ORTHOGRAPHIC PROJECTION REPRESENTS A THREE-DIMENSIONAL OBJECT BY SEVERAL TWO-DIMENSIONAL VIEWS, EACH LOOKING SQUARE-ON AT ONE FACE. THE DEFINING GEOMETRY: · THE PROJECTORS ARE PARALLEL TO ONE ANOTHER, and · PERPENDICULAR (ORTHO-) TO THE PLANE OF PROJECTION. THE CONSEQUENCE, and it is the entire justification for the method: ANY FACE PARALLEL TO THE PROJECTION PLANE APPEARS IN ITS TRUE SIZE AND TRUE SHAPE, WITH NO FORESHORTENING. Contrast PERSPECTIVE projection, where projectors converge on the eye and equal lengths at different distances draw differently. A perspective view LOOKS right and MEASURES wrong; an orthographic view looks flat and measures right. A DRAWING IS FOR MEASURING, SO ORTHOGRAPHIC WINS. THE SIX PRINCIPAL VIEWS, obtained by placing the object inside a glass box and projecting onto each face: FRONT (elevation) — the view showing the most characteristic shape; chosen first, and everything else follows from it TOP (plan) BOTTOM LEFT SIDE (end elevation) RIGHT SIDE REAR IN PRACTICE THREE VIEWS ARE ENOUGH for most objects — FRONT, TOP and one SIDE — because opposite views usually duplicate information. THE RULE IS TO USE THE MINIMUM NUMBER OF VIEWS THAT FULLY DESCRIBES THE OBJECT; a redundant view is another thing to keep consistent through revisions. Some objects need only ONE view plus a note: a flat plate needs a front view and "THK 6" for its thickness; a shaft needs one view because ⌀ tells the reader it is round. CHOOSING THE FRONT VIEW — a genuine skill and often examined: · it should show the most characteristic shape / the most detail · it should show the object in its NATURAL OPERATING POSITION · it should have the FEWEST HIDDEN LINES · the longest dimension is usually horizontal ALIGNMENT IS COMPULSORY, and it is what makes the set readable: THE TOP VIEW SITS DIRECTLY ABOVE OR BELOW THE FRONT VIEW, AND THE SIDE VIEW DIRECTLY BESIDE IT. Views are never shuffled around the sheet for convenience. Because of this, a feature's position can be projected from one view straight into another with a set square — WHICH IS HOW THE THIRD VIEW IS CONSTRUCTED FROM THE FIRST TWO, using a 45° MITRE LINE to transfer depths between the top and side views. SHARED DIMENSIONS — the check that a set of views is consistent: FRONT and TOP share WIDTH FRONT and SIDE share HEIGHT TOP and SIDE share DEPTH IF THOSE THREE AGREE, THE VIEWS ARE MUTUALLY CONSISTENT. This is the fastest way to check your own drawing in an exam.

THE SINGLE MOST IMPORTANT DISTINCTION IN THIS TOPIC, and the one that causes real manufacturing errors when misread. Two perpendicular planes divide space into four QUADRANTS. The object is placed in the FIRST or the THIRD; the second and fourth are not used, because the planes would overlap when unfolded. ── FIRST ANGLE PROJECTION ────────────────────────────────── OBJECT BETWEEN THE OBSERVER AND THE PLANE. observer → OBJECT → plane The view is projected THROUGH the object onto the plane behind it, so THE VIEW APPEARS ON THE FAR SIDE. RESULT AFTER UNFOLDING: · the view from the LEFT is drawn on the RIGHT · the view from ABOVE (top view) is drawn BELOW · the view from the RIGHT is drawn on the LEFT EACH VIEW ENDS UP ON THE OPPOSITE SIDE FROM THE DIRECTION IT WAS TAKEN. USED IN: EUROPE, INDIA, NEPAL, and most of Asia and Africa. THIS IS THE CONVENTION YOU WILL BE EXAMINED ON. ── THIRD ANGLE PROJECTION ────────────────────────────────── PLANE BETWEEN THE OBSERVER AND THE OBJECT. observer → plane → OBJECT The plane is treated as transparent and the view is projected forward onto it, so THE VIEW APPEARS ON THE NEAR SIDE. RESULT AFTER UNFOLDING: · the view from the LEFT is drawn on the LEFT · the view from ABOVE is drawn ABOVE EACH VIEW ENDS UP ON THE SAME SIDE AS THE DIRECTION IT WAS TAKEN — which many people find more intuitive. USED IN: THE UNITED STATES, CANADA, JAPAN, AUSTRALIA. THE MEMORY AID THAT ACTUALLY WORKS: FIRST ANGLE — the object is between you and the paper, so the object "pushes" the view AWAY, to the far side. THIRD ANGLE — the paper is between you and the object, so the view lands on the near side, WHERE YOU ARE LOOKING FROM. THE SYMBOL — mandatory in the title block, and the drawing is ambiguous without it. It is a TRUNCATED CONE (a frustum) shown in two views: FIRST ANGLE THIRD ANGLE ┌───┐ ┌───┐ │ ▻ │ ◁ ▷ ◁ ▷ │ ◁ │ └───┘ └───┘ the small end appears the small end appears on the SIDE AWAY from on the SAME SIDE as the circle view the circle view A cone is used precisely BECAUSE IT IS ASYMMETRIC ALONG ITS AXIS — a symmetric shape would look identical in both systems and could not distinguish them. WHY IT MATTERS BEYOND THE EXAM: READING A FIRST-ANGLE DRAWING AS THIRD-ANGLE PRODUCES A MIRROR-IMAGE PART. Every left-hand feature becomes right-hand. The part looks entirely plausible, passes a dimensional inspection, and does not fit. This is a well-known and expensive failure in international subcontracting, and it is the reason the projection symbol is mandatory rather than optional. THE COMPARISON TABLE: FIRST ANGLE THIRD ANGLE ─────────────────────────────────────────────────────────── arrangement observer-object- observer-plane- plane object plane treated as opaque TRANSPARENT top view drawn BELOW the front ABOVE the front left view drawn on the RIGHT on the LEFT view position OPPOSITE to the SAME as the viewing viewing direction direction used in Europe, India, USA, Canada, Japan, NEPAL Australia ISO status both permitted; the symbol declares which ─────────────────────────────────────────────────────────── SECTION AND AUXILIARY VIEWS, which complete the method: AN AUXILIARY VIEW is projected onto a plane PARALLEL TO AN INCLINED FACE, so that the inclined face — which is foreshortened in all six principal views — is seen in its TRUE SHAPE. Without it, an angled surface could not be dimensioned truthfully, and the true-size guarantee that justifies orthographic projection would be broken for that face.