For cutting the facets the laps are charged with fine washed emery, smoothed with an agate or pebble burnisher, and supplied with water. The work when too small to be held in the fingers is cemented on a small wooden stick to serve as the handle, and the position of the facets is given with the fingers unassisted by any guide. If the facets have to be entirely produced by grinding, the pin, and while the mandrel is rotated, the grinder is swung backwards and forwards in an arc of about one fourth of a circle. The face of the grinder being quite flat, and traversed at right angles to the mandrel, the heads of the screws are ground quite flat, notwithstanding that they are polished very highly.

Sometimes instead of the mandrel being rotated by the flat hand as above described, the instrument has a pulley like a drilling lathe, and is driven with a drill bow; in this case one mandrel only is used, and the screws are fixed in similar grasping apparatus made as small chucks, fitted to the mandrel either by a screw or a plain conical fitting.

Large flat works in cast iron for heavy machinery are in almost all cases wrought in the planing machine, and when they are smoothed it is done with files and rubbers as explained on page 1074. Large flat works in wrought iron are frequently ground on the edges of large stones and finished with files. The flat parts of objects of complex form are worked in the various cutting and paring machines, and the grindstone is seldom resorted to for plane surfaces requiring even moderate accuracy. Sometimes however when a large surface has to be made tolerably level and smooth for appearance alone, the flat side of the grindstone is employed, and the work is traversed across the stone upon slides; this method of using the flat side of the stone is however liable to the objections stated on page 1135.

In a grinding machine constructed by Mr. James Nasmyth and shown in figs. 1084 and 1085,* this difficulty is removed by making the grindstone as an annulus about fifteen inches wide, composed of 12 segments of stone each fitted into a separate radial compartment in a cast iron wheel or chuck about seven feet diameter. The machine is double or possesses two compound grinding stones fixed on the opposite ends of the same shaft, each of the grindstones is provided with separate slides, which are duplicates of each other, and made self-acting. The foundation of the machine is of masonry, and pits are sunk on each side for the lower part of the stones to work in, just the same as for ordinary large grindstones. To the masonry are firmly fixed two cast iron frames a, a, upon which are bolted two plummer blocks for carrying the main shaft, having at each nasmyth's grinding machine for flat surfaces. 188 extremity the large wheels or chucks, each made as a face plate 7 feet diameter, having on the one side 12 radial ribs about 6 inches deep, that extend from the center to the periphery where they terminate in a ring of equal depth. A second concentric ring about 4 feet diameter intersects the ribs, and thus divides the entire chuck into 24 compartments, in the 12 outer of which separate pieces of grindstone s, s, about 14 inches thick are fitted like the stones of an arch, and each is wedged fast between the ribs by a single set screw passing through the outer rim of the chuck. On the top of the cross frames a, are fixed two longitudinal frames b, b, for supporting the bearers c, c, upon which the slides d, are traversed across the faces of the stones. Upon the slides d, are mounted at right angles the slides e, upon which the work is fixed and advanced towards the stone as the grinding proceeds.

* Transcribed from plate 54 of "Buchanan's Mill Work" by Rennie, 1841.

Fig. 1084.

The Production Of Plane Surfaces By Abrasion Part  30036The Production Of Plane Surfaces By Abrasion Part  30037

A self-acting motion is given to the slide d, by which the work being faced is gradually traversed along the bearers c, so as to bring the face of the work in contact with the revolving grindstones. This motion is obtained as follows. Upon the main shaft of the machine is fixed an endless screw, which drives a worm wheel fixed on an upright spindle, communicating by two pairs of bevil wheels and a short horizontal spindle, with a second horizontal spindle running the whole length of the machine, and having at each extremity a small bevil wheel, that leads alternately into two other bevil wheels fitted loosely on the screw of the slide d, which is traversed in opposite directions, accordingly as the one or other wheel is engaged by a central clutch seen at f.

The clutch is shifted for every traverse of the slide d, by means of a rod sliding endlong through two bearings fixed on the front of the bearers c; and upon this rod two pins are fitted that admit of being adjusted to any distance from each other, according to the length of traverse required for the work in hand. A pin fixed on the slide d, is brought by the traverse of the machine in contact with one of the pins on the rod, and slides it endlong, so as to disengage the clutch from the one bevil wheel on the screw of the slide d, and cause it to take into the other and reverse the motion; a counterpoise weight g is fixed on the rod to retain it steady while the clutch is being shifted. The upper slide e, is also provided with a screw for advancing the work towards the stone in steps, as each layer is ground off by the traverse motion.

The softer varieties of stone such as Bath, Caen, and Pens-wick stones, admit of being cut into slabs and smaller pieces with toothed saws, which are sometimes made of a similar form to the cross cutting saws for wood with upright teeth, shown in figs. 640 and 643, Vol. II., but the toothed saws for soft stone are generally made somewhat wider in the middle than those for wood, so as to make the blade more rounding in the direction of its length, and instead of being reciprocated backwards and forwards nearly in a horizontal line, as for cross cutting wood, the toothed saws for stone are used with a swinging stroke, so as to act upon only a moderate portion of the length of the cut at the one instant of time; this is done to reduce the labour and give the saw teeth more penetration. Some of these very soft stones are worked with chisels and gouges similar to those of the carpenter, and they may even be worked into mouldings with planes like those used for hardwood, but this is not generally practised.