_FITTING AND FINISHING._
The fitting or finishing department of engineering establishments
is generally regarded as the main one.
Fitting processes, being the final ones in constructing machinery,
are more nearly in connection with its use and application; they
consist in the organisation or bringing together the results of
other processes carried on in the draughting room, pattern shop,
foundry, and smith shop.
To the unskilled, or to those who do not take a comprehensive view
of an engineering business as a whole, the finishing and fitting
department seems to constitute the whole of machine manufacture--an
impression which a learner should guard against, because nothing
but a true understanding of the importance and relations of the
different divisions of an establishment can enable them to be
thoroughly or easily learned.
Finishing, therefore, it must be borne in mind, is but one among
several processes, and that the fitting department is but one out
of four or more among which attention is to be divided.
Finishing as a process is a secondary and not always an essential
one; many parts of machinery are ready for use when forged or cast
and do not require fitting; yet a finishing shop must in many
respects be considered the leading department of an engineering
establishment. Plans, drawings and estimates are always based on
finished work, and when the parts have accurate dimensions; hence
designs, drawings and estimates may be said to pass through the
fitting shop and follow back to the foundry and smith shop, so
that finishing, although the last process in the order of the
work, is the first one after the drawings in every other sense;
even the dimensions in pattern-making which seems farthest removed
from finishing, are based upon fitting dimensions, and to a great
extent must be modified by the conditions of finishing.
In casting and forging operations the material is treated while in
a heated and expanded condition; the nature of these operations
is such that accurate dimensions cannot be attained, so that both
forgings and castings require to be made enough larger than their
finished dimensions to allow for shrinkage and irregularities.
Finishing as a process consists in cutting away this surplus
material, and giving accurate dimensions to the parts of machinery
when the material is at its natural temperature. Finishing
operations being performed as said upon material at its normal
temperature permits handling, gauging and fitting together of the
parts of machinery, and as nearly all other processes involve
heating, finishing may be called the cold processes of metal
work. The operations of a fitting shop consist almost entirely of
cutting, and grinding or abrading; a proposition that may seem
novel, yet these operations comprehend nearly all that is performed
in what is called fitting.
Cutting processes may be divided into two classes: cylindrical
cutting, as in turning, boring, and drilling, to produce circular
forms; and plane cutting, as in planing, shaping, slotting and
shearing, to produce plane or rectangular forms. Abrading or
grinding processes may be applied to forms of any kind.
To classify further--cutting machines may be divided into those
wherein the tools move and the material is fixed, and those wherein
the material is moved and the tools fixed, and machines which
involve a compound movement of both the tools and the material
acted upon.
There is also a distinction between machine and hand cutting
that may be noted. In machine cutting it is performed in true
geometrical lines, the tools or material being moved by positive
guides as in planing and turning; in hand operations, such as
filing, scraping or chipping, the tools are moved without positive
guidance, and act in irregular lines.
To attempt a generalisation of the operations of the fitting shop
in this manner may not seem a very practical means of understanding
them, yet the application will be better understood as we go
farther on.
Cutting tools include nearly all that are employed in finishing;
lathes, planing machines, drilling and boring machines, shaping,
slotting and milling machines, come within this class. The
machines named make up what are called standard tools, such as are
essential and are employed in all establishments where general
machine manufacture is carried on. Such machines are constructed
upon principles substantially the same in all countries, and have
settled into a tolerably uniform arrangement of movements and parts.
Besides the machine tools named, there are special machines to
be found in most works, machines directed to the performance of
certain work; by a particular adaptation such machines are rendered
more effective, but they are by such adaptation unfitted for
general purposes.
General engineering work cannot consist in the production of
duplicate pieces, nor in operations performed constantly in the
same manner as in ordinary manufacturing; hence there has been much
effort expended in adapting machines to general purposes--machines,
which seldom avoid the objections of combination, pointed out in a
previous chapter.
The principal improvements and changes in machine fitting at the
present time is in the application of special tools. A lathe, a
planing machine, or drilling machine as a standard machine, must
be adapted to a certain range of work, but it is evident that if
such tools were specially arranged for either the largest or the
smallest pieces that come within their capacity, more work could be
performed in a given time and consequently at less expense. It is
also evident that machine tools must be kept constantly at work in
order to be profitable, and when there are not sufficient pieces
of one kind to occupy a machine, it must be employed on various
kinds of work; but whenever there are sufficient pieces of the
same size upon which certain processes of a uniform character are
to be performed, there is a gain by having machines constructed to
conform as nearly as possible to the requirements of special work,
and without reference to any other.
It is now proposed to review the standard tools of a fitting
shop, noticing the general principles of their construction and
especially of their operation; not by drawings nor descriptions to
show what a lathe or a planing machine is, nor how some particular
engineer has constructed such tools, but upon the plan explained
in the introduction, presuming the reader to be familiar with the
names and purposes of standard machine tools. If he has not learned
this much, and does not understand the names and general objects
of the several operations carried on in a fitting shop, he should
proceed to acquaint himself thus far before troubling himself with
books of any kind.
(1.) Why cannot the parts of machinery be made to accurate
dimensions by forging or casting?--(2.) What is the difference
between hand tool and machine tool operation as to truth?--(3.)
Why cannot hand-work be employed in duplicating the parts of
machinery?--(4.) What is the difference between standard and
special machine tools?