_DESIGNING MACHINES._
It will scarcely be expected that any part of the present work,
intended mainly for apprentice engineers, should relate to
designing machines, yet there is no reason why the subject should
not to some extent be treated of; it is one sure to engage more or
less attention from learners, and the study of designing machines,
if properly directed, cannot fail to be of advantage.
There is, perhaps, no one who has achieved a successful experience
as an engineer but will acknowledge the advantages derived from
early efforts to generate original designs, and none who will not
admit that if their first efforts had been more carefully directed,
the advantages gained would have been greater.
It is exceedingly difficult for an apprentice engineer, without
experimental knowledge, to choose plans for his own education, or
to determine the best way of pursuing such plans when they have
been chosen; and there is nothing that consumes so much time, or is
more useless than attempting to make original designs, if there is
not some systematic method followed.
There is but little object in preparing designs, when their
counterparts may already exist, so that in making original plans,
there should be a careful research as to what has been already
done in the same line. It is not only discouraging, but annoying,
after studying a design with great care, to find that it has been
anticipated, and that the scheme studied out has been one of
reproduction only. For this reason, attempts to design should at
first be confined to familiar subjects, instead of venturing upon
unexplored ground.
Designing is in many respects the same thing as invention, except
that it deals more with mechanism than principles, although it
may, and often does include both. Like invention, designing should
always be attempted for the attainment of some definite object laid
down at the beginning, and followed persistently throughout.
It is not always an easy matter to hit upon an object to which
designs may be directed; and although at first thought it may seem
that any machine, or part of a machine, is capable of improvement,
it will be found no easy matter to detect existing faults or to
conceive plans for their remedy.
A new design should be based upon one of two suppositions--either
that existing mechanism is imperfect in its construction, or that
it lacks functions which a new design may supply; and if those who
spend their time in making plans for novel machinery would stop to
consider this from the beginning, it would save no little of the
time wasted in what may be called scheming without a purpose.
After determining the ultimate objects of an improvement, and
laying down the general principles which should be followed in
the preparation of a design, there is nothing connected with
constructive engineering that can be more nearly brought within
general rules than arranging details. I am well aware of how far
this statement is at variance with popular opinion among mechanics,
and of the very thorough knowledge of machine application and
machine operation required in making designs, and mean that
there are certain principles and rules which may determine the
arrangement and distribution of material, the position and relation
of moving parts, bearings, and so on, and that a machine may be
built up with no more risk of mistakes than in erecting a permanent
structure.
Designing machines must have reference to adaptation, endurance,
and the expense of construction. Adaptation includes the
performance of machinery, its commercial value, or what the
machinery may earn in operating; endurance, the time that machines
may operate without being repaired, and the constancy of their
performance; expense, the investment represented in machinery.
The adaptation, endurance, and cost of machines in designing become
resolved into problems of movements, the arrangement of parts, and
proportions.
Movements and strains may be called two of the leading conditions
upon which designs for machines are based: movements determine
general dimensions, and strains determine the proportions and sizes
of particular parts. Movement and strain together determine the
nature and area of bearings or bearing surfaces.
The range and speed of movement of the parts of machines are
elements in designing that admit of a definite determination
from the work to be accomplished, but arrangement cannot be so
determined, and is the most difficult to find data for. To sum up
these propositions we have:--
1. A conception of certain functions in a machine, and some
definite object which it is to accomplish.
2. Plans of adaptation and arrangement of the component parts of
the machinery, or organisation as it may be called.
3. A knowledge of specific conditions, such as strains, the range
and rate of movements, and so on.
4. Proportions of the various parts, including the framing, bearing
surfaces, shafts, belts, gearing, and other details.
5. Symmetry of appearance, which is often more the result of
obvious adaptation than ornamentation.
To illustrate the practical application of what has preceded, let
it be supposed, for example, that a machine is to be made for
cutting teeth in iron racks ¾ in. pitch and 3 in. face, and that
a design is to be prepared without reference to such machines as
may already be in use for the purpose.
It is not assumed that an actual design can be made which by words
alone will convey a comprehensive idea of an organised machine; it
is intended to map out a course which will illustrate a plan of
reasoning most likely to attain a successful result in such cases.
The reader, in order to better understand what is said, may keep in
mind a common shaping machine with crank motion, a machine which
nearly fills the requirements for cutting tooth racks.
