_GENERALISATION OF SHOP PROCESSES._
Having thus far treated of such general principles and facts
connected with practical mechanics as might properly precede, and
be of use in, the study of actual manipulation in a workshop, we
come next to casting, forging, and finishing, with other details
that involve manual as well as mental skill, and to which the term
"processes" will apply.
As these shop processes or operations are more or less connected,
and run one into the other, it will be necessary at the beginning
to give a short summary of them, stating the general object
of each, that may serve to render the detailed remarks more
intelligible to the reader as he comes to them in their consecutive
order.
Designing, or generating the plans of machinery, may be considered
the leading element in engineering manufactures or machine
construction, that one to which all others are subordinate, both
in order and importance, and is that branch to which engineering
knowledge is especially directed. Designing should consist, first,
in assuming certain results, and, secondly, in conceiving of
mechanical agents to produce these results. It comprehends the
geometry of movements, the disposition and arrangement of material,
the endurance of wearing surfaces, adjustments, symmetry; in short,
all the conditions of machine operation and machine construction.
This subject will be again treated of at more length in another
section.
Draughting, or drawing, as it is more commonly called, is a means
by which mental conceptions are conveyed from one person to
another; it is the language of mechanics, and takes the place of
words, which are insufficient to convey mechanical ideas in an
intelligible manner.
Drawings represent and explain the machinery to which they relate
as the symbols in algebra represent quantities, and in a degree
admit of the same modifications and experiments to which the
machinery itself could be subjected if it were already constructed.
Drawings are also an important aid in developing designs or
conceptions. It is impossible to conceive of, and retain in the
mind, all the parts of a complicated machine, and their relation
to each other, without some aid to fix the various ideas as they
arise, and keep them in sight for comparison; like compiling
statistics, the footings must be kept at hand for reference, and to
determine the relation that one thing may bear to another.
In the workshop, the objects of drawing are to communicate plans
and dimensions to the workmen, and to enable a division of the
labour, so that the several parts of a machine may be operated
upon by different workmen at the same time--also to enable
classification and estimates of cost to be made, and records kept.
Drawings are, in fact, the base of shop system, upon which depends
not only the accuracy and uniformity of what is produced, but
also, in a great degree, its cost. Complete drawings of whatever
is made are now considered indispensable in the best regulated
establishments; yet we are not so far removed from a time when
most work was made without drawings, but what we may contrast the
present system with that which existed but a few years ago, when
to construct a new machine was a great undertaking, involving
generally many experiments and mistakes.
Pattern-making relates to the construction of duplicate models
for the moulded parts of machinery, and involves a knowledge of
shrinkage and cooling strains, the manner of moulding and proper
position of pieces, when cast, to ensure soundness in particular
parts. As a branch of machine manufacture, pattern-making requires
a large amount of special knowledge, and a high degree of skill;
for in no other department is there so much that must be left to
the discretion and judgment of workmen.
Pattern-makers have to thoroughly understand drawings, in order to
reproduce them on the trestle boards with allowance for shrinkage,
and to determine the cores; they must also understand moulding,
casting, fitting, and finishing. Pattern-making as a branch of
machine manufacture, should rank next to designing and drafting.
Founding and casting relate to forming parts of machinery by
pouring melted metal into moulds, the force of gravity alone being
sufficient to press or shape it into even complicated forms. As a
process for shaping such metal as is not injured by the high degree
of heat required in melting, moulding is the cheapest and most
expeditious of all means, even for forms of regular outline, while
the importance of moulding in producing irregular forms is such
that without this process the whole system of machine construction
would have to be changed. Founding operations are divided into
two classes, known technically as green sand moulding, and loam
or dry sand moulding; the first, when patterns or duplicates are
used to form the moulds, and the second, when the moulds are built
by hand without the aid of complete patterns. Founding involves a
knowledge of mixing and melting metals such as are used in machine
construction, the preparing and setting of cores for the internal
displacement of the metal, cooling and shrinking strains, chills,
and many other things that are more or less special, and can only
be learned and understood from actual observation and practice.
Forging relates to shaping metal by compression or blows when
it is in a heated and softened condition; as a process, it is
an intermediate one between casting and what may be called the
cold processes. Forging also relates to welding or joining pieces
together by sudden heating that melts the surface only, and then by
forcing the pieces together while in this softened or semi-fused
state. Forging includes, in ordinary practice, the preparation of
cutting tools, and tempering them to various degrees of hardness
as the nature of the work for which they are intended may require;
also the construction of furnaces for heating the material, and
mechanical devices for handling it when hot, with the various
operations for shaping, which, as in the case of casting, can only
be fully understood by experience and observation.
Finishing and fitting relates to giving true and accurate
dimensions to the parts of machinery that come in contact with
each other and are joined together or move upon each other, and
consists in cutting away the surplus material which has to be
left in founding and forging because of the heated and expanded
condition in which the material is treated in these last processes.
In finishing, material is operated upon at its normal temperature,
in which condition it can be handled, gauged, or measured, and
will retain its shape after it is fitted. Finishing comprehends
all operations of cutting and abrading, such as turning, boring,
planing and grinding, also the handling of material; it is
considered the leading department in shop manipulation, because
it is the one where the work constructed is organised and brought
together. The fitting shop is also that department to which
drawings especially apply, and other preparatory operations are
usually made subservient to the fitting processes.
Shop system may also be classed as a branch of engineering work;
it relates to the classification of machines and their parts by
symbols and numbers, to records of weight, the expense of cast,
forged, and finished parts, and apportions the cost of finished
machinery among the different departments. Shop system also
includes the maintenance of standard dimensions, the classification
and cost of labour, with other matters that partake both of a
mechanical and a commercial nature.
In order to render what is said of shop processes more easily
understood, it will be necessary to change the order in which they
have been named. Designing, and many matters connected with the
operation of machines, will be more easily learned and understood
after having gone through with what may be called the constructive
operations, such as involve manual skill.
(1.) Name the different departments of an engineering
establishment.--(2.) What does the engineering establishment
include?--(3.) What does the commercial department include?--(4.)
The foundry department?--(5.) The forging department?--(6.) The
fitting department?--(7.) What does the term shop system mean as
generally employed?