JC Graphics common mistakes
JC Graphics · Understand the traps and how to avoid them.
Constructing a tangent from an external point to a circle by guessing the touch point and drawing a line that merely looks tangent, instead of constructing the right angle in a semicircle on the line joining the point to the centre.
Candidates remember 'tangent touches at 90° to the radius' but forget that the touch point is unknown, so they cannot apply the radius rule directly and fall back on visual approximation.
What to do instead
Drill the standard sequence: join external point P to centre O, bisect PO, draw the semicircle on PO to cut the given circle at T, then join PT. Check by confirming OT is perpendicular to PT with a set square before inking.
Losing accuracy in a geometric construction because arcs are struck with a compass radius set by eye or by measuring rather than by transferring the exact given length, and because construction lines are rubbed out before the final figure is checked.
Candidates rush to produce a clean finished drawing and treat construction arcs as clutter, so they erase the evidence needed to verify the construction and cannot see where accuracy drifted.
What to do instead
Keep all construction arcs light but visible in HB until the end; set every compass radius by placing the point on one given point and the pencil on the other. Verify the finished figure against the given data (equal lengths, equal angles) before darkening any line.
Producing an auxiliary view or true shape by projecting perpendicular to the wrong reference plane, so the resulting view shows the edge of the inclined face rather than its true shape.
Candidates confuse the rule 'project at right angles to the plane you want to see true' with 'project at right angles to the main planes', and they do not check which line in the given views represents the edge view of the surface.
What to do instead
Before drawing, identify the edge view of the inclined face in one of the given views; the auxiliary view is then projected at 90° to that edge line, with all measurements transferred from the adjacent view. Add a one-line written check: 'projection direction is parallel to the surface, perpendicular to its edge view.'
Applying a single scale factor to a drawing when the title block specifies a reduction or enlargement, or mixing up the ratio direction (drawing 1:2 means the drawing is half size, not double).
Scale is often treated as a label rather than an instruction, and the colon notation is read casually under time pressure.
What to do instead
Read the scale before starting and write it at the top of the sheet as a working note, e.g. '1:2 - divide real dimensions by 2'. Convert all given dimensions to drawing dimensions in one pass and mark them on the sheet before drawing.
Incorrectly developing a truncated cylinder or cone because the girth line is divided into unequal or too few parts, and because the cut points are not transferred vertically from the elevation to the development.
Candidates divide the circumference by eye or into a convenient number of parts without equal spacing, and they project cut heights across by eye rather than with a set square from the elevation.
What to do instead
Divide the plan circumference into 12 equal parts using compass stepping around the circle, then project each division up to the elevation and across to the development with a set square. Number the divisions on plan, elevation and development so each height is transferred to the matching point.
Drawing an ellipse with a compass as two arcs, or by freehand, when the question requires a specific construction such as the concentric-circle or rectangle method.
An ellipse looks circular enough that candidates shortcut it, and they do not recognise that the marks are awarded for the construction method and the plotted points.
What to do instead
Identify which method the question names and use it exactly: for concentric circles, draw both circles, divide into 12, draw rays, then drop perpendiculars to the major and minor axes to locate each point. Plot all points, draw a smooth curve through them, and label the major and minor axes.
Ignoring line conventions: using the same weight for outlines, hidden detail, centre lines and projection lines, or drawing centre lines as short dashes without the long-short pattern.
Candidates focus on getting the geometry right and treat line weight as cosmetic, and many practise only with a single pencil grade.
What to do instead
Use a fixed convention on every drawing: thick continuous for visible outlines, thin dashed for hidden detail, thin long-short for centre lines, thin continuous for projection and construction lines. Practise with a 2H for construction, HB for outlines and a 0.5 mm fine liner for centre lines so the hierarchy becomes automatic.
Submitting a drawing with no title block, no scale, no projection symbol and no dimensions, or with dimensions that duplicate or contradict each other.
Candidates regard the drawing as finished once the geometry is complete and see annotation as an afterthought; they also copy every dimension from the brief rather than the ones needed to manufacture the part.
What to do instead
Reserve a title block area on the sheet before starting. Finish every drawing with a checklist: title, scale, projection symbol, material or notes, and a dimension set that gives each feature exactly once, placed outside the outline with the smallest dimensions nearest the view.
Misreading third-angle versus first-angle projection and placing the plan above the elevation when third angle is specified, or placing a left-side view on the wrong side of the front view.
The two systems are mirror images and candidates rely on memory of 'plan on top' without checking which symbol is given.
What to do instead
Anchor on the symbol: in first angle the truncated cone's small end points away from the front view; in third angle the plan sits above the elevation and the view from the left sits on the left. Sketch the symbol in the corner of the sheet first, then lay out the views to match it.
Running out of time because the sheet layout is decided as drawing proceeds, so views overlap, a view is placed where there is no room for its dimensions, or the candidate redraws a whole view after a layout error.
Candidates start at the first given view without planning the bounding box of all views plus dimension space, and they underestimate how long inking and lettering take.
What to do instead
Before drawing anything, calculate the overall width and height of the view block, add space for dimensions and the title block, and lightly box in each view's position. Allocate time explicitly (for example, half the time on construction, a quarter on inking, a quarter on dimensions and checking) and keep to it.