The evening technical school has been rapidly developing during recent years. From a makeshift it is coming to occupy a field distinctly its own. The ambitious man attending an evening technical school is fully the equal of his brother at the day technical school and his worth is being increasingly realized. The foundation subjects mathematics, mechanics, and drawing require particular attention in evening courses, where the time may be somewhat limited and the needs of the student varied. This book has been prepared for Ohio Technical Drawing School students as part of a technical course.

Progress in engineering work of any kind depends upon an intimate knowledge of mechanical drafting as the language of the engineering world. Its possibilities must be understood. The mere drawing of lines and more or less copying of exercises or sketching from a few models is far from the purpose of a drawing course. The value of drawing as one of the working tools to be treasured and used during a lifetime in the most useful of professions, ENGINEERING, should be realized. It is as an aid in the study, and later use of engineering knowledge, that drawing finds its place. These preliminary remarks may serve to explain the makeup of the book. The actual handling of the instruments can best be taught by careful individual instruction of each student, after which false or awkward motions should be immediately corrected. Inefficiency in this respect is one of the most severe handicaps of many "self-made" draftsmen. The treatment of the various subjects is necessarily somewhat brief, as it is intended that personal instruction should be given in each subject.

In the first studies the student is taught to represent each object in strict conformity to the laws of projection. All lines are drawn, all intersections are shown, and invisible surfaces are all indicated by dotted lines. For simple parts such drawings are easily read and they are generally used in the drafting room.

When more complicated pieces are met with or where whole machines or constructions are to be represented, such a method would lead to great confusion and often would produce a drawing which it would be almost impossible to read. The time necessary would be very great even for an expert. In such cases the full lines representing the visible surfaces are shown, but the intersections and invisible surfaces are not all drawn in. The selection of what lines to draw and what lines to leave out is animportant study in itself.

Furthermore there are many representations of parts which are or appear to be violations of orthographic projection, which are used because practice has shown that they convey the idea to the workman more completely or easily. Other representations are used to save the draftsman's time, or in the interests of simplicity. Almost anything which will make a drawing more readily intelligible is justified. This statement must be used with caution, as what will seem plain to a man familiar with the work may not be so plain to the workman or other reader. A drawing has one great purpose, and that is to be useful. To this end lines may be added or left out, shading may be used, or notes may be put on. As an expression in the engineering language each drawing should have only one meaning, and should state that meaning with the least possible chance for misinterpretation. Many of these idiomatic expressions of the engineering language will be considered in the later chapters.

The chapters on Materials and Stresses, Machine Construction, and Estimation of Weights are brief treatments of subjects which are necessary for the making of intelligent drawings. Considerable elementary machine design is included as belonging in a practical treatment of mechanical drafting, for the author does not look with favor upon fine distinctions between "subjects." It is the "usability" which really counts. The subjects are arranged to suit the author's convenience, but they may be taken in a different order if desired. The problems are placed in one chapter at the end of the book, so that a selection may be easily made. These problems are suggestive, and may be amplified by the teacher, who should make a free use of actual shop blueprints and castings.

The author believes that the highest grade work can be done by evening school men, and in an experience of many years has always found that they are ever ready to meet the most exacting requirements when satisfied that what they are receiving is really worthwhile.
 

Classes of Drawings. - The origin of a drawing is of interest, and a knowledge of how drawings are produced is essential. Roughly drawings may be divided into two classes; detail drawings and assembly drawings. These names are sufficiently descriptive in a general way. Drawings are sometimes made from a machine or part by measuring and sketching. The usual source of a detail drawing is the designer's board. Here the whole machine is laid out to scale in a more or less complete manner, the relation of one part to another is shown, and such fixed dimensions as are necessary are determined. The shapes of the various parts as required for strength and motion are worked out and drawn. From such drawings the detail draftsman works and finishes the drawings of the separate parts.

A detail drawing shows each piece separately and completely defines it (Fig. 152). The number of views is determined by what is necessary to show the shape and size of the object. A pin, shaft, or bolt can generally be shown in one view, while a casting may require two, three, or more views together with sectional and auxiliary views. The main views should always be arranged in strict conformity to the rules of projection. The third quadrant is used exclusively for this pm-pose. Auxiliary views and sections may be placed in other positions but explanatory notes should always be used to define them as explained in Chapter X.

The size of paper and the scales to use have been treated in other chapters. Use a scale that will show the object clearly and that will not require crowding of the dimensions. In general it is better not to use more than one scale on the same sheet. To this end large and small pieces would not be put on the same sheet. There are many concerns where each part is drawn on a sheet by itself. The character of the work will determine the practice in this respect.

It is generally well to draw large castings separately and to group small parts together as:

Small Castings,
Bronze and Composition Castings,
Forgings,
Bolts and Screws.

Special Detail Drawings. - Special detail drawings are some- times made for the different classes of workmen. These might be classed as follows:

Pattern Drawings,
Forging Drawings,
Machinist's Drawings,
Stock Drawings.

