CHAPTER VIII
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.
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.
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.”
Fig. 20a.—Patent conical counter shafting.
Fig. 20b.—Patent conical counter shafting.
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]).
Fig. 21.