NC-controlled and in particular CNC-controlled machine tools are well known from the prior art for a wide variety of embodiments. CNC (computerized numerical control) here means that the machine tool numerically controls the tool guide, i.e. by means of a CNC program. The machine tool is equipped with a tool removing material from the workpiece by machining. The control of the tool, in particular a movement and/or an orientation of the tool, is effected using a control device by means of CNC control data of the CNC program. Furthermore, the CNC control data optionally controls an orientation of the workpiece, e.g. by rotating a tool table of the machine tool, on which the workpiece is clamped in the machine tool. All in all, a control by NC programs or CNC control data enables the efficient, flexible, accurate and repeatable machining of a workpiece clamped in the machine tool by means of CNC control data.
In today's prior art, CNC programs and CNC control data are produced or generated in software-supported fashion by means of CAM systems (CAM stands for “computer aided manufacturing”). A generated CNC program here comprises the control data which controls an employed tool relative to a workpiece clamped in the machine tool along a generated tool path to remove material from the workpiece when the tool travels over the path, e.g. by milling or other processes.
The numerical path calculation is here based on geometric parameters and orients itself by the predetermined aspired finished part geometry of the workpiece. Then, the material is removed path for path from the workpiece on the machine tool by means of the generated control data when the tool travels over the generated or calculated tool paths until contour of the finished part has been achieved. In addition, the control data can also comprise data which instructs a tool change, automatic tool changes being optionally carried out while the workpiece is machined. Today's machine tools often enable the conduction of an automatic program-controlled workpiece change in which after the machining of a first workpiece, the first workpiece on a workpiece clamping means of the machine tool is exchanged with a second workpiece to thus machine the second workpiece.
A particular universal and flexible use is possible with CNC-controlled machine tools which comprise at least 5 axes enabling the free movement of the tool in 5 degrees of freedom through the space to remove material from the workpiece. The 5 degree of freedom movements here comprise the 3 spatial degrees of freedom (conventionally three orthogonally controllable spatial degrees of freedom, in particular referred to as the x-axis, y-axis and z-axis) which can be controlled by at least three linear axes, and 2 angular or rotational degrees of freedom which enable any tool orientation. The two angular and rotational degrees of freedom can here be controlled by two or more rotational axes of the machine tool. Today's CNC machine tools having at least 5 axes enable the simultaneous control of the 5 degrees of freedom so as to render possible particularly complex and efficient tool paths relative to a clamped workpiece. In addition, the prior art discloses CNC machine tools having at least 6 axes along which at least 3 axes of translation and at least 3 rotational axes can simultaneously be controlled.
The above described CNC machine tools are universally used in tool construction to produce finished parts having a complex geometry efficiently and precisely by means of machining. This comprises rotationally symmetric finished parts, such as impellers or blisks making great demands on the compliance with a predetermined geometric shape. In mechanical engineering, in particular in shipbuilding, for example, in the environmental technology (e.g. in the case of wind power plants), in aviation and in machine tool manufacture, it may also be necessary to provide transmissions having the most different outputs, for which gear wheels, in particular spur gears and bevel gears, have to be produced according to different demands made on surface finish, tooth contact pattern and rolling characteristics. Here, it is often not absolutely necessary to obtain a large number of items but what matters is rather a high flexibility with respect to the broad range of types, in particular with respect to individual geometries comprising complex flank geometries, tooth flank geometries or blade geometries.
For the production of such finished parts having a base body and at least one flank section protruding from the base body, in particular gear wheels, such as spur gears or bevel gears, blisks or impellers, the prior art discloses special machine tools which are equipped with special tools to produce tooth profiles of gear wheels, such as spur gears or bevel gears, or blade or vane profiles of impellers or blisks in different embodiments.
For the production of a gearing of gear wheels the prior art discloses as special machine tools in particular hob milling machines which are suited to provide a workpiece with a gearing in a generating milling process using hob milling tools. Such hob milling machines are e.g. suited to produce spur gears having a cylindrical base body and tooth flank section protruding therefrom of the gearing or bevel gears having a conical base body and tooth flank sections protruding therefrom of the gearing.
Such special machines, in particular the above described hob milling machines, are cost-intensive as regards purchase and maintenance and the manufacture of individual flank profiles is limited by the shape of the special tools, e.g. the special shape of the cutting edge of the hob milling tools of hob milling machines, which already predefines an achievable or producible tooth and flank geometry. Moreover, the manufacture of individual flank profiles on the above described special machines is limited by the restricted degrees of freedom in a possible relative movement between workpiece and tool.
To achieve a high surface finish it is also optionally necessary to remachine or finish the workpieces after the machining operation on the above described special machines, e.g. on additional, cost-intensive special machines.
