Orthographic projection

The most-repeated orthographic fault is estimating a view by eye instead of projecting from the others. Symptoms: the end-view depth over-projected because not carried from the plan, and the plan left incomplete. Project widths to the plan, heights to the end view, and use a 45-degree mitre line so depths agree. Marks reward projection and completed views.

The first/third-angle distinction shows up as a side view on the wrong side of the front elevation. The rule: first-angle puts the view-from-left on the right and the plan below; third-angle puts it on the left, plan above. Read the projection symbol first -- circles right = first-angle, circles left = third-angle. Misreading the system mirrors the part.

An orthographic drawing communicates through its line conventions, which examiners often find absent or wrong: thick continuous for outlines, thin dashed for hidden detail, thin chain for centre lines, thin continuous for projection lines. Candidates lose easy marks by drawing everything as a plain outline. Match each line type to its meaning.

A fixed wrong rule: that the plan always sits below the front elevation. That is only true in first-angle. In third-angle the plan is drawn above, because the horizontal viewing plane folds up not down. The plan's position is itself a clue: plan below = first-angle, plan above = third-angle. Always dropping the plan underneath draws a third-angle layout wrongly.

Mixing the two systems on one drawing -- some views placed first-angle and others third-angle -- makes the layout impossible to read consistently and mirrors part of the object. Choose one system for the whole drawing, mark it with the projection symbol, and place every view by that system's rule. When reading a drawing, identify the system from the symbol first.

Leaving the projection symbol off removes the one thing that tells the reader whether it is first- or third-angle. Because the two systems place views on opposite sides, an unlabelled drawing can be read as the wrong system and made as a mirror image. Always draw the symbol -- a trapezoid beside two concentric circles, circles right for first-angle, left for third-angle.

Getting the two systems back to front. First-angle projects each view onto the plane beyond it, so the left-view lands on the right and the plan below. Third-angle is the opposite -- the plane sits between viewer and object, so each view is on the near side (left-view on the left, plan above). Symbol: circles left = third-angle, circles right = first-angle.