MACHINE PRINTING--_continued_
Register--Atmospheric Conditions--The Key--The Gripper--
Starting the Machine--Fixing the Stone--Strength of
Colour--Grit--Making Ready--Regulation of Speed.
It is almost impossible to overestimate the importance of register in
lithographic machine printing, and any suggestions which are likely to
be of assistance to the printer in this matter will no doubt be
welcomed.
Variable atmospheric conditions, insufficiently matured paper, or
constitutional defects in the machine, are frequent sources of
inaccurate register. These may be to some extent unavoidable and
therefore beyond the printer's control, but there are numerous other
points which have an important bearing upon the accurate fitting of one
colour or forme with another, and therefore require care and attention.
The following method of procedure is well worth consideration, as it has
decided advantages over many others.
The _key_, or outline forme, to which the colour formes have been set
up, is put into the machine at the beginning of the printing operations.
The exact position of the design on the sheet is arranged, and twenty or
thirty impressions taken on a reliable paper. With these impressions as
a guide it is a comparatively easy matter to register each colour
accurately. This effects a saving both in time and material, and rarely
fails to produce satisfactory results. During the early stages of the
printing, when it is difficult to detect any slight movement of the
stone in the machine, a sheet bearing an impression of the key may be
printed in the usual way, when any variation in register will be
revealed at a glance. The relative positions of the side lay and gripper
seldom receive the consideration they ought to have. The gripper and
side lay should be exactly at right angles to each other, and any
divergence whatever from this rule simply courts disaster. If they form
an acute angle there is a danger of the sheet moving _forward_ a little
as the gripper closes. If, on the other hand, they are fixed at an
obtuse angle, there is a proportionate risk of the sheet falling back as
the gripper closes. If any degree of uniformity could be guaranteed in
these movements, then all would still be well, but unfortunately no such
guarantee can be given, owing to a possible variation in the cutting of
different batches of paper.
[Illustration: FIG. 18.]
Another matter of a similar character and quite as important in its
issues is more directly connected with the gripper.
The type of gripper shown in Fig. 18 is probably the best for general
use. It enables the printer to use two or more pins upon which to rest
his sheet, according to the particular requirements of his work. Two
pins are usually sufficient and answer best, for the following reasons.
It is by no means unusual to find that the paper, trimmed though it may
be, has slightly convex or concave edges, owing either to insufficient
damping or an inaccurate setting of the knife in the guillotine cutting
machine. This can, of course, be avoided, but the point at present under
consideration is one of _possible_ effects. This contingency and its
effect are considerably exaggerated in Figs. 19A and 19B, but for
purposes of illustration the suggestiveness of the two sketches is not
at all too emphatic.
[Illustration: FIG. 19.]
Start the machine with a light pressure, for once the stone is locked up
a certain amount of danger will always exist if at any time it is
necessary to reduce the pressure. The stone may still be held by the
blocks, even after the bed of the machine has been lowered, only to come
down with a snap when pressure is applied. Such a danger might, of
course, be averted by slackening the screws and blocks; but then the
stone would almost certainly move out of position and the registration
of the forme be altered.
Narrow slips of paper folded two or three times, and inserted between
the block and the stone, will often check any tendency the latter may
have to lift when the screws are tightened.
Of the many annoyances associated with the lithographic machine
printer's work, _grit_ is probably the most troublesome, inasmuch as its
presence is almost imperceptible, while its effect is extensive and
often disastrous. Its sharp grains become embedded in the inking-roller
skins, and plough tiny furrows across the printing forme, doing much
damage before the printer realises the presence of any foreign matter on
the inking-rollers. Prevention is a simple matter enough, but a cure is
rarely, if ever, accomplished. Dust the rollers and examine them
carefully before commencing operations, and in this way ensure perfect
cleanliness. It may seem a trifle, but it is none the less an important
one, and perhaps the reader has already realised that "trifles make
perfection, and perfection is no trifle."
[Illustration: FIG. 20A.--Patent conical counter shafting.]
[Illustration: FIG. 20B.--Patent conical counter shafting.]
[Illustration: FIG. 21.]
In _making ready_ on a lithographic printing machine, as in almost every
phase of industrial life, method is the great secret of success. Method
conquers the most stubborn difficulties, and, though it is not at all
times profoundly interesting in its application, yet it more than repays
any monotony it may involve. In the matter of lithographic printing, at
any rate, a few methods of an essentially simple character might be
cultivated with advantage. This chapter is not intended as a complete
record of such methods, but a number of items are discussed herein
which, though simple, are intensely practical, and likely to suggest
more to the reader than is found described in the text.
The question of speed may sometimes seriously handicap progress. It is a
self-evident fact that the solid impression of a heavy poster cannot be
made at the same speed as a light tint in chromo work. Speed cones are
usually fixed to a counter-shaft to regulate the speed of the machine as
required. Figs. 20A and 20B show an improved arrangement of this
character, in which tapering drums A A are substituted for cones, the
belt being moved and held in any position by the screw and forks B and
C. D is the driving pulley which transmits the power to the machine.
Electricity as a motive power for printing machinery is quietly yet
irresistibly winning its way into general favour, and for very cogent
reasons. It is the most convenient form of motive power, and can be
transmitted for long distances without any appreciable loss. It takes up
little space, and almost entirely dispenses with belts and shafting. It
is also essentially economical, because it can be applied to the
smallest press just as easily as to a 60' by 40' poster machine (Fig.
21).