Having assumed a certain work to do, the cutting of tooth racks
¾ in. pitch, and 3 in. face, the first thing to be considered
will be, is the machine to be a special one, or one of general
adaptation? This question has to do, first, with the functions of
the machine in the way of adapting it to the cutting of racks of
various sizes, or to performing other kinds of work, and secondly,
as to the completeness of the machine; for if it were to be a
standard one, instead of being adapted only to a special purpose,
there are many expensive additions to be supplied which can be
omitted in a special machine. It will be assumed in the present
case that a special machine is to be constructed for a particular
duty only.
The work to be performed consists in cutting away the metal between
the teeth of a rack, leaving a perfect outline for the teeth; and
as the shape of teeth cannot well be obtained by an adjustment of
tools, it must be accomplished by the shape of the tools. The
shape of the tools must, therefore, be constantly maintained, and
as the cross section of the displaced metal is not too great, it
may be assumed that the shape of the tools should be a profile of
the whole space between two teeth, and such a space be cut away at
one setting or one operation. By the application of certain rules
laid down in a former place in reference to cutting various kinds
of material, reciprocating or planing tools may be chosen instead
of rotary or milling tools.
Movements come next in order, and consist of a reciprocating
cutting movement of the tools or material, a feed movement to
regulate the cutting action, and a longitudinal movement of the
rack, graduated to pitch or space, the distance between the teeth.
The reciprocating cutting movement being but four inches or less,
a crank is obviously the best means to produce this motion, and
as the movement is transverse to the rack, which may be long and
unwieldy, it is equally obvious that the cutting motion should be
performed by the tools instead of the rack.
The feed adjustment of the tool being intermittent and the amount
of cutting continually varying, this movement should be performed
by hand, so as to be controlled at will by the sense of feeling.
The same rule applies to the adjustment of the rack for spacing;
being intermittent and irregular as to time, this movement should
also be performed by hand. The speed of the cutting movement is
known from ordinary practice to be from sixteen feet to twenty
feet a minute, and a belt two and a half inches wide must move two
hundred feet a minute to propel an ordinary metal cutting tool, so
that the crank movement or cutter movement must be increased by
gearing until a proper speed of the belt is reached; from this the
speed of intermediate movers will be found.
Arrangement comes next; in this the first matter to be considered
is convenience of manipulation. The cutting position should be so
arranged as to admit of an easy inspection of the work. An operator
having to keep his hand on the adjusting or feed mechanism, which
is about twelve inches above the work, it follows that if the
cutting level is four feet from the floor, and the feed handle
five feet from the floor, the arrangement will be convenient for a
standing position. As the work requires continual inspection and
hand adjustments, it will for this reason be a proper arrangement
to overhang both the supports for the rack and the cutting tools,
placing them, as we may say, outside the machine, to secure
convenience of access and to allow of inspection. The position of
the cutting bar, crank, connections, gearing, pulleys, and shafts,
will assume their respective places from obvious conditions, mainly
from the position of the operator and the work.
Next in order are strains. As the cutting action is the source
of strains, and as the resistance offered by the cutting tools
is as the length or width of the edges, it will be found in the
present case that while other conditions thus far have pointed
to small proportions, there is now a new one which calls for
large proportions. In displacing the metal between teeth of
three-quarters of an inch pitch, the cutting edge or the amount of
surface acted upon is equal to a width of one inch and a half. It
is true, the displacement may be small at each cut, but the strain
is rather to be based upon the breadth of the acting edge than the
actual displacement of metal, and we find here strains equal to the
average duty of a large planing machine. This strain radiates from
the cutting point as from a centre, falling on the supports of the
work with a tendency to force it from the framing. Between the rack
and the crank-shaft bearing, through the medium of the tool, cutter
bar, connection, and crank pin, and in various directions and
degrees, this strain may be followed by means of a simple diagram.
Besides this cutting strain, there are none of importance; the
tension of the belt, the side thrust in bearings, the strain from
the angular thrust of the crank, and the end thrust of the tool,
although not to be lost sight of, need not have much to do with
problems of strength, proportion, and arrangement.
Strains suggest special arrangement, which is quite a distinct
matter from general arrangement, the latter being governed mainly
by the convenience of manipulation. Special arrangement deals
with and determines the shape of framing, following the strains
throughout a machine. In the present case we have a cutting strain
which may be assumed as equal to one ton, exerted between the
bracket or jaws which support the work, and the crank-shaft. It
follows that between these two points the metal in the framing
should be disposed in as direct a line as possible, and provision
be made to resist flexion by deep sections parallel with the
cutting motion.
