SPM Lukisan Kejuruteraan common mistakes
SPM Lukisan Kejuruteraan · Understand the traps and how to avoid them.
When constructing a perpendicular bisector or angle bisector, the candidate draws the arcs by eye from roughly-estimated centres instead of setting the compass to a fixed radius and keeping it unchanged, so the bisector line misses the true midpoint or the angle is split unevenly.
Under time pressure candidates treat construction arcs as decoration rather than as the geometric definition of the bisector; they also re-adjust the compass between arcs and lose the equal-radius property that makes the construction valid.
What to do instead
Before drawing, mark the two given points, set the compass to any radius greater than half the distance, and draw the two arcs from each point without changing the setting; join the intersection points of the arcs. For an angle bisector, strike the same arc from the vertex across both arms, then strike equal arcs from those two intersection points. Leave all construction arcs visible and label the intersection points so the marker can see the method, not just the result.
The candidate draws a tangent from an external point to a circle by simply laying the straightedge so the line looks like it touches, without first bisecting the line joining the external point to the centre and using that midpoint as the centre of the construction arc.
The tangent construction depends on the angle in a semicircle being 90 degrees, which is not visually obvious, so candidates skip the auxiliary circle and rely on appearance.
What to do instead
Join the external point to the circle centre, bisect that line to find its midpoint, then draw a semicircle on it. Where the semicircle crosses the given circle are the tangent points; join the external point to each. Check with a set square that the radius to the tangent point is at 90 degrees to the tangent line.
On a three-view orthographic layout, the candidate places the top view directly below the front view instead of above it, or the side view on the wrong side, and does not align the views with projection lines.
Candidates remember 'first angle' or 'third angle' as a label without holding a mental model of the projection box, and they draw each view independently rather than projecting features across from one view to the next.
What to do instead
Fix the convention at the start: in first-angle projection the view from above is placed below the front view and the view from the left is placed on the right; in third-angle the top view sits above and the left view sits on the left. State the symbol used. Draw faint 45-degree mitre lines or horizontal and vertical projection lines from the front view first, then build the other two views only on those lines so every feature height and depth matches.
Hidden detail is either omitted entirely, drawn as a solid line, or drawn as a continuous dashed line that merges with visible outlines, so internal features such as holes and slots cannot be read.
Candidates are focused on the visible outline and treat hidden lines as optional; they also lack control of dash length and gap so the convention breaks down at intersections.
What to do instead
Before inking, list every internal feature and mark where it is hidden in each view. Use short dashes of even length with clear gaps, keep dashes touching at corners, and stop the hidden line where it meets a visible line rather than crossing it. Where a hidden line coincides with a centre line, show the hidden line. Cross-check each view so a hole shown hidden in one view appears as a circle in another.
In isometric drawing the candidate draws all receding edges at 45 degrees, or measures full true lengths along the isometric axes for a non-isometric line, and draws ellipses as freehand ovals with the major axis in the wrong direction.
The 30-degree axes and the 0.816 true-length factor are memorised as numbers without the underlying geometry, and circles in isometric are assumed to be simple ovals rather than ellipses oriented to the face.
What to do instead
Set the two receding axes at 30 degrees to the horizontal using a set square and keep the vertical axis truly vertical. Mark isometric lengths at full scale along the axes, and for lines not on an axis construct the enclosing isometric box and locate endpoints on its faces. For a circle on a top face, draw the isometric square, find its centre, and draw the ellipse with its major axis along the long diagonal of that square; use the four-centre approximate method only when permitted.
When producing a sectional view, the candidate hatches the whole cut area including shafts, bolts, nuts, and pins that are cut longitudinally, and uses the same hatch direction and spacing for adjacent parts.
The rule that standard fasteners and solid cylindrical parts are drawn unsectioned when cut along their axis is easily forgotten, and hatching is treated as shading rather than as a code identifying separate components.
What to do instead
Decide first which parts the cutting plane actually passes through. Draw standard items such as bolts, nuts, rivets, pins, keys, and shafts unsectioned when the plane contains their axis. Hatch only the material actually cut, using thin lines at 45 degrees; reverse the direction or change the spacing for each adjacent part, and never let hatch lines run parallel to an outline.
