The completion of the edges of tools after grinding is effected either upon the Turkey oilstone or one of the family of hone slates described on page 1065. These stones differ exceedingly in quality, some being so hard as scarcely to take any hold of the tool, whilst others are altogether as soft. The latter are best for broad tools, as they cut rapidly, and are then less exposed to being irregularly worn than when used for narrow tools.
On the whole, the preference is given to the Turkey oilstone for ordinary tools, and the yellow German hone for razors and delicate instruments. The Turkey stone being crystalline, is cut into square blocks with the slicer, fed with diamond powder; but the hone slates may be split through their natural fissures into rough parallel blocks; and before use they are ground flat rubbing them on a wide stone, or iron plate, fed with hard and or emery. The stones are afterwards mounted in a wooden stock, as explained on page 1081. In sharpening, as in the majority of mechanical operations, the work becomes a copy of the tool, and a flat oilstone, now he tool, will produce the most correct edge with the least expenditure of time. The oilstone should be kept flat princi-ally by an even distribution of the wear; the stone or iron late must, however, be occasionally resorted to for restoring a level surface.
The oilstone should be moistened with good clean oil not disposed to dry; otherwise it becomes thick, like glue or varnish, and entirely prevents the action of the stone upon the tool.
Soap and water have been recommended for razor hones, but its rapid evaporation is unfavourable to its use.
It is there mentioned that the ultimate angles of the ordinary tools for wood vary from about 25 to 45 degrees, according to the hardness of the wood; and the manner in which the tool is applied. The smallest angle, or about 25 degrees, is used for the spokeshave iron. Paring chisels and gouges are generally sharpened at about 30 degrees, and plane irons at about 35 degrees. Turning chisels and gouges vary from about 30 to 45 degrees. The screw tools and moulding tools for hardwood and ivory are made at from 50 to 60 degrees. Tools for iron and steel have angles of from 60 to 70 degrees; and those for brass and gun-metal from 80 to 90 degrees.
In all cases in which the sharpening of the tools is completed upon the oilstone, the principal part of the material is removed upon the grindstone, at an angle a little less than that forming the ultimate edge of the tool, the greatest differences being made in the tools for soft wood, which only require a moderate degree of strength in their edges, such as the plane irons, paring chisels, and gouges, which are generally ground about 10 degrees more acutely than they are sharpened. In the tools for metal, which require considerable strength in their edges, the difference is not more than about 2 degrees. It is therefore necessary in all cases that the shaft of the tool to be sharpened, should be held at such an angle to the surface of the oilstone, as to place the edge of the tool at the required angle. Thus, if a tool with one bevil only, such as a plane iron, is to be sharpened at an angle of 40 degrees, the shaft of the tool is held at an angle of 40 to the face of the oilstone; but if a tool with two bevils, such as a turning chisel, is to be sharpened at an angle of 40, its shaft must be held at half that angle, or 20 degrees, so as to place the second bevil at the angle of 40. It consequently results, from the tools being placed at two different angles on the grindstone and oilstone respectively, that the chamfer of the tool presents two bevils, the one produced by the grindstone, the other by the oilstone, and which, in the case of the tools for soft wood, are quite distinct, but in the tools for metal gradually slide into each other.
It has been explained at page 1137, that some practice is required to enable the tools to be held steadily upon the grindstone at the proper angle, the same remarks apply to setting tools upon the oilstone; but in the latter case the difficulty is increased by the necessity for rubbing the tools backwards and forwards upon the quiescent stone. With a little care and practice, however, the hands acquire the habit of traversing the tool at the same angle in parallel lines, and which is quite essential, as should a rocking motion be given to the tool in the direction of the bevil, during the stroke, the chamfers, instead of being flat, would become rounded, and the ultimate edge of the tool would be thereby thickened and unsuited for its purpose.
Rectilinear tools that are sharpened upon the one bevil only, require to be laid flat on the face to remove the wire edge; this is done as the last process of setting; the tool should be rubbed upon the face no more than is absolutely necessary, and not in the least degree tilted up, which would produce a second bevil, and greatly increase the angle of the edge, at the same time destroying the accuracy of the face given in the manufacture of the tool.
The method of sharpening a plane iron has been described somewhat in detail at page 496, Vol. II., the peculiar mode of holding the plane iron is there stated as follows: - "The iron is first grasped in the right hand, with the fore finger only above and near the side of the iron, and with the thumb below; the left hand is then applied with the left thumb lapping over the I right, and the whole of the fingers of that hand on the surface of the iron; the edge should be kept nearly square across the oilstone, as when one corner precedes the other, the foremost angle is the more worn." This method of holding the tool gives great steadiness and command of position, and it should be adopted with all rectilinear tools that will admit of its application; as the back of the tool is then firmly supported upon the three fingers of the right hand, assisted by the two thumbs placed beneath, while the pressure is given almost exclusively by the fingers on the top of the blade.