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MECHANICAL DRAWING SELF-TAUGHT:
COMPRISING
INSTRUCTIONS IN THE SELECTION AND
PREPARATION OF DRAWING
INSTRUMENTS,
_ELEMENTARY INSTRUCTION IN PRACTICAL
MECHANICAL DRAWING_;
TOGETHER WITH
EXAMPLES IN SIMPLE GEOMETRY AND ELEMENTARY MECHANISM,
INCLUDING SCREW THREADS, GEAR WHEELS,
MECHANICAL MOTIONS, ENGINES AND BOILERS.
BY JOSHUA ROSE, M.E.,
AUTHOR OF "THE COMPLETE PRACTICAL MACHINIST,"
"THE PATTERN MAKER'S ASSISTANT,"
"THE SLIDE VALVE"
ILLUSTRATED BY THREE HUNDRED AND THIRTY ENGRAVINGS.
PHILADELPHIA:
HENRY CAREY BAIRD & CO.,
INDUSTRIAL PUBLISHERS, BOOKSELLERS AND IMPORTERS,
810 WALNUT STREET.
LONDON:
SAMPSON LOW, MARSTON, SEARLE & RIVINGTON,
CROWN BUILDINGS, 188 FLEET STREET.
1887.
Copyright by
JOSHUA ROSE.
1883.
PHILADELPHIA.
COLLINS, PRINTER
PREFACE.
The object of this book is to enable the beginner to learn to make
simple mechanical drawings without the aid of an instructor, and to
create an interest in the subject by giving examples such as the
machinist meets with in his every-day workshop practice. The plan of
representing in many examples the pencil lines, and numbering the order
in which they are marked, the author believes to possess great
advantages for the learner, since it is the producing of the pencil
lines that really proves the study, the inking in being merely a
curtailed repetition of the pencilling. Similarly when the drawing of a
piece, such, for example, as a fully developed screw thread, is shown
fully developed from end to end, even though the pencil lines were all
shown, yet the process of construction will be less clear than if the
process of development be shown gradually along the drawing. Thus
beginning at an end of the example the first pencil lines only may be
shown, and as the pencilling progresses to the right-hand, the
development may progress so that at the other or left-hand end, the
finished inked in and shaded thread may be shown, and between these two
ends will be found a part showing each stage of development of the
thread, all the lines being numbered in the order in which they were
marked. This prevents a confusion of lines, and makes it more easy to
follow or to copy the drawing.
It is the numerous inquiries from working machinists for a book of this
kind that have led the author to its production, which he hopes and
believes will meet the want thus indicated, giving to the learner a
sufficiently practical knowledge of mechanical drawing to enable him to
proceed further by copying such drawings as he may be able to obtain, or
by the aid of some of the more expensive and elaborate books already
published on the subject.
He believes that in learning mechanical drawing without the aid of an
instructor the chief difficulty is overcome when the learner has become
sufficiently familiar with the instruments to be enabled to use them
without hesitation or difficulty, and it is to attain this end that the
chapter on plotting mechanical motions and the succeeding examples have
been introduced; these forming studies that are easily followed by the
beginner; while sufficiently interesting to afford to the student
pleasure as well as profit.
NEW YORK, _February, 1883_.
CONTENTS.
CHAPTER I.
THE DRAWING BOARD.
The T square 18
The triangles 19
Curves 21
Selecting and testing drawing instruments 22
Lead pencils 23
Mixing India ink 25
The drawing paper 26
Tracing paper 29
The ink 30
Testing and selecting India ink 30
Draftsmen's measuring rules 33
CHAPTER II.
THE PREPARATION AND USE OF THE INSTRUMENTS.
Preparing the lining pen for use 34
The shapes of the lining pen points 35
Oil stoning pen points 36
Preparing the circle pen for use 38
The shape for circle pen points 38
Shaping circle pens for very small circles 39
A form of pen point recently introduced; forming the pen point 39
The method of oil-stoning circle pen points 40
The needle point and pen point 42
How to use the circle pen 43
German instrument to avoid slipping of a needle point 44
How to use the lining pen 45
Applying the ink to the bow-pen 46
Using a straight line or lining pen with a T square 47
CHAPTER III.