There are many advantages to this system where a large number of parts are made as each workman is given only such information as concerns him. As pattern dimensions are used only when the pattern is made or for alterations they complicate the drawing and can better be left off the machinist's drawing. One method is to put the pattern dimensions and information on the tracing in pencil, make several blueprints, and erase the pencil information from the tracing. Gasolene applied with a soft cloth is excellent for this purpose. For forgings two separate drawings will be necessary, one for the blacksmith and one for the machinist. The saving in time will make up for the expense of the extra drawing in most cases.

How to make a Drawing. - A detail drawing is started by first locating the main center lines as shown in Fig. 153 for the necessary views. Next "block in" the fixed dimensions in all views and from them work out the shape of the object. The small circles, fillets, etc. should be drawn last. Figs. 153 to 158 show the drawing for a slide valve in the various stages of making.

After completing the drawing in pencil it is ready to be inked on paper or traced.

Tracing. - Most drawings are now inked on tracing cloth. This is a translucent linen cloth. There are many grades, some nearly transparent. One side of the cloth is generally shiny or glazed and the other dull. Either side may be used but the dull side is to be preferred. The cloth is tacked down over the pencil drawing and the lines inked in as though they were on the cloth. The surface of the cloth should be rubbed over with powdered chalk and then the chalk thoroughly removed. A clean blotter will serve the same purpose. The fine thread running at the edges of the cloth should be torn off before using to prevent wrinkling. As the cloth is absorbent it should be protected from moisture.

Order for inking Lines. - The weight of line to be used has been discussed in the first chapter. First ink the center lines using a fine dot and dash line. The order of inking then is:

1. Small circular arcs and circles,
2. Large circular arcs and circles:
3. Irregular curved lines,
4. Straight horizontal lines.
5. Straight vertical lines.
6. Dotted circular arcs.
7. Dotted lines,
8. Witness and dimension lines.
9. Dimensions notes, title.
10. Section lining.

When a large or complicated drawing is to be inked it is advisable to ink one view at a time or to start only so much as can be completed on the same day. If a view is left uncompleted it will generally be found very difficult to join the various lines, because the cloth is very sensitive to atmospheric changes which cause it to stretch.

Assembly Drawings. - An assembly drawing shows the parts of a machine in their proper relation to one another. There are many kinds of assembly drawings, some of which will be described.

An outline or Setting drawing is frequently made to show the appearance of the machine, give center distances, and overall dimensions. Such drawings are used to illustrate the machine to prospective customers, to lay out the foundation, and for locating the machine in its building. Fig. 159 shows one form of such a drawing.

An Assembly Working Drawing is often made when only a few of the machines are to be constructed. Such a drawing might contain a number of part views or sections. It would be completely dimensioned so that no separate or detail drawings would be required. Fig. 175 shows such a drawing.

Part Assembly Drawings are sometimes made giving a few pieces in their proper relation to each other and either partly or completely dimensioned. When completely dimensioned no further detail drawings are made.

Assembly drawings made to show the sizes, location, and method of fastening pipes and wires are called piping or wiring diagrams or drawings, depending upon how completely they are figured.

Erection Drawings are an important class of assembly drawings. They show the proper order of putting the parts together, dimensions, such as center distances, which must be exact, give the location of oil holes, valves, switches, etc., and methods of making adjustments.

Diagram Drawings are used by many concerns. These comprise a sectional or external view of the whole of the machine upon which the parts can be numbered or named. Such a drawing frequently contains a Ust of the parts, drawing numbers, pattern numbers, materials, weight, and other information.

Outline drawings are often used for catalogs, advertising, and similar purposes. Some of the points to be considered are given in the following list. The one upon which emphasis must be put will depend upon the use to which the drawing is to be put.

1. Get the important points.
2. Sense of proportion.
3. Suggestion.
4. Simplicity (few lines).
5. Record peculiarities in shape or design.
6. Use notes if necessary.
7. Number of machine.
8. Name of manufacturer.
9. Trade names.
10. Use of shading.
11. Not necessarily to scale.

Show Drawings are sometimes made. These are often in the nature of a picture in which the center lines and dimensions are left off (Fig. 160). Line shading as explained in a later chapter is often used. A good effect may sometimes be obtained by mass shading with a soft pencil, using the dull side of the tracing cloth. For more particular work on paper, india ink tinting applied with a brush can be used.

Blueprints. - The object of making tracings is to provide a convenient means for obtaining several copies of the original drawing. The most common method is by the blueprinting process. Blueprint paper is paper which has been coated with iron salts which are sensitive to light. The method of making blueprints is as follows:

Place the tracing with the right side or inked side next to the glass of a printing frame as shown in Fig. 167. Next place a piece of blueprint paper on the tracing with the coated side down. Follow this with the felt pad and close the frame. Expose to the direct sunlight as indicated in Fig. 168. The length of the exposure varies from 30 seconds in strong sunlight with rapid printing paper to three or four minutes under the same conditions with slow printing paper. The time can best be found by trial, as the age of the paper and the brightness of the light all exert an influence. After exposing, the paper should be removed and thoroughly washed. The excess water may be blotted off and the print hung up to dry. New paper has a yellow color on the coated side. After exposure this changes to a gray-bronze except where the lines of the tracing prevent the Ught from reaching it.

Electric light is very generally used in the larger mechanical factories for making blueprints. Machines for this purpose as well as many other methods of duplication are described in drawing supply catalogs to which the reader is referred.