Part 6

If it is a shop drawing or sketch not to be preserved, use detail paper, which is the most economical and will stand a great deal of handling without becoming soiled. If it is a detailed plan, finished drawing or a picture, use the best white drawing paper to be obtained, so that your drawings can be preserved indefinitely without danger of fading, which is due either to the paper being poorly made and discoloring with age, or being of poor fiber and absorbing the ink or color, and the drawing consequently losing its brightness.

After deciding on the size of paper most suitable for the work, then carefully select the paper embracing the most qualities of value for the proposed drawing.

Mechanical Drawing.

In distinction to “free-hand,” mechanical drawing is executed almost entirely by the use of the instruments previously described; hence its other term, instrumental drawing. To define it particularly it may be said that,--

Mechanical drawing is the correct reproduction of any figure or part of a machine, whether of full size or reduced in the proportion of one part to another; it also comprises the art of delineating the interior parts which are hidden from view in solid bodies.

A mechanical drawing is the vehicle for conveying the ideas of the designer to those who are to embody them in wood and metal, and the considerations which should govern its production are those which affect its clearness and legibility or those which facilitate reference to it.

Drawings consist of plans, elevations and sections; plans being views on the top of the object in a horizontal plane; elevations, views on the sides of the object in vertical planes; and sections, views taken on bisecting planes, at any angle through an object.

Drawings in true elevation or in section are based upon flat planes, and given dimensions parallel to the planes in which the views are taken.

Two elevations taken at right angles to each other fix all points, and give all dimensions of parts that have their axis parallel to the planes on which the views are taken; but when a machine is complex, or when several parts lie in the same plane, three and sometimes four views are required to display all the parts in a comprehensive manner.

A man must have either a natural talent for hand-drawing or years of experience, before he can produce a sketch and “dimension” it, fit to work from; hence the elementary character of the examples given for practice. A “pretty” drawing is not expected from a beginner; it should always be borne in mind, that correctness of dimensions and general clearness, rather than elaborate finish, are what will save the battle in the days of competition.

Mechanical drawings should be made with reference to all the processes that are required in the construction of the work, and the drawings should be responsible, not only for dimensions, but for adaptation to fitting, forging, pattern-making, moulding, and so on.

Every part laid down should have something to govern it that may be termed a “base”--some position which, if understood, will suggest size, shape and relation to other parts. Searching after a base for each and every part and detail, the draughtsman should proceed upon a regular system, continually maintaining a test of what is done.

A mechanical drawing consists chiefly of three views:

1. The plan or top view. 2. The side elevation. 3. The end elevation.

In addition to the above, drawings are used to show interior portions of the figure; these are termed “sections,” and they may be taken where any plane crosses another.

NOTE.--The word _elevation_, as applied to mechanical drawings, means simply a view; hence a side elevation is a side view, or an end elevation is an end view.

The word _plan_ is employed in place of the word top; hence a plan view is a top view or a view looking down upon the top of the piece.

A _general_ view means a view showing the machine put together or assembled, while a detail drawing is one containing a detail, as a part of the machine or a single piece disconnected from the other parts of the whole machine.

Penciling.

It is very nearly true, as has been said, that every mechanical drawing is, in the first instance, penciled; if this is so, then more work is done with the pencil than with the pen; therefore the first attention of the student of mechanical drawing should be directed to the following instructions for penciling a drawing.

With all necessary materials in hand, and in good order for the beginning of a drawing, the first thing to do is to pin the paper on the board quite square.

To do this effectively, lay the paper flat and put on the T-square with its head at the left side of the board; slide the square up nearly to the top, and arrange the paper level with the blade; with the right hand hold the paper still and move the square down a little; now, pin the top of the paper with thumb-tacks.

Next, pressing the square lightly to the paper, slide it down to the bottom and pin that part of the paper to the board. The paper must not project outside or over the edges of the board, and the pins or tack-heads should be forced down flush with the paper, so as not to interfere with the free movement of the tee-square up and down the board as occasion may require.

The accuracy of the work depending upon their condition, it is first needful to see that the pencil and pencil compasses are properly sharpened. Reference is made to valuable directions contained on pages beginning with 55, under the heading of “Free-hand Drawing,” to which may be added that--

All lines should be drawn with the pencil slightly inclined in the direction in which it is moved.

Any and all lines not needed in the finished drawing should be erased at one time after the final lines have been determined, for the surface of the paper is soiled very quickly when worked upon after erasures have been made.

