Joinery - Slipstone

If the edge of a sharp chisel be examined under a microscope, it will look like a rough edge of a very blunt saw, with the teeth bending a little upon one side—this bending to one side is important. The burr left by the oil-stone is a very thin piece of the steel, which is bent upwards by the pressure upon the oil-stone; when the chisel is turned over and rubbed front down, a portion of this burr is ground away and the edge above it is slightly bent back — removing the feather edge does not alter this, and the extreme cutting edge remains bent back; this is as it should be, because the front of the chisel rests against the solid wood, and the chips or shavings come away at the back. The reverse is the case with a plane—the shavings come away from the front of the iron, therefore the extreme cutting edge should be bent the reverse way from that of a chisel.

When sharpening a plane-iron, after rubbing the back on the oil-stone, and turning it over to make the burr into the feather edge, the iron should be turned over again and passed lightly once up and down the stone, in order to bend the extreme cutting edge into the right direction, after which the feather edge is removed in the same manner as with a chisel. It should always be borne in mind when sharpening tools that there is what may be termed an invisible bent edge, and that this invisible bent edge should lean towards that side of the tool upon which the shavings or chips will appear when the material, whether wood, ivory, or metal, is cut —the difference is not great but it is perceptible, and for well finished work it is important; besides, it is quite as easy to sharpen tools in the right way as in the wrong way For sharpening gouges thin pieces of oil-stone with rounded edges called “slipstones” are used; these are usually 4 inches to 5 inches long and of various thicknesses; the gouge is held with the left hand, with the convex side of the blade resting upon the edge of the bench, the slipstone is held between the thumb and bent forefinger of the right hand, and rubbed lengthways in the hollow part of the gouge until a burr is formed upon the edge, which is made into a feather edge upon the oil-stone, and then removed with the hand, in the same manner as in the case of a chisel.

A gouge is not an easy tool to sharpen; the slipstone is very liable to slip off at the corners of the tool, which often results in a nasty cut. The amateur is strongly advised to mount his slipstone in a block of wood; he should buy a supstone about 7 or 8 inches long, and use it as if it were a common oil-stone for chisels, etc., but with this difference, that he rubs the concave part of his gouge upon the rounded edge of the slipstone.. An “outside gouge “—that is, a gouge which is ground upon the convex side—is sharpened upon an oil-stone, and the burr converted into a feather edge with a slipstone, but this tool is of little use for general work, whereas gouges sharpened on the concave side are often indispensable.

The angle of the cutting edge of a chisel has been referred to; experience alone can teach what is best. If a chisel be intended to cut a shaving as thin as a piece of tissue paper from a piece of soft deal, or from a cross-grained piece of wood, it is evident that a very fine and sharp edge is necessary; if a chisel with this thin edge be next used to cut across the grain of a hard piece of oak, and a heavy mallet be used in order to cut off big chips, the edge will break; for this kind of work a much thicker and stronger edge is required.

Between these two extremes there is an angle of edge which is most suitable, or, it should be more correctly termed, “theoretically perfect,” for every different piece of wood, and thickness of chips to be cut off, but in practice it would be an utter waste of time to attempt to obtain theoretical perfection; a little common sense is required, which will be assisted by experience. When a chisel has been ground, a small angle of edge can easily be obtained, but after the chisel has been some time in use and thc edge has been worn down with much sharpening upon the oil-stone, a more obtuse angle will gradually become necessary, until, at last, when this angle is becoming too obtuse, and it requires too much time to sharpen the tool upon the oil. stone, the chisel is taken to the grindstone to be ground again.