The sharpening of the cutting edges of planes and chisels is performed primarily on a grindstone or its equivalent an emery grinder, and secondarily on an oilstone.

The implement known as a grindstone consists of a wheel of sandstone mounted on a revolving axle in a trough capable of containing water, its ordinary form being sufficiently familiar to dispense with illustration. There is probably no instrument in the machine shop or factory which pays better for the care bestowed upon it than the grindstone; and considering that nearly every tool, and all edge tools, require it, before they can be used to advantage, or in fact at all, it is somewhat surprising that more attention has not been bestowed on the proper selection of the grit for the purposes for which it is intended. As grindstones are almost constantly in use, their first cost is of little consequence if the quality is calculated to do the work in the shortest time and in the most perfect manner, as more time can be lost on a poor grindstone, badly hung, and out of order, than will pay for a good one every 3 months. This state of things should not continue, as with the great improvements made in the manner of hanging them, and the endless variety of grits to select from, every mechanic should have a grindstone which will not only do its work perfectly, but in the shortest time. This can be accomplished by sending a small sample of the grit wanted to the dealer to select by.

Grindstones are frequently injured through the carelessness of those having them in charge. The grindstone, from being exposed to the sun's rays, becomes so hard as to be worthless, and the frame goes to pieces from the same cause; it will have a soft place in it, caused by a part of it being allowed to stand in water overnight, and the difficulty arising from this cause increases with every revolution of the stone; but as this homely implement is in charge of all the men in the shop in general, and no one in particular, and as the workmen are all too busy to raze it down, double the time is consumed in imperfectly grinding a tool than would be required to do it perfectly if the stone were kept in order by some one, whose business it would be to attend to keeping all the grindstones of the establishment in order. The wages of a man for this duty would be saved in the time and perfection with which the numerous tools could be kept in order for work.

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Most commonly grindstones are made to turn by hand, and necessitate the services of an assistant. It is much better to have one that may be driven by the foot of the operator, with a handle to attach for a second workman to turn it when necessary. When the needs of the workshop will admit of it, the best plan is to have a large grindstone (say 2 ft. diam. or more) for heavy work, and a smaller one (say 9 in. diam.) capable of being fixed to the end of the carpenters' bench and driven by foot power for lighter work. The stone should never be used dry, and with this object a trough is provided for containing water; but the stone must not under any circumstances be allowed to remain immersed in the water when not in use, consequently the water must be drawn off through a bung-hole, or a hinge attached to the trough for lowering it away from the stone, or a prop introduced for supporting the stone-out of reach of the water. A sponge held against the revolving stone by a small rack is useful for preventing the water travelling round with the stone and wetting the handle of the tool and the hands and clothes of the operator. An absolutely essential quality in a grindstone is a true level face.

This may be partially secured by distributing the work over the whole breadth of its surface, so as to wear it away equally all over; but every care in this respect will not suffice to keep it even enough for some tools, and then it must be refaced by means of a steel tool wider than itself. Fig. 365 illustrates an American device (sold by Churchills, Finsbury) for keeping the face of the grindstone constantly true while at work, without interfering with the use of the stone or raising any dust. The main stand or bottom piece a is securely clamped upon the trough close to the face of the stone; then by turning the handwheel b, the threaded roll c is brought into contact with the face of the stone, and allowed to remain so long as it is necessary to produce the desired result. The water is left in the trough as usual. When the thread of the rod c is worn it can be recut. The price of one of these implements suited to a 12-in. stone is 4l.10s. The tool to be ground should be held against the stone in such a way that the bevel or slope of the cutting edge lies flat on the stone, while the handle maintains a horizontal position. The stone should revolve towards the operator, i. e. against the edge.

Usually the trough of the grindstone has high ends or similar means of supporting the tool during the grinding. Such means may take the form of a bevelled block to support the blade, or a notched rest to hold the handle, in either case securing that the grinding is done at the correct angle. Fig: 3GG shows a contrivance for resting the tool, and ensuring its being ground at the desired angle. The plane iron a is held by a clamp screw b in the frame c, while the wheel d revolves on the stone e, and steadies the whole. This rest is sold by Churchills, Finsbury, for 2s. The amount of angle or bevel given to the edge varies with different tools and with the fancies of different workmen. In the case of a plane iron it must always be more acute than the angle formed by the sole of the plane and its mouth. The bevel produced on the grindstone should not be quite flat nor rounding (bulging) but rather hollowed out, a result naturally following from the circular form of the grinding surface of the stone, and varying of course with the size of the stone. Many workmen object to the use of any form of rest for the tool during grinding, as tending to produce a hollow edge - the very thing desired by another class.

Gouges are best held across the stone, as otherwise they are apt to score the surface of the stone. In grinding the jack-plane iron, the cutting edge should be somewhat round, so that the shaving taken off is thicker in the centre than at the edges. The trying iron is also slightly round, but so slight that it is hardly noticeable. The smoothing iron should be a straight line on the cutting edge, with the corners very slightly rounded, but on no account should the edge be curved, though ever so little. These irons are all ground on the back only - that is, the bevel side; the bevel or ground part is about 1/2 in. across. If too long, the iron when working is apt to jump.

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