Pistons for use in internal combustion engines normally comprise a crown portion, a skirt portion and a mounting portion. A gudgeon pin is mounted on the mounting portion and serves to connect the piston to a connecting rod of the engine. The upper surface of the crown portion is arranged to extend across a cylinder in which the piston is mounted for reciprocating movement, being guided in its movement by the skirt portion. The upper surface of the crown portion borders a combustion chamber of the engine and is, therefore, subjected to high forces and temperatures caused by combustion events in the combustion chamber which drive the piston along the cylinder. A side surface of the crown portion of the piston extends generally parallel to the wall of the cylinder and defines one or more grooves extending circumferentially around the piston.

These grooves contain piston rings which seal the gap between the piston and the wall of the cylinder.

Pistons are usually manufactured by first forming a blank. Such blanks are usually cast from metal which is often an alloy of aluminium. The blank forms at least the crown portion of the piston but often also includes the skirt portion and/or the mounting portion in an integral structure. Where the structure is not integral the skirt and/or mounting portions may be formed by further blanks which are secured, eg by welding, to the first mentioned blank. In some pistons the gudgeon pin (also called a wrist-pin) acts to join the crown portion to the skirt portion. The blank or blanks are machined to their final shape.

The high forces and temperatures to which the piston is subjected in service can cause problems of wear or cracking. The crown portion is especially at risk. In many pistons, the upper surface of the crown portion defines a recess which serves to modify the shape of the combustion chamber to improve combustion. Such recesses are referred to as"combustion recesses". It is found that the bounding surfaces of combustion recesses are prone to cracking. In particular, the rim around the opening of such a recess is particularly at risk. The risk of cracking can be reduced by inserting a high fatigue-strength or wear- resistant material into the blank to form at least part of the crown portion, for example the rim of the combustion recess. An insert of such a material may be cast into the blank by placing it in the casting cavity and casting the blank around it, or may be attached to the blank after casting. Both methods of providing an insert are, however, awkward. Another technique for providing increased fatigue strength or wear-resistance at selected locations is to modify the material of the blank locally to give it different properties. Once possibility is to re-melt the material of the blank locally to alter its micro-structure but this is also awkward and difficult to apply to more than a thin surface layer.

An improved method of locally modifying material of a piston blank is disclosed in Intemational Patent Application number PCT/GB01/02835. This method involves inserting a tool into the blank, bringing about rotation of the tool, so that frictional heat is generated thereby plasticising the material adjacent to the tool. Relative translational movement between the blank and the tool is brought about, while the rotation of the tool is continued, so that the tool moves through a region of the blank, thereby plasticising the material in said region and altering its micro-structure. It is also possible to apply an additional material to said region so that the additional material is mixed into the modified material of the blank. The equipment used in this process is similar to that developed by The Welding Institute of Cambridge England for friction welding metal components together, see for example EP 0653265 A.

It is an object of the present invention to provide an improved apparatus for carrying out a method generally as disclosed in International Patent Application Number PCT/GB01/02835. It is a further object of the present invention to provide improvements to said method.

The invention provides an apparatus for use in manufacturing pistons, the apparatus being operable to modify a portion of the material of a piston blank so that said portion of material has different properties to the remainder of said blank, the apparatus comprising material modifying means which comprises a mounting for a tool, inserting means operable to move said mounting to insert of portion of a tool mounted on said mounting into a piston blank, rotating means operable to rotate said tool so that frictional heat is generated where said tool engages the material of the blank thereby plasticising material of the blank adjacent to the tool, and tool moving means operable to move said tool along a path within said blank so that in a region of said blank the material is plasticised and thereby modified; characterised in that the apparatus also comprises transporting means operable to transport a piston blank to and from a working position at which said material modifying means can operate on said blank, the transporting means comprising clamping means operable to clamp the blank in position relative to the transporting means, the transporting means also comprising cooling means operable to cool said blank as it is transported from said working position.

In an apparatus according to the invention, the cooling of the blank is commenced at the earliest possible moment, thereby increasing productivity.

