This article first appeared in Practical Engineering 1940 Vol1 No5. The contents of the article are accurate as of 1940. This article reviews various processes used by Industry and Engineering at the time.
Background to Boring
When a bore has to be finished to very close limits and perfect, circularity is required, as in the case of cylinder bores, and to only a slightly lesser extent the main and connecting-rod bearings, use is made of a boring bar, which is capable of producing a more accurate and truly circular bore than is possible by drilling.
Let us assume that the rough bores which are left in the cylinder block by the cores when the casting is made are to be trued up. As will be seen from an accompanying sketch, the boring bar consists, in its simplest form, of a circular rod which carries a cutter head provided with two hardened steel cutters. The cutter head is also provided with a micrometer adjustment, so that the cutters can be expanded outward against the cylinder walls. The spindle carrying the cutter head is then rotated at a suitable cutting speed and is simultaneously moved downwards in the bore. Consequently, the cutters produce what amounts to an extremely fine screw thread right down the bore, although the pitch of the thread is so small that to all intents and purposes the cylinder walls appear smooth.
Under production conditions the rough boring of the cylinders is usually carried out in multi-spindle machines, in which the crankcase and cylinder-block casting, after the machining operations have been completed, is clamped beneath the spindles of the boring machine. In this case, however, the spindles are usually extended below the cutter head, so that they pass through the bores, forming pilot bars which engage with bushes below the casting, thus keeping the cutter heads accurately centred. When the casting is clamped in place the head of the machine carrying the boring spindles descends close to the top of the cylinder block, while the pilot spindles enter the bores and pass through the bushes in the base of the machine. The cutters then begin to rotate at a speed of approximately 200 r.p.m., while they are fed down through the bores at a speed of roughly 1/2in. per minute.
When the rough boring is completed, several further machining operations may be required on the casting, such as the. boring of the main and camshaft bearings. Finishing main bearings accurately to size and in perfect alignment has always been something of a problem. It is, of course, possible to reamer them in line, and to ensure perfect bedding of the crankshaft in the bearings by subsequent hand scraping of the bearing metal.
Treating Main Bearings
These methods, however, are too slow and expensive for modern production requirements in most cases. As a result, the main bearing housings are bored by a series of cutters, usually of tungsten-carbide alloy, mounted in a heavy boring bar. The cutters are spaced apart at the appropriate intervals to register with the bearing housings. The crankcase is inverted and the boring bar lowered into the upper halves of the bearing housings. The bearing caps are then bolted up and the boring bar rotated. Sometimes the boring bar itself is moved lengthwise while it rotates in order to provide the "stroke." Alternatively, the crankcase casting may be clamped to the bed of the boring machine, which is provided with an automatic feed movement, so that the bearings move across the cutters on the bar, which itself rotates.
When ordinary white metal bearing shells are used, these usually, take the form of die-castings or pressure die-castings, so that they can be produced to very close limits and with a good finish. Nevertheless, before the crankshaft can be assembled in the bearings, finish boring must be carried out; in this case the procedure is similar to the boring of the housings, but the boring bar is usually fitted with diamond cutters. As the cheeks of the bearings also need facing up, it is common for the bar to carry facing cutters in addition to the internal cutters, so that at each end of its stroke these cutters are brought up against alternate cheeks of the bearings. The use of diamond cutters in conjunction with a very slow rate of feed, produces bores which are sufficiently accurate to allow the bearings to be bolted up without hand scraping.
In the case of the connecting-rod bearings, when these are lined with white metal in the conventional manner, the metal is cast into the bearing housings in a die. Following this the bearing caps are removed in order that the oil passages may be drilled in the rod. The caps are then replaced, and the big end is rough bored, using a boring bar of the type first described. Finally, both the gudgeon pin and the big-end bearing are bored simultaneously to the correct size in a tool which ensures absolute parallelism of the two bores.
Thin Shell Bearings
When the engine is fitted with the thin-shell steel-backed bearings which are now widely used on recent models, the bearing housings in the crankcase and connecting-rods are accurately bored to size, while the bearing shells, which are produced by specialist manufacturers, are formed by coating long strips of steel with a special grade of white metal; the total thickness of the strip is usually .052 in. in the case of connecting-rod bearings, and .072in. for most main bearings. The strips are cut into sections and bent to the correct diameter in presses. They are then accurately bored to size in clamps which deliberately distort them to a certain extent, so that the boring takes place with the bearing shells under the same conditions as when they are eventually fitted, to the engine. As is now well known, these shells can be fitted to the crankcase and connecting-rod housings, and the crankshaft will turn freely, but without shake when the bearing caps are bolted up.
Finishing the Cylinder Bores
To return to the cylinder bores, these will need a finishing process. This is left until the rest of the machining operations have been completed, owing to the risk of the finished bores being damaged accidentally, or the possibility of stresses released during the other machining operations resulting in slight distortion of the bores.
