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ENGINEER AND MACHINIST’S DRAWING-BOOK.

PART THIRD.

DRAWING OF MACHINERY BY ORDINARY GEOMETRICAL PROJECTION.

SECTION I.

Introductory Remarks and Definitions.

Mechanical drawing is a particular form of tlie method

of delineation generally by projection. The object of

mechanical drawing is to represent mechanical objects

upon plane surfaces. Every machine has three dimensions,

length, breadth, and thickness ; and these are susceptible

of being exactly defined. In viewing a machine from

an indefinite distance, we may suppose the rays of light

proceeding from it to move parallel to each other; and,

conceiving them to be intercepted upon a plane surface at

right angles to the rays, the representation thus formed

is said to be a projection of the machine upon that plane;

this picture or drawing will convey, so far as it goes,

correct and definite ideas of the structure and shape of

the machine. But, as the form of a machine must, in

virtue of its bulk, be referred to three series of dimensions,

each of them at right angles to the plane of the other, it

follows that any three contiguous planes of an imaginary

cubical space will answer as planes of projection. And

when we remark, further, that the component parts of

machines are, in general, disposed at right angles, it will

be seen that if the planes of the projections of a machine

be made to coincide with the disposition of its parts, its

dimensions will be represented exactly as they are: in

plainer language, there will be little or nothing of that

fore-shortening of form which necessarily occurs in repre-

sentations of natural objects, in virtue of their wavy and

ungeometrical outline.

To illustrate these remarks on geometrical projec-

tions, let us take for example a parallelopiped of which

the outlines are in bona fide planes, meeting at right

angles.

Let a b c (Fig. 170) be a parallelopiped in an upright

position, of which the plane a 6 is horizontal, and the

planes ac,cb, vertical. Let de, d fi dg, be the boundary

planes of a cubical space, in which the body a b c is placed.

Then, the sides of the body being parallel to those planes,

each to each, let the figure of the parallelopiped be pro-

jected upon those planes: for this purpose, draw parallel

lines from the angles of the body perpendicular to the

planes, as indicated by the dot-lines; then there mil

result the projection a c upon the plane df, representing

the surface a c; b' c on the plane d g, being the surface

b c, and a b' on the plane d e, showing the upper surface

a b. As here arranged, the projection a c is called a

front elevation of the object, as it represents the front

aspect; the projection b' c, in like manner, is termed a

side elevationi,; and the projection a' b' a plan of the

object. The line d l is the ground line.

<%. 170.)

ENGINEER AND MACHINIST’S DRAWING-BOOK.

PART THIRD.

DRAWING OF MACHINERY BY ORDINARY GEOMETRICAL PROJECTION.

SECTION I.

Introductory Remarks and Definitions.

Mechanical drawing is a particular form of tlie method

of delineation generally by projection. The object of

mechanical drawing is to represent mechanical objects

upon plane surfaces. Every machine has three dimensions,

length, breadth, and thickness ; and these are susceptible

of being exactly defined. In viewing a machine from

an indefinite distance, we may suppose the rays of light

proceeding from it to move parallel to each other; and,

conceiving them to be intercepted upon a plane surface at

right angles to the rays, the representation thus formed

is said to be a projection of the machine upon that plane;

this picture or drawing will convey, so far as it goes,

correct and definite ideas of the structure and shape of

the machine. But, as the form of a machine must, in

virtue of its bulk, be referred to three series of dimensions,

each of them at right angles to the plane of the other, it

follows that any three contiguous planes of an imaginary

cubical space will answer as planes of projection. And

when we remark, further, that the component parts of

machines are, in general, disposed at right angles, it will

be seen that if the planes of the projections of a machine

be made to coincide with the disposition of its parts, its

dimensions will be represented exactly as they are: in

plainer language, there will be little or nothing of that

fore-shortening of form which necessarily occurs in repre-

sentations of natural objects, in virtue of their wavy and

ungeometrical outline.

To illustrate these remarks on geometrical projec-

tions, let us take for example a parallelopiped of which

the outlines are in bona fide planes, meeting at right

angles.

Let a b c (Fig. 170) be a parallelopiped in an upright

position, of which the plane a 6 is horizontal, and the

planes ac,cb, vertical. Let de, d fi dg, be the boundary

planes of a cubical space, in which the body a b c is placed.

Then, the sides of the body being parallel to those planes,

each to each, let the figure of the parallelopiped be pro-

jected upon those planes: for this purpose, draw parallel

lines from the angles of the body perpendicular to the

planes, as indicated by the dot-lines; then there mil

result the projection a c upon the plane df, representing

the surface a c; b' c on the plane d g, being the surface

b c, and a b' on the plane d e, showing the upper surface

a b. As here arranged, the projection a c is called a

front elevation of the object, as it represents the front

aspect; the projection b' c, in like manner, is termed a

side elevationi,; and the projection a' b' a plan of the

object. The line d l is the ground line.

<%. 170.)