Part 18 (1/2)

(4) It must combine the greatest possible speed with this power.

(5) It must be easily operated, to save the workman.

All these qualities combined are not to be found in any press. .h.i.therto applied to lithography.

II

APPLICATION OF BOOK- AND COPPER-PLATE PRESSES TO LITHOGRAPHY

If we consider the peculiarities of book and copper print, we find a decided difference between them that affects printing importantly.

The letters of book-type are raised, the engraving in copper is depressed. It is evident that the former requires no such power for making impressions as the latter. Therefore the presses are so different that copper plates cannot be printed on a book-press and vice versa.

Now, as the stone combines both the elevated and the depressed principles, the natural idea would be to combine the fundamental principles of both presses as nearly as possible for stone-printing. In book-print, only the types are exposed to the pressure, and in the average printed sheet these are only one fourth part of the entire surface. The remaining white s.p.a.ce is not affected at all by the press.

In the stone, however, the elevation of any part of a design is so slight that the entire surface is affected, and consequently a stone plate offers four times as much resistance. A book-press therefore would print a stone only if it were arranged for a pressure four times greater. Now, for a stone of the size of a letter-sheet the power required to print with one vertical pressure would be five or six hundred hundredweight, a pressure that could be supported only by a thick stone laid very exactly on a perfect foundation.

An ordinary copper-plate press increases the pulling of the paper so much in the case of a stone plate that the impression would be worthless. This pulling is not caused, as in the case of the sc.r.a.per, during the impression itself, as already described, but it is caused before the impression through the endeavor of the cylinder to force the plate along under it. Once the stone is under the cylinder, the paper is not pulled noticeably, because the cylinder glides over the leather much more gently and with much less friction than the sc.r.a.per.

This defect might be corrected:--

(_a_) By supporting the cylinder so that it would come down on the stone only at the point where the print is to begin. But as the stone must be drawn pretty well forward for convenience in inking, this would demand that the cylinder be revolved forward and backward again as far as is needed for the impression, which means a great demand on the strength of the printers, not to count the loss of time.

(_b_) A second way would be to plane off a piece two inches wide from the cylinder at the point where the impression is to begin. The stone could be forced under this s.p.a.ce readily, and when the cylinder revolves, it presses forcibly at once without pulling the paper very much.

(_c_) The press might be fitted with iron wheels with cog teeth to engage similar cogs on the cylinder. This would prevent pulling, but the mechanical work would need to be very accurate.

(_d_) The best arrangement will be the following: Set the upper cylinder so high that the stone can be brought under it without touching. Then bring it down with a screw, or better still, with a lever that can be operated by the foot.

The first figure in the plate showing presses represents about how a copper-plate press is to be fitted for this work. On the whole, this is an ordinary copper-plate press, but the upper roller is set with its two axles or spindles in two iron levers, each of which is fastened to a piece of wood with iron screws one inch thick. Each of these pieces of wood is covered with strong sheet iron and can be adjusted higher or lower with two screws or with underlay of pasteboard. This is necessary that the press may be adjusted to varying pressures. The two other ends of the two levers, in which the cylinder sits, can be raised or lowered, so that the cylinder also can rise or sink. Now two springs or two weights are so adjusted that the cylinder with the levers always remains elevated. To force it down on the stone, an iron beam enters both sides of the press with two pegs so adjusted that when the beam is turned ninety degrees the levers are depressed at least two inches. As the cylinder is about in the middle of the two levers, it will thus be depressed one inch, which suffices to permit the stone to pa.s.s under it freely while it is elevated and gives the greatest pressure when it is depressed. However, the upper cylinder must not be one inch distant from the stone, but at the most only one fourth inch, for the remaining s.p.a.ce of three fourths inch is required to provide margin for the elasticity of the various materials, and also to give margin for increased pressure whenever demanded.

On one end of the iron beam with the two pegs is an arm or lever which is joined to a thin stick with a treadle. This tread is so arranged that it remains elevated of itself. If the pressure is to reach sixty or more hundredweight, it must not be fastened directly to the treadle, but a second lever is required which is affixed to the side of the press.

Without going into tedious detail I cannot further describe this press.

Mechanicians will understand me readily and perhaps be able to add many improvements. My belief is that a copper press so arranged would diminish all danger of squas.h.i.+ng and pulling the impression, furnish powerful pressure, permit overlays of felt or fine cloth, and make possible considerable facility and celerity, which is a great advantage, because impressions always are better if too much time is not lost between inking and printing.

To safeguard the stone against cracking in such a press, the following points are to be noted:--

(1) The stone must be ground very true on the under side as well as the upper.

(2) Both cylinders must be perfectly true, and care is to be taken particularly that one cylinder is not thin toward the middle and the other thick, as this would easily crack the stone lengthwise.

The board on which the stone rests must be equally true and uniformly thick. At the same time it must be very thin, only one half inch thick at most. It will get very heavily squeezed during the printing, and the more the impression approaches the centre, the more concave will it become. The parts farthest from the point of pressure then resist unduly if the board is thick, and thus become the chief cause of cracking the stone. If the rollers are very true and the stone is very uniform, it is almost impossible to crack it if it is pa.s.sed between the two rollers without a board underneath. If the board is thin, it is as if it were not there.

I believe that competent mechanicians can improve the present presses greatly.

III

LITHOGRAPHIC PRESSES USED HITHERTO