_MECHANICAL DRAWING._
Machine-drawing may in some respects be said to bear the same
relation to mechanics that writing does to literature; persons
may copy manuscript, or write from dictation, of what they do not
understand; or a mechanical draughtsman may make drawings of a
machine he does not understand; but neither such writing or drawing
can have any value beyond that of ordinary labour. It is both
necessary and expected that a draughtsman shall understand all the
various processes of machine construction, and be familiar with the
best examples that are furnished by modern practice.
Geometrical drawing is not an artistic art so much as it is a
constructive mechanical one; displaying the parts of machinery
on paper, is much the same in practice, and just the same in
principle, as measuring and laying out work in the shop.
Artistic drawing is addressed to the senses, geometrical drawing is
addressed to the understanding. Geometrical drawing may, however,
include artistic skill not in the way of ornamentation, but to
convey an impression of neatness and completeness, that has by
common custom been assumed among engineers, and which conveys to
the mind an idea of competent construction in the drawing itself,
as well as of the machinery which is represented. Artistic effect,
so far as admissible in mechanical drawing, is easy to learn, and
should be understood, yet through a desire to make pictures, a
beginner is often led to neglect that which is more important in
the way of accuracy and arrangement.
It is easy to learn "how" to draw, but it is far from easy to
learn "what" to draw. Let this be kept in mind, not in the way of
disparaging effort in learning "how" to draw, for this must come
first, but in order that the objects and true nature of the work
will be understood.
The engineering apprentice, as a rule, has a desire to make
drawings as soon as he begins his studies or his work, and there
is not the least objection to his doing so; in fact, there is a
great deal gained by illustrating movements and the details of
machinery at the same time of studying the principles. Drawings if
made should always be finished, carefully inked in, and memoranda
made on the margin of the sheets, with the date and the conditions
under which the drawings were made. The sheets should be of uniform
size, not too large for a portfolio, and carefully preserved, no
matter how imperfect they may be. An apprentice who will preserve
his first drawings in this manner will some day find himself in
possession of a souvenir that no consideration would cause him to
part with.
For implements procure two drawing-boards, forty-two inches long
and thirty inches wide, to receive double elephant paper; have the
boards plain without cleets, or ingenious devices for fastening
the paper; they should be made from thoroughly seasoned lumber, at
least one and one-fourth inches thick; if thinner they will not be
heavy enough to resist the thrust of the T squares.
It is better to have two boards, so that one may be used for
sketching and drawing details, which, if done on the same sheet
with elevations, dirties the paper, and is apt to lower the
standard of the finished drawing by what may be called bad
association.
Details and sketches, when made on a separate sheet, should be to a
larger scale than elevations. By changing from one scale to another
the mind is schooled in proportion, and the conception of sizes and
dimensions is more apt to follow the finished work to which the
drawings relate.
In working to regular scales, such as one-half, one-eighth,
or one-sixteenth size, a good plan is to use a common rule,
instead of a graduated scale. There is nothing more convenient
for a mechanical draughtsman than to be able to readily resolve
dimensions into various scales, and the use of a common rule for
fractional scales trains the mind, so that computations come
naturally, and after a time almost without effort. A plain T
square, with a parallel blade fastened on the side of the head,
but not imbedded into it, is the best; in this way set squares can
pass over the head of a T square in working at the edges of the
drawing. It is strange that a draughting square should ever have
been made in any other manner than this, and still more strange,
that people will use squares that do not allow the set squares to
pass over the heads and come near to the edge of the board.
A bevel square is often convenient, but should be an independent
one; a T square that has a movable blade is not suitable for
general use. Combinations in draughting instruments, no matter what
their character, should be avoided; such combinations, like those
in machinery, are generally mistakes, and their effect the reverse
of what is intended.
For set squares, or triangles, as they are sometimes called, no
material is so good as ebonite; such squares are hard, smooth,
impervious to moisture, and contrast with the paper in colour;
besides they wear longer than those made of wood. For instruments,
it is best to avoid everything of an elaborate or fancy kind;
such sets are for amateurs, not engineers. It is best to procure
only such instruments at first as are really required, of the
best quality, and then to add others as necessity may demand; in
this way, experience will often suggest modifications of size or
arrangement that will add to the convenience of a set.
