Mechanical Drawing Self-Taught Part 16

_EXAMPLES IN LINE-SHADING AND DRAWINGS FOR LINE-SHADED ENGRAVINGS._

Although in workshop drawings, line-shading is rarely employed, yet where a design rather than the particular details of construction is to be shown, line-shading is a valuable accessory. Figure 295, for example, is intended to show an arrangement of idle pulleys to guide belts from one pulley to another; the principle being that so long as the belt pa.s.ses to a pulley moving in line with the line of rotation of the pulley, the belt will run correctly, although it may leave the pulley at considerable angle. When a belt envelops two pulleys that are at a right angle to each other, two guide pulleys are needed in order that the belt may, in pa.s.sing to each pulley, move in the same plane as the pulley rotates in, and the belt is in this case given what is termed a quarter twist.

It will be observed that by the line-shading even the twist of the belt is much more clearly shown than it would be if left unshaded.

An excellent example of shading is given in Figure 296, which is extracted from the _American Machinist_, and represents a cutting tool for a planing machine. The figure is from a wood engraving, but the effect may be produced by lines, the black parts being considered as simply broad black lines.

[Ill.u.s.tration: Fig. 295.]

The drawings from which engravings are made are drawn to conform to the process by which the engraving is to be produced. Drawings that are shaded by plain lines may be engraved by three methods. First, the drawing may be photo-engraved, in which process the drawing is photographed on the metal, and every line appears in the engraving precisely as it appears in the drawing.

[Ill.u.s.tration: Fig. 296.]

For this kind of engraving the drawing may be made of any convenient size that is larger than the size of engraving to be produced, the reduction of size being produced in the photographing process. Drawings for photo-engraving require to have the lines jet black, and it is to be remembered that if red centrelines are marked on the drawing, they will be produced as ordinary black lines in the engraving.

The shading on a drawing to be photo-engraved must be produced by lines, and not by tints, for tints, whether of black or of colors, will not photo-engrave properly.

It is generally preferred to make the drawing for a photo-engraving larger than the engraving that is to be made from it, a good proportion being to make the drawing twice the length the engraving is to be. This serves to reduce the magnitude of any roughness in the lines of the drawing, and, therefore, to make the engraving better than the drawing.

The thickness of the lines in the drawing should be made to suit the amount of reduction to be made, because the lines are reduced in thickness in the same proportion as the engraving is reduced from the drawing. Thus the lines on an engraving reduced to one-half the dimensions of the drawing would be one-half as thick as the lines on the drawing.

Drawings for photo-engraving should be made on smooth-faced paper; as, for example, on Bristol board; and to make the lines clean and clear, the drawing instruments should be in the best of condition, and the paper or Bristol board quite dry. The India rubber should be used as little as possible on drawings to be photo-engraved, because, if used before the lines are inked in, it roughens the surface of the paper, and the inking lines will be less smooth and even at their edges; and for this reason it is better not to rub out any lines until all the lines have been inked in. If used to excess after the lines have been inked in it serves to reduce the blackness of the lines, and may so pale them that they will not properly photo-engrave.

To make a drawing for an engraver in wood it would be drawn directly on the face of the box-wood block, on which it is to be engraved. The surface of the block is first whitened by a white water color, as Chinese white. If the drawing that is to be used as a copy is on sufficiently thin paper, its outline may be traced over by pencil lines, and the copy may then be laid face down on the wood block and its edges held to the block by wax, the pencilled lines being face to the block.

The outline may then be again traced over with a pencil or pointed instrument, causing the imprint of the lead pencil lines to be left on the whitened surface of the block. If the copy is on paper too thick to be thus employed, a tracing may be made and used as above; it being borne in mind that the tracing must be laid with the pencilled lines on the block, because what is the right hand of the drawing on the block is the left hand in the print it gives. The shading on wood blocks is given by tints of India ink aided by pencilled lines, or of course pencilled lines only may for less artistic work be used. Another method is to photograph the drawing direct upon the surface of the wood block; it is unnecessary, however, to enter into this part of the subject.

The third method of producing an engraving from a drawing is by means of what is known as the wax process. Drawings for this process should be made on thin paper, for the following reasons: The process consists, briefly stated, in coating a copper plate with a layer of wax about 1/32 inch deep, and in drawing upon the wax the lines to compose the engraving, which lines are produced by means of tools that remove the wax down to the surface of the copper.

The plate and wax are then placed in a battery and a deposit of copper fills in the lines and surface of the wax, thus forming the engraving.

