When edged tools become dulled through repeated whettings, or through being brought into contact with metal, they must be ground, Fig. 85. Grinding is a rather difficult task for beginners to learn to do well. Beginners ought, however, to learn to whet their edged tools.
Fig. 85. Grinding a Chisel.
Fig. 86. Whetting a Chisel.
Fig. 87. Starting Position in Whetting.
Whetting consists in rubbing the tool backward and forward, Fig. 86, taking care to hold the tool at one and the same angle. This angle may be determined as follows: place a little oil on the stone, and placing the iron as in Fig. 87, gradually raise the handle until the oil can be seen to press out from under the cutting edge, Fig. 86. To raise the handle any higher would result in a blunt edge in whetting.
Not to raise it high enough to expel the oil would result in the whetting being done at the heel of the bevel, which would do no good.
Oil is used upon a whetstone to mix with and remove the little particles of steel which, otherwise, would clog the pores of the stone. A chisel has its edge ground straight across.
A plane iron for general manual training purposes is ground straight across but is whetted slightly rounded as in Fig. 88.
Whetting usually causes a wire edge to be turned up on the face of a chisel or plane-iron. This wire edge may be detected by rubbing the fingers along the face out over the edge. To remove this edge, strop the tool upon a piece of leather upon which has been placed a slight coating of oil and emery dust. Hold the tool first as in Fig. 89, then as in Fig. 90, alternating rapidly from one position to the other as the stropping proceeds.
Fig. 88. Shape of Jack-plane Iron. Exaggerated..
Fig. 89. First Stropping Position.
Fig. 90. Second Stropping Position.
Fig. 91. Thumb-nail Test for Sharpness.
Fig. 92. Mechanics' Test for Sharpness.
There are a number of ways of telling whether an edge is sharp or not. One way is to draw the edge over the thumb-nail as in Fig. 91. If the tool is sharp it can be felt "taking hold." If the edge is not sharp it will simply slide over the nail.
A more delicate test, the one used by carpenters, is to make the same kind of a test but using the ball of the thumb, Fig. 92. Judgment is required in this latter test or a cut thumb will be the result. Do not use a finger. The thumb is calloused and when the sharp edge "takes hold" it is cutting in this callous.