—When edged tools become rounded over by repeated whettings or when they are nicked too deeply for the oilstone to remove the nicks, the grindstone is needed to cut the metal to the proper angle. [Fig. 101] shows the manner of holding the chisel upon the stone. The tool must be held firmly and at the same angle. This angle will depend upon the temper of the tool and the kind of wood to be cut, whether hard or soft, soft wood allowing the use of a sharper angle. On plane-irons the length of bevel or grind should be three-sixteenths or one-fourth of an inch; on the chisels, three-eighths or one-half an inch. The tool should not be kept in the middle of the stone but should be moved from right to left and vice versa across it as the grinding proceeds, that the surface of the stone may be worn as evenly as possible.
20° to 25°
Fig. 101.
The pressure of the left hand should be so applied that the stone shall cut straight across the blade. Examine the tool often, being careful to replace it each time as nearly as possible at the same angle. [Fig. 102] shows the flat bevel which is to be obtained, also the rounded effect caused by frequent changing of the angle at which the tool is held.
Angle
20° to 25°
Fig. 102.
Grindstones are usually turned towards the tool because in doing so they will cut faster.
Water is caused to flow on the stone for two reasons: To keep the edge of the tool from being burned or softened by the heat which friction would generate, and to wash off the particles of steel and stone, thus keeping the cutting surface clean that it may cut the more freely.