MATSEC SEC Graphical Communication cheat sheet

First-angle projection (the ISO/European convention used in Malta): the object sits between the observer and the projection plane, so the plan is drawn directly below the front elevation and the left end elevation is placed on the right.

Third-angle projection: the projection plane sits between the observer and the object, so the plan appears above the front elevation and the right end elevation is placed to its right.

The three regular views stay aligned: plan and end elevation share the same depth, and a 45° mitre line transfers that depth from the plan across to the end elevation.

Line hierarchy: thick continuous for visible outlines, thin dashed for hidden detail, thin chain for centre lines, thin continuous for dimension, hatching and projection lines.

Typical line widths are 0.7 mm for outlines and 0.35 mm for thin lines, a 2:1 ratio; hidden dashes run about 3–4 mm with 1 mm gaps.

Dimensions are written in millimetres with no unit symbol; extension lines start about 1 mm clear of the object and run about 2 mm past the dimension line.

The front view is chosen to show the most characteristic shape with the fewest hidden lines; height appears in elevations, width in elevation and plan, depth in plan and end elevation.

Section views: the cutting plane is a thick chain line with arrows giving the viewing direction; hatch at 45° with 2–4 mm spacing and omit hidden detail behind the cut.

Isometric axes are 120° apart: one vertical and two at 30° above the horizontal; equal lengths are stepped off along all three axes at the same scale.

True isometric projection foreshortens every axis to 0.816 of true length; isometric drawing uses full lengths along the axes and is therefore about 22% larger.

A circle of diameter D on an isometric face becomes an ellipse with major axis 1.22D and minor axis 0.71D; the major axis is perpendicular to the axis normal to that face.

Only lines parallel to the isometric axes are measurable directly; any other line must be set out from the coordinates of its endpoints along the three axes.

Isometric views normally omit hidden lines but keep centre lines where they are needed to locate circles and arcs.

Cavalier oblique: the front face is drawn true shape and the receding axis is usually at 45° to full scale; cabinet oblique draws the receding axis at half scale.

The oblique receding axis is normally 30°, 45° or 60°; 30° reduces overlap of faces while 45° gives the most natural-looking depth.

In oblique projection circles on the front face remain true circles, while circles on receding faces become ellipses set out from the enclosing square.

Perpendicular bisector: equal arcs from both ends with radius greater than half the line; the line through the two arc intersections is the locus of points equidistant from the ends.

Bisect an angle with an arc from the vertex cutting both arms, then equal arcs from those two points; the line from the vertex through their intersection is the bisector.

Construct 60° by stepping the radius around a circle or by an equilateral triangle; obtain 30° and 15° by repeated bisection, and 90° by the semicircle-on-diameter construction.

Divide a line into n equal parts using an auxiliary line at any angle, n equal steps along it, a line from the last step to the endpoint, then parallels through the other steps.

Regular hexagon: step the radius six times around the circumscribing circle. Regular pentagon: central angle 72°, or the standard diagonal construction.

Tangent from an external point: draw a circle on the midpoint of the centre-to-point line as diameter; its intersections with the given circle are the tangent points.

Loci: fixed distance from a point gives a circle, from a line gives two parallel lines, equidistant from two points gives the perpendicular bisector, from two lines gives the angle bisectors.

Ellipse: the sum of focal distances is constant and equals the major axis; parabola: distance to the focus equals distance to the directrix; cycloid: path of a point on a rolling circle.

Development needs true lengths: rotate a sloping edge in an auxiliary view until it is parallel to the projection plane before stepping its length onto the development.

Right prism lateral development: a rectangle whose width equals the base perimeter and whose height equals the prism height, plus the true-shape end faces.

Right cylinder: a rectangle of width πD and height h plus two circles; for a truncated cylinder, points are projected from the section to set out the curve.

Right cone: a sector of radius equal to the slant height l, with arc length πD, so the sector angle equals 360° × r / l.

Pyramid: every face is a triangle; find the true length of each slant edge by rotation, then triangulate the development face by face.

The true shape of a section is obtained in an auxiliary view taken perpendicular to the cutting plane, projecting the cut points from plan and elevation.

A sphere is non-developable; its surface is approximated by gore or lune strips, or by the zone method, with a small accepted error.

Curves of interpenetration are found by locating points common to both surfaces using auxiliary views or intermediate section planes, then joining them with a smooth curve.

Sheet sizes: A0 is 841 × 1189 mm and each later size halves the area - A1 594 × 841, A2 420 × 594, A3 297 × 420, A4 210 × 297 mm.

The title block carries drawing title, name, scale, date, drawing number and the first- or third-angle projection symbol.

Scale notation: 1:1 full size; 1:2, 1:5, 1:10, 1:20 reducing; 2:1, 5:1, 10:1 enlarging; the chosen scale is always stated in the title block.

Freehand sketching: block in overall proportions with light construction lines first, draw long strokes from the shoulder, then darken the final outlines.

Freehand ellipses: draw a light enclosing rectangle or centre lines, mark the four quadrant points and join each pair with a smooth single stroke.

Presentation graphics include bar charts, pie charts, pictograms, flow charts and Gantt charts, each needing a title, scale or key, and labelled axes.

Lettering uses vertical or 15° right-sloping capitals at least 2.5 mm high, with light guidelines keeping height and spacing consistent.

Standard line types: continuous thick for visible edges, continuous thin for dimensions, leaders and hatching, dashed for hidden detail, chain for centre lines, thick chain for cutting planes.