One pair each of three and one-half inch and five inch compasses,
two ruling pens, two pairs of spring dividers, one for pens and one
for pencils, a triangular boxwood scale, a common rule, and a hard
pencil, are the essential instruments for machine-drawing. At the
beginning, when "scratching out" will probably form an item in the
work, it is best to use Whatman's paper, or the best roll paper,
which, of the best manufacture, is quite as good as any other for
drawings that are not water-shaded.
In mounting sheets that are likely to be removed and replaced,
for the purpose of modification, as working drawings generally
are, they can be fastened very well by small copper tacks driven
along the edges at intervals of two inches or less. The paper can
be very slightly dampened before fastening in this manner, and if
the operation is carefully performed the paper will be quite as
smooth and convenient to work upon as though it were pasted down;
the tacks can be driven down so as to be flush with, or below the
surface of, the paper, and will offer no obstruction to squares.
If a drawing is to be elaborate, or to remain long upon a board,
the paper should be pasted down. To do this, first prepare thick
mucilage, or what is better, glue, and have it ready at hand, with
some slips of absorbent paper an inch or so wide. Dampen the sheet
on both sides with a sponge, and then apply the mucilage along the
edge, for a width of one-fourth or three-eighths of an inch. It
is a matter of some difficulty to place a sheet upon a board; but
if the board is set on its edge, the paper can be applied without
assistance. Then, by placing the strips of paper along the edge,
and rubbing over them with some smooth hard instrument, the edges
of the sheet can be pasted firmly to the board, the paper slips
taking up a part of the moisture from the edges, which are longest
in drying. If left in this condition, the centre will dry first,
and the paper be pulled loose at the edges by contraction before
the paste has time to dry. It is therefore necessary to pass over
the centre of the sheet with a wet sponge at intervals to keep the
paper slightly damp until the edges adhere firmly, when it can
be left to dry, and will be tight and smooth. In this operation
much will be learned by practice, and a beginner should not be
discouraged by a few failures. One of the most common difficulties
in mounting sheets is in not having the gum or glue thick enough;
when thin, it will be absorbed by the wood or the paper, or is too
long in drying; it should be as thick as it can be applied with a
brush, and made from clean Arabic gum, tragacanth, or fine glue.
Thumb-tacks are of but little use in mechanical drawing except
for the most temporary purposes, and may very well be dispensed
with altogether; they injure the draughting-boards, obstruct the
squares, and disfigure the sheets.
Pencilling is the first and the most important operation in
draughting; more skill is required to produce neat pencil-work than
to ink in the lines after the pencilling is done.
A beginner, unless he exercises great care in the pencil-work of
a drawing, will have the disappointment to find the paper soon
becoming dirty from plumbago, and the pencil-lines crossing each
other everywhere, so as to give the whole a slovenly appearance. He
will also, unless he understands the nature of the operations in
which he is engaged, make the mistake of regarding the pencil-work
as an unimportant part, instead of constituting, as it does, the
main drawing, and thereby neglect that accuracy which alone can
make either a good-looking or a valuable one.
Pencil-work is indeed the main operation, the inking being merely
to give distinctness and permanency to the lines. The main thing in
pencilling is accuracy of dimensions and stopping the lines where
they should terminate without crossing others. The best pencils
only are suitable for draughting; if the plumbago is not of the
best quality, the points require to be continually sharpened, and
the pencil is worn away at a rate that more than makes up the
difference in cost between the finer and cheaper grades of pencils,
to say nothing of the effect upon a drawing.
It is common to use a flat point for draughting pencils, but a
round one will often be found quite as good if the pencils are
fine, and some convenience is gained by a round point for free-hand
use in making rounds and fillets. A Faber pencil, that has
detachable points which can be set out as they are worn away, is
convenient for draughting.
For compasses, the lead points should be cylindrical, and fit into
a metal sheath without paper packing or other contrivance to hold
them; and if a draughtsman has instruments not arranged in this
manner, he should have them changed at once, both for convenience
and economy.