LINES AND CURVES.
Explanation of simple geometrical terms; radius; explanation of
conventional dotted lines 48
A line at a right angle to another; a point; parallel lines 49
A line produced; a line bisected; a line bounding a circle; an arc of a
circle; segments of a circle; the chord of an arc; a quadrant of a
circle 50
A sector of a circle; a line tangent to a circle; a semicircle; centre of
a circle; axis of a cylinder; to draw a circle that shall pass through
three given points 51
To find the centre from which an arc of a circle has been struck; the
degrees of a circle 52
The protractor 53
To find the angle of one line to another 54
To find the angles of three lines one to the other 55
Acute angles and obtuse angles 57
Triangles; right angle triangle; obtuse angle triangle; equilateral
triangle; isosceles triangle 58
Scalene triangle; a quadrangle; quadrilateral or tetragon 59
Rhomboid; trapezoid; trapezium 60
The construction of polygons 61
The names of regular polygons 62
The angles of regular polygons; the ellipse 63
Form of a true ellipse 69
The use of a trammel for drawing an ellipse 72
To draw a parabola mechanically 73
To draw a parabola by lines 74
To draw a heart cam 75
CHAPTER IV.
SHADOW LINES AND LINE-SHADING.
Section lining or cross-hatching 77
To represent cylindrical pieces one within the other; to represent a
number of pieces one within the other 78
To represent pieces put together and having slots or keyways through
them. 79
Effects of shading or cross-hatching 80
Lines in sectional shading or cross-hatching made to denote the material
of which the piece is composed--lead, wood, steel, brass, wrought
iron, cast iron 81
Line-shading 82
The shade line to indicate the shape of piece; representation of a
washer 83
A key drawn with a shade line; shade line applied to a nut; a German
pen regulated to draw lines of various breadths 84
Example of line-shading in perspective drawing, shown in a pipe threading
stock and die 85
A cylindrical pin line-shaded; two cylindrical pieces that join each
other; a lathe centre; a piece having a curved outline 86
Line-shading applied to a ball or sphere; applied to a pin in a socket
shown in section 87
A piece of tube, where the thickness of the tube is shown; where the
hollow or hole is seen, the piece shown in section; where the body is
bell-mouthed and the hollow curve shown by shading 88
Example of line-shading to denote the relative distances of various
surfaces from the eye 89
Line-shading to denote that the piece represented is of wood; shade-lines
being regular or irregular 90
CHAPTER V.
MARKING DIMENSIONS.
Examples in marking dimensions 91
CHAPTER VI.
THE ARRANGEMENT OF DIFFERENT VIEWS.
The different views of a mechanical drawing; elevation; plan; general
view; a figure to represent a solid cylinder 94
To represent the different sides of a cube; the use of a cross to denote a
square 95
A triangular piece requires two or three views 96
To represent a ring having hexagon cross section; examples; a rectangular
piece in two views 98
The position of the piece when in its place determines the name of the
view in the drawing 103
View of a lever 105
Best method of projecting one view from another; the two systems of
different views of a piece 106
CHAPTER VII.
EXAMPLES IN BOLTS, NUTS AND POLYGONS.
To represent the thread of a small screw 112
A bolt with a hexagon head 113
United States standard sizes for forged or unfinished bolts and nuts 116
The basis of the Franklin Institute or United States standard for bolts
and nuts; hexagonal or hexagon heads of bolts 118
Comparison of hexagon and square heads of bolts; chamfers 120
Without chamfer; best plan for view of both square and hexagon heads 123
Drawing different views of hexagon heads 125
To draw a square-headed bolt; to draw the end view of a hexagon head 125
Use of the triangle to divide circles 129
Scales giving the length of the sides of polygons 135
To find what a square body which measures one inch on each side measures
across the corners; to find what diameter a cylindrical piece of
wood must be turned to which is to be squared, and each side of which
square must measure an inch 136
To find a radius across corners of a hexagon or a six sided figure, the
length of a side being an inch 138
To draw a stud 142
To pencil in a cap nut; pencilling for a link having the hubs on one side
only 145
Link with hubs on both sides; pencil lines for a double eye or a knuckle
joint 146
Double eye or knuckle joint with an offset; a connecting rod end 147
A rod end with a round stem 148
A bolt with a square under the head 149
Example in which the corner where the round stem meets the square
under the head is sharp; a centre punch giving an example in
which the flat sides gradually run out upon a circle, the edges
forming curves 150
CHAPTER VIII.
