BACKGROUND OF THE INVENTION
The invention relates to a method for working or forming a workpiece by flow-forming, particularly for the manufacture of internal teeth on the workpiece, in which a cup-shaped preform or blank is axially fixed against a tool chuck and pressed by means of at least one rolling member onto the tool chuck and the blank is rotated relative to the rolling member.
Numerous different methods are used in the non-cutting manufacture of internal teeth by flow-forming. Thus, DE 196 36 567 A1 (cf. FIG. 4) describes a manufacturing method called climb-stretching. In the latter a blank is radially centered and fixed by an axial pressure against the tool chuck. The feed movement of the spinning rollers takes place from the tailstock side, so that the spinning rollers set to the external diameter of the workpiece reduce the external diameter of the blank and press the material into the profile on the circumference of the tool chuck and thereby roll it out in the axial direction.
DE 196 36 567 A1 (cf. FIG. 5) describes the manufacture of internal teeth by radial rolling in. The blank is radially centred and axially fixed on the base side, so that in the case of a radial infeeding of the spinning rollers the displaced material radially and axially penetrates the profile of the tool chuck.
In another method variant (cf. DE 196 36 567 A1, FIG. 6), the preform or blank is radially centred and axially fixed on both sides, so that on a radial infeeding of the spinning rollers the displaced material penetrates and fills the tool chuck profile.
Finally, in the case of a climb-stretching with axial stop member (cf. DE 196 36 567 A1, FIG. 7), the blank is radially centred and axially fixed on the base side of the blank. In the axial direction the tool chuck carries a stop member against which the material flows during flow-forming and which stops said material as soon as, through the feed movement, the spinning rollers set to an external diameter of the workpiece are moved from the tailstock side against the blank and displace the material. In this method the material can only penetrate in the radial direction into the tool chuck profile and can increase in diameter against the frictional resistance of the stop member face.
In all these methods the radial bulging problem arises, being caused by the mutual spacing of the spinning rollers, which occurs due to the geometrical dimensions of said spinning rollers with their bearings. The consequence is a reversed loading in the tool chuck profiles during working or forming.
It is also common to all these methods that at the start of forming the radial resistance of the material from the ring-shaped transition area between the base and wall of the blank must be overcome. To this must be added the support of the wall of the blank on the toothed crests of the tool chuck. Thus, varying forming forces due to the different resistance during the forming of the blank lead to different workhardnesses within the shaped profiles in the workpiece. As constant forming forces are a prerequisite for constant tolerance ranges on the workpiece profile, a fluctuating tolerance range must be expected. This is a method-caused, serious disadvantage during the manufacture of workpieces requiring precise dimensioning in accordance with the aforementioned methods.
SUMMARY OF THE INVENTION
Thus, the object of the invention is to provide a method in which, for obtaining a high-quality workpiece, the forming resistance of the blank is virtually constant in the vicinity of the inner profile during flow-forming.
In the aforementioned method, this object is achieved according to the invention in that the at least one rolling body at the start of forming at the open end of the blank is pressed against the cylinder wall area thereof and is guided in forming contact in axial relative movement over the cylinder wall area to the base of the blank. As a result of the start of flow-forming at the open side of the blank, throughout the forming path a constant material cross-section is available for shaping. As a result the necessary forming force remains substantially constant, so that there is a constant tolerance range of the formed profile. The axial relative movement can be effected by an axial movement of the blank with respect to an axially stationary rolling body, by an axial movement of the rolling body with respect to an axially stationary blank or by a combined movement of the rolling body and the blank.
According to the invention, a reliable profiling of the inside of the blank is achieved in that axial teeth are shaped on the cylinder wall area with an axial length smaller than the length of the cylinder wall area and that the shaped internal teeth have a clearly defined spacing from the base. In the flow-forming according to the invention from the open side of the blank, material can be moved in the direction of the base. According to the invention, a free space is provided between the desired internal teeth and the base, in which the axially displaced material can be received without any undesired compression. The at least one rolling body thereby overtravels the cylinder wall of the blank not completely to the base, but instead stops at a clearly defined, axial distance in front of the base area. With a through, axial construction of the outer profile on the tool chuck, it is e.g. possible to provide an adequate free space. It is alternatively possible to have a special, ring-shaped groove on the tool chuck adjacent to the base of the fixed blank. This ensures that a necessary, axial length of the internal teeth can be reliably flowformed on the workpiece.
According to the invention, it is preferable to form a transition contour with a wall thickness increase in a corner area between the base and the cylinder wall area. In the corner area between the substantially radially directed base and the substantially axially directed cylinder wall area, the maximum bending load occurs if the formed workpiece is e.g. used as a toothed gear part. As there is no clearly defined shape development of the internal teeth on the inside of the corner area, a reliable, high strength is achieved by a specially constructed transition contour with a wall thickness increase compared with the shaped cylinder wall area. The transition area preferably has a conical contour, which tapers from the base to the cylinder wall area.
If, according to a preferred variant the blank is formed or worked by a plurality of rolling bodies arranged in ring-like manner around the blank and mounted in rotary manner in a cage in each case, the blank can be supported and simultaneously formed by the geometrically maximum number of spinning rollers, rolls or balls on the circumference during the rotation thereof. The rolling bodies to be formed orbit the blank in planet-like manner, when contacting and forming the latter.
In the known spinning rollers, due to the separate mounting and control of the spindles as a result of the axial displacement of the rollers, one roller always commences the shaping process, so that there is necessarily a tool deflection until further axially displaced rollers come into use. As a result of this alternating deflection of the forming tool, a uniform loading and a self-centring are not possible. However, in the present variant of the inventive method, the force is uniformly transferred symmetrically via an outer race of a bearing to all the rollers. All the rollers are simultaneously involved in the forming process. The inner tool can be independently centred and is uniformly loaded.
The blank is appropriately moved in an axial relative movement through the ring-like rolling body arrangement and is pressed by the rolling bodies onto the spinning or tool chuck. Alternatively, the rolling bodies mounted in the cage can be radially infed.
If, in preferred manner, the blank is formed by conical spinning rolls, which roll in an arrangement inclined to the blank rotation axis in a conical outer race, an improved centring can be achieved on introducing the blank into the rolling arrangement. Due to an axial displacement and positioning of the rolling bodies with the cage, a radial positioning and setting of the rolling bodies is possible.
The preform or blank is appropriately fixed between two axially movable drive spindles of a flow-forming machine and on one of the drive spindles is provided the tool chuck and on the other drive spindle a centring device, the blank is initially arranged and kept centred on the drive spindle with the centring device and for fixing the blank to the tool chuck the drive spindles are moved axially against one another. The movement unit formed by the two drive spindles and the blank can then be moved for producing a feed movement, so that rapid fixing and forming occur.
Preferably, the centring device is constituted by a centring pin, on which the blank is mounted in accurately fitting manner with a hub opening. The centring pin and hub opening can form a clearance fit or a slight press fit, which ensures that the blank is initially fixed on one drive spindle. For fixing the blank to the tool chuck, the facing drive spindle is axially introduced into the cup, the base area of the blank being fixed between the two drive spindles. The two mutually moved drive spindles form a unit, which is then preferably moved with the workpiece through an axially fixed forming device with the at least one rolling body.
The blank can in each case be moved with the open side towards the forming tool, i.e. the open side of the blank faces the large diameter of the conical reception opening formed by the rolling bodies.
The spacing between the rollers is reduced to a minimum as a result of the large number of rollers. Thus, the radial bulge of the blank and therefore the reversed loading on the tool chuck profiles are reduced to a minimum and a uniform material hardening is obtained.