Ink used in drawing should always be the best that can be procured;
without good ink a draughtsman is continually annoyed by an
imperfect working of pens, and the washing of the lines if there
is shading to be done. The quality of ink can only be determined
by experiment; the perfume that it contains, or tinfoil wrappers
and Chinese labels, are no indication of quality; not even the
price, unless it be with some first-class house. To prepare ink,
I can recommend no better plan of learning than to ask some one
who understands the matter. It is better to waste a little time in
preparing it slowly than to be at a continual trouble with pens,
which will occur if the ink is ground too rapidly or on a rough
surface. To test ink, a few lines can be drawn on the margin of
a sheet, noting the shade, how the ink flows from the pen, and
whether the lines are sharp; after the lines have dried, cross
them with a wet brush; if they wash readily, the ink is too soft;
if they resist the water for a time, and then wash tardily, the
ink is good. It cannot be expected that inks soluble in water can
permanently resist its action after drying; in fact, it is not
desirable that drawing inks should do so, for in shading, outlines
should be blended into the tints where the latter are deep, and
this can only be effected by washing.
Pens will generally fill by capillary attraction; if not, they
should be made wet by being dipped into water; they should not be
put into the mouth to wet them, as there is danger of poison from
some kinds of ink, and the habit is not a neat one.
In using ruling pens, they should be held nearly vertical, leaning
just enough to prevent them from catching on the paper. Beginners
have a tendency to hold pens at a low angle, and drag them on their
side, but this will not produce clean sharp lines, nor allow the
lines to be made near enough to the edges of square blades or set
squares.
In regard to the use of the T square and set squares, no useful
rules can be given except to observe others, and experiment until
convenient customs are attained. A beginner should be careful of
adopting unusual plans, and above all things, of making important
discoveries as to new plans of using instruments, assuming that
common practice is all wrong, and that it is left for him to
develop the true and proper way of drawing. This is a kind of
discovery which is very apt to intrude itself at the beginning of
an apprentice's course in many matters besides drawing, and often
leads him to do and say many things which he will afterwards wish
to recall.
It is generally a safe rule to assume that any custom long and
uniformly followed by intelligent people is right; and, in the
absence of that experimental knowledge which alone enables one to
judge, it is safe to receive such customs, at least for a time, as
being correct.
Without any wish to discourage the ambition of an apprentice to
invent, which always inspires him to laudable exertion, it is
nevertheless best to caution him against innovations. The estimate
formed of our abilities is very apt to be inversely as our
experience, and old engineers are not nearly so confident in their
deductions and plans as beginners are.
A drawing being inked in, the next things are tints, dimension,
and centre lines. The centre lines should be in red ink, and pass
through all points of the drawing that have an axial centre, or
where the work is similar and balanced on each side of the line.
This rule is a little obscure, but will be best understood
if studied in connection with a drawing, and perhaps as well
remembered without further explanation.
Dimension lines should be in blue, but may be in red. Where to
put them is a great point in draughting. To know where dimensions
are required involves a knowledge of fitting and pattern-making,
and cannot well be explained; it must be learned in practice. The
lines should be fine and clear, leaving a space in their centre
for figures when there is room. The distribution of centre lines
and dimensions over a drawing must be carefully studied, for
the double purpose of giving it a good appearance and to avoid
confusion. Figures should be made like printed numerals; they are
much better understood by the workman, look more artistic, and
when once learned require but little if any more time than written
figures. If the scale employed is feet and inches, dimensions to
three feet should be in inches, and above this in feet and inches;
this corresponds to shop custom, and is more comprehensive to the
workman, however wrong it may be according to other standards.
In sketches and drawings made for practice, such as are not
intended for the shop, it is suggested that metrical scales be
employed; it will not interfere with feet and inches, and will
prepare the mind for the introduction of this system of lineal
measurement, which may in time be adopted in England and America,
as it has been in many other countries.
In shading drawings, be careful not to use too deep tints, and
to put the shades in the right place. Many will contend, and not
without good reasons, that working drawings require no shading;
yet it will do no harm to learn how and where they can be shaded:
it is better to omit the shading from choice than from necessity.
Sections must, of course, be shaded--not with lines, although I
fear to attack so old a custom, yet it is certainly a tedious and
useless one: sections with light ink shading of different colours,
to indicate the kind of material, are easier to make, and look
much better. By the judicious arrangement of a drawing, a large
share of it may be in sections, which in almost every case are the
best views to work by. The proper colouring of sections gives a
good appearance to a drawing, and conveys an idea of an organised
machine, or, to use the shop term, "stands out from the paper." In
shading sections, leave a margin of white between the tints and
the lines on the upper and left-hand sides of the section: this
breaks the connection or sameness, and the effect is striking; it
separates the parts, and adds greatly to the clearness and general
appearance of a drawing.