SCREW THREADS AND SPIRALS.
Screw threads for small bolts with the angles of the thread drawn in, and
the method of doing this 152
A double thread; a round top and bottom thread such as the Whitworth
thread; a left hand thread; to draw screw threads of a large
diameter 156
Drawing the curves for screw threads 157
To draw the United States standard thread 160
To draw a square thread 162
Form of template for drawing the curves of threads 165
To show the thread depth in a top or end view of a nut; to draw a spiral
spring 166
To obtain an accurate division of the lines that divide the pitch 167
CHAPTER IX.
EXAMPLES FOR PRACTICE.
A locomotive spring; a stuffing box and gland; working drawings of a
coupling rod; dimensions and directions marked; a connecting rod
drawn and put together as it would be for the lathe, vise, or erecting
shop 169
Drawings for the blacksmith 172
A locomotive frame 174
Reducing scales 175
Making a drawing to scale 177
CHAPTER X.
PROJECTIONS.
A spiral wound around a cylinder whose end is cut off at an angle 178
A cylindrical body joining another at a right-angle; a Tee for
example 180
Other examples of Tees 181
Example of a cylinder intersecting a cone 186
A cylindrical body whose top face if viewed from one point would appear as
a straight line, or from another a circle 188
CHAPTER XI.
DRAWING GEAR WHEELS.
Names of the curves and lines of gear teeth 193
How to draw spur wheel teeth 194
Professor Willis' scale of tooth proportions 195
The application of the scale 197
How to find the curve for the tooth face 198
To trace hypocycloides for the flanks of teeth 200
Sectional view of a section of a wheel for showing the dimensions through
the arms and hub 202
To draw an edge view of a wheel; rules for drawing the teeth of wheels;
bevel gear wheels 203
The construction to find the curves 204
To draw the arcs for the teeth 205
To draw the pitch circle of the inner and small end of the pinion
teeth 206
One-half of a bevel gear and an edge view projected from the same 207
A pair of bevel wheels shown in section; drawing of a part of an Ames
lathe feed motion; small bevel gears 208
Example in which part of the gear is shown with teeth in, and the
remainder illustrated by circles; drawings of part of the feed
motion of a Niles horizontal tool work boring mill 209
Three bevel gears, one of which is line-shaded; the construction of oval
gearing; Professor Rankine's process for rectifying and subdividing
circular arcs 210
Various examples of laying out gear wheels 214
CHAPTER XII.
PLOTTING MECHANICAL MOTIONS.
To find how much motion an eccentric will give to its rod 223
To find how much a given amount of motion of a long arm will move the
short arm of a lever 224
Example of the end of a lever acting directly on a shoe; a short arm
having a roller acting upon a larger roller 225
A link introduced in the place of the roller to find the amount of motion
of the rod; a lever actuating a plunger in a vertical line, to find
how much a given amount of motion of the long arm will actuate the
plunger 226
Two levers upon their axles or shafts, the arms connected by a link and
one arm connected to a rod 227
A lever arm and cam in one piece on a shaft, a shoe sliding on the line,
and held against the cam face by the rod, to find the position of the
face of the shoe against the cam 228
To find the amount of motion imparted in a straight line to a rod,
attached to an eccentric strap 229
Examples in drawing the cut off cams employed instead of eccentrics on
river steamboats in the Western and Southern States. Different views
of a pair of cams 232
The object of using a cam instead of an eccentric 234
Method of drawing or marking out a full stroke cam 237
Illustration of the lines embracing cut off cams of varying limits of
cut-off 240
Part played by the stroke of the engine in determining the conformation
of cut-off cams; manner of finding essential points of drawings of
cutoff cams 241
A cam designed to cut off the steam at five-eighths of the piston
stroke 244
Three-fourths and seven-eighths cams 246
Necessary imperfections in the operations of cut-off cams 247
Drawing representing the motion which a crank imparts to a connecting
rod 249
Plotting out the motion of a shaper link quick return 250
Plotting out the Whitworth quick return motion employed in machines 253
Finding the curves for moulding cutters 257
CHAPTER XIII.
