The inside of the heater itself is often lined with lead, a material which is not liable to oxidize, and which can very readily be cleaned.
When a beater has been running for some time the knives of the roll and the bed-plate become worn and so far reduced that they must be taken out and re-cut. The bed-plate is removed, firmly fixed in the bed of a planing machine, and the edges trimmed by means of a chisel, so as to cut the knives at the proper angle.
The roll and bed-plate are shown in section in Fig. 32. Fig. 33 is a plan of a bed-plate, and Fig. 34 illustrates the manner in which the knives are fixed.
FIG. 32.
It will be seen that the knives in the bed-plate are placed so that they do not lie parallel with those of the roll. This arrangement imitates to some extent the action of a pair of scissors. Occasionally the knives are slightly bent, so as to form a very obtuse angle. Bed-plates so fitted are called {121} “knee-plates.” They are largely used in America, but not much in this country.
FIG. 33.
FIG. 34.