An apparatus according to the invention may also comprise tool heating means operable to heat a tool mounted on said mounting to a pre-determined temperature and to maintain said tool at said temperature. This enables the tool to enter the blank at a predetermined temperature which can be selected to optimise the rate at which blanks can be operated on by the material modifying means. In particular, the tool can be heated to a temperature in the range 200 to 400°C, preferably 250 to 350°C. This has the advantages that the frictional heat generated does not have to heat the tool and that the tool can be inserted more quickly with reduced risk of breaking the tool. In one alternative, the apparatus also comprises positioning means operable to move said mounting between a working position thereof in which it is positioned to operate on a blank, and a tool heating position at which said tool heating means can operate on a tool mounted on said mounting. However, the tool heating means may alternatively be moveable to and from a tool heating position of the tool at which the tool rests between operations. Said tool heating means may comprise an inductance coil.

The transporting means may comprise a turntable operable to transport a blank to and from a plurality of positions, the positions comprising a loading position at which the blank can be loaded on to the turntable and clamped by said clamping means, and the working position of the blank at which the blank can be operated on by the material modifying means.

The transporting means may comprise a turntable on which the clamping means is mounted. The turntable is operable to transport a blank to and from a plurality of positions, the positions comprising a loading position at which the blank can be loaded on to the turntable and clamped by said clamping means, and a working position of the blank at which the blank can be operated on by the material modifying means. The positions may also comprise a hole-forming position, located between said loading position and said working position, and the apparatus then comprises hole-forming means operable on a blank at the hole-forming position to form a hole in said blank at a location therein at which said tool is inserted at said working position. The hole-forming means may comprise drilling means operable to drill a hole in each blank which has a diameter equal to 10 to 70% of the smallest diameter of the portion of the tool which enters the blank. Preferably, the diameter of the hole is 30 to 60% of said smallest diameter.

In order to increase productivity by reducing the amount of the frictional heat generated which is used to heat the material of the blank, an apparatus according to the invention may also comprise blank heating means operable to heat a blank to a pre-determined temperature before it is operated on by said material modifying means. Preferably, the blank is heated to 100 to 250°C, more preferably to 150 to 200°C.

The apparatus may also comprise means operable to quench blanks after they have been operated on by the material modifying means. This may comprise first quenching means operable to rapidly cool the blank initially and second quenching means operable to complete the cooling. The first quenching means may operate on the blank while it is being transported from said working position.

Said tool moving means may be operable to move said tool along said path in the blank with the rotational axis of the tool inclined relative to a normal to the surface of the blank at an angle of up to 5 degrees backwardly of the direction of movement. Inclining the tool in this way is found to reduce the possibility of the formation of voids in the modified material. Preferably, the inclination is up to 2 degrees.

In an apparatus according to the invention, said tool moving means may be operable to move said tool at a speed of 30 to 1000mm/minute, preferably 300 to 700mm/minute, and the rotating means may be operable to rotate said tool at 100 to 1000rpm, preferably 200 to 500rpm.

The portion of said tool which is inserted into the blank may be generally frusto-conical in shape, having a ratio between its smallest diameter and its largest diameter of between 1.1 and 2.0, preferably between 1.1 and 1. 8.

The invention also provides a method of manufacturing a piston for use in an internal combustion engine, the method comprising transporting a blank to a working position, clamping said blank in position, modifying material in a region of the blank by inserting a tool into a blank, rotating the tool within the blank so that frictional heat is generated plasticising the material of the blank adjacent to the tool, and causing relative translational movement between the blank and the tool while continuing the rotation of the tool so that the tool moves through the region of the blank, characterised in that the method also comprises transporting the blank away from said working position while cooling said blank.

Preferably in a method according to the invention, the method also comprises heating said tool to a pre-determined temperature prior to inserting it into said blank, and maintaining said tool at said pre-determined temperature until it is inserted into said blank. Where the method also comprises forming further blanks and modifying material in a region of each blank, the method may also comprise maintaining said tool at said pre-determined temperature between insertions of the tool into the blanks.

There now follows detailed descriptions, to be read with reference to the accompanying drawings, of apparatuses which are illustrative of the invention and of methods which are illustrative of the invention in its method aspects.

