The Technique of FURNITURE MAKING

Projections commonly used

ORTHOGRAPHIC (ORTHOGONAL) PROJECTION

The standard method of representing a three­dimensional object in two-dimensional form, i. e. as a scale drawing on one plane, is by ortho - graphic projection in which related views give the various aspects, viz.:

Plan Representation of the object on the horizontal plane, i. e. looking down.

Front and side elevations Representation of the object on the vertical plane, i. e. level with the eyes.

Section The object cut through either on the horizontal or the vertical plane to show details which would not be visible in simple outlines of plan and elevation. Vertical sections are usually included in a side elevation projected from the front elevation, and horizontal sections included in the plan, although if the object is complex several such sectional drawings may be necessary.

All these drawings are to a convenient scale, but full - or half-size details are necessary for free-hand curves where a compass radius cannot be stated, or where the particular detail calls for amplification.

They can be projected from either the first dihedral angle (first angle drawings) or the third dihedral angle (third angle drawings), and 331:1 gives a typical layout using the first angle and drawn to a scale of 1/8 (11/2 in to the foot) or to metric scale. The details given show, at 331:lB, the front elevation of a simple wooden plinth with a circular recess on the face for an inlaid metal plaque; 331:1 A is the plan of the plinth showing a stepped housing for a statuette; and 331:lC a vertical side section on the centre-line XY (331:1 A). Full-size details of the stepped housing and the circular recess are shown in 331:lD and 331:lE and no further information should be necessary. Where the drawings are of complex objects, however, it may be necessary to add a three-dimensional project for clarification.

First-angle projection

Where a horizontal plane intersects a vertical plane (VP) dihedral angles are formed, of which both the first and third angles are used in orthographic projection. Where the first

dihedral angle is used (332:1) the object is placed between the observer and the vertical plane (332:2), thus the front elevation is seen first (the arrow points the direction), the side elevation is projected from it by folding back the side vertical plane on the vertical trace (VT), and placing it alongside the front elevation. The horizontal plane is then folded down on the horizontal trace (HT) XY, known as the XY line, and therefore appears below the front elevation.

Third-angle projection

This method is becoming increasingly popular, although it hardly matters which method is adopted provided the drawing is quite clear as to what it is intended to represent. In third angle projection the object is placed behind the vertical plane (332:3), the plan is seen first, the front elevation below it, and the side elevation projected alongside the front elevation.