Fig. 3166.
The cutter head is formed triangular, as in [Fig. 3166], carrying three knives. The knives are set at an angle to the axis of the cutter bar or cutter head. When the knives are at an angle, they take their cut gradually, and the cutting action is more continuous, which diminishes the vibration of the machine, and causes the finished surface to be smoother. Furthermore, the knives take a shearing cut, and therefore cut more easily and freely.
In some practice the knives are made spiral, but spiral knives are difficult to bed properly to the cutter head, and also difficult to grind. The cutter head is made of a solid mild centre steel forging, and runs in phosphor bronze journals, in which it has about 1⁄8 inch end play, which tends to distribute the oil along the bearing. It is driven by a pulley at each end, the pulleys seating on a cone.
The amount of skew is about 3⁄4 inch for a cutter head carrying a knife 30 inches long, and about 3⁄8 inch for a cutter head whose knives are 10 or 12 inches long.
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Fig. 3167.
[Figs. 3167] and [3168] represent a machine in which there are three feed rolls and one delivery roll, all being driven.