The working lines and other lines that are to be removed should be erased when the drawing is ready to finish and before its outlines have been strengthened, in order that the final lines may be left in perfect condition.

To show where the lines meet or terminate it is needful that all pencil lines pass the actual ending place, making a distinct intersection. This does not apply to “inking in” the lines, but rather to prevent the over-drawing of the ink lines, because the edge of the rule and the pen itself obstruct or partly cover the view of the line, it is very liable to pass over or beyond the required point in inking the lines, which must not occur.

In the preliminary operation of producing a regular mechanical or instrumental drawing, it is necessary to make a “sketch,” in pencil, of the object to be represented. The AMERICAN MACHINIST has given in a few words the order to be followed, in effecting the best results; we quote, as follows:

“In making a free-hand sketch of an object from the model it is well to observe the following order: Look the model over carefully and determine the number of views necessary to illustrate it fully, drawing the same, free-hand, in their proper relation to each other, on sketching paper. Look the sketch over carefully to see that nothing has been omitted, and put on dimension lines, after which scale the model carefully and put on dimensions. Do not put in the dimensions at the same time the dimension lines are drawn; have all the dimension lines in place before attempting to insert dimensions.

“Follow the same order in making the drawing with instruments as was used in making the sketch; that is, draw the views in their proper relation to each other, put in dimension lines, then dimensions, and lastly notes and title. If section drawing is made, do not draw section lines in pencil.”

POINTS TO BE OBSERVED IN SKETCHING.

1. Especial attention is to be paid to outlines--edges of plane surfaces are lines; when a line is made it represents the edge or outside of something.

2. Learn to be accurate before being rapid.

3. A sketch should be intelligible to any one, even if they are unacquainted with drawing.

4. Horizontal and vertical lines and a few curves will enable one to make almost any simple sketch.

5. It must be also remembered in making drawings from actual measurement that the instruments are not in the first place employed; the rough sketch is first made and then it is converted into a drawing. The draughtsman makes a rough sketch entirely by the hand and eye, measures the various parts, and jots down the measurements in his sketch; after this he reduces the whole to the desired scale, and proceeds to make his mechanical drawing.

6. Let the sketch book be the constant companion of the student; it may be advantageously filled with outlines of machine or other work suitable for preservation, to be made into finished drawings, or for reference. Sketches are often valuable for reference as aids in originating new designs.

7. A sketch, when possible, should have all the dimensions written upon it, but--

8. Sketches in shop practice should not take the place of working-drawings; the latter have a check upon them in being drawn to a scale--hence the figures written upon them and dimensions by scale must agree.

9. Place title and date on each sketch--no matter how seemingly unimportant--for future reference.

10. Practice sketching at every favorable opportunity. There is no necessity for detail at first--simply the outlines of the article and its parts.

11. Sketch-books, with paper bound in cloth covers, are utilized for bold, off-hand sketches by experienced draughtsmen, but a single sheet of paper, used on both sides, is not unworthy of service in an emergency--or even the blank side of a letter may be available. Sketching-blocks, or paper “pads,” 4 × 6, or more, in size, and containing 48 sheets, are sold by stationers, and are found to be most convenient to have in hand and for practical use. Portfolio-envelopes, made of extra length paper (manila) are useful in filing away sketches and drawings. The size 10¹⁄₂ × 15 is used for United States Patent Office drawings.

The function of the pencil--in mechanical drawing--is to make a path for the pen to follow. If it were possible to make a drawing with all its lines ending at the proper place, at the first time, there would be no necessity for using the pencil. One is obliged, however, so to use the pencil that all lines pass beyond the actual ending place, thus making a distinct point for the drawing pen to stop at.

The pencil should be pressed to the paper just enough to make a clean, fine line, and no more; once over the path is sufficient, if the line is visible and true.

To sharpen drawing pencils, 1, use a fine file, after taking off enough of the wood with a knife; 2, make a conical point for the free-hand drawing pencils and a chisel point for ruling and marking distances.

Pencil compasses are instruments where one leg is provided with a pencil point. Fig. 209 shows the mode of manipulation of those shown in fig. 180 and fig. 181.

The pencil compass is held by the projection above the joint between the thumb and first finger, which enables it to be rotated by a movement of the finger without causing any undue pressure on the points. Should much pressure be applied, there is a tendency to force the point or center through the paper, making an ugly center mark; at the same time the pressure tends to break off the pencil point.

These few illustrations from fig. 212 to fig. 219 are made designedly simple, so that they may be utilized in “free-hand” work, for which they are good practice, as well as serving for examples in mechanical drawing.

