The rods here mentioned are those which are cylindrical along nearly their whole lengths; and the remarks principally refer to the mid-portions of joint-rods, some classes of simply formed connecting-rods, and gland-rods.

Joint-Rods

A joint-rod having a boss at each end, resembling Fig. 957, is centred with regard to the portion which has the least metal to be removed, according to rules given in a previous chapter. It may happen that the straight mid-part remains to be turned after the bosses and their joint-holes are finished. But in general it is more convenient to turn the mid-part at the first commencement of the paring, and to next line the bosses to show their lengths, while the truly turned mid-part is in vee-blocks on a table. By this mode it is convenient to use the centres of the recesses for adjusting the scriber-point, in order to mark the centres of the boss-faces, and, consequently, the centres of the holes also.

If the turning of a rod's mid-part is the first operation, the bosses are rough, and perhaps without any hole; therefore a carrier of any sort can be fixed to either boss without risk of damaging it. But when the bosses are shaped, and the holes also, the carrier which is to rotate the rod must be carefully attached. Such a boss can be gripped with a carrier similar to the one in Fig. 956; and it is attached by means of two flat smooth packing blocks, one on each boss-face and both in the hole of the carrier. Grippers are also used consisting of holdfast plates and bolts. The simplest one of this class consists merely of a plate and bolt such as are seen attached to the rod in Fig. 958, the plate being inclined towards the lathe-chuck, to conveniently engage with the driver. Fig. 959 represents a boss gripped with two plates and two bolts; in this case it is not necessary to put either bolt through the hole in the boss, both bolts being put through the holes which are suitably located in the plates.

Slide-Rods

The slide-valve rods here noticed are small and simply formed rods, which are principally parallel, these instructions being chiefly directed to the production of the cylindrical parts. The remarks apply also to piston-rods of simple form. Further details will be given as we proceed to the consideration of the paring operations for large rods.

The cylindrical parts of rods are easily produced by the aid of the long traverse motion with which every engineer's lathe is supplied. But it is necessary to ascertain whether the lathe to be used is in a proper condition for parallelism previous to commencing a parallel portion, especially if the diameter of the rod to be turned is but very little greater than the finished diameter. In such a piece the slide-rest tool is liable to enter the metal too far, and make it too small in some part, if the lathe is not properly adjusted to parallelism before beginning the turning. This adjustment need not be effected for a rod that has plenty of metal to spare, until the tool has been once along the rod, after which both ends of the turned portion are measured, and the poppet-head is shifted accordingly.

When it occurs that a number of cylindrical parts require turning at one lathe, all of them should be first turned with vee-tools until near the specified diameters, allowing only about a sixtieth of an inch to be cut off each one to complete it. After the entire number have been thus treated, the use of vee-tools should be discontinued, the author's method of finishing consisting in finally turning the several pieces with springy tools, although they may be only about an inch, or less, in diameter.