Fig. 209.—Two Pieces of Wood Scribed Together.
GRINDING AND SHARPENING GOUGES.
A chisel edge is the same shape as the edge of a plane iron, and is produced in the same way, and there is no need to treat chisels separately here. Information, however, on keeping gouges in order may be welcome. They seldom require grinding, but this, when necessary, must be done with great care, as the bevel must follow the curve of the tool to ensure good work, Fig. 205 shows the gouge applied to the stone; it must be constantly turned backwards and forwards, as shown by the arrows. Fig. 206 illustrates the manner of holding the tool whilst grinding. The same rule as regards turning the gouge applies when sharpening it upon the oilstone; this is shown by the arrows at a (Fig. 207); if it were held like a chisel, as b, a good result would never be obtained—that is, the sweep of the tool would not be correct, as shown at c. After rubbing upon the flat stone, it may be necessary for some purposes to sharpen the gouge from the inside. This is done by means of an oilstone slip. The tool is held in the left hand, b, as in Fig. 208, and the slip, held in the right hand, is rubbed up and down the gouge, taking care to keep the slip pressed quite flat against the groove, so as not to turn the edge. The reason for having the gouge edge so sharpened for ordinary use is evident from the following explanation of the use of a gouge sharpened the reverse way; that is, with the bevel inside the groove. Fig. 209 shows two pieces of wood fitted or scribed together. Figs. 210 and 211 are the piece marked (A) removed, the more plainly to illustrate the application of the gouge. At Fig. 212 is seen a scribing gouge b, showing by a section that it is ground on the inside, thus enabling it to cut straight down, as a; this is impossible with the ordinary gouge ground upon the outside, as Fig. 213 plainly shows.
Fig. 210.—Scribed Piece of Wood.
Fig. 211.—Scribed Piece of Wood.
Fig. 212.—Gouge ground Inside.
Fig. 213.—Gouge ground Outside.
HAND SAWS.
THE SAW AS A TOOL.
THE saw cannot be classified with any other tool. The hammer is a tool for consolidating material; the splitting axe, although usually regarded as an edge tool, is generally a hammer with a wedge pane, and is used for dividing material in the line of the fibres. The true edge tool in its most elementary form is the chisel, and by various added contrivances the chisel becomes the shears and the plane; but saws, which are to be discussed in this chapter, differ from all these. They can scarcely be called derivatives from these, unless the knife be regarded as the connecting link; for sometimes it is used as a chisel and at other times as a saw. All these tools, including the knife, cut or work in the direction of the grain or fibre, but the saw is essentially a tool for use across or at right angles to this fibre, although custom and convenience have arranged the saw for use with the fibre. Even then it is only because the fibres are not straight and parallel. When they are so, as in lath wood, then the saw is not employed. It is true that in such work as the felling of timber the axe is used across the grain, and therefore at right angles to the length of the fibre; yet if the action of the forester be observed, it will be seen that the direction of his blow is not that of the line of separation. He goes at his work indirectly when using the axe, directly when using the saw. These and the following remarks on the theory of the saw’s action and application are taken from Rigg’ Cantor Lectures, to which reference has been made already.
PRINCIPLES OF THE SAW’S ACTION.
For the purpose of separating a bundle of fibres, an edge drawn across will cut the surface fibres only; this is insufficient, for a saw is required to separate fibres below a surface. This separation must be a cutting (not a tearing) action. Looking at the work of a single cutting edge, it will be noticed that, although the continuity of the fibre is destroyed, the separated ends are still interlaced amongst the other fibres. To obtain a piece removable as by a small narrow chisel, it will be requisite to make a second cut parallel to the first. This being done, there is the short piece, retained in position by adhesion only, which must be removed, for the room it occupies is that in which the back of the cutting edge must move. To slide, as it were, a narrow chisel along and cut it out is more simple in suggestion than in execution; for instance, the absence of any guide would cause great difficulty. To draw a pointed cutting edge along the same deepening line needs a very steady hand and eye. To increase the number of cutting edges, and form, as it were, a linear sequence of them, may give a partial guidance. Instead of having two parallel cutters these cutters may be externally parallel but internally oblique to the line of cut, as shown in the sectional and exaggerated view of saw teeth (Fig. 214), in which the portions of wood A B D and E C D have been removed by the gradual penetration of the oblique arms; not only have they been cut, but they have been carried forward and backward and removed, leaving a clear space behind them of the width A E. But a portion within the oblique arms is left, this consisting of particles of woody fibre adhering to each other only by the glutinous or gummy matter of the timber, and not cohering. If the breadth A E is not too large, the whole of the heap would be rubbed away by the power exerted by the workman, and both power and material are economised by narrowing A E. The resultant saw kerf is shown in section by Fig. 215. The active portion of a saw has three edges, of which the lower or horizontal one only is operative, for the tool rides upon the fibres and divides them, and the sloping parts remove the hillock. To act thus, the lower edges would be required to be sharpened at A and E, so as to clear a way for the metal to follow. The resistance to the downward pressure, required to cause the cutting segments to penetrate vertically, is the breadth of the tooth, for it rides upon a number of fibres and divides them by sliding over; the complete action requires not only downward pressure for the cut, but also horizontal pressure for the motion, the latter both in the advance and withdrawal of the tool. These two pressures being at right angles do not aid each other, and will require the use of the workman’s two hands. The compounding of these pressures will give freedom to at least one hand. For the present, assume that the two pressures to be compounded are equal, then the simple operation is to employ one pressure making (say) an angle of 45° with the horizontal line of thrust. If the tool becomes a single-handed one, and relies for its operation upon thrust or tension in one direction only (say thrust), then cutting edges on the back portions of the teeth are useless, and had better be removed.
Fig. 214.—Diagram showing Principle of Saw’s