=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 shifted 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 finishing 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 finishing 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 finishing 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 finishing 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 finishing 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 finishing 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-bushing _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.
The cutter for the inner bearings is shown at _c_.
[Ill.u.s.tration: Fig. 13. Facing and Turning f.l.a.n.g.e of Differential Gear Casing]
After the bearings are bored, the circular table is turned 90 degrees and the work is moved closer to the spindle (as shown in Fig. 13) for facing f.l.a.n.g.e _F_ at right angles to the bearings. Circular f.l.a.n.g.es of this kind are faced in a horizontal boring machine by a special facing-arm or head _H_. For this particular job this head is clamped directly to the spindle sleeve, but it can also be clamped to the spindle if necessary. The turning tool is held in a slotted toolpost, and it is fed radially for turning the side or face of the f.l.a.n.g.e, by the well-known star feed at _S_. When this feed is in operation the bent finger _E_ is turned downward so that it strikes one of the star wheel arms for each revolution; this turns the wheel slightly, and the movement is transmitted to the tool-block by a feed-screw. The ill.u.s.tration shows the tool set for turning the outside or periphery of the f.l.a.n.g.e. This is done by setting the tool to the proper radius and then feeding the work horizontally by shifting the work-table along the bed. By referring to Fig. 12 it will be seen that the facing head does not need to be removed for boring, as it is attached to the spindle driving quill and does not interfere with the longitudinal adjustment of the spindle. This facing head is also used frequently for truing the f.l.a.n.g.es of cylinders which are to be bored, and for similar work.
[Ill.u.s.tration: Fig. 14. Example of Work requiring Boring and Milling]
Fig. 14 shows another example of work which requires boring and milling.
This casting is mounted on a fixture which is bolted to the main table.
In this case the circular table is not necessary, because the work can be finished without swiveling it around. After the boring is completed the edge _E_ is trued by the large-face milling cutter _M_ bolted to the spindle sleeve. The irregular outline of the edge is followed by moving the table crosswise and the spindle vertically, as required.
=Fixture for Cylinder Lining or Bushing.=--A method of holding a cylinder lining or bushing while it is being bored is shown in Fig. 15.
The lining _L_ is mounted in two cast-iron ring-shaped fixtures _F_.
These fixtures are circular in shape and have flat bases which are bolted to the table of the machine. On the inside of each fixture, there are four equally s.p.a.ced wedges _W_ which fit into grooves as shown in the end view. These wedges are drawn in against the work by bolts, and they prevent the lining from rotating when a cut is being taken. This form of fixture is especially adapted for holding thin bronze linings, such as are used in pump cylinders, because only a light pressure against the wedges is required, and thin work can be held without distorting it. If a very thin lining is being bored, it is well to loosen the wedges slightly before taking the finishing cut, so that the work can spring back to its normal shape.
[Ill.u.s.tration: Fig. 15. Cylinder Lining mounted in Fixture for Boring]
[Ill.u.s.tration: Fig. 16. Detrick & Harvey Horizontal Boring Machine of the Floor Type Boring Engine Bed Casting]
=Horizontal Boring Machine of Floor Type.=--The type of horizontal boring, drilling and milling machine, shown in Fig. 16, is intended for boring heavy parts such as the cylinders of large engines or pumps, the bearings of heavy machine beds and similar work. This machine can also be used for drilling and milling, although it is intended primarily for boring, and the other operations are usually secondary. This design is ordinarily referred to as the "floor type," because the work-table is low for accommodating large heavy castings. The spindle _S_ which drives the boring-bar, and the spindle feeding mechanism, are carried by a saddle. This saddle is free to move vertically on the face of column _C_ which is mounted on transverse ways extending across the right-hand end of the main bed. This construction permits the spindle to move vertically or laterally (by traversing the column) either for adjusting it to the required position or for milling operations. The spindle also has a longitudinal movement for boring. There is an outer bearing _B_ for supporting the boring-bar, which also has lateral and vertical adjustments, so that it can be aligned with the bar.
The work done on a machine of this type is either clamped directly to the large bed-plate _A_ (which has a number of T-slots for receiving the heads of the clamping bolts) or, in some cases, a special fixture may be used or an auxiliary table. Boring machines of this same general construction are built in many different sizes. The main spindle of the machine ill.u.s.trated is driven by a motor located at the rear of the vertical column _C_, the motion being transmitted to the spindle through shafts and gearing. The casting _D_, shown in this particular ill.u.s.tration, is for a steam engine of the horizontal type, and the operation is that of boring the cylindrical guides or bearings for the crosshead. These bearings have a diameter of 15-3/4 inches and are 37-3/4 inches long. In boring them, two roughing cuts and one finishing cut are taken. The end of the casting, which in the a.s.sembled engine bears against the cylinder, is then faced by means of a regular facing arm.
After removing the boring-bar the table _E_ of the special fixture on which the casting is mounted is turned one quarter of a revolution. A large milling cutter 24 inches in diameter is next mounted on the spindle of the machine, and one side of the main bearing, as well as the pads for the valve-rod guide-bar brackets, are milled. The table is then revolved and the opposite side of the main bearing is milled in the same way, the table being accurately located in the different positions by an index plunger _F_ which engages holes on the under side. The spindle is now moved upward to allow the table to be turned so as to locate the bearing end of the frame next to the headstock of the machine. The milling cutter is then used to machine the inside and top surfaces of the main bearing. By turning the fixture and not changing the position of the casting after it is bolted into place, the various surfaces are machined in the correct relation to one another without difficulty. This is a good example of the work done on horizontal boring machines of the floor type.