The method of a fixed grinder may also be resorted to for grinding the internal cone, when the works admit of being chucked in the lathe, and the opening is of sufficient size for the admission of a rigid grinder; but conical holes are seldom so large as to admit of a revolving lap, and extreme accuracy is less frequently required in the preparation of the internal cone, as the very minute errors incidental to the ordinary process of grinding, will be partially corrected by the final grinding together of the two cones to ensure contact.
Internal cones are generally ground upon solid grinders, mostly formed of tin cast upon an iron rod and turned to the corresponding form. The best works are completed with brass grinders which from being harder retain their forms longer unimpaired and therefore leave the holes more accurate.
For short conical holes in small objects such as rings or detached collars, the grinder is mounted in the lathe, generally between centers, and the work is passed over the rod before the screw of the popit head is adjusted, but sometimes the grinder is made as a chuck to screw at once upon the lathe mandrel; this arrangement allows of the work being more readily removed. In either case the work is applied in just the same manner as for grinding the external cone. The work when small is held in the fingers, and at frequent intervals is allowed to be carried partly round by the grinder, so as continually to change its position, to compensate for any irregularity of direction in holding the work. When the hole is large and the friction is so great that the object cannot be held steadily in the hands, it is fixed in a clamp such as fig. 1112, or in the center of a pulley to serve as a handle.
Long conical holes, such as axletree boxes, are sometimes ground upon the spring grinder fig. 1120, which may be viewed as a combination of figs. 1116 and 1117, but made to screw directly upon the lathe mandrel after the manner of a chuck, and closed by two or three binding screws; the elasticity of the spring suffices for keeping the halves of the grinder distended, and the work grasped in a clamp with a double-ended lever is applied in the same manner as small objects, the workman standing in front of the grinder, the binding screws of which are gradually slackened with the progress of the work, so as to avoid the necessity for employing more than one grinder.
The conical collars of hardened steel generally employed for the bearings of lathe mandrels, as will be adverted to in the succeeding volume, are required to be made not only as accurately as possible to the same angle and diameter as the cone that is to work within the collar itself, but the axes of both bearings should also be strictly in a line with each other. In the mandrel the axes of the two cones are placed straight, almost without the possibility of error, by turning both cones in the lathe from the same centers, but a less direct mode is from necessity resorted to for ensuring the straightness of the axes of the two bearings, which are sometimes both made as detached rings, or collars of steel, fitted into cylindrical holes in the lathe head; at other times, the mandrel works in a collar and center screw. The parallelism of the holes for the reception of the bearings is obtained by boring both holes at the one fixing, with the cutter bar described at page 569, Vol. II. The collars are turned singly in the lathe to the required cone, but a little smaller in diameter than the finished size, they are then fixed upon a mandrel revolving truly in the lathe, and the exterior turned to fit the holes in the lathe head, the steel collars are afterwards hardened and driven in. This method places the axis of the conical collar so nearly in a line with the second bearing, that the trifling correction necessary for position, is brought within the limits of the grinding necessary for fitting the mandrel into the collar, and which is effected with a grinder made nearly as a copy of the mandrel, so far as the two bearings are concerned, the one of which serves as a guide for the position of the grinder while the other bearing is being ground.
The mandrels of small lathes are usually made to work at the back end in a conical center, and at the front, through a conical collar the smaller diameter of which is outwards, and consequently in correcting the collar after it has been fixed in the lathe head, the grinder has to be inserted from the inside, between the two bearings; the form of grinder usually employed for this purpose is represented in fig. 1121. A center screw having a cylindrical fitting in the back upright of the lathe head, is used for keeping the grinder straight, and the square end of the rod upon which the grinder is cast, passes through the conical collar and is received in the square hole chuck of a lathe, by which the grinder is driven; while the end traverse for advancing the collar lengthways upon the grinder is given by the back center screw, which is supported by the popit head of the lathe employed for driving the grinder. The center screw on which the grinder revolves is screwed into a clamp, fixed on the lathe head being ground, so that the advance of this screw through its clamp traverses the lathe head upon the grinder, which revolves in one position, while the lathe head is shifted to and fro, and twisted round at all angles, to maintain a continual change in the relative positions of the grinder and work.
Sometimes instead of driving the grinder with continuous motion by the lathe, a pulley fixed on the middle of the rod of the grinder, is used to work the grinder by hand as usual; at other times a cord is wound around the pulley and led to a spring fixed overhead like a pole lathe, so as to revolve the grinder alternately backwards and forwards.
Large lathe mandrels are usually made to work through two conical collars in order to allow of wheels being fixed on the back end of the mandrel. The two conical collars are mostly ground separately in the first instance, the same as in correcting the cylindrical collars of traversing mandrels. The grinder is then made as in fig. 1122, with a conical grinder of tin or brass fixed upon an iron rod, the opposite end of which is turned cylindrically, and traverses through a conical plug having a central cylindrical hole that is fitted into the back collar, and serves as a guide for traversing the grinder in a straight line, while the front collar is being ground; the back collar is afterwards ground in the same manner, a plug being fitted to the front collar as a guide.
The principal errors having been removed with the single cone grinders, the collars are further corrected in the same manner with a grinder having two brass cones, made exactly as a counterpart of the mandrel, and supplied with flour emery and water. Finally the mandrel itself is ground into the collars, first with very fine emery and oil, and lastly with oil alone for the final polish.
When the works are so large as not to be perfectly under control in the horizontal position, or that the weight of the grinder would be liable to cause the lower side of the collars to be ground in excess, the lathe head is placed vertically, and a cord attached to the grinder is passed over a pulley above and led to a counterpoise, to sustain the principal weight of the grinder. By this arrangement the irregularities of fitting in the cones can be more readily appreciated by the sense of feeling, which is principally depended upon for the condition of the work.