The present invention relates to grinding machines in general, and more particularly to improvements in grinding machines whose grinding wheel or wheels can treat and/or form relatively complex surfaces, for example, surfaces bounding the grooves at the roots of turbine blades or the like. Such surfaces often exhibit several curvatures, i.e., two or more convex and/or concave portions having different radii of curvature.
It is already known to treat the surfaces which surround the grooves at the roots of turbine blades by resorting to grinding machines wherein one or more horizontal spindles for grinding wheels are movable up or down and the workpiece is mounted on a table or an analogous support for movement along a horizontal path and substantially at right angles to the axes of the spindles. It is further known to indirectly mount the work holder on the table in such a way that the workpiece can move with reference to the table and in parallelism with the axes of the spindles. The grinding wheel or wheels in such a machine can readily treat a rather complex surface or two or more surfaces by moving the work holder with as well as relative to the table. For example, such composite movements of the work holder can be carried out to machine grooves in one end portion of a turbine blade or a like workpiece. As a rule, the working surfaces of grinding wheels are their peripheral surfaces, and such surfaces can machine grooves into a workpiece which is moved with the table substantially tangentially of the grinding wheels. If the work holder is simultaneously caused to move in a direction which is parallel to the axes of the spindles, each grinding wheel provides the workpiece with a horizontal groove, and the depth of such grooves can vary in response to simultaneous movement of the work holder in parallelism with the axes of the spindles. Thus, the just outlined mode of regulating the operation of the grinding machine can entail the formation of grooves which are bounded in part by parallel surfaces and in part by convex or concave surfaces (in the deepmost regions or the grooves).
Additional possibilities of forming and/or treating complex surfaces in a grinding machine of the above outlined type are presented by movability of the spindles at right angles to their axes, i.e., toward and away from the base or bed which carries the table. This renders it possible to impart a certain curvature to the surfaces which would be flat or would remain flat in the absence of any upward or downward movement of the grinding wheels while the work holder is moved with and/or relative to the table. However, the just described movements of the spindles at right angles to their axes can only take place while the work holder is held against movement in parallelism with the axes of the spindles, i.e., the grooves which are formed during such movement of the spindles are again bounded, at least in part, by flat surfaces. In other words, one cannot provide a workpiece with a surface which is a portion of a concave or convex spherical surface, i.e., which exhibits different curvatures in different directions.
On the other hand, the formation or treatment of surfaces which are more complex than those which can be treated or formed in presently known grinding machines is often desirable or necessary, especially in connection with the making and/or treatment of turbine blades or the like. Thus, the root portion of each of a certain type of turbine blades must be provided with a first set of grooves at one side and with a mirror symmetrical second set of grooves at the other side of the blade. The surfaces which flank the grooves are rather complex, i.e., a first part of each groove must be bounded by a concave surface portion, another part of the same groove must be bounded by a convex surface portion, the concavity or convexity need not be constant, etc. In accordance with heretofore known practice, such complex surfaces are formed and/or treated by resorting to so-called cup wheels or face wheels. The radii of the working surfaces on such grinding wheels match the radii of curvature of the treated surfaces. This presents serious problems when the radius of curvature of a surface to be treated or formed is relatively large or very large, i.e., it is necessary to employ large-diameter cup wheels or face wheels with attendant pronounced increase in complexity, bulk and initial as well as maintenance cost of the grinding machine. Therefore, it is not possible to increase the diameter of a grinding wheel at will because this would entail a prohibitive increase of the cost of the grinding machine and of the products, at least for a large number of applications.
German Pat. No. 705,817 discloses a method of grinding arcuate surfaces on the teeth of chucks. The method must be practiced by resorting to a specially designed grinding wheel in lieu of a cup wheel, namely, a grinding wheel whose marginal portion is configurated with a view to allow for penetration into the deepmost regions of tooth spaces without undesirable contact between the working surface of the grinding wheel and the remaining portion or portions of surface bounding the tooth space which receives a portion of the grinding wheel. The grinding wheel is pivoted while it extends into a tooth space so that the workpiece rolls along the surface which flanks the tooth space all the way from the one to the other end of the tooth space. Such pivoting of the grinding wheel would present many problems in a grinding machine which is designed for mass treatment of workpieces, i.e., the controls for pivotal movements of one or more grinding wheels and their spindles would contribute excessively to the complexity, cost and sensitivity of the machine. The complexity (with attendant increase of proneness to malfunction) is especially pronounced if the machine for the practice of the method disclosed in the German patent is to machine arcuate (such as convex, concave, multi-convex or multi-concave) surfaces with a high or very high degree of accuracy and reproducibility.
British Pat. Nos. 1,253,238 and 1,349,864 disclose multiple-spindle peripheral grinding machines with inclined work holders. Such work holders are movable on a table along horizontal paths and at right angles to the axes of the spindles. The purpose of the patented machines is to allow for treatment of plane surfaces in the slots of jaws for use in universal chucks.
East German Pat. No. 135,358 discloses a grinding machine for pilgrim rolls of rolling mills. The machine employs a control system which changes the position as well as the orientation of the grinding wheel. This contributes to complexity of the machine, the same as in accordance with the teaching of the aforementioned German Pat. No. 705,817. Therefore, such machine is not suited for the treatment of roots of turbine blades or analogous complex workpieces with a requisite degree of accuracy as well as at a reasonable cost.