Part 18 (1/2)
=Cylinder Boring.=--Fig. 9 ill.u.s.trates the use of a cutter-head for cylinder boring. After the cylinder casting is set on the platen of the machine, the boring-bar with the cutter-head mounted on it is inserted in the spindle. The bar _B_ has a taper shank and a driving tang similar to a drill shank, which fits a taper hole in the end of the spindle. The cutter-head _C_ is fastened to the bar so that it will be in the position shown when the spindle is s.h.i.+fted to the right, as the feeding movement (with this particular machine) is to be in the opposite direction. The casting _A_ should be set central with the bar by adjusting the work-table vertically and laterally, if necessary, and the outer support _F_ should be moved close to the work, to make the bar as rigid as possible.
The cylinder is now ready to be bored. Ordinarily, one or two roughing cuts and one finis.h.i.+ng cut would be sufficient, unless the rough bore were considerably below the finish diameter. As previously explained, the speed and feed must be governed by the kind of material being bored and the diameter of the cut. The power and rigidity of the boring machine and the quality of the steel used for making the cutters also affect the cutting speed and feed. As the finis.h.i.+ng cut is very light, a tool having a flat cutting edge set parallel to the bar is ordinarily used when boring cast iron. The coa.r.s.e feed enables the cut to be taken in a comparatively short time and the broad-nosed tool gives a smooth finish if properly ground.
[Ill.u.s.tration: Fig. 9. Cylinder mounted on Horizontal Machine for Boring]
The coa.r.s.e finis.h.i.+ng feed is not always practicable, especially if the boring machine is in poor condition, owing to the chattering of the tool, which results in a rough surface. The last or finis.h.i.+ng cut should invariably be a continuous one, for if the machine is stopped before the cut is completed, there will be a ridge in the bore at the point where the tool temporarily left off cutting. This ridge is caused by the cooling and resulting contraction and shortening of the tool during the time that it is stationary. For this reason independent drives are desirable for boring machines.
Facing arms are attached to the bar on either side of the cylinder for facing the f.l.a.n.g.es after the boring operation. The turning tool of a facing arm is fastened to a slide which is fed outward a short distance each revolution, by a star-wheel that is caused to turn as it strikes against a stationary pin. By facing the f.l.a.n.g.es in this way, they are finished square with the bore.
When setting a cylinder which is to be bored it should, when the design will permit, be set true by the outside of the f.l.a.n.g.e, or what is even better, by the outside of the cylinder itself, rather than by the rough bore, in order that the walls of the finished cylinder will have a uniform thickness. The position of very large cylinders, while they are being bored, is an important consideration. Such cylinders should be bored in the position which they will subsequently occupy when a.s.sembled. For example, the cylinder for a large horizontal engine should be bored while in a horizontal position, as the bore is liable to spring to a slight oval shape when the cylinder is placed horizontal after being bored while standing in a vertical position. If, however, the cylinder is bored while in the position in which it will be placed in the a.s.sembled engine, this trouble is practically eliminated.
There is a difference of opinion among machinists as to the proper shape of the cutting point of a boring tool for finis.h.i.+ng cuts, some contending that a wide cutting edge is to be preferred, while others advocate the use of a comparatively narrow edge with a reduced feed. It is claimed, that the narrow tool produces a more perfect bore, as it is not so easily affected by hard spots in the iron, and it is also pointed out that the minute ridges left by the narrow tool are an advantage rather than a disadvantage, as they form pockets for oil and aid in lubricating the cylinder. It is the modern practice, however, to use a broad tool and a coa.r.s.e feed for the light finis.h.i.+ng cut, provided the tool does not chatter.
The type of machine tool used for boring cylinders, and also the method of procedure is determined largely by the size of the work and the quant.i.ty which is to be machined. The turret lathe, as well as horizontal and vertical boring mills, is used for this work, and in automobile factories or other shops where a great many cylinders are bored, special machines and fixtures are often employed.
[Ill.u.s.tration: Fig. 10. Boring a Duplex Cylinder on a Horizontal Machine]
=Boring a Duplex Gasoline Engine Cylinder.=--The method of holding work on a horizontal boring machine depends on its shape. A cylinder or other casting having a flat base can be clamped directly to the platen, but pieces of irregular shape are usually held in special fixtures. Fig. 10 shows how the cylinder casting of a gasoline engine is set up for the boring operation. The casting _W_ is placed in a fixture _F_ which is clamped to the machine table. One end of the casting rests on the adjustable screws _S_ and it is clamped by set-screws located in the top and sides of the fixture. There are two cylinders cast integral and these are bored by a short stiff bar mounted in the end of the spindle and having cutters at the outer end. A long bar of the type which pa.s.ses through the work and is supported by the outboard bearing _B_, could not be used for this work, because the top of each cylinder is closed.
When one cylinder is finished the other is set in line with the spindle by adjusting the work-table laterally. This adjustment is effected by screw _C_, and the required center-to-center distance between the two cylinders can be gaged by the micrometer dial _M_ on the cross-feed screw, although positive stops are often used in preference. After the first cylinder is bored, the dial is set to the zero position by loosening the small knurled screw shown, and turning the dial around.
The feed screw is then rotated until the dial shows that the required lateral adjustment is made, which locates the casting for boring the second cylinder. The end of the casting is also faced true by a milling cutter. Ordinarily, milling cutters are bolted directly to the spindle sleeve _A_ on this particular machine, which gives a rigid support for the cutter and a powerful drive.
[Ill.u.s.tration: Fig. 11. Cylinder turned around for Machining Valve Seats]
The next operation is that of boring and milling the opposite end of the cylinder. This end is turned toward the spindle (as shown in Fig. 11) without unclamping the work or fixture, by simply turning the circular table _T_ half way around. This table is an attachment which is clamped to the main table for holding work that must be turned to different positions for machining the various parts. Its position is easily changed, and as the work remains fixed with relation to the table, the alignment between different holes or surfaces is a.s.sured, if the table is turned the right amount. In this case, the casting needs to be rotated one-half a revolution or 180 degrees, and this is done by means of angular graduations on the base of the table. The ill.u.s.tration shows the casting set for boring the inlet and exhaust valve chambers. The different cutters required for boring are mounted on one bar as shown, and the casting is adjusted crosswise to bring each valve chamber in position, by using the micrometer dial. The single-ended cutter _c_ forms a shallow circular recess or seat in the raised pad which surrounds the opening. The cover joint directly back of the cylinders is finished by milling.
[Ill.u.s.tration: Fig. 12. Boring Differential Gear Casing]
=Examples of Boring, Radial Facing and Milling.=--Another example of boring, in which the circular table is used, is shown in Fig. 12. The work _W_ is a casing for the differential gears of an automobile. It is mounted in a fixture _F_ which is bolted to the table. The casting has round ends, which are clamped in V-blocks, thus aligning the work. This fixture has a guide-bus.h.i.+ng _G_ which is centered with the bar and cutter in order to properly locate the casting. There is a bearing at each end of the casing, and two larger ones in the center. These are bored by flat cutters similar to the style ill.u.s.trated at _A_ in Fig. 3.