This invention is concerned with a method of manufacturing cams, in particular cams for mounting on camshafts used to operate the valves of an internal combustion engine.

Traditionally, camshafts have been cast as one piece, ie the shaft and all the cams mounted thereon are cast in one operation. However, some camshafts are made by making the shaft and the cams separately, and mounting the cams on the shaft. Clearly, such cams can be individually cast but this is inefficient when compared with casting a plurality of cams in one operation.

It is an object of the present invention to provide a method of manufacturing cams which enables a plurality of individual cams to be cast in one casting operation.

The invention provides a method of manufacturing cams, characterised in that the method comprises assembling a plurality of split dies into a stack with frangible members interposed between the dies, each die defining a cavity in the shape of a cam and each frangible member defining a restricted orifice providing communication between adjacent cavities, filling said cavities with molten metal, the metal passing from cavity to cavity through said orifices, allowing said metal to solidify, removing said dies from the solidified metal, breaking said frangible members away from the solidified metal, and separating individual cams from the solidified metal.

In a method in accordance with the invention, cams are cast in one operation and can be handled as a unit prior to separation, eg the cam surfaces can be ground before separation. Separation can be by cutting or chopping through the narrow sprues of metal which solidify in said restricted orifices.

Conveniently, the frangible members, which may be 2mm to 4mm in thickness in a direction longitudinally of the stack and may be made of sand, are received in locating recesses in the dies. The dies may also have co-operating locating recesses and projections arranged to ensure correct assembly of each die and further co-operating recesses and projections arranged to retain each die in the stack.

Since cams are often formed with a hole therethrough into which a shaft is inserted, a core member may be positioned so that it extends through the cavities and the restricted orifices before said cavities are filled with molten metal. Thus, the metal is cast around the core which can then be withdrawn to leave a hole through each cam. The core member may be generally cylindrical. The core member may be made of graphite or be a glass rod.

It is common practice to harden the surfaces of cams by chilling the surfaces as the cams are solidified. Such chilling causes formation of white iron (iron carbides in a pearlite matrix) in surface regions of the cams whereas grey iron (graphite flakes in a pearlite matrix) is formed away from said surface regions. To achieve this, the dies, preferably, provide heat sinks having a thermal capacity sufficient to chill the surfaces of the cams. Alternatively, the dies may be cooled by water flowing through them or be contact with a member through which water flows.

Preferably, the cavities are filled with molten metal with the stack arranged vertically and the cavities are filled from the bottom.

There now follows a detailed description, to be read with reference to the accompanying drawings, of a method of manufacturing cams which is illustrative of the invention.

In the drawings:

Figure 1 is a vertical cross-sectional view of apparatus used in the illustrative method;

Figure 2 is a perspective view of a die half of the apparatus shown in Figure 1;

Figure 3 is a perspective view of an alternative form frangible member to that shown in Figure 1; and

Figure 4 is a vertical cross-sectional view of a portion of an apparatus similar to that shown in Figure l, which apparatus comprises frangible members of the form shown in Figure 3.

The illustrative method of manufacturing cams comprises assembling a plurality of split metal dies 10 into a vertical stack, with frangible members 12 interposed between the dies 10. In Figure 1, only five dies 10 and members 12 are shown but, in practice, a much higher number would be used.

The dies 10 are all of the same construction and each comprises two co-operating die halves 10a (one of which is shown in Figure 2) . Each die half 10a is generally in the shape of half of a disc and is bounded by a semi-circular outer surface 14, a flat surface 16 which extends radially from one end of the outer surface 14, a further flat surface 16 which extends radially from the other end of the outer surface 14, and a die cavity-defining surface 18 which joins inner ends of the flat surfaces 16. The outer

surface 14 need not be semi-circular but may be any desires shape, eg half of a regular hexagon. One of the flat surfaces 16 has a V-shaped projection 20 therefrom, and the other surface 16 has a V-shaped recess 22 therein. When the flat surfaces 16 of the two die halves 10a of a die 10 are brought into engagement, the projection 20 of each die half 10a enters the recess 22 in the other die half. Thus, the projections 20 and the recesses 22 provide co¬ operating locating recesses and projections arranged to ensure correct assembly of each die 10. In this condition, the surfaces 18 of the two halves 10a co-operate in defining a cavity 24 in the shape of a cam. The cavity 24 is open at the top and bottom and is of constant horizontal cross-section.

