Several mills have been introduced in America in which the bark is sawn or rasped by toothed discs like circular saws, but these are only capable of dealing with barks of a brittle nature, and are immediately choked by tough materials like the bark of the mimosa or oak. A better form of mill, but one which is, to some extent, subject to the same disadvantage, is the “shaving-mill,” in which blades are fixed like plane-irons upon a disc, cones or cylinder, and are rotated at a high speed against the material which is fed against them by toothed rollers at such an angle that the shavings are cut diagonally to the grain. These shaving-mills are largely in use in America for hemlock-bark, with which they are particularly successful. The principle of the machine is exactly the same as that of the machines used in cutting oakwood, quebracho, and the different dye woods. One type of shaving-mill is illustrated in Fig. 74. [Illustration: FIG. 74.--Shaving Mill.] It frequently happens that the material is delivered from the mill in a very unequal state of division, and it is sometimes necessary to screen it and thus separate the coarser portion either for use in the leaches or for re-grinding, while the finer portion is more suitable for “dusting.” With disintegrators, which deliver the bark with considerable impetus, the screening can be accomplished by placing a screen diagonally below the mill, through which the finer parts are projected. It is, however, essential that this screen should be quite smooth on its upper surface and very strong, as ordinary wire gauze is immediately cut through by the impact of the material. What are called “locked wire screens” in which the wires are supported by being actually twisted round the transverse bars are very suitable. Where the circumstances will not permit of screening in this way, cylindrical rotating screens, or nearly horizontal screens vibrated by an eccentric may be used. The latter are cheaper to erect and have the advantage that they take up less room, and by having lengths of wirework or perforated steel of different coarseness, the material may be separated into more than one degree of fineness. [Illustration: FIG. 75.--Bark-Breaker.] Oak-bark as it is taken off the trees is usually in lengths of perhaps three feet, and it is necessary to cut or break it into smaller fragments before it can be ground in most of the machines just described. This is frequently done by hand by chopping the bark into pieces about four inches long, and the operation is known as “hatching.” Machines on the principle of the chaff-cutter, consisting of a fly-wheel with curved blades radially attached to it, are sometimes used. Instead of “hatching” it, the bark is frequently broken by passing through toothed rollers fitting into each other, and often attached to the mill; the construction of this machine will be readily understood from Fig. 75.