Fig. 629.
Fig. 630.
The cutting edge is said to have "lead" in proportion to the distance it is in advance of the cap-iron. The cap can be set some little distance from the edge for the jack-plane, as far as an eighth of an inch, but with the fore-plane and smoothing-plane it must be set quite close to the edge, the distance varying according to the character of the wood. The more crooked or cross-grained the wood, the nearer the dull iron is brought down towards the edge of the sharp one. The nearer the edge, the smoother the result, but the harder to work the plane.
Fig. 631.
Something more than the break-iron is required, however, to insure breaking the shavings. There must be an angle, against which they can be broken, close in front of the cutting edge and above the shaving. This angle is the forward edge of the mouth or slot in the sole through which the iron projects (Fig. 631). Thus the width of the mouth makes a difference in the smoothness of the surface, for a narrow mouth is necessary to ensure the shaving being readily broken by the cap. With a wide mouth, the shaving will not be broken by the cap in time, because there is no corner against which to break it.
With straight-grained wood this does not make so much difference, but with crooked and broken grain narrowness of mouth is quite essential to a smooth surface, provided that the opening is wide enough to allow the shaving to pass through freely. Rough and knotty wood requires the mouth very narrow and the iron set very fine (i.e., projecting but very little from the sole) and the cap quite near the edge.
The modern iron planes have simple appliances for setting or adjusting the projection of the iron from the sole and thus regulating the thickness of the shaving. If, however, you are obliged to use the old-fashioned wooden planes, you raise the iron in the same way that you loosen it for removal, by lightly tapping on the top of the fore end of the stock, keeping hold of the plane with the left hand so as to prevent the iron falling through if loosened too much. When the iron is raised enough, fix it in place by tapping on top of the "chip" (Fig. 621, e) or wedge which holds it in place. To lower the cutting edge, loosen as before and, checking the edge with the finger, let it project the required distance, which you can tell about by looking along the sole (Fig. 632), and fix in place by tapping the "chip" as before. This is the process used in removing the iron for sharpening and replacing it, the chip being removed as well as the iron. Any carpenter will show you the operation. Always hold the plane in the left hand in all these adjusting operations. Do not strike or tap any part of it while it rests on the bench or on anything solid.