Fig. 1128.
The common form of the smaller sizes of boring bar is that shown in [Fig. 1127]. a a being the bar, d d the lathe centres, b the cutter passing through a slot or keyway in the bar, and c a key tapered (as is also the back edge of the cutter) to wedge or fasten the cutter to the bar. It is obvious that, if the cutter is turned up in the bar, and is of the exact size of the hole to be bored, it will require to stand true in the bar, and will therefore be able to cut on both ends, in which case the work may be fed up to it twice as fast as though only one edge were performing duty. To facilitate setting the cutter quite true, a flat and slightly taper surface should be filed on the bar at each end of the keyway, and the cutter should have a recess filed in it, as shown in [Fig. 1128], the recess being shown at a, and the edges b b forming the diameter of the cutters. The backing off is shown at c, from which it will be observed that the cutting duty is performed by the edge c, and not along the edge b, further than is shown by the backing off. The recess must be made taper, and to fit closely to the flat places filed on the bar. Such a cutter, if required to be adjustable, must not be provided with the recess a, but must be left plain, so that it may be made to extend out on one side of the bar to cut any requisite size of bore; it is far preferable, however, to employ the recess and have a sufficient number of cutters to suit any size of hole, since, as already stated (there being in that case two cutting edges performing duty), the work may be fed up twice as fast as in the former case, in which only one cutting edge operates.
Fig. 1129.
Messrs. Wm. Sellers and Co. form the cutters for their celebrated car wheel boring bar machine as in [Fig. 1129], the bottom or plain edge performing the cutting. By this means the recess to fit the bar is not reduced in depth from sharpening the tool. The tool is sharpened by grinding the ends of the lower face as shown by the unshaded parts, and the cutter is said to work better after the cutting part has begun to be oblique from grinding.
The cutter is hardened at the ends and left soft in the middle, so that the standard size of the cutter may be restored when necessary, by pening and stretching the soft metal in the middle. These cutters will bore from 50 to 250 car wheels, without appreciable reduction of size.
The description of bar shown in [Fig. 1127] may be provided with several slots or keyways in its length, to facilitate facing off the ends of work which requires it. Since the work is fed to the cutter, it is obvious that the bar must be at least twice the length of the work, because the work is all on one side of the cutter at the commencement, and all on the other side at the conclusion of the boring operation. The excessive length of bar, thus rendered necessary, is the principal objection to this form of boring bar, because of its liability to spring. There should always be a keyway, slot, or cutter way, near to the centre of the length of the bar, so as to enable it to bore a hole as long as possible in proportion to the length of the boring bar, and a keyway or cutter way at each end of the bar, for use in facing off the end faces of the work.