Each die 10 has an upper surface which has a generally-cylindrical locating recess 26 therein which surrounds the upper opening to the cavity 24. This recess 26 is circular, in plan view, and is defined partially by each die half 10a. This recess 26 is 3mm deep and is arranged to receive a frangible member 12 which is a close fit therein.

The upper surface of each die 10 also has an annular projection 28 around the outer periphery thereof. This projection 28 is defined partially by each die half 10a. The lower surface of each die 10 has an annular recess 30 which is arranged to receive the projection 28 of the next die 10 below said die in the stack. The recesses 30 and the projections 28 provide co-operating recesses and projections arranged to retain each die 10 in the stack.

The frangible members 12 are all of the same construction and are in the shape of a flat circular disc of the same thickness and diameter as the recess 26 so that a member 12 fits snugly in the recess 26. Specifically, the members 12 are cast from foundry sand. Each member 12

has a central circular hole 32 therethrough which is bounded by a V-shaped inwards projection of the member 12 so that the hole 32 has its smallest diameter at a central level in the thickness of the member 12. In the stack, each member 12 defines a restricted orifice (provided by the hole 32) providing communication between adjacent cavities 24. The members 12 are 3mm in thickness in the direction of the stack.

To assemble the stack, two die halves 10a are brought together to form a die 10 with projections 20 in recesses 22, a frangible member 12 is positioned in the recess 26 formed by that die 10, a further die 10 is positioned on top of the first-mentioned die 10 with the projection 28 of the first-mentioned die 10 located in the recess 30 of the further die 10, and a further frangible member 12 is positioned in the recess 26 of the further die 10. This process is continued until the stack contains the desired number of dies 10. The stack can be assembled within a tubular collar (not shown) to prevent the halves 10a from separating.

When the stack of dies 10 is complete, the illustrative method continues by positioning a generally- cylindrical core member 34 so that it extends through all the cavities 24 and the restricted orifices 32. Specifically, a ceramic flow-off bush 36 is positioned on top of the uppermost member 12. This bush 36 is funnel- shaped and has an open top and an open bottom which communicates with the orifice 32. A support 38 from which the core member 34 depends is lowered vertically so that the core member 34 enters the bush 36 and all the cavities 24 and the support 38 rests on the bush 36. The core 34 may be made of graphite.

Next, in the illustrative method, the cavities 24 of the dies 10 in the stack are filled with molten metal. To

do this, the lower opening of the lowermost cavity 24 is brought in to communication with the in-gate 40 (details of which are not shown as they are conventional) . The in-gate 40 is connected to a runner 42 to which molten metal can be supplied through a pouring bush 44 and a vertical down sprue 46 which is longer than the height of the stack of dies 10. Metal poured into the pouring bush 44 enters the lowermost cavity 24 from the in-gate 40 and passes upwardly from cavity 24 to cavity 24 through the holes 32 until the metal enters the flow-off bush 36.

Next, in the illustrative method, the molten metal is allowed to solidify. The relatively large volume of the dies 10 gives them sufficient thermal capacity to chill the surfaces of the cams formed by the surfaces 18. Shrinkage in the metal as it cools is compensated for by metal flowing back into the cavities 24 from the bush 36.

When the metal has solidified, the illustrative method continues by removing the dies 10 from the solidified metal by splitting the die halves 10a apart. Next, the illustrative method comprises breaking the frangible members 12 away from the solidified metal. The core member 34 is also removed by breaking it out of the metal. This leaves a stack of cams joined by short narrow tubular sprues which were defined by the holes 32. This stack of cams can now, if desired, be subjected as a unit to further treatment, eg surface grinding.

The illustrative method is completed by separating the individual cams from the solidified metal by chopping through said tubular sprues.

Figures 3 and 4 illustrate a variation of the illustrative method in which the core member 34 is not used. In this case, the holes 32 in the frangible members

12 are smaller in diameter and solid sprues are created between the cams.