METHOD OF FORMING STEEL BALL BLANKS, IN PARTICULAR FOR

ROLLING BEARINGS

TECHNICAL FIELD The present invention relates to a method of forming steel ball blanks, in particular for rolling bearings .

BACKGROUND ART

As known, a steel ball for rolling bearings is made from material shaped as a cylinder, which is obtained by drawing a cylindrical steel bar and cutting such bar in pieces of the required length.

Such cylinder is subjected to pressing, either to cold forming or to warm forming according to the diameter of the ball to be obtained and, consequently, of the dimensions of the starting cylinder.

Such pressing consists in a compression along the axis of the cylinder and allows to plastically deform the cylinder to obtain a blank having a shape approximating that of the final ball. The shape of the blanks obtained using the known processes described above differs from the desired spherical shape in three zones: an annular protuberance is evident along the equatorial line of the blank, while a small circular edged flattening is evident on both poles of the blank.

The material in excess along the equatorial line and the poles must be eliminated to obtain the desired spherical shape, therefore in the known production processes, the pressed blank is subjected to a removal operation of materials, for example flashing followed by a soft grinding operation. A thermal hardening treatment of the resulting blank follows and a series of standard operations complete the machining cycle.

The known solutions described above are poorly satisfactory, above all for big size balls (i.e. for balls having diameter larger than 13 millimetres) , because the amount of material in excess to be removed from the blanks during the flashing and grinding operations for obtaining the required ball shape is relatively high, typically approximately 18-20% by weight. Therefore, this causes a high waste of material which has heavy repercussions both on the cost of the starting material and on the fixtures, spaces, times and costs needed to dispose of the removed material in excess.

Furthermore, as mentioned above, a removal operation of materials is required between pressing and grinding with consequent high machining times and energy costs . DISCLOSURE OF INVENTION

It is the object of the present invention to provide a method for forming steel ball blanks, in particular for rolling bearings, which allows to simply solve, or at least limit, the drawbacks illustrated above .

According to the present invention, a method is provided for forming steel ball blanks, in particular for rolling bearings, as defined in claim 1.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the present invention, it will now be described a preferred embodiment only by way of non-limitative example, and with reference to the accompanying drawing, which schematically illustrates the steps of a method for forming steel ball blanks, particularly for rolling bearings, according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the accompanying figure, it is indicated by 1 a steel cylinder which extends along an axis 2 and has a straight generating line cylindrical side surface 3. The cylinder 1 is plastically deformed to obtain a ball blank 4, which is then subjected to a thermal hardening treatment in a machine G of the conventional type, followed by other conventional manufacturing process operations, not described in detail, to obtain, at the

end, a ball 5.

Cylinder 1, preferably, is defined by a steel bar segment (not shown) which is obtained by drawing in a machine D of conventional type and is then cut in a machine C, also of the conventional type, to obtain segments of the required length.

In particular, the diameter and the axial length of the cylinder 1 are chosen beforehand according to the diameter of the ball 5 to be obtained. The cylinder 1 is subjected to a radial plastic deformation operation, to reduce the transversal section of an intermediate axial portion 6, and a subsequent axial plastic deformation operation. The radial plastic deformation operation is performed so as to obtain an essentially cylindrical blank Ia, whose intermediate portion βa still has a transversal circular section, although reduced. In particular, the radial plastic deformation is defined by a radial compression by rolling or by upsetting in a machine R of the conventional type.

In machine R, the forming tools (not shown) which plastically deform the portion 6 are chosen with a shape so as to obtain a portion 6a having an outwardly concave curved profile generating line 7. The axial ends of the cylinder 1 are not concerned by the radial pressure of

"" O — ^*

the forming tools and therefore remain cylindrical also in the blank Ia, where they are indicated by reference numeral 8 and present reciprocally equal axial height. The radial depth of the plastic deformation of the portion βa, with respect to the initial side surface 3, is axially increasing from the ends 8 towards the centre of the blank Ia.

The axial length of the portion 6a concerned by the plastic deformation, between the ends 8, the curving of the generating line 7 (and therefore of the forming bodies of the machine R) and consequently the radial depth of the plastic deformation with respect to the surface 3 are established beforehand, according to the diameter of the ball 5 to be obtained and so as to optimise the quality of the ball blank 4.

The axial compression for obtaining the ball blank 4 is defined by cold forming the blank Ia in a machine P of the conventional type comprising dies (not shown) which have an appropriate shape so as to provide the correct curvature of the external surface 9 of the ball blank 4.

The surface 9 of the ball blank 4 exiting from the machine P comprises two essentially spherical caps, without squeezing of the poles and reciprocally in a separate way by an "equator" line 10, which extends

continuously on the surface 9 and lays on an ideal plane Q orthogonal to axis 2.

Line 10 essentially does not present any significant radial width nor axial width compared to the radial protrusions which are instead present on the equator in the known solutions made without rolling.

In particular, by advantageously applying the method for the production of so-called "big" balls 5, that is balls having diameter larger than 13 millimetres, the percentage by weight of material in excess of the blank 4 with respect to that of the ball 5 is only approximately 2-3%.

The presence at the equator of a simple line 10 instead of evident protrusions, the disappearance of the squeezing of the poles and the reduction of material in excess with respect to the known solutions are due exactly to the radial plastic deformation before pressing. Indeed, the material of the blank Ia tends to be radially removed from axis 2 along plane Q during rolling, but in a smaller amount with respect to that which would occur by pressing the cylinder 1 without rolling: in particular, the fact of obtaining a simple line 10 is essentially due to the fact that, at the beginning of pressing, the material in portion 6a is in a radially more inner position with respect to that of

the portion 6.

Since the quantity of material in excess with respect to the final ball is relatively small and the shape of the blank 4 is extremely close to the spherical shape of the ball 5, the thermal hardening treatment may be performed directly after cold forming.

Therefore, with respect to the known solutions without rolling, a flashing and a soft grinding operations are eliminated, with consequent saving of process times and energy costs needed to perform such operations.

Furthermore, the limited waste of material with respect to the known solutions obtained by pressing steel cylinders without rolling allows, on one hand, to reduce starting material purchasing costs and, on the other hand, to contain fixtures, spaces and therefore costs for recycling the material in excess removed from the ball blank 4.

Furthermore, even if the diameter of the ball 5 to be obtained is high, warm forming is not indispensable and cold forming is sufficient with consequent considerable saving of energy.

It is finally apparent that changes and variations can be made to the method here described with reference to the accompanying drawing without departing from the

scope of protection of the present invention, as defined in the accompanying claims.

In particular, the reduction of the cross-section of the portion 6 may be obtained differently than shown by way of example.

Furthermore, the power and characteristics of the machines R and P currently available on the market allow to apply the method described above to balls of up to 120 millimetres of diameter, but it is not excluded that, in the future, balls of large diameters may also be machined.