In the drawings: Figure 1 is a diagrammatic perspective view of material modifying means of a first or of a second illustrative apparatus operating on a piston blank ; Figure 2 is a diagrammatic plan view of the first illustrative apparatus; Figure 3 is a similar view to Figure 2 but of the second illustrative apparatus; Figure 4 is a diagrammatic side elevation view, on an increased scale, of a tool for use in either the first or second illustrative apparatus, also showing tool heating means of the apparatus; Figure 5 is a diagrammatic side elevation view of an alternative tool to that shown in Figure 4, also showing a blank in which the tool is inserted.

Figure 6 is a diagrammatic vertical cross-sectional view taken through clamping means of the first or the second illustrative apparatus; and Figure 7 is a diagrammatic horizontal cross-section view taken through the clamping means shown in Figure 6.

The first illustrative apparatus 10 shown in Figure 2 is for use in manufacturing pistons for use in internal combustion engines. The apparatus 10 is operable to modify a portion of the material of a piston blank 12 so that said portion of material has different properties to the remainder of said blank 12.

Specifically, the blank 12 is cast from aluminium-silicon alloy. The blank 12 see Figure 1 incorporates a crown portion 12a, a skirt portion 12b and a mounting portion 12c. The mounting portion 12c comprises two bosses which define aligned bores for receiving opposite ends of a gudgeon pin. The crown portion 12a has a upper surface 12d which is to define a combustion recess. A portion 14 of the material of the blank 12 which is to form the rim of the combustion recess is modified by the operation of the apparatus 10 on the blank 12.

Specifically, the portion 14 is given a finer micro-structure than the remainder of the material of the blank.

The apparatus 10 comprises transporting means 16 (see Figure 2). The transporting means 16 is operable to transport a blank 12 to and from a working position 18 at which material modifying means 20 of the apparatus 10 can operate on said blank to modify the material 14. The transporting means comprises a turntable 22 which is mounted for turning movement about a vertical axis, the reciprocating turning movement being through 180 degrees. The transporting means also comprises two sets of clamping means 24 mounted on the turntable 22 at diametrically opposed positions thereon. Each set of clamping means 24 (see Figures 6 and 7) comprises a support 24a in the form of an upwardly-tapering block of generally rectangular horizontal cross-section.

The support 24a is shaped so that it fits snugly into the interior of a piston, ie within the skirt portion 12b and between the mounting portions 12c. The piston blank 12 is supported by an upper surface 24b of the support 24a which engages a lower surface of the crown portion 12a of the blank 12. Each set of clamping means also comprises two side clamps 24c each of which is generally semi- circular in horizontal cross-section (see Figure 7). Each side clamp 24c is mounted on a slideway 24d for linear movement towards and away from the support 24a, the movement being achieved by operation of a piston and cylinder assembly 24e associated with each side clamp 24c. Each side clamp 24c has a horizontal projection 24f arranged to enter the bore of a mounting portion 12c of a blank 12 and to fit snugly therein. The operation of each clamping means 24 will now be described. When the clamping means 24 is at a loading and unloading position 26 of the turntable 22 and the side clamps 24c are at their furthest distance from the support 24a, a piston blank 12 is moved vertically downward onto the support 24a by a robot 27 so that the support fits snugly within the blank as before described. Then, the side clamps 24c are simultaneously moved towards the support 24a until they engage side surfaces of the blank 12, this bringing the projections 24f into the bores of the mounting portion 12c. The clamping means 24 has now clamped the blank 12 firmly in position relative to the turntable 22, the orientation being determined by the shape of the support 24a and by the projections 24f. The clamping means 24 continues to clamp the blank 12 while it is transported to the working position 18, operated on by the material modifying means 20, and transported back to the loading and unloading position 26. Then, the above-described procedure is reversed to unclamp and remove the blank 12 from the turntable 22.

The transporting means 16 also comprises cooling means 28 operable to cool a blank 12 as it is transported from the working position 18 to the loading and unloading position 26. The cooling means 28 comprises passageways 28a (see Figure 6) which are formed in the side clamps 24c. The passageways 28a are arranged in a upper portion of the side clamps 24c which, when the clamping means 24 is clamping a blank 12, is adjacent to the crown portion 12a of the blank. However, it is possible to provide the passageways throughout the side clamps 24c and similar passageways could be incorporated into the support 24a.

