MACHINE PRINTING
The Printing Machine--The Halligan--Some Mechanical
Phases--Speed--Pressure--Levelling the Stones--Cylinder
Brake--Inking Rollers--Damping.
Concerning the structural qualities of the various types of lithographic
printing machines now in use, much might be written and divers opinions
expressed. In this respect, however, it would be invidious to suggest
that one maker's machines were better than another's, and such would be
the natural trend of a discussion on these lines. The machines all have,
it is true, many points in common where comparisons would be legitimate
and easy. Yet, on the other hand, they each possess distinct advantages
which will no doubt appeal to the printer individually, in proportion to
their suitability or otherwise for his particular work. Conviction will
follow experience in these matters, and any decision arrived at after
this fashion may be regarded as a useful and valuable acquisition.
The illustration on page 35 (Fig. 13) gives a fair general idea of the
modern lithographic machine.
Fig. 14 illustrates a somewhat novel type of lithographic printing
machine, in which the gripper is entirely dispensed with, the sheet
being held to gauges by the operator until caught between the small
cylinder and the stone, when pressure is immediately applied. The stone
is simply blocked up in the bed of the machine and the position of the
print on the paper assured by moving the gauges. This useful little
jobbing machine is a decided innovation, and the simplicity of its
construction is only equalled by the precision of its movements.
[Illustration: FIG. 13.]
Lithographic machine printing presents many peculiar features, each one
of which requires careful and constant attention for their successful
operation. Some of its purely mechanical aspects--the care of the
machine and its accessories, together with their various functions and
applications--offer a wide scope for resourcefulness and ability of a
high order. The primary purpose of the machine itself was undoubtedly to
accelerate the reproductive power of lithography from a commercial point
of view; and throughout the entire course of its development the aim of
the engineer has been to produce a printing machine with an
ever-increasing capacity for reproduction. It does not follow, however,
that the printer's responsibility has been proportionately increased.
Mechanical appliances have now so far superseded hand labour that, apart
from a thorough knowledge of the principles of lithography, which is in
itself essential, successful lithographic machine printing is largely
due to resourcefulness, alert perception, and a skilful blending of
mechanical and technical knowledge.
[Illustration: FIG. 14.]
Passing over the vast amount of detail which is usually and almost
invariably associated with machine printing, but which offers little
that is new to the practical worker, it might be advantageous to discuss
a few points which are too often overlooked.
Speed, as has already been pointed out, is a very important factor in
lithographic machine printing. It has become quite a necessity, and
everything which conduces to it should receive the most careful
consideration. Economy of power is too seldom regarded as a standard of
efficiency in the printer. At any rate, as far as this is concerned it
is doubtful if he fully realises the effect of what may appear to him
as insignificant matters. A little pressure more or less on the stone
may be in itself a mere trifle, so also would be a careless arrangement
of the inking rollers or indiscriminate damping of the stones, yet, when
taken together, what a considerable waste of power they might cause;--a
waste which is altogether unnecessary and could easily be obviated by
care and forethought. Excessive pressure is frequently resorted to in
order to "bring up" an impression which is defective owing to some error
of judgment in its preparation. It undoubtedly secures the desired
effect, but at what a cost! There is a proportionately heavier drag on
the machine and a greater strain on its most vital parts. The following
view of this matter may be regarded as somewhat exaggerated, but it is
by no means an uncommon state of affairs, and will at least serve to
emphasise the importance of this point. It is a popular fallacy to
suppose that in adjusting the litho-stone to the bed of the printing
machine it should be made _perfectly level_. As a matter of fact a much
easier and more satisfactory impression can be made from a stone which
is worked just a little higher at the front or gripper edge than at the
back, and for this reason. The drag on the cylinder as it makes the
impression is appreciably greater at the back than at the front, and
when the pressure is heavy it has a tendency to leave the back edge with
a very decided jerk. The remedy is obvious and simple. As already
suggested, the stone should be set in the machine with the least
possible inclination towards the front. This adjustment is easily
effected by a judicious arrangement of a few layers of brown paper. Just
think for a moment of the effect likely to be produced by such a jerk or
jar, which would under ordinary working conditions occur from twelve to
fourteen times per minute whilst the machine was in motion! Abnormal
pressure would of course intensify the strain, and sooner or later
produce results of a decidedly disastrous character. Under the most
favourable conditions this continual springing would tend to move the
stone out of position, and thus affect the register of one forme with
another.
Another certain result of this condition of things is, that the sharp
pressure on the back edge of the stone would almost certainly cause an
appreciable indentation in the cylinder covering. This would eventually
cut through, or at least interfere with the working of a larger sheet at
some future time.
The mechanism for raising or lowering the lithographic stone in the
machine for the adjustment of pressure is comparatively simple (Fig.
15).
[Illustration: FIG. 15.]
There are two screws similar to A which pass right through the feet of
the stone carriage B B. A movement of the screws will therefore cause a
corresponding movement of the stone carriage on the blocks or inclines C
C. The lock-nut D holds the screw securely once the pressure is
adjusted.
So few printers really understand the proper adjustment of a cylinder
brake that some information concerning it will no doubt prove
acceptable. In the first place, a continuous action brake which can be
released at certain intervals is most suitable. It holds the cylinders
perfectly rigid whilst the machine is running free, and applies a
sufficient check at the points required. The intermittent movement
referred to is obviated in various ways. Fig. 13 shows an example of one
which is both simple and effective. It might be well also to explain the
principle and purpose of the cylinder brake. It is almost impossible to
cut mechanical gearing which will run easily and yet be entirely free
from _slogger_. Consequently the revolution of a printing machine
cylinder would be more or less jerky unless steadied in some way. This
is especially the case when it reaches the stone, and, owing to the
pressure applied, lifts a little in the gearing. A recognition of this
simple matter will enable an intelligent workman to arrange the brake
action with judgment and effect.
[Illustration: FIG. 16A.]
A comparison of the old arrangement of inking rollers (Fig. 16A) with
the new (Fig. 16B) is in itself an object lesson in this question of
power and its economical application. It is but reasonable to suppose
that the power required to move a set of rollers arranged in the
old-fashioned manner (Fig. 16A) will be infinitely greater than that
which would be needed for such an arrangement as shown in Fig. 16B.
[Illustration: FIG. 16B.]
Pursuing this matter still further, the question of indiscriminate
damping presents itself. Granted that the influence here is an indirect
one, yet it is a cause which frequently leads to an undesirable finish.
Every printer knows something of the effect produced by excess of water
upon printing inks. It hardens and stiffens them by accelerating
oxidisation. In course of time their free working on the rollers is
interfered with, and loss of power is by no means the worst result. Weak
and impoverished impressions, abnormal wear and tear of the printing
forme, and excessive saturation of the paper may follow.
In lithography generally, and in lithographic machine printing
particularly, the damping of the stone is a matter which requires
constant and careful attention; any arrangements for this purpose should
therefore be as effective as possible. The damping rollers should be
thoroughly cleaned each day, in order to remove any scum or grease which
may have been collected from the printing forme.
[Illustration: FIG. 17.]
The arrangement of damping rollers shown in Fig. 17 is a decidedly
practical one. The upper roller consists of metal, usually brass or
zinc. It collects any accumulation of ink or scum from the actual
dampers, and can be cleaned at any time without serious interference
with the progress of the work. Its adoption, however, has not been very
general, although it would be difficult to ascribe any good reasons for
such a fact.