There are on the market two or three well known machines that are adaptations of the stop-cylinder principle to a small press with automatic feeding device.
The two-revolution machine is now generally accepted as the standard type of cylinder printing machine for general work. It derives its name from the fact that the cylinder makes two revolu tions for each sheet printed. During one revolution the sheet is printed, of ter which the cylinder rises clear of the forme to allow the return of the carriage to deliver the sheet on to the flyers. In this type of machine the cylinder is not actuated by the bed. Both cylinder and bed are driven independently but in unison with each other.
The inking mechanism is on the slab principle and the sheets are fed from the crown of the cylinder, a feature being the method of delivery of the printed sheet which is done in such a way that the newly printed work does not come into contact with anything that would be likely to smear the wet ink. This style of machine is capable of producing an average of about 2,000 copies per hour. One well-known machine is an adaptation of the two-revolution principle to a small press with automatic feeding attachment. The two-revolution make of machine is made with two cylinders, either for "perfecting," i.e., printing both sides of the sheet, or for printing two colours before the sheet is delivered. These machines have two type beds, each with separate inking equipment. The demands for fine work, coupled with the need for speed have brought this style of machine to an extremely high degree of efficiency and accuracy.
A large number of small cylinder machines are built with the object of giving a large output, but quality and speed do not usually go together. Hand feeding of printing machines is rapidly being superseded by automatic feeding. There are two distinct types of machines for this purpose. One is known as a "pile feeder" where a large stack of paper is placed in a contrivance which automatically rises as the sheets are removed from the top by means of suction, placed on the feed board, and carried to the lays on to the printing machine. In the event of the sheet not reaching the lays, the machine stops. The other type is known as the "continuous feeder." In this case the sheets are "put up" a few at a time and while the printing machine is running they are "combed out" and mechanically controlled until they reach the lays, and should they not do so, the machine automatically stops.
The plates have to be curved to suit the cylinders and are secured by means of clamps. Where only one size of plate is printed on the machine, as in newspapers, two clamps are fixtures and the other two are movable in order to provide for the secure locking up of the plates. Where plates of various sizes are printed the plate cylinder is spirally grooved, into which movable clamps are fitted to suit the size of the plates to be printed.
There are two types of rotary machine: one of which prints single sheets (on one side), and the other where the paper is fed from a reel, printed on both sides and folded before delivery.
The inking mechanism on a rotary consists of an ink duct, a distributing drum, and a series of rollers to carry the ink from the drum to the rollers which ink the plates. The speed of a sheet-fed rotary being beyond the capacity of hand-feeding, makes automatic feeding an essential. On the reel-fed, or, as it is sometimes called, the web-fed perfecting rotary press, the travel of the paper through the machine and the folding of the pages are entirely automatic. Both sheet-fed and reel-fed rotaries are used for fine colour printing from half-tone plates.
The most effective multi-colour printing machines are those which consist of a large impression cylinder with f our separate plate cylinders each with its own mechanism for inking. The plate cylinders are half the circumference of the impression cylinders and two sheets may be printed (on one side) at each revolution of the cylinder. The sheet is taken from the feed board by means of grippers attached to the cylinder, is carried under the first plate cylinder (where a yellow impression is printed on the sheet), then under the second set of plates (where a red impression is made on the top of the yellow impression), the sheet passing on to the two remaining plate cylinders (where blue and black impressions respectively are superimposed on the top of the previous colours), and the sheet is delivered with the pic ture completed in its full range of colours and tints. Electrotype plates are used for printing, these having to be curved to suit the circumference of the plate cylinders, it being impossible to use "overlays" on the impression cylinder. The impressions from the four sets of plates take place on the same section of the cylinder. As each set of plates requires a different "make ready," the plates are "bumped" (the surface graded in height according to the tones in the plate).