EXAMPLES IN LINE-SHADING AND DRAWING FOR LINE-SHADED
ENGRAVINGS.
Arrangement of idle pulleys to guide bolts from one pulley to another;
representation of a cutting tool for a planing machine 264
Drawings for photo-engraving 267
Drawing for an engraver in wood; drawings for engravings by the wax
process 268
Engraving made by the wax process from a print from a wood engraving;
engravings of a boiler drilling machine 269
CHAPTER XIV.
SHADING AND COLORING DRAWINGS.
Coloring the journals of shafts; simple shading; drawing cast-iron,
wrought iron, steel and copper 277
Points to be observed in coloring and shading; colored drawings to be
glued around their edges to the drawing board; to maintain an even
shade of color; mixing colors 278
To graduate the depth of tint for a cylindrical surface 279
The size and use of brushes; light in shading; example for shading a
Medart pulley 280
Brush shading 281
To show by the shading that the surfaces are highly polished;
representation of an oil cup; representation of an iron planing
machine 282
Example in shading of Blake's patent direct acting steam pump 284
Example of shading an independent condenser 288
CHAPTER XV.
EXAMPLES OF ENGINE WORK.
Drawings of an automatic high speed engine; side and end views of the
engine; vertical section of the cylinder through the valve face 289
Valve motion; governor 292
Pillow box, block crank-pin, wheel and main journal 294
Side and edge view of the connecting rod 295
A two hundred horse power horizontal steam boiler for a stationary engine;
cross sectional view of the boiler shell 296
Side elevation, end view of the boiler, and setting 297
Working drawings of a one hundred horse power engine; plan and side
view of the bed plate, with the main bearing and guide bars; cross
sections of the bed plate; side elevation of the cylinder, with end
view of the same 299
Steam chest side and horizontal cross section of the cylinder; steam chest
and the valves; cam wrist plate and cut-off mechanism; shaft for the
cam plate; cross head; side view and section through the centre of
the eccentric and strap 301
Construction of the connecting rod 303
INDEX 305
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|Transcriber's note: In this text $T$ indicates a larger capital letter.|
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MECHANICAL DRAWING
SELF-TAUGHT.
CHAPTER I.
_THE DRAWING BOARD._
A Drawing Board should be of soft pine and free from knots, so that it
will easily receive the pins or tacks used to fasten down the paper. Its
surface should be flat and level, or a little rounding, so that the
paper shall lie close to its surface, which is one of the first
requisites in making a good drawing. Its edges should be straight and at
a right angle one to the other, and the ends of the battens B B in
Figure 1 should fall a little short of the edge A of the board, so that
if the latter shrinks they will not protrude. The size of the board of
course depends upon the size of the paper, hence it is best to obtain a
board as small as will answer for the size of paper it is intended to
use. The student will find it most convenient as well as cheapest to
learn on small drawings rather than large ones, since they take less
time to make, and cost less for paper; and although they require more
skill to make, yet are preferable for the beginner, because he does not
require to reach so far over the board, and furthermore, they teach him
more quickly and effectively. He who can make a fair drawing having
short lines and small curves can make a better one if it has large
curves, etc., because it is easier to draw a large than a very small
circle or curve. It is unnecessary to enter into a description of the
various kinds of drawing boards in use, because if the student purchases
one he will be duly informed of the kinds and their special features,
while if he intends to make one the sketch in Figure 1 will give him all
the information he requires, save that, as before noted, the wood must
be soft pine, well seasoned and free from knots, while the battens B
should be dovetailed in and the face of the board trued after they are
glued and driven in. To true the edges square, it is best to make the
two longest edges parallel and straight, and then the ends may be
squared from those long edges.
