This article first appeared in Practical Engineering 1940 Vol1 No3. The information contained within the article is accurate as of 1940. The article informs the reader of developments in Engineering at the time.
The manufacture of pistons is largely becoming a matter for specialists, as with so many components of the modern car. A manufacturer may purchase pistons in a completely finished form; the larger firms, however, receive the pistons in a partly finished state, and carry out the final machining operations themselves, so that the pistons can be closely matched with the cylinders. For the sake of clearness, however, let us take a typical sequence of operations from the initial casting of the piston to its finished form.
Sequence of Operations
Experimental aluminium pistons and special types are often sand cast. The majority of production pistons, however, are die-cast in metal moulds. The core, as explained in an earlier chapter in this series, is made in several sections so that it can be dismantled and re-moved after the casting has cooled. The “risers," or lengths of metal adhering to the piston, which are the result of the molten aluminium solidifying in the pouring passages, are broken off while the metal is still in a semi-plastic state. The piston is then “fettled" and is ready for machining.
The preliminary turning of the casting is usually carried out in high-speed , automatic lathes, three or more cutters generally being employed simultaneously, in order to turn the outside diameter of the piston skirt and machine the crown. The truing-up of the lower edge of the piston skirt is carried out by inserting the piston in a cup which is gripped in the chuck of the lathe; accuracy at this stage ensures that the piston will be correctly located when boring the gudgeon-pin holes and turning the ring grooves, since in most cases the lower flange of the skirt is used to register the piston during these operations.
The gudgeon-pin holes may be drilled in a vertical drilling machine having two or more spindles, so that several pistons can be dealt with at once. Alternatively, the rough bores which are left by the dummy gudgeon-pin bar which passes through the bosses of the casting in the mould may be reamed out, a vertical drilling machine again being used. The bores are brought to within .020in. and .030in. of the required diameter, finishing operations being carried out later.
Holding the Piston in the Lathe
Turning the piston to nearly its final diameter, roughing and finishing of the piston-ring grooves, and finish-turning the skirt ready for grinding may be performed simultaneously by a number of tools in the lathe. An interesting method of gripping the piston in the lathe is often used. A dummy gudgeon pin is passed through the gudgeon-pin bores, and is gripped by a clamp which, operated by a compressed-air cylinder, pulls the lower flange of the piston firmly against a locating plate on the rotating head-stock of the lathe.
A number of minor machining operations are necessary at this stage, such as chamfering the lower edges of the oil control ring grooves on some types of piston, and the turning of a recess in the outer ends of the gudgeon-pin bores when a circlip is used to position the gudgeon pin. An interesting method of carrying this out is the use of two small circular-saw cutters mounted at the correct distance apart on a vertical spindle. The spindle is fed centrally through the gudgeon-pin bores until the correct position is reached, and is then moved sideways so that the rotating cutters cut a groove to the correct depth. The table carrying the piston is rotated at the same time, so that the groove is cut around the interior of the gudgeon-pin holes.
When oil-return holes have to be drilled in or below the scraper ring grooves a vertical drilling machine may be used, but where a large number of pistons have to be dealt with it is more usual to arrange the necessary number of drilling spindles, each operated by its own electric motor, radially around the piston, which is mounted at the centre; so that the oil holes are drilled simultaneously and automatically.
Finishing to Size
So far, the machining operations have been . fairly straightforward. The method of finishing the piston to size, however, varies considerably with different designs. When a truly circular and parallel skirt is required, the piston can be turned in a lathe with the aid of a diamond-tipped tool, which imparts a very high finish. Alternatively, the final finishing can be carried out on a centreless grinding machine, which combines extreme accuracy with rapid production.
The designer, however, often sets the production engineer a more difficult problem. Consider, for instance, the type of piston which is both oval and tapered, now widely used on modern engines. At first sight it would seem something of a problem to grind the piston quickly and accurately to size, and at the same time to reduce expense by cutting down the number of operations necessary. In practice, however, the oval and taper effect is produced simultaneously at one operation in the grinding machine.
Briefly, the skirt of the piston runs true, being centred by the tailstock of the grinding wheel. The crown of the piston is carried by the driven headstock, which runs slightly eccentric under the action of a cam, the effect being to move the head of the revolving piston towards and away from the face of the grinding wheel. Consequently, through being thrust in and out from the face of the wheel in this manner the crown of the piston is ground oval; the effect, however, decreases as the grinding wheel passes down the length of the piston, so that the skirt remains truly circular,the piston tapering through-out its length.
Using a Diamond Tool
The finish-boring of the gudgeon-pin holes follows. As this must be an extremely accurate operation,, the final cut is usually taken by a diamond tool. The precise height of the gudgeon-pin holes determines the height of the piston, and thus the compression ratio, while the alignment of the two bores affects the assembly of the piston and connecting rod when the engine is finally erected.
