Application of Taper Attachment.—Practical examples of lathe work, which illustrate the use of the taper attachment, are shown in [Figs. 11] and [12]. [Fig. 11] shows how a taper hole is bored in an engine piston-head, preparatory to reaming. The casting must be held either in a chuck C or on a faceplate if too large for the chuck. The side of the casting (after it has been “chucked”) should run true, and also the circumference, unless the cored hole for the rod is considerably out of center, in which case the work should be shifted to divide the error. The side of the casting for a short space around the hole is faced true with a round nose turning tool, after which the rough-cored hole is bored with an ordinary boring tool t, and then it is finished with a reamer to exactly the right size and taper.
Fig. 12. Taper Attachment Set for Turning Taper End of Piston-rod
This particular taper attachment is set to whatever taper is given on the drawing, by loosening nuts N and turning slide S until pointer P is opposite that division on the scale which represents the taper. The attachment is then ready, after bolt H and nuts N are tightened, and clamp C is fastened to the lathe bed. The hole is bored just as though it were straight, and as the carriage advances, the tool is gradually moved inward by the attachment. If the lathe did not have a taper attachment, the taper hole could be bored by using the compound rest.
The hole should be bored slightly less than the finish size to allow for reaming. When a reamer is used in the lathe, the outer end is supported by the tailstock center and should have a deep center-hole. The lathe is run very slowly for reaming and the reamer is fed into the work by feeding out the tailstock spindle. The reamer can be kept from revolving, either by attaching a heavy dog to the end or, if the end is squared, by the use of a wrench long enough to rest against the lathe carriage. A common method is to clamp a dog to the reamer shank, and then place the tool-rest beneath it to prevent rotation. If the shank of a tool is clamped to the toolpost so that the dog rests against it, the reamer will be prevented from slipping off the center as it tends to do; with this arrangement, the carriage is gradually moved along as the tailstock spindle is fed outward. Some reamers are provided with stop-collars which come against the finished side of the casting when the hole has been reamed to size.
After the reaming operation, the casting is removed from the chuck and a taper mandrel is driven into the hole for turning the outside of the piston. This mandrel should run true on its centers, as otherwise the outside surface of the piston will not be true with the bored hole. The driving dog, especially for large work of this kind, should be heavy and stiff, because light flexible clamps or dogs vibrate and frequently cause chattering. For such heavy work it is also preferable to drive at two points on opposite sides of the faceplate, but the driving pins should be carefully adjusted to secure a uniform bearing on both sides.
The foregoing method of machining a piston is one that would ordinarily be followed when using a standard engine lathe, and it would, perhaps, be as economical as any if only one piston were being made; but where such work is done in large quantities, time could be saved by proceeding in a different way. For example, the boring and reaming operation could be performed much faster in a turret lathe, which is a type designed for just such work, but a turret lathe cannot be used for as great a variety of turning operations as a lathe of the regular type. There are also many other classes of work that can be turned more quickly in special types of machines, but as more or less time is required for arranging these special machines and often special tools have to be made, the ordinary lathe is frequently indispensable when only a few parts are needed; in addition, it is better adapted to some turning operations than any other machine.
[Fig. 12] illustrates how a taper attachment would be used for turning the taper fitting for the crosshead end of an engine piston-rod. Even though this taper corresponds to the taper of the hole in the piston, slide S would have to be reset to the corresponding division on the opposite side of the central zero mark, because the taper of the hole decreased in size during the boring operation, whereas the rod is smallest at the beginning of the cut, so that the tool must move outward rather than inward as it advances. The taper part is turned practically the same as a cylindrical part; that is, the power feed is used and, as the carriage moves along the bed, the tool is gradually moved outward by the taper attachment.
If the rod is being fitted directly to the crosshead (as is usually the case), the approximate size of the small end of the taper could be determined by calipering, the calipers being set to the size of the hole at a distance from the shoulder or face side of the crosshead, equal to the length of the taper fitting on the rod. If the crosshead were bored originally to fit a standard plug gage, the taper on the rod could be turned with reference to this gage, but, whatever the method, the taper should be tested before turning too close to the finished size. The test is made by removing the rod from the lathe and driving it tightly into the crosshead. This shows how near the taper is to size, and when the rod is driven out, the bearing marks show whether the taper is exactly right or not. If the rod could be driven in until the shoulder is, say, 1/8 inch from the crosshead face, it would then be near enough to finish to size by filing. When filing, the lathe is run much faster than for turning, and most of the filing should be done where the bearing marks are the heaviest, to distribute the bearing throughout the length of the taper. Care should be taken when driving the rod in or out, to protect the center-holes in the ends by using a “soft” hammer or holding a piece of soft metal against the driving end.