions, with the tangent and normal at a point on the locus omitted or drawn by eye.
The candidate under-estimates how many positions are needed for a smooth curve and does not know that the instantaneous centre of rotation gives the tangent and normal.
What to do instead
Divide the generating circle into twelve equal parts, step the same number of divisions along the straight path using the chordal length (approximately pi times diameter divided by twelve), and plot every position. At any point, the normal passes through the point of contact with the straight line (the instantaneous centre) and the tangent is perpendicular to it.
In a development of a truncated or inclined solid, the candidate transfers lengths straight from the elevation, where they are foreshortened, instead of finding true lengths, and seams are placed on the shortest or most awkward edge.
The candidate assumes any line on the drawing is measurable, forgetting that only lines parallel to a projection plane appear at true length.
What to do instead
Use an auxiliary view or a rotation construction to obtain the true length of every slant edge, then set each one out individually from the base line of the development. Divide the base circle into twelve equal parts and step them along. Place the seam on the shortest edge or a plain generator, and state where it is on the drawing.
The section is hatched in the view where the cutting plane appears as a line rather than producing the true shape in an auxiliary view, and the hatching is at random angles, unevenly spaced or carried across the background.
The candidate equates 'section' with 'shade the cut area' and does not realise that the true shape only appears in a view perpendicular to the cutting plane.
What to do instead
Project perpendicular to the cutting plane to obtain the true shape, transfer the intersection points from the plan at constant distance from the plane, then hatch only the cut material: thin lines at 45 degrees, evenly spaced (about 3 mm), same direction on the same part, and discontinued where lettering or dimensions cross.
Freehand sketches are produced with a ruler and eraser, starting from one corner and working outward, so proportions are wrong and lines are short, hairy strokes built up from the wrist.
The candidate equates sketching with untidy drawing and believes accuracy requires instruments, so no proportional envelope is set up before detail is added.
What to do instead
Block in a light bounding box to the overall proportions first, then centre lines, then main divisions, then detail, all freehand. Draw long lines in single strokes from the shoulder or elbow, rotating the sheet rather than the wrist, and do not erase: correct with a second confident line.
Presentation sheets lack a border, title block, scale and units; line weights are uniform across outlines, dimensions and construction; dimension lines cross each other or the object, hidden edges are dimensioned, and symbols such as the diameter and radius marks are missing.
The candidate considers conventions to be decoration and focuses all effort on geometry, so the drawing cannot be read or manufactured from.
What to do instead
Set up the sheet first: border, title block with title, name, scale and date, and a line-weight legend. Dimension outside the outline on thin lines, with the dimension line about 8 mm clear of the view, arrowheads touching the extension lines, and figures reading from the bottom or right. Give diameter and radius with their symbols and state units once in the title block.