The passageways 28a are arranged so that each extends only in one of the side clamps 24c. The cooling means also comprises pumping means (not shown) operable to circulate cooling water through the passageways 28a. Specifically, the pumping means does not operate until the material modifying means 20 has completed its operation on a blank 12 but then operates until the blank is removed from the tumtable 22. In other words, the cooling means 28 does not operate on blanks 12 as they are loaded, transported to the working position 18, and operated on by the material modifying means 20, but operates to cool the blanks 12 as they are transported from the working position 18 to the loading and unloading position 26.

The material modifying means 20 comprises a mounting 30 for a tool 32.

The tool may have various different shapes but examples are shown in Figures 4 and 5. The tool 32 shown in Figure 4 comprises three co-axial portions, specifically an upper cylindrical portion 32a by which the tool 32 is clamped in the mounting 30, a central cylindrical portion 32b of greater diameter than the portion 32a and having a lower surface which is provided with ribs 34, and a lower frusto-conical portion 32c which projects from the centre of the lower surface of the portions 32b and tapers away from the portion 32b. Figure 5 shows a tool 32 which is essentially the same as the tool shown in Figure 4 but has different dimensions. The material modifying means 20 also comprises rotating means 35 operable to rotate said mounting 30, and therefore a tool mounted on the mounting 30, about a central axis of the tool. The rotating means is in the form of an electric motor and is operable to rotate the tool at between 100 and 1000rpm.

The material modifying means also comprises inserting means 36 operable to move the mounting 30 and the rotating means 35 to insert a portion of a tool, such as the tool 32 mounted on the mounting 30 into a blank 12 which is'clamped by the clamping means 24 at the working position 18. The inserting means 36 comprises a servo mechanism 37 arranged to move the mounting 30 and the rotating means 35 vertically. The inserting means 36 also comprises a second servo mechanism 39 operable to move the servo mechanism 37, and hence the mounting 30, horizontally in a first direction. The inserting means also comprises a third servo mechanism 41 operable to move the second servo mechanism 39 horizontally in a second direction which is perpendicular to said first direction. It will be apparent that, by appropriate operation of the servo mechanisms 37,39 and 41, the mounting 30 and a tool mounted thereon can be moved in three dimensions to position the tool over a specific point in a blank 12 at the working position 18 and to lower the tool into said blank. The servo mechanisms 37,39 and 41 may take many forms but, in this case, are hydraulic cylinders operating under numerical control. The servo mechanisms 37,39 and 41 also provide tool moving means of the material modifying means 20 which is operable to move a tool mounted on the mounting 30 along a path within a blank 12. The path may be of any desired shape in three dimensions but preferably begins in a portion of a blank 12 which will subsequently be machined away in the manufacture of the piston. Preferably, the path also ends in a portion of the blank 12 which will be machined away.

In the operation of the material modifying means 20, the servo mechanisms 37,39 and 41 are operated to insert the tool into a blank 12 while the tool is being rotated by the rotating means 35. Specifically, the tool 12 is inserted into a hole formed in the blank 12 at a pre-determined location. The hole is drilled into the blank and has a diameter equal to 10 to 70% of the smallest diameter of the portion 32c of the tool. As the tool enters the blank, the tool engages the material of the blank and, due to frictional heat generated by the tool rotating in contact with the material, the material plasticises, thereby softening and allowing the tool to fully enter the material of the blank. As shown in Figure 5, the central portion of the tool 32b also enters the material of the blank to a small extent so that the ribs 34 engage the material and also generate frictional heat. While the rotation of the tool continues, the servo mechanisms 39 and 41 then move the tool along said path within the blank, the material ahead of the blank being plasticised so that passage of the tool is possible. The tool is moved at a speed of 30 to 1000mm per minute. The plasticised material cools after passage of the tool and then has a modified micro-structure from the remainder of the material of the blank 12. Specifically, the modified material has a finer grain structure giving it improved properties. When the tool reaches the end of the path, the servo mechanism 37 operate to remove it from the blank.

As shown in broken line in Figure 5, in some cases, the tool 32 is moved along the path with its rotational axis inclined relative to the surface of the blank 12 at an angle of up to 5 degrees. The direction of inclination is backwardly relative to the direction of movement along the path. Thus, viewing Figure 5, the tool moves to the left and the inclination of the rotational axis is to the right.

