Part 4 (1/2)

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).

[Ill.u.s.tration: FIG. 15.]

There are two screws similar to A which pa.s.s 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.

[Ill.u.s.tration: 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-fas.h.i.+oned manner (Fig. 16A) will be infinitely greater than that which would be needed for such an arrangement as shown in Fig. 16B.

[Ill.u.s.tration: 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 sc.u.m or grease which may have been collected from the printing forme.

[Ill.u.s.tration: FIG. 17.]

The arrangement of damping rollers shown in Fig. 17 is a decidedly practical one. The upper roller consists of metal, usually bra.s.s or zinc. It collects any acc.u.mulation of ink or sc.u.m 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.

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 a.s.sistance to the printer in this matter will no doubt be welcomed.

Variable atmospheric conditions, insufficiently matured paper, or const.i.tutional 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.

[Ill.u.s.tration: 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 ill.u.s.tration the suggestiveness of the two sketches is not at all too emphatic.