Now if the drawing is made on thin paper, the engraver coats the surface of the drawing with a dry red pigment, and with a pointed instrument traces over the lines of the drawing, which causes them to leave a red imprint on the surface of the wax, and after the drawing is removed the engraver cuts these imprinted lines in the wax. If the drawing is on thick paper, this method of transferring the drawing to the wax cannot be used, and the engraver may take a tracing from the drawing and transfer from the tracing to the wax. It is obvious, also, that for wax engravings the drawing should be made of the same size that the engraving is required to be, or otherwise the tracing process described cannot be used. Figure 297 represents an engraving made by the wax process from a print from a wood engraving, and it is obvious that since all the lines drawn on the wax sink down to the surface of the copper plate, the shading is virtually composed of lines, the black surfaces being where the lines have been sufficiently close together and broad to remove all the wax enclosed within those surfaces.

[Ill.u.s.tration: Fig. 297.]

[Ill.u.s.tration: Fig. 298.]

The wax process is, however, more suitable for engravings in plain outline only, and is especially excellent when the parts are small and the lines fall close together; as, for example, in Figures 298 and 299, which are engravings of a boiler drilling machine, and were produced for the _American Machinist_ by tracing over a wood engraving from London, "Engineering" in the manner already described. The fineness and cleanness of the lines in the wax process is here well ill.u.s.trated, the disposition of the parts being easily seen from the engraving, and easily followed in connection with the following description:

The machine consists of two horizontal bed-plates A 1 and A 2, made with $V$ slides on top, and placed at right angles to each other. Upon each of the bed-plates is fitted a vertical arm B 1 and B 2, each of which carries two saddles, C 1 and C 2, these being each adjustable vertically on its respective arm by means of rack and pinion and hand wheels D 1 and D 2. The saddles are balanced so that the least possible exertion is sufficient to adjust them. The vertical arms, B 1 and B 2, are cast each with a round foot by which the arms are attached to the square boxes E 1 and E 2, which are fitted to the $V$ slides on the horizontal beds A 1 and A 2, and are adjustable thereon by means of screw and ratchet motion F 1 and F 2. Each of the square boxes has cast on it a small arm G 1 and G 2, carrying studs upon which run pinions gearing into the circular racks at the foot of the vertical arms. The square boxes have each a circular groove turned in the top to receive the bolts by which the vertical arms are connected to them, and thus the vertical arms, and with them the drill spindles N 1 and N 2, are adjustable radially with the boiler--the adjustment being effected by means of the pinions and circular racks. The pinions are arranged so that they can be worked with the same screw key that is used for the bolts in the circular grooves.

The sh.e.l.l to be drilled is placed upon the circular table H, which is carried by suitable framework adjustable by means of screw on a $V$ slide I, placed at an angle of 45 with the horizontal bed-plates. By this arrangement, when the table is moved along I, it will approach to or recede from all the drills equally. J 1 and J 2 are girders forming additional bearings for the framework of the table. The bed-plates and slides for the table are bolted and braced together, making the whole machine very firm and rigid. Power is applied to the machine through the cones K 1 and K 2, working the horizontal and vertical shafts L 1 and L 2, etc. On the vertical shafts are fitted coa.r.s.e pitch worms sliding on feather keys, and carried with the saddles C 1 and C 2, etc. The worms gearing with the worm wheels M 1 and M 2 are fitted on the sleeves of the steel spindles N 1 and N 2. The spindles are fitted with self-acting motions O 1 and O 2, which are easily thrown in and out of gear.

The machine is also used for turning the edge of the f.l.a.n.g.es which some makers prefer to have on the end plates of marine boilers. The plates are very readily fixed to the circular table H, and the edge of the f.l.a.n.g.e trued up much quicker than by the ordinary means of chipping.

When the machine is used for this purpose, the cross beam P, which is removable, is fastened to the two upright brackets R 1 and R 2. The cross beam is cast with $V$ slides at one side for a little more than half its length from one end, and on the opposite side for the same length, but from the opposite end. The $V$ slides are each fitted with a tool box S 1 and S 2, having a screw adjustment for setting the tool to the depth of cut, and adjustable on the $V$ slides of the cross beam to the diameter of the plate to be turned. This arrangement of the machine is also used for cutting out the furnace mouths in the boiler ends. The plate is fastened to the circular table, the centre of the hole to be cut out being placed over the centre of table; one or both of the tool boxes may be used. There is sufficient s.p.a.ce between the upright brackets R 1 and R 2, to allow that section of a boiler end which contains the furnace mouths to revolve while the holes are being cut out; the plate belonging to the end of a boiler of the largest diameter that the machine will take in for drilling. The holes cut out will be from 2 feet 3 inches in diameter and upwards. Power for using the turntable is applied through the cone T. The bevel wheels, worms, worm wheels, and pinions for driving the tables are of cast steel, which is necessary for the rough work of turning the f.l.a.n.g.es.

