When the irons are well screwed up, the cap-iron is slightly bent inwards by the pulling of the screw, with the effect that the smooth inner surface of the lower end is kept very hard down on the face of the cutting iron, preventing shavings from running up between the two irons. This must on no account happen.

The edge of a plane-iron is not straight, but slightly curved from end to end, as in fig. 50, in which, however, the curve is much exaggerated. It will be noticed that the corners are rounded off also. The object of this is to prevent the corners from cutting and making sharp marginal edges down each cut. The block or body of a plane is invariably made of some hard wood, which will not warp or twist. In order to ensure this desirable quality it is desiccated. Beech, although when green liable to warp, is, if prepared by being subjected to steam, the most suitable wood from which to make the bodies of planes.

The brown and red varieties are the most reliable, and the very best, clear, straight-grained pieces are used.

Fig. 49. - The screw a can be passed through the slot at b, and moved down in the direction of the arrow, as may be required.

Fig. 50.

The annual ring should be nearly parallel to the face, and the silver grain at right angles to it, or as nearly so as possible.

Fig. 51 shows the end section of a plane with medullary ray and annual ring.

The shrinkage of the timber, which, to some extent, must occur even in seasoned wood, it must be remembered, always takes place to a greater extent across the silver grain than parallel to it, so that the sides, a b, will contract, while the top, c, and the sole, d, will be much less affected.

The selection of the outside of the tree will assist to lessen the contraction, as the angle of inclination of the medullary rays is not so apparent as further in towards the heart. Although the outside of the tree should be chosen, no sap-wood should be taken.

Of course the very best seasoned bodies may occasionally get untrue on the sole from shrinkage as well as from wear.

They must then be re-shot and be well rubbed with linseed oil to prevent fresh shrinkage.

The escapement of the plane is a V-shaped mortice, with one nearly upright side towards the front of the plane, and the other sloping at 45° to the face.

In this is cut another shallow mortice, to hold the head of the screw of the cap-iron, and allow the cutting iron to lie flat on the back portion of the escapement.

The sides of the mortice, as will be seen in fig. 52, are nearly upright, a diminishing slot in both sides at the back giving a shoulder to hold the wedge in. The wedge is the means of holding the iron tightly in its place. It varies in shape in different planes, but that given in fig. 53 is the general form of the wedge of the jack, trying, and smoothing planes. The points should be kept sharp, to prevent shavings from catching between them and the iron.

Fig. 51.

At the bottom of the mortice is the narrow mouth, which runs nearly across the face of the plane. The length and breadth of this opening vary in different planes, but, generally speaking, the narrower the mouth the more accurate will be the work performed, as the sole of the plane under pressure holds the wood down and prevents the splitting before mentioned, and the closer the face of the plane is to the edge of the iron, the more effectually is this done. In short, when the iron is in its place, ready for use, only sufficient room should be left for the shaving to pass through.

Fig. 52. - Plan and sectional elevation of the escapement of a plane. The elevation is on a b, and the plan shows in one portion the position of the mouth.

Fig. 53.

When a plane is re-shot, i.e., planed true on the face after warping or wearing, the mouth necessarily becomes wider, and if the operation of re-shooting is repeated several times, the plane must be re-mouthed. To do this a piece of boxwood or beech, of the shape shown in fig. 54, is let into the face, but it is advisable to leave such a task to a tool-maker unless the teacher is very skilful.

These general characteristics of planes do not apply of course to those intended for special and peculiar purposes, like the moulding plane, rebate plane, bull nose, plough, chariot, fillister, and others. These planes, though made on the same principles as those used in this course, are required only by workmen. As they are not necessary for manual instruction, this work has not been burdened with a description of their details.

The great point of difference between them and the kinds here required, however, is in the bull nose, chariot, and shoulder plane, and consists in the turning over of the iron, so that the face is downwards when in use, and in these no cap-iron is used.

The iron in these planes is placed at a very acute angle to the face, only 15° to 20°. This would be impossible if the back of the iron were down, remembering that though the iron in these planes have a somewhat finer cutting angle, it is still 30°. The cap-iron is not required, because the mouth is so narrow that splitting is practically impossible. In these special planes the mouth is sometimes 1/60 of an inch in breadth, or even less.

Fig. 54. - Plan and section of re-mouthed plane.