At one time it was common for cylinder blocks to be left out in the open for several months so that they underwent a slow "weathering" process, which effectively relieved any internal strains set up during casting. Although one or two makers still adhere to this process, careful heat treatment, designed to "normalise" the casting before machining, is usually relied upon to remove any stresses from the metal. As an instance of the care taken to avoid distortion, however, several makers carry out the final finishing of the cylinder bores with a dummy cylinder head tightened down, since it has been found that the mere strain of the cylinder holding-down nuts is sufficient in some cases to distort the bores to a slight extent.
The method by which the final mirror finish is put on the bores varies with different manufacturers. The bores may be ground, using a rapidly rotating abrasive wheel mounted on the end of a long spindle. The spindle passes through an eccentric sleeve in a feed bar which rotates slowly around the bore of the cylinder, and at the same time is traversed up and down the cylinder walls. By means of a micrometer adjustment on the eccentric sleeve, the grinding wheel can be moved outward against the wall, so that the required cut is obtained.
By this means an accurate and very high polished surface is obtainable. An alternative method is honing. In this case the cutting medium consists of a series of carborundum strips spaced vertically around the circumference of a carrier. A micrometer adjustment on the carrier enables the diameter at which the slips work to be set. The complete assembly is driven by an electric motor through a universal joint.
The hone is moved up and down in the cylinder by a “stroking" lever, winch in the majority of cases is automatically operated. When trueing or finishing an individual cylinder, however, as after reboring, a manually-operated stroking lever is usually provided. As the work precedes the micrometer setting is altered in order to enlarge the diameter of the hone; a limit stop, however, ensures that the cylinder bore is not enlarged beyond the correct diameter.
As opposed to grinding or honing, fine boring is a highly accurate method of finishing the cylinders which has been widely adopted since its introduction in 1931. In the most common types of fine boring machine several castings are clamped to the work table simultaneously, beneath the spindle of the boring machines. Each electric motor usually drives two spindles, so that on a four-cylinder casting, two bores in each block are dealt with at once. The spindle speed is usually in the neighbourhood of 300 r.p.m., while the boring bars are fed down at a speed of 2in. per minute. The fact that the cutters make over 600 revolutions for every inch of travel down the bore ensures an extremely fine finish. Many authorities, moreover, maintain that the microscopic screw thread effect which still remains, has valuable properties in retaining an oil film on the bores during the running-in period.
When the boring operation is completed on the first two cylinders of each block, the castings are slid lengthwise in order to bring the spindles above the remaining pairs of cylinders. When these have been bored the castings are usually drawn forward over lamp bulbs in order that the finish of the cylinder walls can be examined. In a machine of this type extreme precautions are taken to ensure perfect alignment of the boring bars. Long bearings above the single-point tool are generally used in this case to provide adequate piloting.
Occasionally, after fine boring, the final operation on the cylinder walls consists of rolling. This has been carried out on Morris engines, for instance, for many years. During this Operation the work is clamped beneath a four or six-spindle drilling head. The rolling tools mounted on each of the spindles have rollers which are slightly barrel-shaped. These are pressed against the cylinder walls, and each tool, rotating at a speed of 500 r.p.m., is traversed once up and down the bore at a speed of about 10in. per minute, when the operation is complete.
As a final check on the accuracy of the bores, a micrometer gauge is generally used. In the majority of large factories, however, the hand-operated gauge has given place to the Solex pneumatic micrometer, in which sensitive jets of air provide the measuring device. The principle on which the pneumatic micrometer operates has already been described in Practical Engineering.
The Pneumatic Micrometer
It will be remembered that compressed air is admitted to a central tube immersed in a cylinder filled with water. As the surplus air bubbles from the lower end of the tube and escapes, the air pressure at the upper end of the tube is always constant, being governed by the depth of the water in the cylinder, and is independent of variations in the supply pressure. The air is fed at this constant pressure to a pipe attached to the measuring plunger. This has two diametrically opposed air jets, and the ease with which the air can escape from these depends on the clearance between the plunger and the bore. If the escape of the air is restricted, pressure will be built up in the tube, and will be communicated to a calibrated glass measuring tube. This is connected at its upper end to the constant air supply, and at its lower end to the water-filled cylinder.
Consequently, the height of the column of liquid in the tube is a direct measure of the clearance between the jets and the bore, since any resistance to the escape of air from the measuring jets will build up pressure in the top of the tube and force the column of liquid down.
This micrometer is extremely sensitive and has the effect of magnifying variations in diameter by 9,000 times. A graphic illustration of the extent of this magnification is gained when it is realised that a pin head so enlarged would measure 16yds. across. By rotating the plunger, or raising or lowering it, an oval or taper bore can be detected accurately to within 1-10,000th part of an inch.
The pneumatic micrometer can also be applied to measuring the diameter of other parts after final boring or grinding, a typical instance being the checking of the bores of the special shock absorber cylinders used on the independent suspension system of Vauxhall cars. Owing to the necessity of checking the alignment as well as the diameter of the bores in the case of main and camshaft bearings, however, it is more usual in this case to use accurate cylindrical gauges which must be a close sliding fit in the bearings.