[Illustration: Fig. 1.]
THE $T$ SQUARE.
Drawing squares or T squares, as they are termed, are made of wood, of
hard rubber and of steel.
There are several kinds of T squares; in one the blade is solid, as it
is shown in Figure 5 on page 20; in another the back of the square is
pivoted, so that the blade can be set to draw lines at an angle as well
as across the board, which is often very convenient, although this
double back prevents the triangles, when used in some positions, from
coming close enough to the left hand side of the board. In an improved
form of steel square, with pivoted blade, shown in Figure 2, the back is
provided with a half circle divided into the degrees of a circle, so
that the blade can be set to any required degree of angle at once.
[Illustration: Fig. 2.]
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
THE TRIANGLES.
[Illustration: Fig. 5.]
Two triangles are all that are absolutely necessary for a beginner. The
first is that shown in Figure 3, which is called a triangle of 45
degrees, because its edge A is at that angle to edges B and C. That in
Figure 4 is called a triangle of 60 degrees, its edge A being at 60
degrees to B, and at 30 degrees to C. The edges P and C are at a right
angle or an angle of 90 degrees in both figures; hence they are in this
respect alike. By means of these triangles alone, a great many straight
line drawings may be made with ease without the use of a drawing square;
but it is better for the beginner to use the square at first. The manner
of using these triangles with the square is shown in Figure 5, in which
the triangle, Figure 3, is shown in three positions marked D E F, and
that shown in Figure 4 is shown in three positions, marked respectively
G H and I. It is obvious, however, that by turning I over, end for end,
another position is attained. The usefulness in these particular
triangles is because in the various positions shown they are capable of
use for drawing a very large proportion of the lines that occur in
mechanical drawing. The principal requirement in their use is to hold
them firmly to the square-blade without moving it, and without
permitting them to move upon it. The learner will find that this is best
attained by so regulating the height of the square-blade that the line
to be drawn does not come down too near the bottom of the triangle or
edge of the square-blade, nor too high on the triangle; that is to say,
too near its uppermost point. It is the left-hand edge of the triangle
that is used, whenever it can be done, to produce the required line.
[Illustration: Fig. 6.]
CURVES.
To draw curves that are not formed of arcs or parts of circles,
templates called curves are provided, examples of these forms being
given in Figure 6. They are made in wood and in hard rubber, the latter
being most durable; their uses are so obvious as to require no
explanation. It may be remarked, however, that the use of curves gives
excellent practice, because they must be adjusted very accurately to
produce good results, and the drawing pen must be held in the same
vertical plane, or the curve drawn will not be true in its outline.
DRAWING INSTRUMENTS.
It is not intended or necessary to enter into an elaborate discussion of
the various kinds of drawing instruments, since the purchaser can obtain
a good set of drawing instruments from a reputable dealer by paying a
proportionate price, and must _per force_ learn to use such as his means
enable him to purchase. It is recommended that the beginner purchase as
good a set of instruments as his means will permit, and that if his
means are limited he purchase less than a full set of instruments,
having the same of good quality.
All the instruments that need be used in the examples of this book are
as follows:
A small spring bow-pen for circles, a lining pen or pen for straight
lines, a small spring bow-pencil for circles, a large bow-pen with a
removable leg to replace by a divider leg or a pencil leg, and having an
extension piece to increase its capacity.
