This invention relates to machine tools and, more particularly, to computer controlled machine tool contouring systems.
The use of digital computers to direct and control machine tools in the production of non-round contours is well known in the art, particularly for machine tools utilizing a generally cylindrical cutting element and wherein the workpiece support table is positioned in relation to the cutting element in response to computer generated control signals. Non-round contours which may be produced by such contouring systems particularly include the interior surfaces of rotary engine chambers (e.g. the epitrochoid-shaped contours of Wankel engine chambers), and also the inner and outer surface of various cams. The end uses for such contoured workpieces often require precision of contour within 0.001 inches. To obtain contours with this precision, the control portion of the contouring system must accommodate continuing wear of the cutting element. In addition, the relative velocity of the point of contact of the grinding wheel with respect to the work surface must be substantially constant.
Generally, contouring systems for non-round contour generation use a complex cam-following apparatus for guiding the cutting element against the workpiece. In such systems, the cam-following apparatus provides a controlled movement of the workpiece relative to the cutting element along a suitable path to accommodate changes in radius of the grinding tool and the desired feedrate. However, in the grinding of complex non-round contours, it is difficult and correspondingly expensive to produce a mechanism (such as a tapered cam follower) for achieving the desired cutting element wear compensation. Further, in view of the substantial dynamic forces on the tracking and drive cams during high speed grinding operations, such contouring systems are subject to severe grinding speed limitations resulting from the deleterious effects of those forces on the cam surfaces. Further, although the cam-following contouring systems often do provide for a nominally constant surface velocity of the workpiece with reference to the contour of the cutting element by providing a separate surface velocity control cam, the surface velocity may vary substantially from the nominal value. Consequently, localized regions of heating occur on the workpiece during low speed portions of the cutting cycle and result in damage to the workpiece surface. In addition, the localized heating causes thermal expansion of the workpiece which must be compensated for in the generation of the cam tracks in order to provide precision contouring. It will be noted that the latter effect is difficult to effectively offset for complex non-round contours.
A further limitation on cam-following contouring systems is the relative inflexibility of such systems in that only a specific contour (designed into the system by the cam) may be produced without requiring system modification.
It is further known in the art to utilize digital computers for the control of machine tools wherein the computer generates control signals for directing the relative movement of the cutting element with respect to the workpiece so that the central axis of the cutting element is positioned to pass through a predetermined series of spatial points. The computer in such systems may be used to first calculate the coordinates of these predetermined spatial points in a manner accommodating a current radius of the cutting element, a required offset for achieving a desired feedrate, and a desired surface velocity, and then to generate appropriate signals to the associated positioning systems. However, the workpiece positioning systems are generally limited in the speed to which they can respond to the computer generated control signals. The limitation restrains this type of contouring system to surface velocities of less than 20 feet per minute or rotational velocities of less than 10 RPM.