Lastly, proportions; having estimated the cutting force required
at one ton, although less than the actual strain in a machine of
this kind, we proceed upon this to fix proportions, beginning with
the tool shank, and following back through the adjusting saddle,
the cutting bar, connections, crank pins, shafts, and gear wheels
to the belt. Starting again at the tool, or point of cutting,
following through the supports of the rack, the jaws that clamp it,
the saddle for the graduating adjustment, the connections with the
main frame, and so on to the crank-shaft bearing a second time,
dimensions may be fixed for each piece to withstand the strains
without deflection or danger of breaking. Such proportions cannot,
I am aware, be brought within the rules of ordinary practice by
relying upon calculation alone to fix them, and no such course is
suggested; calculation may aid, but cannot determine proportions
in such cases; besides, symmetry, which cannot be altogether
disregarded, modifies the form and sometimes the dimensions of
various parts.
I have in this way imperfectly indicated a methodical plan of
generating a design, as far as words alone will serve, beginning
with certain premises based upon a particular work to be performed,
and then proceeding to consider in consecutive order the general
character of the machine, mode of operation, movements and
adjustments, general arrangement, strains, special arrangement, and
proportions.
With a thorough knowledge of practical machine operation, and
an acquaintance with existing practice, an engineer proceeding
upon such a plan, will, if he does not overlook some of the
conditions, be able to generate designs which may remain without
much modification or change, so long as the purpose to which the
machinery is directed remains the same.
Perseverance is an important trait to be cultivated in first
efforts at designing; it takes a certain amount of study to
understand any branch of mechanism, no matter what natural capacity
may be possessed by a learner. Mechanical operations are not
learned intuitively, but are always surrounded by many peculiar
conditions which must be learned _seriatim_, and it is only by an
untiring perseverance at one thing that there can be any hope of
improving it by new designs.
A learner who goes from gearing and shafts to steam and hydraulics,
from machine tools to cranes and hoisting machinery, will not
accomplish much. The best way is to select at first an easy
subject, one that admits of a great range of modification,
and if possible, one that has not assumed a standard form of
construction. Bearings and supports for shafts and spindles, is a
good subject to begin with.
In designing supports for shafts the strains are easily defined and
followed, while the vertical and lateral adjustment, lubrication
of bearings, symmetry of supports and hangers, and so on, will
furnish grounds for endless modification, both as to arrangement
and mechanism.
In making designs it is best to employ no references except such
as are carried in the memory. The more familiar a person is
with machinery of any class, the more able he may be to prepare
designs, but not by measuring and referring to other people's
plans. Dimensions and arrangement from examples are, by such a
course, unconsciously carried into a new drawing, even by the most
skilled; besides, it is by no means a dignified matter to collect
other people's plans, and by a little combination and modification
produce new designs. It may be an easy plan to acquire a certain
kind of proficiency, but will most certainly hinder an engineer
from ever rising to the dignity of an original designer.
Symmetry, as an element in designs for machinery, is one of those
unsettled matters which may be determined only in connection with
particular cases; it may, however, be said that for all engineering
implements and manufacturing machinery of every kind, there should
be nothing added for ornament, or anything that has no connection
with the functions of the machinery.
Modern engineers of the abler class are so thoroughly in accord in
this matter of ornamentation, both in opinion and practice, that
the subject hardly requires to be mentioned, and it will be no
disadvantage for a learner to commence by cultivating a contempt
for whatever has no useful purpose. Of existing practice it may be
said, that in what may be called industrial machinery, the amount
of ornamentation is inverse as the amount of engineering skill
employed in preparing designs.
A safe rule will be to assume that machinery mainly used and seen
by the skilled should be devoid of ornament, and that machinery
seen mainly by the unskilled, or in public, should have some
ornament. Steam fire engines, sewing machines, and works of a
similar kind, which fall under the inspection of the unskilled, are
usually arranged with more or less ornament.
As a rule, ornament should never be carried further than graceful
proportions; the arrangement of framing should follow as nearly
as possible the lines of strain. Extraneous decoration, such as
detached filagree work of iron, or painting in colours, is so
repulsive to the taste of the true engineer and mechanic that it is
unnecessary to speak against it.
(1.) Name some of the principal points to be kept in view in
preparing designs?--(2.) Why should attempts at designing be
confined to one class of machinery?--(3.) What objection exists
to examining references when preparing designs?