The spring bow-pen should have a stiff spring, and should be opened out
to its full capacity to see that the spring acts well when so opened
out, keeping the legs stiff when opened for the larger diameters. The
purchaser should see that the joint for opening and closing the legs is
an easy but not a loose fit on the screw, and that the legs will not
move sideways. To test this latter, which is of great importance in the
spring bow-pencil as well as in the pen, it is well to close the legs
nearly together and taking one leg in one hand and the other leg in the
other hand (between the forefinger and thumb), pushing and pulling them
sideways, any motion in that direction being sufficient to condemn the
instrument. It is safest and best to have the two legs of the bow-pen
and pencil made from one piece of metal, and not of two separate pieces
screwed together at the top, as the screw will rarely hold them firmly
together. The points should be long and fine, and as round as possible.
In very small instruments separate points that are fastened with a screw
are objectionable, because, in very small circles, they hide the point
and make it difficult to apply the instrument to the exact proper point
or spot on the drawing.
The joints of the large bow or circle-pen should also be somewhat stiff,
and quite free from side motion, and the extension piece should be
rigidly secured when held by the screw. It is a good plan in purchasing
to put in the extension piece, open the joint and the pen to their
fullest, and draw a circle, moving the pen in one direction, and then
redraw it, moving it in the other direction, and if one line only
appears and that not thickened by the second drawing, the pen is a good
one.
The lead pencil should be of hard lead, and it is recommended that they
be of the H, H, H, H, H, H, in the English grades, which corresponds to
the V, V, H, of the Dixon grade. The pencil lines should be made as
lightly as possible; first, because the presence of the lead on the
paper tends to prevent the ink from passing to the paper; and, secondly,
because in rubbing out the pencil lines the ink lines are reduced in
blackness and the surface of the paper becomes roughened, so that it
will soil easier and be harder to clean. In order to produce fine pencil
lines without requiring a very frequent sharpening of the pencil it is
best to sharpen the pencil as in Figures 7 and 8, so that the edge shall
be long in the direction in which it is moved, which is denoted by the
arrow in Figure 7. But when very fine work is to be done, as in the case
of Patent Office drawings, a long, round point is preferable, because
the eye can see plainer just where the pencil will begin to mark and
leave off; hence the pencil lines will not be so liable to overrun.
[Illustration: Fig. 7.]
[Illustration: Fig. 8.]
In place of the ordinary wood-covered lead pencils there may be obtained
at the drawing material stores pencil holders for holding the fine,
round sticks of lead, and these are by far the best for a learner. They
are easier to sharpen, and will slip in the holder, giving warning when
the draftsman is pressing them too hard on the paper, as he is apt to
do. The best method of trimming these leads, as also lead pencils after
they have been roughly shaped, is with a small fine file, holding the
file still and moving the pencil; or a good piece of emery paper or sand
paper is good, moving the pencil as before.
All lines in pencilling as in inking in should begin at the left hand
and be drawn towards the right, or when triangles are used the lines are
begun at the bottom and drawn towards the top or away from the
operator. The rubber used should not be of a harsh grade, since that
will roughen the face of the paper and probably cause the ink to run.
The less rubbing out the better the learner will progress, and the more
satisfaction he will receive from the results. If it becomes necessary
to scratch out it is best done with a penknife well sharpened, and not
applied too forcibly to the paper but somewhat lightly, and moved in
different and not all in one direction. After the penknife the rubber
may sometimes be used to advantage, since it will, if of a smooth grade,
leave the paper smoother than the knife. Finally, before inking in, the
surface that has been scraped should be condensed again by rubbing some
clean, hard substance over it which will prevent the ink from spreading.
The end of a paper-cutter or the end of a rounded ivory handled drawing
instrument is excellent for this purpose.
[Illustration: Fig. 9.]
[Illustration: Fig. 10.]
It is well to use the rubber for general purposes in such a way as to
fit it for special purposes; thus, in cleaning the sheet of paper, the
rubber may be applied first, as in Figure 9, as at A, and then as at B,
and if it be moved sideways at the same time it will wear to the form
shown in Figure 10, which will enable it to be applied along a line that
may require to be rubbed out without removing other and neighboring
lines. If the rubber is in the form of a square stick one end may be
bevelled, as in Figure 11, which is an excellent form, or it may be made
to have a point, as in Figure 12. The object is in each case to enable
the rubber action to be confined to the desired location on the paper,
so as to destroy its smooth surface as little as possible.