One common method of boring the gudgeon-pin holes is to clamp several pistons to a work table between two banks of boring bars. One row of spindles is fitted with hardened steel cutters which leave the gudgeon-pin bore a few thousandths of an inch under size, while the other bank of spindles carry diamond tools. The work table to which the pistons are clamped is first moved towards the hardened steel cutters. When these have passed right through the bores the work table moves across towards the other spindles carrying the diamond-finishing tools.
Sometimes duplex boring heads are fed into position from both sides of the piston simultaneously. Alternatively, the boring can be carried out to within a tolerance of .002in., and the required size obtained by hand reaming to ensure a perfect fit for the gudgeon pin.
Final Surface Treatment
After finish-turning or grinding, a final surface treatment is frequently applied to the pistons. This may take the form of anodising, which consists of the formation of a thin film of aluminium oxide on the surface of the piston by an electrolytic process. This gives the pistons a special fine crystalline surface which tends to retain an oil film. The anodised surface, moreover, is dielectric—that is to say, non-conducting— and consequently electrical effects which result from piston movements, and which have been found to have an adverse effect on the cylinder walls, are no longer present.
As opposed to anodising, some makers favour tin plating, the tin being deposited electrolytically. The layer is extremely thin, and is chiefly designed to prevent "scuffing" and picking up" of local high spots on the piston during the running-in period.
Finally, the piston must be checked for diameter. To-day the measuring is often carried out with the aid of a Solex pneumatic micrometer. In this case the piston is inserted in an accurate cylindrical gauge in the walls of which are the air jets or measuring nozzles of the pneumatic micrometer. The rate at which the air escapes past the piston and the walls of the measuring pot is reflected by the height of the column of liquid in the calibrated glass gauge of the micrometer.
Alternatively, the pistons may be passed through accurately machined circular gauge rings, the gauges varying slightly in size within specified limits so that the pistons can be, graded. An interesting development of this practice is- the measuring device used for the steel-skirted pistons manufactured at the Morris Works for Wolseley and some Morris engines. These pistons are a push fit in the cylinders, and the pressure required to push them through a lapped cylindrical die or measuring pot is determined by placing the pot on the table of a weighing machine.
The piston is pushed through a pot by a rod which is moved vertically downward by a hand-operated rack and pinion gear. While the piston is forced slowly through the pot the scale of the weighing machine must not show variation of more than l-1/2lb. on either side of the specified load.
Machining the Gudgeon Pins
The accuracy with which the piston is machined or the gudgeon-pin holes bored is of little value unless the gudgeon pins themselves are produced to equally close limits. Usually the pins are made from solid bar stock, as opposed to tubing, in order to minimise any likelihood of the presence of slag in the metal. The majority of gudgeon pins for cars use are made from case-hardening mild steel. More highly stressed pins are made from steel to which a small percentage of nickel is added, while chrome vanadium steel is used for small pins on motorcycle or other high-speed engines. Gudgeon pins for aircraft or racing engines are usually made from case-hardening nickel chrome steel.
The bars of steel are first cut into blanks of the correct length, the automatic machine often having as many as four spindles carrying circular cutters, arranged vertically above one another so that four blanks are produced simultaneously. Nearly 1,000 blanks an hour-can be turned out in a machine of this type. The blanks are then carburised, either by using a case-hardening compound or by treating them in a salt bath furnace as explained in an earlier article. After carburising, the bulk of the metal is drilled out of the bore of the blanks, while the ends are finishing in double grinding machines.
The pins are then given the necessary heat treatment, and after leaving the furnace are discharged down a chute into an oil quenching bath. A second heat treatment follows in which the "case" is refined and hardened, oil or water being used according to the steel under treatment. Finally, a third heat treatment tempers the pins.
Finish-grinding is usually carried out in a centreless grinding machine, while the bores are finished by broaching. There is an interesting reason for this. When the bore is broached, any tool marks run the length of the bore instead of around its inside diameter, as is the case if a reamer is used. Consequently, any likelihood of fracture through fatigue is thus minimised. Any external or internal grooving or tapering which is required is carried out on a grinding machine with stones of the required shape. The pins are then lapped in a special machine to give a high finish.
Before the pistons are fitted with rings, which are usually purchased by the car manufacturer in a finished state, and the gudgeon pins are inserted, each piston is weighed and graded according to its weight as well as its size. Meanwhile, the gudgeon pins are checked, tested for hardness, and their fit tested in the pistons. The complete assembly of piston, gudgeon pin and connecting rod is weighed and again checked in a special jig in order to ensure that the connecting rod bearings, connecting rod and piston skirt are in perfect alignment.