The round shape of the stone causes the bevel of the tool to be concave if held steadily in one position, as shown in Fig. A, Plate 4. Fig. B shows the incorrect result if the blade is not held evenly on the stone. It can be readily seen that the latter result will not make a very sharp cutting edge.
Care should be taken when grinding not to round the corners of the tool.
The theory of the cutting edge of the tool is the same as that of the wedge; the thinner the wedge the easier it is to drive it. However, the wedge, as well as the tool, must be thick enough to stand the strain of being driven into the wood, or the material which is to be split or cut. Too long and thin a bevel, while sharp at first, soon loses its edge through usage, while too blunt an edge makes the tool unsatisfactory to work with.
Plate 4.
The grindstone leaves the tool edge rough, or with a wire edge, as it is called. This roughness is removed on the oil stone. One or two drops of thin stone oil should be placed on the stone and the tool placed bevel side flat on the surface of the stone. Work with a circular motion, bearing on the tool with uniform pressure. Turn the blade over, bevel side up, seeing that the blade lies perfectly flat on the stone. Work with a similar motion. Repeat these operations until the blade is as sharp as desired. Wipe the oil from the tool and test by drawing the blade lightly across the thumb. If the blade clings to the skin it will be found sharp enough.
Fig. C, on [Plate 4], shows the correct and incorrect methods of oil-stoning the tool blade. Always wipe the stone dry after using, as the oil will dry and gum up the grinding surface if not kept clean.
Tool grinding is an important and rather difficult operation at first and skill comes only with continued practice.