Figs. 212, showing a spool or bobbin, exhibit three views, viz.: front elevation or plan, and section; both are drawn with simple lines, the end elevation by circles.

Figs. 213 and 214 are two side views of a hexagon head bolt. Figs. 216 and 217 are a square head bolt--two side views and end view. Figs. 218 and 219 are a front and edge view of a forked or double joint.

Figs. 215 show three views of a file handle; the front view and section are practice for compound curves and curved lines meeting straight ones; all these are capable of being produced by instruments.

Moreover, many of the views and illustrations used to instruct and explain machine tools and other devices in other parts of the volume, are drawn so that they may be used also as examples in advanced instrumental practice. This is a “hint” to the diligent and painstaking student worthy of remembering.

The designing and drawing of arcs and whole circles occupy a large proportion of space in nearly all mechanical drawings. The making of a complete circle is a matter of no great difficulty, but the beginning and termination of parts of circles require both judgment and considerable practice.

To aid the student these two illustrations of circles are introduced. To draw fig. 220 with a pencil, using the upper edge of the blade of the T-square as a guide, draw a center line, _A B_, mark on it a distance of 4 inches, space this into half inches, using the dividers and making the points with it; then with the pencil compasses or bow pencil, which must be held as shown on page 209, and rotated from left to right, or clockwise, draw a series of circles through these points, tangent to one another or all touching at _A_, care being taken that the pencil lines exactly meet at _A_, and also cut the divided points as shown in the illustration. For fig. 221 divide the center line as before, and draw the semi-circles on it _A B_, _B C_, meeting at _B_, and _C D_, _D E_, etc.

Now from center _B_ draw circles _A C_, _C E_, meeting in _C_, and so on with the circles, arcs or segments; success in drawing this figure depends on the correct spacing of the center line in the first instance into equal parts.

The T-square should be used for drawing horizontal lines only. Its head should always be placed upon the left edge of the board. Vertical lines should be drawn by the use of a triangle placed upon the T-square and not by means of the T-square only; because the edges of a board are seldom at right angles to each other, and the blade of the T-square is often not at right angles to the head, so that lines at right angles to each other will not result from the use of the T-square upon all edges of the board. Only the upper edge of the T-square should be used, as the edges are often not quite straight or parallel.

The 45° triangle has two angles of 45° and one of 90°. The 30° and 60° triangle has an angle of 30°, one of 60°, and one of 90°. By placing these triangles upon the T-square, lines at any of these angles with a vertical or horizontal line may be drawn.

Drawings finished in ink are much more effective and desirable than pencil drawings; but as a good inked drawing cannot be made except upon an accurate pencil drawing, students should begin with the pencil, and should not use ink until they are able to produce satisfactory results in pencil.

Projection.

The word projection means to throw forward, and in ordinary machine drawing it is the projecting or throwing forward of one view from another view.

In drawings the lines in one view or plan may be availed of to find those of others of the same object, and also to find their shape or curvature as they would appear in the other representations; this is called projection-drawing.

Fig. 222 is the illustration as shown in fig. 212 on page 143, with the addition of dotted projection line, which illustrates the method of throwing forward the section and the end view of the object; these two views are procured from the plan or first figure, as shown in fig. 222.

Fig. 224 represents the square bolt and nut shown in fig. 216, and the mode of projecting is similarly shown by dotted lines.

Fig. 223 shows file handle shown in fig. 215 and the mode of projecting.

The principles upon which “projection” in drawing is based, are illustrated in the following examples and text: As a real object can be scaled with a foot rule, so a drawing must permit of scaling and measuring. This measuring may take place as with the real object in full size or the drawing may, for the sake of convenience, be reproduced and measured in a reduced scale, as half, or in still smaller sizes. Sometimes it may prove convenient to enlarge the drawing to twice the natural size of the object, as a means of making it stand out more clearly than the real size would accomplish.

For practical purposes, it is productive of economy of time to mark the dimensions of height and width or depth on the drawing in figures, to avoid the scaling. This marking of the dimensions is best done at the time of making the drawing, while the conception of the object is clear.

To convey a correct impression of the object, all lines that are marked to be of equal length should appear equally long on the drawing and be capable of being scaled to such equal length; for this, it must be assumed that the eye of the observer is equally distant from all points of a plane through the nearest point, or one of the axes of the object, and that the lines of sight are all parallel to each other and square to this plane.

