_GAUGING IMPLEMENTS._
Among the improvements in machine fitting which have in recent
years come into general use, is the employment of standard gauges,
by means of which uniform dimensions are maintained, and within
certain limits, an interchange of the parts of machinery is
rendered possible.
Standard gauging implements were introduced about the year 1840,
by the celebrated Swiss engineer, John G. Bodmer, a man who for
many reasons deserves to be considered as the founder of machine
tool manufacture. He not only employed gauges in his works to
secure duplicate dimensions, but also invented and put in use
many other reforms in manipulation; among these may be mentioned
the decimal or metrical division of measures, a system of detail
drawings classified by symbols, the mode of calculating wheels by
diametric pitch, with many other things which characterise the best
modern practice.
The importance of standard dimensions, and the effect which a
system of gauging may have in the construction of machines, will
be a matter of some difficulty for a learner to understand. The
interchangeability of parts, which is the immediate object in
employing gauges, is plain enough, and some of the advantages at
once apparent, yet the ultimate effects of such a system extend
much farther than will at first be supposed.
The division of labour, that system upon which we may say our
great industrial interests are founded, is in machine fitting
promoted in a wonderful degree by the use of gauging implements. If
standard dimensions can be maintained, it is easy to see that the
parts of a machine can be constructed by different workmen, or in
different shops, and these parts when assembled all fit together,
without that tedious and uncertain plan of try-fitting which was
once generally practised. There are, it is true, certain kinds
of fitting which cannot well be performed by gauges; moving flat
surfaces, such as the bearings of lathe slides or the faces of
steam engine valves, are sooner and better fitted by trying them
together and scraping off the points of contact; but even in such
cases the character of the work will be improved, if one or both
surfaces have been first levelled by gauging or surface plates.
In cylindrical fitting, which as before pointed out, constitutes
the greater part in machine fitting, gauges are especially
important, because trial-fitting is in most cases impossible.
Flat or plane joints nearly always admit of adjustment between the
fitted surfaces; that is, the material scraped or ground away in
fitting can be compensated by bringing the pieces nearer together;
but parallel cylindrical joints cannot even be tried together
until finished, consequently, there can be nothing cut away in
trying them together. Tapering, or conical joints, can of course be
trial-fitted, and even parallel fits are sometimes made by trial,
but it is evident that the only material that can be cut away in
such cases, is what makes the difference between a fit too close,
and one which will answer in practice.
As to the practical results which may be attained by a gauging
system, it may be said that they are far in advance of what is
popularly supposed, especially in Europe, where gauges were first
employed.
The process of milling, which has been so extensively adopted in
the manufacture of guns, watches, sewing-machines, and similar work
in America, has, on principles explained in the chapter on milling,
enabled a system of gauging which it is difficult to comprehend
without seeing the processes carried on. And so important is the
effect due to this duplicating or gauging system, that several
important branches of manufacture have been controlled in this way,
when other elements of production, such as the price of labour,
rent, interest, and so on, have been greatly in favour of countries
where the trying system is practised.
As remarked, the gauging system is particularly adapted to,
or enabled by milling processes, and of course must have its
greatest effect in branches of work directed to the production
of uniform articles, such as clocks, watches, sewing-machines,
guns, hand tools, and so on. That is, the direct effect on the
cost of processes will be more apparent and easily understood in
such branches of manufacture; yet in general engineering work,
where each machine is more or less modified, and made to special
plans, the commercial gain resulting from the use of gauges is
considerable.
In respect to repairing alone, the consideration of having the
parts of machinery fitted to standard sizes is often equal to its
whole value.
Machinery subjected to destructive wear, and to be operated at a
distance from machine shops--locomotive engines for example--if not
constructed with standard dimensions, may, by the detention due to
repairing, cause a loss and inconvenience equal to their value; if
a shaft wheel bearing, or even a fitted screw bolt is broken, time
must be allowed to make the parts new; and in order to fit them,
the whole machine, or such of its details as have connection with
the broken parts, must be taken to a shop in order to fit by trial.
The duplicate system has gradually made its way in locomotive
engineering, and will no doubt extend to the whole of railway
equipment, as constants for dimensions are proved and agreed upon.
The gauging system has been no little retarded by a selfish and
mistaken opinion that an engineering establishment may maintain
peculiar standards of its own; in fact, relics of this spirit
are yet to be met with in old machines, where the pitch of
screw-threads has been made to fractional parts of an inch, so that
engineers, other than the original makers, could not well perform
repairing, or replace broken parts.
One of the effects of employing gauges in machine fitting is to
inspire confidence in workmen. Instead of a fit being regarded
as a mysterious result more the work of chance than design, men
accustomed to gauges come to regard precision as something both
attainable and indispensable. A learner, after examining a set of
well fitted cylindrical gauges, will form a new conception of what
a fit is, and will afterwards have a new standard fixed in his mind.
The variation of dimensions which are sensible to the touch at
one ten-thousandth part of an inch, furnishes an example of how
important the human senses are even after the utmost precision
attainable by machine action. Pieces may pass beneath the cutters
of a milling machine under conditions, which so far as machinery
avails will produce uniform sizes, yet there is no assurance of the
result until the work is felt by gauges.
The eye fails to detect variations in size, even by comparison,
long before we reach the necessary precision in common fitting.
Even by comparison with figured scales or measuring with rules, the
difference between a proper and a spoiled fit is not discernible by
sight.
Many of the most accurate measurements are, however, performed
by sight, with vernier calipers for example, the variation being
multiplied hundreds or thousands of times by mechanism, until the
least differences can be readily seen.
In multiplying the variations of a measuring implement by
mechanism, it is obvious that movable joints must be employed; it
is also obvious that no positive joint, whether cylindrical or
flat, could be so accurately fitted as to transmit such slight
movement as occurs in gauging or measuring. This difficulty is in
most measuring instruments overcome by employing a principle not
before alluded to, but common in many machines, that of elastic
compensation.
A pair of spring calipers will illustrate this principle. The
points are always steady, because the spring acting continually
in one direction compensates the loose play that may be in the
screw. In a train of tooth wheels there is always more or less
play between the teeth; and unless the wheels always revolve in
one direction, and have some constant resistance offered to their
motion, 'backlash' or irregular movement will take place; but if
there is some constant and uniform resistance such as a spring
would impart, a train of wheels will transmit the slightest motion
throughout.
The extreme nicety with which gauging implements are fitted seems
at first thought to be unnecessary, but it must be remembered
that a cylindrical joint in ordinary machine fitting involves a
precision almost beyond the sense of feeling, and that any sensible
variation in turning gauges is enough to spoil a fit.
Opposed to the maintenance of standard dimensions are the
variations in size due to temperature. This difficulty applies
alike to gauging implements and to parts that are to be tested; yet
in this, as in nearly every phenomenon connected with matter, we
have succeeded in turning it to some useful purpose. Bands of iron,
such as the tires of wheels when heated, can be 'shrunk' on, and a
compressive force and security attained, which would be impossible
by forcing the parts together both at the same temperature.
Shrinking has, however, been almost entirely abandoned for such
joints as can be accurately fitted.
(1.) How may gauging implements affect the division of
labour?--(2.) In what way do standard dimensions affect the value
of machinery?--(3.) Why cannot cylindrical joints be fitted
by trying them together?--(4.) Under what circumstances is it
most important that the parts of machinery should have standard
dimensions?--(5.) Which sense is most acute in testing accurate
dimensions?--(6.) How may slight variations in dimensions be made
apparent to sight?