Technical Drawings—the requirements of, nted of conventions. Orthographic Projection—principles of. Isometric Projection —its uses and defects. Oblique I'rojection—-various modifications. Perspective I'rojection — what it is. Freehand Sketches- -advantages of. Practical Geometry- -its use to the draughtsman. Working Drawings—essentials of. Workshop Drawing
Technical Drawing is, in eifect, the language of the workshop and drawing office, the means by which the ideas or intentions of the designer are conveyed to the constructor in a much more definite and understandable manner than can be done by the most elaborately written or spoken directions.
It will be readily understood that, to avoid confusion, universally accepted methods or con\entions must be adhered to in preparing such drawings, otherwise the user would be in the same position as one trying to read a book written in a foreign language ; even the latitude allowed the artist or pictorial draughtsman in depicting his emotions or impressions would render technical drawings useless for their purpo.-e. We must speak in a common language to be understood by all.
But the requirements of the constructive arts are so numerous and varied that even within the circumscribed sphere of the building trades several different types or classes of drawing are necessary. Each of these will be dealt with as fully as may be needful for elementary work a i
in tho succeeding chapters, anil the descriptions given here are rather in the nature of a summary of the advantages, and disadvantages-or at least the limitations of each kind—than an instruction in their preparation.
Orthographic or Perpendicular Projection.—This is the commonest and most useful method of producing drawings, hut its meaning is not always obvious to the inexperienced, for it is a graphic language that generally has to be acquired step by step. In this form of drawing, which is usually adopted for architectural " plans" of buildings and for working drawings, every part is drawn as if immediately in front of the observer, no allowance for distance in the parts of the object from the observer being made ; it is the unaccustomed appearanc e which results when this method is used that is so confusing to the uninitiated, but the procedure is essential if direct measurements and correct angles arc to be obtained trom the drawings. In this kind of drawing the observer is supposed to stand directly in front of the object, and to see all its parts upon that face at the same time, and it will be obvious that, if this is so, he cannot see more than one front or surface at a time; ¡Consequently, in depicting any solid, as many " projections " or views have to be made as the solid has sides or surfaces. As, often, the several sides do not possess distinctive features by which they can be readily identified, a conventional or generally-agreed-upon set of terms are used to desenlie these views, mainly based upon their relative position to the ground. Thus the surface or side which is supposed to rest upon, or is parallel to, the surface of the ground is termed the Plan ; that surface which is at right angles to the ground—■ i.e. vertical—is termed the Elevation, and as there arc only three dimensions to any solid—viz. length, breadth and thickness- -it is obvious that we can depict any regular solid by three views or projections showing these dimensions. One of these will be the Plan, the other two will be Elevations, which are distinguished as circumstances dictate,
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