[Ill.u.s.tration: Fig. 299. (Page 275.)]

As to the practical results of using the machine, the drills are driven at a speed of 340 feet per minute at the cutting edges. A jet of soapsuds plays on each drill from an orifice 1/32 in. in diameter, and at a pressure of 60 lbs. per square inch. A joint composed of two 1-inch plates, and having holes 1 and one-eighth in. in diameter, can be drilled in about 2-1/2 minutes, and allowing about half a minute for adjusting the drill, each drill will do about 20 holes per hour. The machine is designed to stand any amount of work that the drills will bear. The time required for putting on the end of a boiler and turning the f.l.a.n.g.e thereon (say 14 feet diameter) is about 2-1/2 hours; much, however, depends on the state of the f.l.a.n.g.es, as sometimes they are very rough, while at others very little is necessary to true them up. The time required for putting on the plate containing the furnace mouths and cutting out three holes 2 feet 6 in. in diameter, the plate being 1 and one-eighth in. thick, is three hours. Of course, if several boilers of one size are being made at the same time, the holes in two or more of these plates can be cut out at once. The machine is of such design that it can be placed with one of the horizontal bed-plates (say A 1), parallel and close up to a wall of the boiler shop; and when the turning apparatus is being used, the vertical arm B 2 can be swiveled half way round on its square box E 2, and used for drilling and tapping the stay holes in marine boiler ends after they are put together; of course sufficient room must be left between bed-plate A 2, and the wall of boiler shop parallel with it, to allow for reception of the boiler to be operated upon.

It would obviously be quite difficult to draw such drawings as in Figures 298 and 299 on thin paper, so as to enable the drawing to be traced on the wax direct by the process before described, unless indeed the draftsman had considerable experience in fine work; hence, it is not uncommon to make the drawing large, and on ordinary drawing paper. The engraver then has the drawing photographed on the surface of the wax, and works to the photograph. The letters of reference in wax engravings are put in by impressing type in the wax, and in this connection it may be remarked that the letters I and O should not be used on drawings to be engraved by the wax process, unless they are situated outside the outlines of the drawing, because the I looks so much like part of a dotted line that it is often indistinguishable therefrom, while the O looks like a circle or an ellipse.

CHAPTER XIV.

_SHADING AND COLORING DRAWINGS._

The shading or coloring of drawings by tints is more employed in large drawings than in small ones, and in Europe than in the United States; while on the other hand tinting by means of line-shading is more employed in the United States than in Europe, and more on small drawings than on large ones.

Many draftsmen adopt the plan of coloring the journals of shafts, etc., with a light tint, giving them the deepest tint at the circ.u.mference to give them a cylindrical appearance. This makes the drawing much clearer and takes but little time to do, and is especially advantageous where the parts are small or on a small scale, so that the lines are comparatively close together.

For simple shading purposes black tints of various degrees of darkness may be employed, but it is usual to tint bra.s.s work with yellow. Cast iron with India ink, wrought iron with Prussian blue, steel with as light purple tint produced by mixing India ink, Prussian blue and a tinge of crimson lake. Copper is tinted red. On plane surfaces an even tint of color is laid, but if the surfaces are cylindrical they are usually colored deeper at and near the circ.u.mference, and are tinted over the colors with light tints of India ink to show their cylindrical form.

If a drawing is to be colored or shaded with India ink the paper should be glued all around its edges to the drawing board, and then dampened evenly all over with a sponge, which will cause the paper to shrink and lay close to the surface of the drawing board. If, in applying a color or a tint, the color dries before the whole surface is colored, the color will not be of an equal shade; hence it is necessary before applying the color to dampen the surface, if it is a large one, so that the color at one part shall not get dry before there has been time to go over the whole surface; a more even depth of color is attained by the application of several coats of a light tint, than with one coat, giving the full depth of color. But if the paper is not allowed to dry sufficiently between the coats, or if it has been made too wet previous to the application of the colors, it will run in places, leaving other hollows into which the color will flow, making darker-colored spots. To avoid this the paper may be dried somewhat by the application of clean blotting paper.

To maintain an even shade of color, it is necessary to slightly stir up the color each time the brush is dipped into the color saucer or palette, especially when the coloring is composed of mixed colors, because the coloring matter is apt to separate from the water and sink to the bottom.

