Short cylinders are in many cases ground by hand instead of in the lathe, the work is then fixed horizontally in the vice, and the workman stands in front of the cylinder, and twists the grinder about half way round backwards and forwards, and at the same time traverses it to and fro lengthways of the cylinder, varying the direction of the stroke as much as possible every time, and occasionally twisting the cylinder partly round in the vice, in order to expose it more equally to the action of the grinder, which is fitted upon the cylinder, and applied to reduce the high points in succession, just the same as in the lathe process. This method is less rapid than grinding in the lathe, but is more under control, as the resistance offered by trifling irregularities, is more easily appreciated when the work is at rest, than when it is revolving, and from the constant change of the path of the grinder, the cylinder is less liable to be marked with rings.
When the cylinder terminates at the one end in a collar or projection, it is rather difficult to grind the work square in the corner, partly owing to the angle of the grinder being worn away. In this case tin is generally used instead of lead for the grinder, which is also made narrow in order to allow of as much of the traversing or screwing action as possible, and partly avoid the liability of the grinder to become more enlarged at the ends, than in the middle.
Large cylindrical works, such as rollers, present too much surface friction to be ground between clamps, and for those purposes which only require moderate accuracy, the works are left sufficiently true from the lathe, and the surfaces are polished as explained on page 1072. Works requiring a little more accuracy are sometimes smoothed with a grinder, made by casting a lump of lead upon the cylinder to embrace about one-third of its circumference, and weighing from one to two hundred weight, a bar of iron 3 or 4 feet long is inserted in the loam mould at the time of casting, in order to serve as the handle. The roller is made to revolve in the lathe, and the grinder, mounted upon the roller, is traversed backwards and forwards by the handles, the weight of the grinder supplying the pressure.
Many cylindrical works, such as lathe mandrels, gages for the diameters of holes, flatting rollers for thin gold wire, and other similar objects, that are required to possess considerable accuracy and durability, are made of steel, and afterwards hardened, in which latter process they are liable to become distorted, as explained in Vol. I. Chap. XII. Section IV.
The above class of works, which require the greatest possible exactness of form, are usually ground before hardening, with the clamps, fig. 1112, but which method is not sufficiently accurate for the final correction of the best works, as the grinders have a constant tendency to wear of an oval figure, and also to become rounded in the direction of their length, from the outer edges being more rapidly worn than the middle, this partly arises from the absence of a sufficient guide to ensure the grinder being traversed parallel to the axis of the cylinder, as the shortness of the grinder allows the handles of the clamp to be imperceptibly twisted from the square position, in opposite directions with every stroke.
To avoid this liability as much as possible, in works requiring tolerable exactness, the grinder is made as long as admissible, and the handles very short, in order to reduce the leverage as much as possible, the finishing clamps being sometimes no longer than is required for the binding screws, which are only tightened so far that the grinder just touches the highest points of the cylinder, and allows of its being traversed with very moderate force, so that small inequalities may be detected by the sense of feeling, and in this manner a sufficient approach to correctness for many works is readily attained. But although with careful management the work may be made tolerably circular by this method, it is deficient of any correctional process that can be relied upon for the absolute straightness of the cylinder.
A more accurate method for the best works, is to mount the cylinder upon the lathe, and apply a fixed grinder in the sliding rest, exactly like a blunt turning tool. The grinder is made of lead, copper or iron, supplied with fine emery, and adjusted so as just to touch the highest points of the cylinder as it revolves with moderate velocity. The grinder is traversed from end to end of the cylinder by the sliding rest, and as the highest points are gradually reduced, the grinder is set forward to remove the next series of prominencies and so on. By this method the true circular form may be at once attained, and the parallelism of the cylinder will depend upon the perfection of the slide, and the accuracy with which it is adjusted. In some few instances a diamond point mounted in the sliding rest, and traversed with a very slow motion, has been similarly employed, for correcting hardened steel rollers requiring great accuracy, but the method is tedious, and scarcely better than the fixed grinder supplied with emery.
Another method very nearly as accurate and much more expeditious than the fixed grinder, is the analogous employment of a rapidly revolving lap, mounted on the sliding rest, and gradually traversed along the cylinder, which at the same time slowly revolves upon the lathe. The best lathe mandrels are frequently corrected in this manner after hardening. The iron or copper laps generally employed for this purpose measure from 6 to 9 inches diameter, and about five-eighths of an inch thick; they are driven at a velocity of about 300 to 400 revolutions per minute, and the mandrel makes about 20 revolutions in the same time.
The cylindrical rollers used in paper-making machinery, for pressing the single sheet of paper as it is produced by the machine, require that the two surfaces should fit each other with great accuracy, in order that the rollers may act uniformly upon the paper, and the surfaces at the same time are required to be very smooth, that they may impart a finished surface to the paper.