[Illustration: Fig. 11.]
[Illustration: Fig 12.]
For simple cleaning purposes, or to efface the pencil lines when they
are drawn very lightly, squares of sponge-rubber answer admirably, these
being furnished by the dealers in drawing materials.
A piece of bread will answer a similar purpose, but it is less
convenient.
For glazed surface paper, as Bristol-board, the smoothest rubber must be
used, the grade termed velvet rubber answering well.
THE DRAWING PAPER.
Whatever kind of drawing paper be used it should be kept dry, or the
ink, however good it may be, will be apt to run and make a thick line
that will not have the sharp, clean edges necessary to make lines look
well.
Drawing paper is made in various qualities, kinds, and forms, as
follows: The sizes and names of paper made in sheets are:
Cap, 13 x 16 inches.
Demy, 20 x 15 "
Medium, 22 x 17 "
Royal, 24 x 19 "
Super Royal, 27 x 19 "
Imperial, 30 x 21 "
Elephant, 28 x 22 "
Columbier, 34 x 23 "
Atlas, 33 x 26 "
Theorem, 34 x 28 "
Double Elephant, 40 x 26 "
Antiquarian, 52 x 31 "
Emperor, 40 x 60 "
Uncle Sam, 48 x 120 "
the thickness of the sheets increasing with their size. Some sheets of
paper are hot pressed, to give a smoother surface, and thus enable
cleaner-edged lines to be drawn.
[Illustration: Fig. 13.]
For large drawings paper is made in rolls of various widths, but as
rolled paper is troublesome to lay flat upon the drawing board, it is
recommended to the learner to obtain the sheets, which may be laid
sufficiently flat by means of broad headed pins, such as shown in Figure
13, which are called thumb tacks. These are forced through the paper
into the board at each corner, as in Figure 14 at _f_. On account of the
large diameter of the stems of these thumb tacks, which unduly pierce
and damage the board, and on account also of their heads, by reason of
their thickness, coming in the way of the square blade, it will be
found preferable to use the smallest sizes of ordinary iron tacks, with
flat heads, whose stems are much finer and heads much thinner than thumb
tacks. The objection to ordinary tacks is that they are more difficult
to remove, but they are, as stated, more desirable for use.
[Illustration: Fig. 14.]
[Illustration: Fig. 15.]
If the paper is nearly the full size of the board, it does not much
matter as to its precise location on the board, but otherwise it is best
to place it as near the left-hand edge of the board as convenient, as is
shown in Figure 14.
The lower edge, D, Figure 15, of the paper, however, should not be
placed too near the edge, A, of the board, because if the end P of the
square back comes down below the edge of the board, it is more difficult
to keep the square back true against the end of the board.
The paper must lie flat upon and close to the surface of the board, and
a sufficient number of tacks must be used to effect this purpose.
Drawings that are to be intricate, or to contain a great many lines, as
a drawing of an engine or of a machine, are best pasted or glued all
around the edges of the paper, which should first be dampened; but as
the learner will scarcely require to make such drawings until he is
somewhat familiar with and well practised in the use of the instruments,
this part of the subject need not be treated here.
TRACING PAPER.
For taking tracings from drawings tracing paper or tracing cloth is
used. They require to be stretched tightly and without wrinkles upon the
drawing. To effect this object the mucilage should be thick, and the
tracing paper should be dampened with a sponge after it is pasted. It
must be thoroughly dry before use, or the ink will run.
Tracing cloth must be fastened by pins or thumb tacks, and not dampened.
The drawing should be made on the polished side of the cloth, and any
coloring to be done should be on the other side, and done after the
tracing is removed from the drawing.
THE INK.
India ink should always be used for mechanical drawing: First, because
it lies upon and does not sink into the paper, and is, therefore, easily
erased; and, secondly, because it does not corrode or injure the drawing
instruments.
India ink is prepared in two forms--in the stick and in a liquid form.