So, also, in mixing colors it is best to apply the end of the color to the surface of the palette and not to apply the brush direct to the cake of color, because the color is more completely mixed by contact with the palette than it can be by the brush, which may retain a speck of color that will, unless washed out, make a streak upon the drawing.

To graduate the depth of tint for a cylindrical surface, it is best to mix several, as, say three depths or degrees of tint, and to first use the darkest, applying it in the direction in which the piece is to be shaded darkest. The width this dark application should be is obviously determined by the diameter of the piece. The next operation is to lighten or draw the part, line or streak thus dark colored, causing it to get paler and paler as it approaches the axial line of the piece or cylinder. This lightening is accomplished as follows: The dark streak is applied along such a length of the piece that it will not dry before there has been time to draw it out or lighten it on the side towards the axis. A separate brush may then be wetted and drawn along the edge of the dark streak in short strokes, causing the color to run outwards and become lighter as it approaches the axis. It will be found that during this process the brush will occasionally require washing in water, because from continuous contact with the dark streak the tint it contains will darken. When the first coat has been laid and spread or drawn out from end to end of the piece, the process may be repeated two or three times, the most even results being obtained by making the first dark streak not too dark, and going over the drawing several times, but allowing the paper to get very nearly dry between each coat. In small cylindrical bodies, as, say 1/4 inch in diameter, the darkest line of shadow may be located at the lines representing the diameter of the piece, but in pieces of larger diameter the darkest line may be located at a short distance from the line that denotes the diameter or perimeter on the shadow or right-hand side of the piece, as is shown in many of the engravings that follow. It is obvious that if a drawing is to have dimensions marked on it, the coloring or tinting should not be deep enough to make it difficult to see the dimension figures.

The size of the brush to be used depends, of course, upon the size of the piece to be shaded or colored, and it is best to keep one brush for the dark tint and to never let the brush dry with the tint in it, as this makes it harsh. In a good brush the hairs are fine, lie close together when moistened, are smooth and yet sufficiently stiff or elastic to bend back slightly when the pressure is removed. If, when under pressure and nearly dry, the hairs will separate or the brush has no elasticity in it, good results cannot be obtained. All brushes should be well dried after use.

The light in shading is supposed to come in at the left-hand corner of the drawing, as was explained with reference to the shade line.

Excellent examples to copy and shade with the brush are given as follows:

Figure 300 represents a Medart pulley, constructed by the Hartford Steam Engineering Company; the arms and hub are cast in one piece, and the rim is a wrought iron band riveted to the arms, whose ends are turned or ground true with the hub bore. The figure is obviously a wood engraving, but it presents the varying degrees of shade or shadow with sufficient accuracy to form a good example to copy and brush shade with India ink.

Figure 301 represents a similar pulley with a double set of arms, forming an excellent example in perspective drawing, as well as for brush-shading.

[Ill.u.s.tration: Fig. 300.]

In brush-shading as with line-shading, the difficulties increase with an increase in the size of the piece, and the learner will find that after he has succeeded tolerably well in shading these small pulleys, it will be quite difficult, but excellent practice to shade the large pulley in Figure 302.

One of the princ.i.p.al considerations is to not let the color dry at the edges in one part while continuing the shading in another part of the same surface, hence it is best to begin at the edge or outline of the drawing and carry the work forward as quickly as possible, occasionally slightly wetting with water edges that require to be left while the shading is proceeding in another direction.

[Ill.u.s.tration: Fig. 301.]

When it is required to show by the shading that the surfaces are highly polished, the lighter parts of the shading are made to contain what may be termed splashes of lighter and darker shadow, as in Figure 303, which represents an oil cup, having a bra.s.s casing enclosing a gla.s.s cylinder, which appears through the openings in the bra.s.s sh.e.l.l.

Figure 304 represents an iron planing machine whose line-shading is so evenly effected that it affords an excellent example of shading. Its parts are similar to those shown in the iron planer in Figure 297, save that it carries two sliding heads, so as to enable the use, simultaneously, of two cutting tools.

[Ill.u.s.tration: Fig. 304. (Page 282.)]

[Ill.u.s.tration: Fig. 302.]

A superior example in shading is shown in Figures 305 and 306, which represent a plan and a sectional view of the steam-cylinder of a Blake's patent direct-acting steam-pump. The construction of the parts is as follows: A is the steam-piston, H 1 and H are the cylinder steam-pa.s.sages; M is the cylinder exhaust port.