The Cap. A supplementary iron, or "cap," shown by c, Fig. 92, is fastened to most plane-irons. Its use is well illustrated by the two sections, Figs. 93 and 94. The single iron will do smooth work as long as the grain of the wood is favorable, as shown at a. When the grain becomes obstinate, as at b, the shaving, by running up on the iron, acquires a leverage which causes it to split in advance of the cutting edge, below the reach of which it breaks, leaving a surface extremely rough. The office of the cap is to break the shaving as soon as possible after it is cut, Fig. 94, and thus prevent a gain of leverage on its part.

Fig. 92

Fig. 93

Fig. 94

The distance at which the cap is set from the edge of the iron, must vary with the thickness of the shaving taken. For a smooth-plane or a fore-plane, a thirty-second of an inch is frequently not too close, while for a jack-plane an eighth of an inch may not be too great a distance.

A cutting iron and cap together are frequently spoken of as a "double iron."

73. Narrowness of Mouth in a plane is the chief element in the production of smooth surfaces. If, in Fig. 94, that portion of the stock in advance of the iron, marked c, were wanting, the shaving, having nothing to hold it down, would rarely be broken, notwithstanding the presence of the cap. A wide mouth would produce a similar effect. This being true, whatever other conditions there may be, the wider the mouth is, the less frequently the shaving will be broken and, in obstinate grain, the rougher will be the work.