In fig. 225 these lines of sight are seen as directed toward one side of a cube or block; it will be readily understood, however, that in this way nothing is visible and accessible for scaling and dimensioning except this front face of the block, thus, a determination of the dimensions of only height and width would be possible, while the dimension of depth is entirely undetermined.

It is thus necessary to get a view of the block from another side; the direction in which it will be most instructive to obtain additional views is in the direction of the breadth and of the length and square to the lines of sight of the first view.

Fig. 226 shows how the lines of sight would strike the object in the three directions. If these lines should be rays of light, some of them would pass by the body until they squarely strike the large plane surfaces, _I_, _II_, _III_; naturally the rays of light on the faces of the object will be retained, and cannot strike the plane surfaces, thus leaving dark shadows of exactly the same outlines as the block; these would be exact drawings of the faces, and if by some means they can be fixed and retained on the plane they can be completely measured and dimensioned.

This throwing forward of the outline of the object in different views on the planes is called projection of the object, and furnishes a highly important means of fixing the outlines and dimensions in the three main directions of height, width and depth; evidently, the light rays passing by the front face may not all reach the plane of projection, but they may be retained by protruding parts of the object behind the front face. These protruding parts naturally would also be projected on the plane in the same manner as the main body of the block.

These projections of the protruding part are plainly visible in plane _II_ and plane _III_, while the part would not be drawn in outline in plane _I_. It may be imagined, however, that the greater thickness of the body in the direction of the protruding part would intensify also the shadow, thus outlining the face of the protruding part in plane _I_.

It is apparent that these three projections are all needed, but as drawing is all done in one single plane, the three projections will for the sake of convenience have to be brought into a single plane. This can be realized if plane _II_ is swung around axis _O Z_ and plane _III_ around axis _O Y_, until all three surfaces are in one single plane, which would then appear as shown in fig. 227.

It is also possible to assume transparent planes in front of the body and extend the parallel lines of sight forward instead of backward. Thus an outline picture will be created on each of the three planes _I_, _II_, _III_ in fig. 228, in a manner similar to fig. 226. For drawing purposes, all three views again have to be brought into a single plane, which is done by swinging _II_ around _O Z_, and _III_ around _O Y_ in the same manner as fig. 227 was evolved from fig. 226.

It will be noted that in fig. 229 plane _III_ is now above _I_ and plane _II_ on the left-hand side of _I_, while in fig. 227 they were below and at the right-hand side of plane _I_. As the swinging of the top and side planes takes place around the edges of the front plane _I_ two systems may thus be distinguished, according to the position of plane _I_ in regard to the object. Fig. 226 thus represents the system of backward projection, while fig. 228 represents the system of forward projection.

Either system can have, however, the plane _II_ at the right- or left-hand side edge, while plane _III_ may be attached to the top or bottom edge of plane _I_; it is readily understood that a number of combinations are possible for each system, as it is not necessary to adhere absolutely to one rule. The system of forward projection is the one generally practiced and further examples are all executed by this system, meaning that the planes are always between the observer and the object.

For the clearness of the drawing, it is desirable to have all corners, edges and outlines appear in such solid lines as they appear to the eye. If, therefore, certain sharply defined outlines occur on one side more than on the opposite one, it is most desirable to take the view against the side that has the most definitely marked outlines.

If the opposite side should show a number of wholly different features, it may prove even desirable to show this side also, thus gaining four views instead of the usual three, and so obtain a more complete understanding of the shape of the object, besides giving increased facilities for dimensioning each part of the body.

This additional view may be taken from the sides, as well as up or down, thus making a maximum of five views possible by which the outside of the object may be delineated.

Fig. 230 shows why it may be desirable to take five views of a block that has a receding space of different outlines in each of the side and top and bottom faces.

It is not necessary, however, to resort to five views in such a case as is represented in fig. 230, as the only differing feature, the circular space in _III_ bottom, might be shown in _III_ top by dotted lines, and the difference of _II_ right might be shown in _II_ left, also by dotted lines. It is desirable to show four or five views only where great complication and consequent lack of clearness through numerous dotted lines would result from having a less number of views.

So far the projections and views have only represented the outlines of the object; it may often be desirable, however, to show central holes or other perforations or variations of sections; in this case it is possible to imagine the object cut in slices, by planes, through certain well defined axes, or other lines of distinctive importance, and then take a view of this sectional plane with its newly created intersections or sharply marked outlines; thus, a section may often take the place of the third, fourth or fifth view to great advantage.