Cylindrical works in metal, of small diameter, and considerable length, such as slender rods, are difficult to be turned of strictly uniform diameter, because from their weakness they are liable to spring away from the turning tool. This liability is, to a considerable extent, counteracted by the application of supports called back stays, or sliding guides, which will be adverted to in the succeeding volume; but, nevertheless, after slender rods have been turned as nearly uniform in diameter as possible, they still retain numerous irregularities, which, although not observable to the eye, may be readily detected by passing the rod between the fingers.
Shorter or thicker cylindrical rods, that are too rigid to spring from the tool, are nevertheless liable to slight irregularities arising from imperfections in the slides of the lathe in which they are turned, hard and soft places in the metal, and also the wear of the tool, which, although small, is quite appreciable in works of moderate length. These circumstances combined, render the attainment of a perfect cylindrical rod by turning a matter of considerable practical difficulty; and consequently, it is usual, after the work has been turned as true as possible, to reduce the minute errors by grinding, and which, at the same time, serves to give the work a more highly finished appearance. The above classes of work which only require a moderate degree of accuracy are usually ground between lead or tin grinders of counterpart form, supplied with emery and water, or oil, and fixed in iron clamps that supply the pressure, and serve as handles for the application of the grinders.
Figs. 1111, and 1112, represent in two views a grinding clamp suitable for cylindrical rods of medium size. The two halves of the clamp are connected at each end by two binding screws, b, and the clamps are curved in the middle, so that when they are separated about one quarter of an inch by the set screws, s, they may present in the center a cylindrical aperture about one inch larger in diameter than the cylinder to be ground.
For casting the lead grinders within the iron clamps, the set screws are withdrawn, and the binding screws slackened, so as to leave an opening of about one quarter of an inch between the flat faces of the clamps, which are then placed edgeways upon a flat block, and a short cylinder or core of the same diameter as the cylinder to be ground, is placed in the center of the circular aperture, two parallel slips of wood or iron sufficiently wide to be grasped between the flat faces of the clamps, are then placed in contact with the sides of the core, so as to divide the opening into two parts, and are firmly pinched by the binding screws. Melted lead is now poured in to fill up the cavities, and form two grinders, each a little less than the semicircle, and the cylindrical faces of which are counterparts of the metal core. The inside surfaces of the clamps are left rather rough, in order that they may the better hold the lead; and, with the same view, a few radial holes are sometimes drilled in the clamps, or otherwise the edges of the opening are chamfered off, in order that the lead may be cast with a projection on each side to prevent the grinders from shifting endways.
To keep the core in the center of the clamp while the lead is being poured, a hole is sometimes bored in the block upon which the clamps are laid. At other times the grinders are cast at once upon the rod to be ground; in this case the rod is fixed vertically in a vice, and a hole is bored through a piece of wood, which is slipped on the rod, and luted with clay to make a close joint.
In grinding a long cylindrical rod the work is mounted in the lathe, and the grinder is fitted upon one part of the rod by first closing the clamps with the binding screws b, the set screws s are then advanced to partially sustain the pressure of the binding screws, and separate the clamps just sufficiently to allow the rod to revolve within the grinder, with as much friction as can be conveniently overcome by the hands applied to the extremities of the lever or handle. The lathe is then put in rotation, and the grinder is traversed backwards and forwards, throughout the length of the rod, with a screwing action somewhat as in boring a hole with an auger. If the grinder were slowly traversed straight along the rod, the latter would not be so uniformly ground, and when finished it would be marked with rings, partly owing to the emery not being equally distributed over the surface of the work. The grinder is first applied to those parts of the rod which present the greatest amount of friction, and when the resistance becomes lessened at the most prominent parts of its length, the grinding clamps are closed upon the rod, by withdrawing the set screws and tightening the binding screws. The clamps thus admit of being gradually adjusted, so as to serve almost as a gage for the parallelism of the rod, and successively reduce the parts of largest diameter, until the grinder slides smoothly and with uniform resistance from end to end of the cylinder.
The grinder should be as nearly as possible the counterpart of the desired cylinder, but in the course of work the grinder becomes irregularly enlarged, while at the same time the cylinder is gradually although slightly reduced; the closing of the clamps partially compensates for the difference of diameter, but not for the irregularity of wear, and consequently two grinders are usually required for the completion of the cylinder. The principal errors are removed with the first, and when the rod has been rendered tolerably cylindrical, and very nearly of the required diameter, a second grinder is cast for finishing the work.
When the rods are required to be of precise diameter, for sliding through bearings and similar purposes, they are turned slightly larger than the finished size, and gradually reduced by grinding, until upon trial they are found to fit with sufficient precision into the hole in which they are intended to work. Sometimes brass or gun-metal grinders made in halves, connected by screws, and bored out to the exact diameter, are employed for the final adjustment of cylindrical works required to be of definite diameter, but the method is scarcely trustworthy, as the grinders are themselves rapidly abraded, and soon become enlarged, unless they are very sparingly employed.