The servo mechanisms 37,39 and 41 also provide positioning means of the apparatus 10 operable to move the mounting 30, a tool mounted thereon, and the rotating means 35 between the working position 18 and a tool heating position 40 at which heating means 42 can operate on a tool mounted on the mounting 30. Specifically, the heating means comprises of inductance coil into which the lower portion 32c of the tool is inserted (see Figure 4). The tool heating means is operable to heat a tool to a pre-determined temperature between 200 and 400°C.

The apparatus 10 has associated with it hole-forming means 44, blank heating means 46, a first quenching means 48 and a second quenching means 50. These associated apparatuses are indicated diagrammatically in Figure 2.

Specifically, the hole-forming means 44 is provided by a drill, the blank heating means 46 is provided by an oven, the first quenching means 48 is provided by an array of nozzles from which cooling liquid can be sprayed on to a blank, and the second quenching means 50 is provided by a bath in which a blank can be immersed.

In the manufacture of a piston by the first illustrative method in which the apparatus 10 is used, piston blanks 12 which have been cast are past to the hole-forming means 44 where the hole into which the tool 32 is inserted is drilled into the surface 12d of the crown portion 12a. Next, the blank is passed to the blank heating means 46 where it is heated to a pre-determined temperature between 100 and 250°C. Next, the heated blank 12 is loaded on to the turntable 22 at the loading and unloading position 26 and clamped by the clamping means 24. Next, the tumtable 22 is rotated through 180 degrees to bring the blank 12 to the working position 18. The servo mechanisms 37,39 and 41 operate to bring the tool 32 from the tool heating means 42. The blank is then operated on by the heated tool 32 so that a region of the material of the blank 12 is modified by the operation of the material modifying means 20. Specifically, the region extends around the portion of the material which will form the rim of a combustion recess of the finished piston. Once the tool 32 has been withdrawn from the blank 12, the cooling means 28 begins to operate and cools the blank as the turntable is rotated through 180 degrees in the reverse direction to bring the blank back to the loading and unloading position 26, while the cooling continues. The blank 12 is then unloaded from the turntable 22 then passed to the first quenching means 48 where it is sprayed to cool it further and then it is passed to the second quenching means 50 where it is further quenched in a tank.

The second illustrative apparatus 60 is shown diagrammatically in Figure 3. The apparatus 60 is identical in many respects to the apparatus 10 and like reference numbers are used for identical parts without further description. The apparatus 60 differs from the apparatus 10 in the position of the hole-forming means 44 thereof, in the position of the first quenching means 48 thereof, and in the construction and operation of the turntable 22 thereof. Specifically, the tumtable 22 of the apparatus 60 has four sets of clamping means 24 mounted thereon at 90 degree intervals and is arranged to rotate stepwise in one direction to bring blanks 12 clamped by one of the sets of clamping means, successively to four positions. These four positions are the loading and unloading position 26, a hole-forming position 62 located between the positions 26 and 18, the working position 18, and a quenching position 64 located between the positions 18 and 26 (see Figure 3). The hole-forming means 44 is positioned to operate on a blank at the position 62 and the first quenching means 48 is positioned to operate on a blank at the position 64. Thus, the hole into which the tool 32 is inserted is drilled into the blank at the position 62 after the blank has been clamped by the clamping means 24 and the first quenching takes place while the blank is still clamped.

The apparatuses 10 and 60 can both be used in a method in accordance with the invention. Such a method comprises modifying material in a region of the blank 12 by utilising the material modifying means 20, ie by inserting a tool 32 into the blank 12, rotating the tool within said blank so that frictional heat is generated plasticising the material of the blank adjacent to the tool, and causing relative translational movement between the blank and the tool while continuing the rotation of the tool so that the tool moves through the region of the blank.

The method also comprises heating said tool to a pre-determined temperature prior to inserting it into said blank, and maintaining said tool at said pre- determined temperature until it is inserted into said blank.

Preferably, a method according to the invention also comprises forming further blanks and modifying material in a region of each blank, characterised in that the method also comprises maintaining said tool at said pre-determined temperature between insertions of the tool into the blanks.

In modifications of the illustrative methods using the illustrative apparatuses 10 and 60, it is possible to further modify the material of the blank 12 by inserting an additional material into the material platicised by the frictional heat. For example, a quantity of the additional material may be inserted into a recess in the blank or may be cast as an insert into the blank.