The edge tools hitherto described consist of a single steel blade, with a cutting edge of various descriptions, and a handle for one or both hands. The inclination of the edge to the surface of the wood may thus be altered at will, as the circumstances of the case require. Narrow surfaces, or surfaces of generally circumscribed area, may thus be levelled and smoothed to a certain extent (though not perfectly) by the knife, the axe, the chisel, etc.; but when long and broad surfaces have to be made absolutely smooth, we require an edge-tool which, by attacking in the first place all the elevations, and by always cutting equally deep on a plane surface (i.e., by always removing shavings of the same thickness), finally reduces the surface to one uniform level.
The plane is the tool which fulfils these requirements. In the plane, the steel blade called the plane-iron is wedged tightly into a parallelopiped-shaped wooden block, called the plane stock, which is formed in various ways for various purposes. The edge of the blade extends slightly beyond the under side of the block.
The plane is used not only in the dressing of plane surfaces, but also in the preparation of all surfaces on which straight lines can be drawn in at least one direction; e.g., in smoothing the surface of cylindrical and conical objects, etc. Consequently, many different kinds of planes are required.
All planes, however, consist of two principal parts: the sole or stock, and the iron. The stock is formed of hard, tough, straight-fibred wood in the form of a parallelopiped, the under side of which, the sole, glides over the work when the tool is used. The best wood is elm, beech, pear, or boxwood, which has been well seasoned to prevent warping. The plane is worked with both hands. The front part of Swedish planes is often provided with a rest for the hand, called the horn. The larger kind of planes have a handle behind the iron.
The plane-iron is placed obliquely in a hole in the stock, called the socket (Figs. 54 and 56), with its edge extending a very little beyond the sole, and it is secured by a wooden wedge. It is made of iron, with a steel front. In shape it resembles a wedge, the thicker end of which is sharpened. The wedge-like shape gives the required thickness and strength to the sharpened end, leaves more room towards the upper end, and also helps to keep the plane-iron firmly in its place when the edge comes against hard knots in the wood and the pressure tends to force the iron upwards.
To form the edge, the plane-iron is ground on the posterior or bevelled edge. This forms an angle of from 20° to 25° with the front face of the plane-iron. The former angle is suitable for loose fibred timber; the latter for hard or knotty wood. The edge must not be too thin, for if so, the iron will fly, i.e., become jagged. The iron is generally placed in the socket at an angle of 45° to the plane of the sole, with the bevelled edge downwards.
Angle of the edge of the plane-iron and its position in the stock.
It occasionally happens, e.g., in small American planes with iron stocks, that the bevelled edge of the plane-iron is turned upwards at an angle of 25° to the plane of the sole. It may also be mentioned, in passing, that in planes manufactured for special purposes, e.g., planing particularly hard kinds of wood, the irons are placed at an angle of 50°, 55°, 60°, or even 90°.
As indicated above, the plane acts by removing thicker or thinner shavings, according as the plane-iron extends more or less beyond the sole. In working with the knife it is always possible to alter the position of the edge in order to prevent its cutting in the same direction as the fibres run, which would tear them, and render the surface uneven. But it is not always possible to guide the stationary plane-iron in this way. Hence in cross-grained wood, or in timber where the fibres lie parallel with the surface, the plane has a tendency to split or tear them, and the resistance offered by the torn fibres is often so great that the plane cannot be driven forward. The fibres also, by their elasticity, tend to drag the iron downwards. To prevent the fibres tearing in front of the iron, provision must be made (1) for breaking them off at once, and (2) for bringing at the same time pressure to bear on them from above, just over the edge of the iron, by means of which their elasticity may be diminished or wholly neutralised. The first object is attained by placing a cover above the iron, the effect of which is to break off the fibres as quickly as they are detached; the second, by reducing the set or opening in front of the iron as much as is compatible with the free passage of the shavings through it.
Fig. 54. Trying JPlane. 1/6. A stock, B handle, C socket, D D cheeks, E wedge, F cover, G iron, H boss.
A rectangular opening in the iron, enlarged and rounded at one end, admits the screw of the cover, and permits of its adjustment. The lower end of the cover is curved, with the concave side inwards, and it terminates in a sharp edge. "When the screw is tightened this sharp edge must lie close against the surface of the iron (see Fig. 55). If the slightest space is left the shavings will force their way through. The other side of the cover must be carefully rounded to permit the shavings to glide freely over it. The edge of the cover should be very near the edge of the iron. In finishing up a surface, and plain jointing, the distance should be about 1/32 inch, and about double that distance in cases where coarser shavings may be removed. The distance between the socket and the edge of the plane in front should be about 1/16 inch for fine planing, and not more than 3/16 inch for coarser work.