MATSEC IM Engineering Drawing and Graphical Communication common mistakes
MATSEC IM Engineering Drawing and Graphical Communication · Understand the traps and how to avoid them.
Drawing hidden detail as continuous thick lines, or omitting hidden lines entirely when a feature is invisible in a view, so that a hole or slot behind a face disappears from the drawing.
Candidates copy the visible outline of the object from the pictorial view without reasoning about which edges are obscured in each direction of sight; line-type conventions are memorised as a list rather than applied per view.
What to do instead
Before drawing each view, list the object's edges and classify each as visible, hidden or absent for that viewing direction. Draw hidden edges as evenly spaced dashed lines of consistent dash length, and check that every feature shown in one view appears in the other views in some line type.
Producing three views that do not align: the front and plan are not vertically projected, or the side view is placed at an arbitrary position instead of being projected at 45 degrees from the plan.
Each view is drawn as a separate picture rather than as one projection system, and the candidate does not use construction lines to transfer heights and depths between views.
What to do instead
Set out the three-view layout with faint construction lines first: project width across to the side view and depth down to the plan, using a 45-degree mitre line to carry depth into the side view. Draw outlines only after the projection grid is complete, then erase construction lines.
Dimensioning the drawing instead of the object: placing dimensions inside the outline, omitting the overall length, repeating the same dimension in two views, or leaving a chain of dimensions that does not close on the overall size.
Candidates dimension what is easiest to measure on their own drawing rather than what a manufacturer needs, and they do not distinguish size dimensions from location dimensions.
What to do instead
Apply the rule that each feature is dimensioned once, in the view where it appears as a true shape or clearest outline, placed outside the outline. Include overall length, width and height, and check that no dimension is duplicated between views and no dimension is missing from a chain.
Using the wrong arrowhead and termination style: arrowheads drawn as open V shapes or oversized filled triangles, extension lines touching the object outline, and dimension lines crossing other dimension lines or extension lines.
Arrowheads and lines are drawn freehand at speed without a template or sharp pencil, and the conventions about gaps between extension lines and the object are not practised.
What to do instead
Draw arrowheads about 3 mm long and 1 mm wide, filled solid, with the point touching the dimension line. Leave a small visible gap between the object outline and the start of each extension line, extend the extension line slightly past the dimension line, and arrange dimensions in rows from smallest to largest so lines do not cross.
Constructing tangents, bisectors and regular polygons by eye or by trial measurement with a rule, rather than by the specified geometric construction, so the point of tangency is visibly wrong or the polygon is not truly regular.
Candidates reach for a scale or estimate because they cannot recall the construction sequence, and small errors in a construction are not checked at the end.
What to do instead
Learn and rehearse the standard sequences (perpendicular bisector, angle bisector, tangent from an external point, tangent arc between two lines, polygon in a circle by dividing the diameter). Always mark the point of tangency and verify it lies on the line joining centres for tangent arcs.
Drawing an isometric or oblique pictorial with incorrect axis angles or with lengths scaled along the receding axes, giving a distorted shape whose isometric lines are not parallel to the 30-degree axes.
The difference between true isometric projection and isometric drawing, and between isometric and oblique, is confused, and non-isometric lines are measured directly instead of being located by their endpoints.
What to do instead
Set the isometric axes at 30 degrees to the horizontal and the vertical axis at 90 degrees, and measure true lengths only along these axes. Locate non-isometric lines by plotting their endpoints on the axes, and draw ellipses for circles using the four-centre construction in the correct isometric plane.
Section views that show the cutting plane but omit or wrongly hatch the cut material: hatching running outside the cut area, hatch lines at inconsistent spacing or direction, or no differentiation between adjacent parts in an assembly section.
The purpose of a section (to reveal internal shape and remove hidden lines) is not understood, so the hatching is treated as decoration and the cut face is not identified from the cutting-plane line.
What to do instead
Draw the cutting-plane line with thick ends and arrows showing viewing direction, then hatch only the material actually intersected by the plane, using thin lines at 45 degrees and consistent spacing. In assemblies, reverse the hatch direction or change spacing between adjacent parts, and omit hidden lines behind the cut unless needed for clarity.
Tolerances and fits written without meaning: a limit given as a single number, a tolerance with no unit or datum, or a geometric tolerance frame missing the datum reference or the tolerance zone symbol.
Candidates copy a nominal size and add an arbitrary plus-or-minus value, and the structure of a feature control frame (symbol, tolerance, datum) has not been learned.
What to do instead
Write limits as an upper and lower value or as a nominal with a symmetric or asymmetric deviation, always with units stated once. For geometric tolerancing, complete the frame in order: characteristic symbol, tolerance value with diameter symbol where the zone is cylindrical, then datum letters, and ensure every datum letter used is identified on the drawing.
Assembly drawings that do not identify parts: no item numbers with balloons, no parts list, and no indication of how components locate relative to one another, so the drawing cannot be read as an assembly.
The candidate treats the assembly as a single complex shape to copy rather than as a set of components with relationships, and leaves the labelling to the end when there is no space left.
What to do instead
Plan the sheet with space reserved for a parts list in the corner. Balloon each component with a leader to its outline, number items to match the parts list (item, description, quantity, material), and show mating features, centre lines and fasteners so the fit between parts is clear.
Charts and graphs with unlabelled axes, no units, a broken or non-zero baseline that exaggerates differences, or a title that does not state what is plotted, so the graphic misleads rather than communicates.
Presentation graphics are treated as a drawing exercise and the communicative purpose (accurate comparison, readable scale) is not considered; scale choice is copied from a template without thought.
What to do instead
Label both axes with quantity and unit, state the scale, choose a chart type that matches the data (bar for categories, line for trends, pie only for parts of a whole), start value axes at zero unless a break is explicitly marked and justified, and give a title that names the quantities and the source.