Cylindrical parts in the plane of sections, such as shafts and
bolts, should be drawn full, and have a 'round shade,' which
relieves the flat appearance--a point to be avoided as much as
possible in sectional views.
Conventional custom has assigned blue as a tint for wrought iron,
neutral or pale pink for cast iron, and purple for steel. Wood is
generally distinguished by "graining," which is easily done, and
looks well.
The title of a drawing is a feature that has much to do with its
appearance, and the impression conveyed to the mind of an observer.
While it can add nothing to the real value of a drawing, it is so
easy to make plain letters, that the apprentice is urged to learn
this as soon as he begins to draw; not to make fancy letters, nor
indeed any kind except plain block letters, which can be rapidly
laid out and finished, and consequently employed to a greater
extent. By drawing six parallel lines, making five spaces, and
then crossing them with equidistant lines, the points and angles
in block letters are determined; after a little practice, it
becomes the work of but a few minutes to put down a title or other
matter on a drawing so that it can be seen and read at a glance in
searching for sheets or details.
In the manufacture of machines, there are usually so many sizes
and modifications, that drawings should assist and determine in
a large degree the completeness of classification and record.
Taking the manufacture of machine tools, for example: we cannot
well say, each time they are to be spoken of, a thirty-six inch
lathe without screw and gearing, a thirty-two inch lathe with screw
and gearing, a forty-inch lathe triple geared or double geared,
with a twenty or thirty foot frame, and so on. To avoid this it
is necessary to assume symbols for machines of different classes,
consisting generally of the letters of the alphabet, qualified by
a single number as an exponent to designate capacity or different
modifications. Assuming, in the case of engine lathes, A to be the
symbol for lathes of all sizes, then those of different capacity
and modification can be represented in the drawings and records
as A¹, A², A³, A⁴, and so on, requiring but two characters to
indicate a lathe of any kind. These symbols should be marked in
large plain letters on the left-hand lower corner of sheets, so
that the manager, workman, or any one else, can see at a glance
what the drawings relate to. This symbol should run through the
time-book, cost account, sales record, and be the technical name
for machines to which it applies; in this way machines will always
be spoken of in the works by the name of their symbol.
In making up the time charged to different machines during their
construction, a good plan is to supply each workman with a slate
and pencil, on which he can enter his time as so many hours or
fractions of hours charged to the respective symbols. Instead of
interfering with his time, this will increase a workman's interest
in what he is doing, and naturally lead to a desire to diminish the
time charged to the various symbols. This system leads to emulation
among workmen where any operation is repeated by different persons,
and creates an interest in classification which workmen will
willingly assist in.
When the dimensions and symbols are added to a drawing, the next
thing is pattern or catalogue numbers. These should be marked in
prominent, plain figures on each piece of casting, either in red
or other colour that will contrast with the general face of the
drawing. These numbers, to avoid the use of symbols in connection
with them, must include consecutively all patterns employed in the
business, and can extend to thousands without inconvenience.
A book containing the pattern record should be kept, in which
these catalogue numbers are set down, with a short description
to identify the different parts to which the numbers belong, so
that all the various details of any machine can at any time be
referred to. Besides this description, there should be opposite the
catalogue of pattern numbers, ruled spaces, in which to enter the
weight of castings, the cost of the pattern, and also the amount of
turned, planed, or bored surface on each piece when it is finished,
or the time required in fitting, which is the same thing. In this
book the assembled parts of each machine should be set down in a
separate list, so that orders for castings can be made from the
list without other references. This system is the best one known
to the writer, and is in substance a plan now adopted in many of
the best engineering establishments. A complete system in all
things pertaining to drawings and classifications should be rigidly
adhered to; any plan is better than none, and the schooling of the
mind to be had in the observance of systematic rules is a matter
not to be neglected. New plans for promoting system may at any
time arise, but such plans cannot be at any time understood and
adopted except by those who have cultivated a taste for order and
regularity.
In regard to shaded elevations, it may be said that photography has
superseded them for the purpose of illustrating completed machines,
and but few establishments care to incur the expense of ink-shaded
elevations. Shaded elevations cannot be made with various degrees
of care, and in a longer or shorter time; there is but one standard
for them, and that is that such drawings should be made with great
care and skill. Imperfect shaded elevations, although they may
surprise and please the unskilled, are execrable in the eyes of a
draughtsman or an engineer; and as the making of shaded elevations
can be of but little assistance to an apprentice draughtsman, it is
better to save the time that must be spent in order to make such
drawings, and apply the same study and time to other matters of
greater importance.
