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.
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 Mechanical appliances have now so far superseded hand labour that, apart from a thorough knowledge of the principles of lithography, which is in itself essential, successful lithographic machine printing is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.
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]).
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.
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].
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.
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.