In order to solve the problems of the above mentioned special machines, in particular the hob milling machines, for the production of gear wheels, in particular spur gears or bevel gears, blisks or impellers, it is useful to produce such gear wheels, in particular spur gears or bevel gears, blisks or impellers, on a CNC-controlled is machine tool comprising at least 5 axes.
This enables the use of standard tools for the production of these finished parts, the most complex geometries, in particular the most complex flank profiles, being enabled for gear wheels, such as spur gears or bevel gears, blisks or impellers or other workpieces, by the high flexibility and the broad field of application of a machine tool controllable in at least 5 degrees of freedom.
A process for generating control data for controlling a workpiece on a machine tool comprising at least 5 axes and serving for machining a workpiece for the production of a predetermined finished part having a base body and at least one flank section protruding from the base body is described in the article “Auf einfachem Weg zu guten Zähnen—Zahnräder mit hoher Qualität auf Standardmaschinen fräsen” [the easy way to good teeth—mill high-quality gear wheels on standard machines] by Hans-Peter Schossig (published in the journal Werkstatt und Betrieb, Carl Hanser Verlag, Munich, Germany, 2007 edition, No. 4/28, pages 28-32, ISSN 0043-2792).
This above mentioned article describes a process for the production of gear wheels by means of a machine tool comprising 5 axes, in particular in the test run for the production of a bevel gear pairing having a surface finish of quality 6 according to DIN 3965. In the described process, all necessary parameters of the gearing according to DIN standard are initially inputted. This corresponds to fundamental geometry parameters of the finished part geometry of the finished part. For this purpose, it is e.g. also possible to input quantitative data on a desired tooth contact pattern with a predetermined or required tooth shape or further data on a desired convexity in individual areas or over the entire tooth flank.
These fundamental geometry parameters are typed in a computer terminal and then a mathematical description of the desired tooth geometry is generated in the computer by mathematical and/or numerical calculations. By means of a CAD/CAM system, an NC program is generated based on the calculation result according to which the 5-axis machine tool can produce the desired finished part using standard tools, in particular e.g. a known end milling cutter. A similar process is also shown in WO 2008/133517 A1, for example.
In the above described processes, the finished part, in particular the flank surfaces of the flanks, is shaped by a milling operation, e.g. using an end milling cutter or another rotationally symmetric milling tool in a milling operation. This milling operation is first simulated mathematically on a computer and the surface or flank surface of the gearing is approximated (approached) on the computer by means of a CAD system. However, such an approximation using a CAD system leads to deviations between the designed and the model flank surface or tooth flank geometry subject to the output of the approximation module or e.g. also the care exercised by the user of the CAD system.
However, high-quality known CAD/CAM systems (e.g. CATIA, UGS, EUKLID, Tebis, HyperMill) or special CAM systems (e.g. MAX-5, MAX-AB MAX-SI from Concepts NREC) provide functions for generating control programs for milling cutters having cylindrical and convex shapes.
In this connection, it is for the very production of tooth flanks of gear wheels, in particular spur gears or bevel gears, that the flank surfaces are approximated so as to be constructed for the above-mentioned special machine tools to produce gear wheels or bevel gears. Here, the surface of a tooth flank of the gear wheel to be produced or a blade of an impeller is described by so-called isoparameter curves (e.g. U-V curves) which mathematically describe an internal design of the surface or a geometric shape of the surface, The position of the isoparameter curves relative to one another and in relation to the position of the aspired gearing shape strongly depends on an arrangement of predetermined data inputs for describing the finished part geometry, in particular data inputs in the CAD system. According to the prior art, a course of the isoparameter curves along the predetermined side flank surface of an aspired gearing shape is not arranged in uniform rows perpendicularly to the tooth base of the gearing of the gear wheel. Thus, the generated isoparameter curves do not orient themselves by the root of the tooth flanks or a course of the tooth base.
When the tool paths are produced or generated for the machining of the tooth flanks or blade flanks using a CAD/CAM system known from the prior art on the basis of the above described, mathematical description generated by approximation of the side or flank surfaces, the tool or an orientation of the tool is arranged along the isoparameter curves of the surfaces to be hobbed.
Here, the problem arises that when the tool travels over the tool path, an irregular tool orientation or positioning of the tool relative to the surface to be machined can result, which can lead to disadvantageous pivoting and unsteady rotational axis movements of the machine tool and to a resulting unsteady movement of the tool along the path. In addition, this can lead to a disadvantageous inclination of the tool relative to the flank course or optionally relative to a hobbing tool guide or to a variation of the inclination of the tool relative to a moving direction along a tool path along the flank surface to be shaped. Moreover, an irregular positioning of the tool can result in a situation unfavourable as regards machining, which can cause an insufficient surface finish, geometrical errors and also a possibly increased wear of the tool.