It is not assumed that shaded elevations should not be made, nor
that ink shading should not be learned, but it is thought best to
point out the greater importance of other kinds of drawing, too
often neglected to gratify a taste for picture-making, which has
but little to do with practical mechanics.
Isometrical perspective is often useful in drawing, especially in
wood structures, when the material is of rectangular section, and
disposed at right angles, as in machine frames. One isometrical
view, which can be made nearly as quickly as a true elevation,
will show all the parts, and may be figured for dimensions the
same as plane views. True perspective, although rarely necessary
in mechanical drawing, may be studied with advantage in connection
with geometry; it will often lead to the explanation of problems
in isometric drawing, and will also assist in free-hand lines that
have sometimes to be made to show parts of machinery oblique to
the regular planes. Thus far the remarks on draughting have been
confined to manipulation mainly. As a branch of engineering work,
draughting must depend mainly on special knowledge, and is not
capable of being learned or practised upon general principles or
rules. It is therefore impossible to give a learner much aid by
searching after principles to guide him; the few propositions that
follow comprehend nearly all that may be explained in words.
1. Geometrical drawings consist in 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.
2. Drawings in true elevation or in section are based upon flat
planes, and give dimensions parallel to the planes in which the
views are taken.
3. 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.
4. 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 unnecessary expense in fitting, forging, pattern-making,
moulding, and so on.
5. Every part laid down has something to govern it that may be
termed a "base"--some condition of function or position which, if
understood, will suggest size, shape, and relation to other parts.
By searching after a base for each and every part and detail, the
draughtsman proceeds upon a regular system, continually maintaining
a test of what is done. Every wheel, shaft, screw or piece of
framing should be made with a clear view of the functions it has
to fill, and there are, as before said, always reasons why such
parts should be of a certain size, have such a speed of movement,
or a certain amount of bearing surface, and so on. These reasons
or conditions may be classed as _expedient_, _important_, or
_essential_, and must be estimated accordingly. As claimed at the
beginning, the designs of machines can only in a limited degree be
determined by mathematical data. Leaving out all considerations
of machine operation with which books have scarcely attempted to
deal, we have only to refer to the element of strains to verify the
general truth of the proposition.
Examining machines made by the best designers, it will be found
that their dimensions bear but little if any reference to
calculated strains, especially in machines involving rapid motion.
Accidents destroy constants, and a draughtsman or designer who does
not combine special and experimental knowledge with what he may
learn from general sources, will find his services to be of but
little value in actual practice.
I now come to note a matter in connection with draughting to
which the attention of learners is earnestly called, and which,
if neglected, all else will be useless. I allude to indigestion,
and its resultant evils. All sedentary pursuits more or less give
rise to this trouble, but none of them so much as draughting. Every
condition to promote this derangement exists. When the muscles
are at rest, circulation is slow, the mind is intensely occupied,
robbing the stomach of its blood and vitality, and, worse than all,
the mechanical action of the stomach is usually arrested by leaning
over the edge of a board. It is regretted that no good rule can
be given to avoid this danger. One who understands the evil may
in a degree avert it by applying some of the logic which has been
recommended in the study of mechanics. If anything tends to induce
indigestion, its opposite tends the other way, and may arrest
it; if stooping over a board interferes with the action of the
digestive organs, leaning back does the opposite; it is therefore
best to have a desk as high as possible, stand when at work, and
cultivate a constant habit of straightening up and throwing the
shoulders back, and if possible, take brief intervals of vigorous
exercise. Like rating the horse-power of a steam-engine, by
multiplying the force into the velocity, the capacity of a man
can be estimated by multiplying his mental acquirements into his
vitality.
Physical strength, bone and muscle, must be elements in successful
engineering experience; and if these things are not acquired at the
same time with a mechanical education, it will be found, when ready
to enter upon a course of practice, that an important element, the
"propelling power," has been omitted.
(1.) What is the difference between geometric and artistic
drawing?--(2.) What is the most important operation in making a
good drawing?--(3.) Into what three classes can working drawings
be divided?--(4.) Explain the difference between elevations
and plans.--(5.) To what extent in general practice is the
proportion of parts and their arrangement in machines determined
mathematically?