Fig. 739.
There are several objections to this form of machine, as will be seen when comparison is made with [Fig. 739], which represents a special cylinder boring lathe, designed and constructed by William Sellers and Co., of Philadelphia, Pennsylvania. The boring bar is here supported in two heads, and is hollow, the feed screw for traversing the head carrying the boring cutters being within the bar. The feed is effected through the medium of the train of gearing shown at the end. The two face plates shown which drive the boring bar, also carry two slide rests which are used to face off the ends of cylinders while the boring bar is in operation, these slide rests being operated by a star feed, acting on the principle described with reference to [Fig. 589]. The boring bar in this case being driven from each side of the work the torsion due to the strain of the cut is divided between the two halves of the bar; or in other words, when a boring bar is driven from one end the strain due to the cut falls upon that part of the bar that lies between the boring-head and the point at which the bar is driven; but when the bar is driven from each end then the strain is divided between the two ends, causing a bar of a given strength to operate more steadily and take a heavier cut for roughing, and a smoother one for finishing. A greater advantage, however, is that it gives to the bar a rigidity, enabling it to carry a cutter having a long cutting edge without chattering, thus allowing a very coarse finishing feed, which will finish a bore with less wear to the tool edge (and therefore more parallel) because for a given amount of work the cutting-edge is under duty for a less period of time, the cutting speed remaining the same, or even slower than would be desirable for a fine feed. The driving-cone, which is shown to be below the boring-bar, is so situated to accomplish two objects, which are to operate the two face plates by a shaft having two pinions (within the bed) gearing with the circumferential teeth on the face plates, and to operate at the same time the table (shown on the bed between the face-plates) to which the cylinder is bolted.
In a boring machine it is of the utmost consequence that the bar shall be as free from vibration as possible, while lost motion, or looseness from wear, is especially to be avoided. By carrying the bar in two bearings, as it were, the wear is greatly reduced.
The duty of facing the cylinder ends is sometimes done by facing cutters carried in the head. Such cutters, however, must have a cutting edge equal to the breadth of the surface faced by them, because the cutter cannot be fed radially to its cut. Furthermore, the cut is carried by the bar at a considerable leverage, and as a result it is very difficult indeed to make the radial faces true or even nearly true, the cutter dipping into the softer parts of the iron or into spongy places if there are any. In any event springing away from its cut, resisting it until forced to cut, and then cutting deeper than should be, so that on a finished surface it is often apparent to the eye where the cutter began and left off. When, however, the radial faces are operated upon by a slide rest, as in the Sellers machine, the tool is more firmly held, and may be fed radially to the cut, producing true faces, and saving a great deal of time in making the cylinder cover joints, as well as in the boring and facing operations.
Fig. 740.
[Fig. 740] represents a double boring and facing lathe by G. A. Gray, Junior, of Cincinnati, Ohio. Two driving heads are provided, each having a main spindle, but holding the boring bar after the manner of an ordinary lathe, and within each spindle is another capable of longitudinal traverse. The main spindle is provided with a head corresponding to a slide rest and carrying a cutting tool for facing purposes, the feed being obtained by means of a star-feed. The work is bolted to the carriage and fed to the cut for boring purposes. It is provided with an automatic feed and also with hand feed. When facing is to be done the carriage may be firmly locked to the lathe shears.