Dimension lines are drawn as part of the object outline, arrowheads point outward with the value outside the extension lines for large dimensions, and the same dimension is repeated on more than one view.
Candidates copy the look of a finished drawing without understanding that each dimension should appear once, on the view that shows the feature most clearly, and they confuse extension lines with object lines.
What to do instead
Keep dimension lines thin and separate from outlines, with a small gap between the object and the start of each extension line. Place the value above a horizontal dimension line and to the left of a vertical one, reading from the bottom or right of the sheet. Dimension each feature once, on the view where it is true shape, and omit a dimension that can be derived from others unless it is a reference dimension in brackets.
The candidate writes a tolerance as a single plus-or-minus value when the fit calls for unilateral tolerances, omits the limit dimensions, or gives a surface finish symbol without the required roughness value and machining process.
Tolerance and surface texture are taught as symbols to copy rather than as instructions to the manufacturer, so candidates leave out the parts that tell the machinist what range is acceptable and how the surface must be produced.
What to do instead
For each toleranced feature, state the basic size and the permitted deviation, using bilateral or unilateral form as the design requires, and check that the resulting limits give the intended clearance or interference. Add the surface texture symbol with the roughness value in the correct position, the process note where needed, and the machining allowance if any. Verify that every toleranced dimension can be inspected with the equipment named.
In an assembly drawing the candidate draws every component in full detail, or leaves out the parts list and item numbers, and the bill of materials does not match the numbered balloons in the drawing.
Candidates treat the assembly view as a bigger version of a detail drawing rather than as a communication of how parts fit together, and they add balloons after finishing the drawing without reconciling them to the list.
What to do instead
Draw the assembly in its working position, showing the components in section or outline as needed for clarity, and omit hidden detail that is not required to understand the assembly. Number each component with a leader and balloon, then build the parts list with the same item numbers, description, quantity, and material. Cross-check every balloon against the list and every listed item against a balloon before submitting.
The candidate works in a single layer with default line weights, draws at arbitrary scale in model space, and does not set up a title block or drawing template before starting, so the plotted sheet has inconsistent line weights and a scale that cannot be verified.
CAD is used as a faster pencil: candidates focus on geometry and ignore layer standards, line-weight conventions, and the paper-space or layout settings that control what actually prints.
What to do instead
Before drawing, create or load a template with named layers for outlines, hidden lines, centre lines, dimensions, and text, each with its correct line weight and colour. Draw full size in model space, then set the scale in the layout or plot dialog and confirm the plotted line weights. Fill in the title block with drawing number, scale, projection symbol, and revision, and plot a check copy to verify line weights and text height at the final scale.
The candidate uses the wrong projection symbol for the views shown, mixes first- and third-angle conventions on one sheet, or applies a scale without stating it, and ignores standard sheet sizes and title-block fields.
Standards are memorised as isolated facts rather than as a consistent set of rules applied to the whole sheet, and the projection symbol is drawn from memory without checking it against the actual view arrangement.
What to do instead
Choose the projection convention at the outset, draw the corresponding truncated-cone symbol, and arrange all views to match it. State the scale in the title block and use a standard sheet size. Check the drawing against a short checklist: projection symbol, view arrangement, line types, dimension style, units, title block, and revision. Keep a one-page summary of the relevant standard clauses at the desk and refer to it during the check.
The candidate handles sharp tools carelessly, leaves a compass point or set square overhanging the desk edge, or uses a craft knife toward the body, and does not clear swarf or offcuts before starting a new drawing.
Workshop habits are formed casually and safety instruction is treated as a preamble rather than as part of the drawing process, so risks are not recognised at the moment they arise.
What to do instead
Before starting, clear the board, secure the drawing sheet, and check that compass points and blades are sharp and stored closed. Cut away from the body on a cutting mat, keep blades retracted when not in use, and never leave sharp items overhanging the desk edge. At the end, return tools to their places, dispose of offcuts, and report damaged equipment rather than using it.