Planes are used chiefly for reducing broad and long surfaces to perfectly smooth, flat, or curved forms, as may be required, or for making continuous grooves and mouldings of a more or less complicated nature with or across the grain, and in straight or curved directions.

When a small piece of wood is to be removed to reduce the surface to any desired level, a chisel or gouge will fulfil the purpose, but when the surface is large, it is clear that the larger and more regular the thickness of the piece of wood removed at each cut, the more easily will the task be performed by the pupil. The plane fulfils this requirement. A wide blade, practically similar to a chisel blade, is fitted into a block of wood in such a manner that its cutting edge just appears through the perfectly true surface of the face or sole of the block, at such an angle as will allow it to cut the timber to be made true in its course over the surface. When the blade is well pressed down it will thus cut just as much of the wood off, as the depth of the blade showing through the sole; and inasmuch as the block of the plane regulates the depth of the cut made by the blade, so it will enable the blade to take more effect on portions of the objective timber which are in relief than on parts which may be depressed. Fig. 46 gives an exaggerated case of uneven wood being acted on by the plane. It is natural that in order to facilitate this reduction of the more prominent irregularities first, a long plane is more reliable than a short one, and this is the reason that jack planes and trying planes have long bodies. Though these planes are useful in reducing timber to a good level surface, absolute smoothness is impossible, for the cut of a plane-iron makes a long trough-like, though very shallow, groove. The fibres, too, in the motion of the plane become somewhat broken, and cause slight roughness in places.

Fig. 46.

So long, then, as the physical power of the operator is not overtaxed, the wider the plane-iron for flat work the better, as it will leave fewer ridges and shallower grooves. Short planes, because they can be pressed down so much closer to the surface of the wood than long ones, are useful in overcoming the slight roughness left from the long trying, and jack planes, but by reason of their tendency to plane the wood hollow in the length, they should only be used sparingly.

The angle at which a plane-iron should be sharpened is, in nearly every case, 35°, the face being flat and the back sloping.

If the edge is thicker it will not cut satisfactorily, and too much force will be necessary. If it is less than 35°, it is apt to break or turn on encountering a hard substance like a knot. In all the planes necessary to the course of manual training in this book, the iron is placed in the block with the back towards the wood. The angle of inclination of the iron to the sole of the plane is regulated by the degree of density of the timber to be planed.

It is obvious that a less angle than 35° would be impossible, as the back of the cutting angle would then be flat on the surface of the timber to be planed; and even this angle is impracticable, as the iron would not then cut at all, but simply slip over the surface of the wood.

The iron is, therefore, usually placed at 45° to the sole of the plane, and this will be found the most serviceable angle, for experience teaches that a greater angle than 45° would, except under special and rarely obtained conditions, cause the iron to become more scraping than cutting in its action, leaving the surface rougher than is the case when placed at the usual angle.

In planing there is at all times a tendency for the wood to split in front of the cut being made, because of the lifting of the fibres split by the action of the iron as the plane progresses. If allowed to proceed unchecked, this would have the effect of causing irregularities in front of, and slightly below, the plane in which the edge of the blade is cutting.

This is especially the case when planing against the grain, i.e., when the fibres dip into the wood away from the operator.

The sketch (fig. 47) shows clearly the natural splitting action, and what would be the result of using a single iron when planing against the grain. The cap-iron is intended to prevent this running split in front of the edge, and by snapping the fibres and sharply diverting the course of the shaving immediately it begins to run up the face of the cutting iron, it prevents the tendency to splitting from becoming serious, and causing any marked obstruction in the path of the plane. Fig. 48 shows the action of the cap-iron, and fig. 49 the method of securing the cutting and cap irons to each other.

Fig. 47. - Section of plane fitted with single iron, and showing the tearing of fibres.

Fig. 48. - Section of plane fitted with double iron, showing breaking action of cap-iron on the fibres.

The cap-iron is bent slightly at the bottom end, and the inside at this end is finished off smooth and true.