For holes of moderate length requiring considerable accuracy, the grinder of the same diameter as the required hole is in some instances mounted on the end of a revolving spindle, made to slide endways through cylindrical bearings under the control of a lever, and the object to be ground is fixed quite stationary. The bearings through which the spindle of the grinder slides, are required to be carefully adjusted so as to be quite central and parallel with the axis of the hole, or the latter will be ground either oval or oblique, and the spindle should be somewhat smaller than the grinder, to allow of the latter being traversed entirely through the hole to equalize the diameter.

Short cylindrical holes, such as ring gages for the diameters of works, admit of being very accurately ground by mounting the work to revolve rapidly in the lathe, and applying a fixed grinder of smaller diameter than the hole, held in the slide rest, and employed exactly in the counterpart manner to the fixed grinder for external cylinders described on page 1237. Sometimes the grinder is made of soft iron or copper, but a circular lump of lead cast on the end of a bar of iron usually serves as the grinder, it is mounted in the slide rest, which is carefully adjusted as for turning a cylinder, and the grinder supplied with emery is brought in contact with one side of the hole, traversed entirely through it, and gradually advanced sideways to reduce the high points in succession, and enlarge the hole exactly to the required size.

External and internal cylinders are frequently fitted together by grinding them in contact for the final adjustment. When the works are of hardened steel, they are first separately corrected for the distortion of hardening, and brought so near to the same size, that the cylinder will just enter the hole about one-eighth of an inch with stiff friction. A small quantity of the finest flour emery mixed with a little oil is then smeared over both surfaces, and the cylinder is gradually worked in, first with a circular motion only, until it is entered about half an inch, and then with a screwing action backwards and forwards, just as in grinding out the hole with a solid grinder. A pulley or a double-ended lever fixed on the end of the cylinder serves as the handle.

It is necessary to keep the surfaces plentifully supplied with emery and oil, as should they be allowed to become dry, they would be liable to heat from the friction, which then becomes so great as to tear the surfaces of the metal, and in extreme cases will even cause them to hold so firmly together that they can only be separated by blows of a hammer applied on the end of the cylinder, to the evident destruction of the cylindrical surfaces. The grinding with emery is only continued until the cylinder will just slide through the hole with uniform resistance; the surfaces are then thoroughly cleaned, and worked in the same manner for a short time with oil alone, which serves to remove the last traces of the emery and put a final polish on the work.

When one cylinder has to be ground into two holes, as for the bearings of a screw mandrel lathe, the two holes are first corrected with the grinder at the one process, as stated on page 1247, but the mandrel itself is ground into each hole separately, as a very small amount of grinding suffices to fit the two cylindrical surfaces together when properly prepared; and if the mandrel were ground into both holes at the same time, the accuracy of fitting could not be so delicately felt, and in all probability one of the holes would be ground so far larger than be cylinder as to allow of a little side play or shake in the fitting.

Emery may be employed for grinding together works of hardened steel without risk of the grinding powder becoming embedded in the surfaces of the work, as the hardness of the steel will not allow the emery to penetrate sufficiently deep to be permanently retained. With soft iron more care is required to entirely remove the last particles of emery, which if permitted to remain would convert the rubbing surfaces into grinders, and they would mutually abrade each other, to the rapid destruction of the fitting. For this reason emery is rejected for grinding together brass works, and pumice-stone powder is employed, as from its greater softness and friability it is less liable to become embedded in the metal, and may be washed away with oil. But in all cases the grinding together of soft metals should be avoided as much as possible, and when resorted to, the grinding powder should be afterwards thoroughly removed from the rubbing surfaces.