When the hole is so short that it would not serve as a guide for the grinder, the latter is mounted to revolve in a lathe, and the work held by the hands is traversed endways on the grinder, and at intervals is allowed to be partly carried round by the friction, so as continually to place the work in different angular positions, which serve to prevent the hole from being ground either oblique, or more on one side than the other. With very short holes, care is required to traverse the work quite square on the grinder, as if it be twisted in the direction of the hole, the latter will be ground larger at the ends than in the middle.

In all cases of grinding cylindrical holes, there is great risk of enlarging the two ends, partly owing to the work being twisted, and partly to the emery cutting more keenly on its first entry into the hole, and becoming crushed before it reaches the middle. This evil is sometimes partly avoided by making the grinder much shorter than the cylinder, the grinder may then be applied to the middle of the hole for a longer period. The grinding of long holes is at all times however a process of considerable uncertainty, from the absence of any guide for the straightness of the work, and consequently, except for hardened steel, the principal reliance for accuracy is placed upon the boring and broaching tools, and a slight grinding is only occasionally resorted to for the purpose of smoothing the surfaces or fitting cylindrical works together.

Fig. 1115 is used for grinding two cylindrical holes of unequal diameter on the same line, as in the case of a screw mandrel lathe head, the front bearing of which is usually made of a larger diameter than that at the back, and both are made cylindrical in order to allow of the longitudinal traverse of the mandrel through the bearings in cutting a screw. Both holes are ground at the same time, as the distance between the bearings causes them to serve as a guide to ensure the holes being ground parallel to each other. For the same reason when holes of equal diameter have to be ground for the reception of a cylindrical rod or shaft, the grinder is made sufficiently long to grind both holes at the one process; and in a similar manner when one hole only has to be ground, advantage is taken of any hole in the same line that may be used as a guide, and the grinder is made with a cylinder to fit the second hole, which is not supplied with emery.

Fig. 1115.

The Production Of Cylindrical Surfaces By Abrasion 30063

The grinder fig. 1116 is made in two halves to allow of the power of expansion, it consists of two semicylindrical rods of iron, fitted to each other either by steady pins, or two projections at the end of the one bar, within which the second bar is fitted. They are held together by 3 or 4 binding screws, placed at equal distances, passing freely through the one bar and tapped into the other for the purpose of closing the grinder and reducing its diameter. The bars are separated by intermediate set screws, tapped through the one bar and bearing against the opposite. The lead to constitute the grinder is cast upon the bars in much the same manner as for the grinding clamps, fig. 1112, two thin slips of wood being inserted between the bars to divide the mould in two parts.

Fig. 1116.

The Production Of Cylindrical Surfaces By Abrasion 30064

The mould for casting cylindrical grinders is frequently a block of wood bored with a hole of the required diameter, but sometimes a temporary mould is made of a sheet of stout paper wrapped around a cylinder of suitable size, and bound with string; the cylinder is afterwards removed. The lead should be only of a moderate heat at the time of pouring, or the casting will be liable to be honey-combed, or filled with air bubbles, even if the mould be quite dry, and if it be damp, the fluid metal may be forcibly driven out. The heat of the melted lead is therefore tested with a piece of paper thrust below the surface, and when it is cooled just sufficiently to avoid burning the paper, the lead is poured into the mould, and when cold the grinder is turned to the proper diameter.

The spring grinder, fig. 1117, is used for grinding out short holes in works that admit of being mounted in the lathe, and principally for those holes that do not extend entirely through the object, and therefore do not admit of the preceding forms of grinders being employed. The two rods of the grinder when left to themselves spring open like the blades of sheep shears, and thus maintain a constant pressure upon the sides of the hole in which they are inserted. For casting this grinder the rods are tied nearly close together with a piece of string, and inserted in a smooth metal mould of the same diameter as the hole to be ground, which itself is often used as the mould, as this grinder is usually left from the casting, and not afterwards turned; the grinder is finally divided lengthways with a saw.

Fig. 1117.

The Production Of Cylindrical Surfaces By Abrasion 30065

The angular manner in which the rods separate is rather objectionable, but nevertheless with careful management it answers moderately well for holes but little larger in diameter than itself, as the angular difference for small openings is so slight as to be scarcely appreciable. A solid grinder is sometimes used for stopped holes, but whatever form of grinder may be employed, it is difficult, with small deep holes, to grind the work cylindrical close up to the bottom of the hole, and which is also very liable to become enlarged at the open end, consequently the grinder is always required to be shorter than the depth of the hole to be ground, and to be kept towards the bottom, the amount of end traverse being only just sufficient to avoid the formation of rings.

The cylinders of steam engines are usually considered to be left sufficiently smooth from the boring machines, such as fig. 517, page 571, Vol. II.; sometimes, however, they are smoothed by grinding them with a heavy mass of lead, cast upon the middle of a long rod, to the same curve as the inside of the cylinder, which itself in most cases serves as the bottom of the mould. The cylinder is laid on its side, and the grinder supplied with emery and oil is traversed backwards and forwards by hand, the cylinder being occasionally twisted round so as to bring every portion successively beneath the grinder.