Milling method

[00002] This application claims the priority of German application 102 19 012.7, filed Apr. 27, 2002, the disclosure of which is expressly incorporated by reference herein.

[00003] This invention relates to a milling method for the manufacture of components composed of difficult-to-cut materials for turbomachines, aircraft and spacecraft.

[00004] A milling method of this kind is known from U.S. Pat. No. 6,077,002. De facto, this patent provides rough machining by so-called full-cut parallel-stroke milling. In this process, a number of laterally offset grooves (rows) are generally milled, the machining of the respectively outermost grooves including the production of the side-wall contours. For this purpose, at least one milling tool which cuts at the tip and at the circumference is used, i.e. an appropriate end milling cutter. Deeper channels, including the multiple lateral groove offset, are machined in a number of feed steps in the longitudinal direction of the milling cutter. The most important area of application for this method are integrally bladed rotor disks, referred to as “blisks” (Bladed Disks), for gas turbines. In the case of the high-strength, difficult-to-cut materials customary for rotors in the construction of gas turbines, the said method has serious disadvantages, such as a low cutting volume per unit time, high tool wear, high costs and thus, ultimately, low economic viability.

[00005] Faced with this situation, it is the object of the invention to propose a milling method for difficult-to-cut materials which is clearly superior to the abovementioned method according to all important criteria.

[00006] This object is achieved by the claimed features The crucial difference is to be regarded as the fact that, in addition to the centric rotation about its longitudinal centre line, the tool performs an eccentric orbiting motion about a defined orbital axis. The opposite senses of the rotation and the orbiting motion result in downcut milling, which reduces stresses on the tool and the workpiece. Owing to superimposition of a translational advance motion on the eccentric orbiting motion, the path curve of the centre of the tip of the milling cutter corresponds to a cycloid, i.e. to the curve of a point connected firmly to a body rolling on a flat or curved path, it being possible for the radius of the point to be different from the rolling radius. To enable the milling tool to hug non-vertical side walls, i.e. side walls that are not parallel to the orbital axis, in an optimum manner, the axis of the milling tool can be swivelled periodically during each orbit to give a wobbling motion about a defined point on the axis in the region of the tip of the milling cutter, the axis temporarily being parallel to the side wall tangents. With side walls parallel to the orbital axis, this wobbling is not necessary. With the milling strategy according to the invention, it is possible to achieve machining from a solid with finishing parameters, i.e. with a high cutting speed, moderate tool loading, reduced wear and the possibility of a large down feed. The cutting volume per unit time exceeds the known roughing methods many times over, increasing economic viability accordingly.

[00007] Preferred refinements of the milling method according to the main claim are reflected in the dependent claims.

[00013] The component 7, here, by way of example, in the form of an axial-type gas turbine rotor to be provided with integral blades, is viewed approximately in the longitudinal direction of the milling tool 1 in FIG. 1. In the machining condition illustrated, a recess 8 is first of all produced between two opposite side walls 9, 10. In the final condition, a multiplicity of recesses 8 will be distributed uniformly over the periphery of the component, each separated by an identical number of blades. The recesses 8 then form the flow channels, while the side walls 9, 10 form the pressure and suction faces, respectively, of the blades. To facilitate understanding, blade profiles 11, 12 have been drawn in FIG. 1, 11 being intended to be the profile at the blade tip and 12 the profile at the blade root. It is clear from this that the side faces 9, 10 are three-dimensional surfaces with varying radii of curvature. This is intended to indicate that even very demanding contours can be produced with the milling method according to the invention. This is made possible by the special kinematics, i.e. forms of motion of the milling tool 1, relative to the component 7, it being possible also to move the component. The milling tool 1 in the form of an end milling cutter with a cylindrical outer contour rotates clockwise and centrically about its own axis, as indicated by means of an arrow in FIG. 1. In addition, the milling tool 1 performs an eccentric orbiting motion 13, which is indicated by a circle drawn as a solid line. This orbiting motion 13 is performed anticlockwise, resulting in what is referred to as downcut milling by the milling tool 1. The diameter of the orbiting motion 13 is varied in such a way that the whole width of the recess is covered, if appropriate minus a wall allowance on both sides for a subsequent final machining operation. Another form of motion is a translational advance motion 14 on a straight and/or curved path. The advance v per orbiting motion 13 is indicated by two dashed circles offset in direction 14. During the advance motion 14, the orbital axis of the milling tool 1 can also be swivelled three-dimensionally if required. This is necessary when producing radial compressors and turbines, for example.

[00014] Further details can be seen from FIG. 2. The milling tool 1 in the illustration is a so-called spherical cutter with a hemispherical tip contour 5, the centre of which is denoted by M. The down feed t for an initial machining operation is shown, as are two further feed positions. The maximum possible ratio of the down feed to the milling-tool diameter D should be about 2. FIG. 2 furthermore shows the axis R of the milling tool 1, the orbital axis K of the eccentric motion of the tool and the diameter dk of the orbiting motion, which, to be precise, here applies only to the tip 3 of the milling cutter. Since neither of the side walls 9, 10 of the recess 8 is parallel to the axis K of revolution, i.e. perpendicular to the base of the recess 8, an additional wobbling motion is imposed on the milling tool 1. This takes place periodically during each orbit around the orbital axis K. As the milling tool 1 approaches the side wall 9, its axis R is swivelled through an angle α relative to the centre M and, as it approaches the side wall 10, it is swivelled through an angle β. The angles are chosen in such a way that the circumference of the milling tool machines the respective side wall as close as possible to the contour. The angles α and β can be location-dependent, i.e. vary with the advance, and can also be depth-dependent, i.e., vary with the down feed. In the case of simple recesses with vertical side walls, the said wobbling motion is not necessary. With increasing distance from the tip of the milling cutter in the direction of the shank, the wobbling motion leads to a distortion of the orbiting motion about the axis K in the direction of an ellipse or a similar closed curve. If the side walls to be machined have a constant slope relative to the vertical at all points, it would also be conceivable to make the axis R of the milling tool 1 “wobble” at a constant angle on an imaginary conical surface with the orbital axis K as the axis of the cone.

[00015] FIG. 3 shows a milling tool 2 with an alternative tip contour in a highly simplified representation. Instead of a hemisphere—as in FIG. 2—the tip 4 of the milling cutter initially has a flat area 6 around the axis extending at a transition radius rs towards the circumference of the cutter. Fundamentally, the only important point is that there should be no notch-generating angles in the region of the tip of the milling cutter. The exact contour of the tip is to be optimized in each individual case.

[00016] FIG. 4 shows the milling method as it may be used for milling any of pocket-type recesses or interspaces, identified by reference number 8′, adjoining flange surfaces, raised portions, and connection stubs on gas turbine housings, or by milling such pocket-type recesses or interspaces 8′ on structural elements of aircraft and spacecraft, such as ribs, frames, spars, stringers, and paneling.

[00017] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.