The stick ink is mixed in what are termed saucers, or cabinet saucers,
one being placed above the other, so as to exclude the dust from
settling in it, and also to prevent the rapid evaporation to which it is
subject.
The surface of the saucer should be smooth, as any roughness grinds the
ink too coarsely, whereas the finer it is ground or mixed the easier it
will flow, the less liability to clog the instruments, and the smoother
and more flat it will lie upon the paper. In mixing the ink only a small
quantity of water should be used, the stick of ink being pressed
_lightly_ upon the saucer and moved quickly, the grinding being
continued until the ink is mixed quite thickly. This will grind the ink
fine as it is mixed, and more water may be added to thin it. It is best,
however, to let the ink be somewhat thick for use, and to keep it
covered when not in use; and though water may be added if it gets too
thick, yet ink that has once dried should not be mixed up again, as it
will not work so well after having once dried.
Of liquid inks the Higgins ink is by far the best, being quite equal to
and much more convenient for use than the best stick ink.
The difference between a good and an inferior India ink lies chiefly in
the extent to which the lamp-black, which is the coloring matter, forms
with the water a chemical solution rather than a mechanical mixture. In
inferior ink the lamp-black is more or less held in suspension, and by
prolonged exposure to the air will separate, so that on being spread the
solid particles will aggregate by themselves and the water by itself.
This explains why draughtsmen will, after the ink has been exposed to
the air for an hour or two, add a drop of mucilage to it; the mucilage
thickening the solution, adding weight to the water, and deferring the
separation of the lamp-black.
A good India ink is jet black, flows easily, lies close to, does not
stand upon or sink into the paper, and has an even lustre, the latter
being an indication of fineness. The more perfect the incorporation of
the lamp-black with the water the easier the ink will flow, the less
liable it is to clog the instruments, the more even and sharp the edges
of the lines, and the finer the lines that may be drawn.
Usually India ink can only be tested by actual trial; but since it is
desirable to test before purchasing it, it may be mentioned that one
method is to mix a little on the finger nail, and if it has a "bronzy"
gloss it is a good indication. It should also spread out and dry without
any tendency to separate.
The best method of testing is to mix a very little, and drop a single
drop in a tumbler of clear water. The best ink will diffuse itself over
the surface, and if the water is disturbed will diffuse itself through
the water, leaving it translucent and black, with a slight tinge of
bronze color. A coarser ink will act in a similar manner, but make the
water somewhat opaque, with a blue-black, or dull, ashy color. A still
coarser ink will, when diffused over the surface of the water, show fine
specks, like black dust, on the surface. This is readily apparent,
showing that the mixture of the ink is not homogeneous.
When it is an object to have the lines of a drawing show as black as
possible, as for drawings that are to be photo-engraved, the ink should
be mixed so thickly as to have a tendency to lift when a body, such as a
lead pencil, is lifted out of it. For Patent Office drawings some will
mix it so thickly that under the above test it appears a little stringy.
The thicker the ink can be used the better, because the tendency of the
carbon to separate is less; and it is for this reason that the test
mentioned with a tumbler of water is so accurate. When ink is to be used
on parchment, or glossy tracing-paper, it will flow perfectly if a few
drops of ox-gall be mixed with it; but on soft paper, or on bristol
board, this will cause the ink to spread.
For purposes of measurement, there are special rules or scales of steel
and of paper manufactured. The steel rules are finely and accurately
divided, and some are of triangular form, so that when laid upon the
paper the lines divided will lie close to the paper, and the light will
fall directly on the ruled surface. Triangular rules or scales are
therefore much superior to flat ones. The object of having a paper rule
or scale is, that the paper will expand and contract under varying
degrees of atmospheric moisture, the same as the drawing paper does.
Figure 16 represents a triangular scale, having upon it six different
divisions of the inch. These are made in different patterns, having
either decimal divisions or the vulgar fractions. Being made of steel,
and nickel-plated, they are proof against the moisture of the fingers,
and are not subject to the variation of the wooden scale.
[Illustration: Fig. 16.]