The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking. Paul N. Hasluck
is possible to make it. This is due to the crushing stress having been too great to be confined to the immediate neighbourhood of the cutting edge, so that it has disturbed the material for some little distance round it. This difficulty is overcome, practically, by giving a compound motion to the chisel, drawing it sideways as well as thrusting it forward. In this way the jaggedness—which always exists, however unapparent—of the edge acts like a saw upon the fibres, and thus relieves the crushing stress, with the result that in making the cut less force is expended. The benefit of this action is more pronounced in the case of the knife than in that of the chisel, as the former permits of a freer sideways motion.
USING CHISEL BEVEL DOWNWARDS.
When obstacles render it necessary to use the chisel as in Fig. 131, an increased effort is required to make the cut. The vertical component e now is opposed by the resistance to crushing of the amount of material covered by the oilstone bevel. Since this is small, the material will be considerably compressed; and if the oilstone bevel be thrust along whilst it is held in a horizontal position, the crushing of the material as the bevel moves will add a downward-acting vertical component to the motion, and so the paring will increase in thickness and the surface will not be cut horizontally. To prevent this, the chisel must be so inclined that the oilstone bevel takes the direction x, and thus the downward motion due to the crushing is exactly balanced by the upward motion due to the direction x. But this crushing cannot take place without absorbing work; so that a paring of given thickness requires much more force when produced in this way than with the method shown by Fig. 130. An additional reason why more force is required is that only the horizontal component of A is now available for actually cutting, whilst the vertical component of A goes to increase the force acting against D, and therefore increases the crushing action. Moreover, the component f is greater, owing to the increased angle of the cutting edge. The double-bent tools used by the carver have the oilstone bevel merged into the grindstone bevel, so as to give an extended bearing surface, and therefore produce less crushing, and so require a smaller expenditure of power.
Fig. 132.—Equilibrium of Forces in using Knife.
THE ACTION OF A KNIFE.
So far as the actual cutting is concerned, the action of the knife is very similar to that of the chisel indicated in Fig. 130; but in the manner in which the force A is applied to the tool an important difference exists. In Fig. 132 the force A is again shown; but in this case it is obtained by the muscular resistance to the forces, B and C. Now, since C + A equals B (for it is a case of parallel forces, and the sum of the forces acting in one direction must equal the sum of those acting in the opposite direction), it is evident that B alone is greater than A. In the case of the chisel, the muscular effort is that required to produce the force A; but in the case of the knife, muscular effort equal to B + C has to be produced; and, of these forces, one alone is greater than A. Under these conditions, it is the advantage obtained by the sawing motion that enables the knife to compete with the chisel. When the thumb is extended along the back of the knife, there is virtually a strutted cantilever (Fig. 133), and the strain on the muscles is then not so great. In almost every case, however, the use of the knife results in the maximum of effort and the minimum of effect, and as a workshop tool it cannot compete with the chisel, though experience has shown it to have many advantages from an educational standpoint.
Fig. 133.—Thumb used as Strut in Cutting with Knife.
Figs. 134 to 136.—Firmer Chisels.
CHISELS THAT ARE REQUISITE.
A few firmer chisels (Figs. 134 to 136) will be necessary. These are made in various sizes, ranging from about 1/8 in. to 1 1/2 in. in width. As the whole set is not a necessity, a few of convenient size may be purchased, adding to them as occasion requires. The following sizes of chisels will perhaps be found the most serviceable: 1/4 in., 1/2 in., 3/4 in., and 1 in. The firmer chisel is used by both carpenter and joiner for cutting away superfluous wood by thin chips. It is a strong chisel, with an iron back and a steel face (the best are made of cast steel); and it is used with the aid of a mallet. It is more generally used than any other for all kinds of hand chiselling—such as paring, for purposes of fitting, etc. The stouter kinds, being strong enough to resist the blow of a mallet, are also used for broad and shallow mortises. A long paring chisel is shown by Fig. 137. Figs. 138 and 139 show a tool known as a mortise chisel; it is much stronger than the firmer chisel, and is used only for mortising. The oilstone bevel should almost form one with the grindstone bevel as shown; otherwise the resistance offered by the wood to an obtuse oilstone facet not only adds to the labour, but causes the tool to slide away from its work. A lock mortise chisel of great strength is shown by Fig. 140.
Fig. 137.—Long Paring Chisel.
Fig. 138.—Mortise Chisel.
HOW CHISEL EDGES SHOULD BE SHAPED.
Edge tools used in wood working are more sensitive to ill-treatment than those used for working metal, and of the latter those which are operated by hand are relatively more sensitive than those that are actuated by machine, the reason being, of course, that the more delicate the nature of the work, the more readily is the action of the tool felt. Many tools in daily use in factory machines are badly formed, so causing a great waste of power that could not be tolerated if they were hand worked, owing to the excess of energy required. In many cases, tools do not cut, or they cut badly, because the wedge form is impaired. The chisel, and all chisel-like tools, should be ground with one facet only, not with several. This applies alike to the chisel for wood working and that for metal cutting, to the gouge, the axe, adze, knife, razor, plane-iron, spokeshave, and others. The reason why it must be so in order to develop the full efficiency of these tools is apparent from the accompanying illustrations. Thus, comparing Figs. 141 and 142, which are slightly exaggerated for the purpose of illustration, the former shows how a chisel-like tool ought to be ground, the concavity of the facet exactly corresponding with the curve of the stone upon which it is ground. Such grinding requires some practice in the case of broad chisels and plane-irons, the tendency in unskilful hands being to produce a succession of facets like Fig. 142, due to the slipping up and down of the tool on the revolving stone. But the advantages of the form shown by Fig. 141 are very great.