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Development of Hard Turning towards a Micron Accuracy Capability Process for Serial Production


Based on investigations in which the main error drivers on part quality were identified, especially improvements in the tooling sector have become necessary. This comprises the development of appropriate grinding techniques for the manufacture of high precision cutting edges with optimum micro-geometry such as edge hones and chamfers or combinations of both. Test on insert clamping and tool design for high precision tools have revealed the necessity for improvements also in this sector. Investigations were performed in the optimum design of the insert pocket and the performance of tool holder made of high-density material. Beyond this internally cooled tool holders were manufactures for testing. Another focus lay on the development of tool holders for machining of small bores and the test of their performance. Design guidelines have been set up for the development and manufacture of suitable tooling. New prototype holders have been developed, which are made of high-density material and which perform well also in industrial applications. Prototype holders, which have an internal cooling, led to a reduction of the thermal expansion of the tool during cutting. New tools for the machining of small bores have been developed, which allow better qualities and a more reliable hard turning process in that field of application. Also the improvements made regarding the cutting edge preparation are taken into account at these tools.
Feasibility study on tool wear monitoring in hard turning process, including evaluation of several type of commercial available tool wear monitoring system and development of force and temperature monitoring system for hard turning process, including system design, interface and software. The system allows evaluating the state of tool wears in high precision hard turning via the measurement of the process forces. The force measurement and evaluation takes place in process.
Development and manufacture of the Hembrug Slantbed-Mikroturn 50 CNC high precision lathe especially for small work pieces. The development has been focused on the following design criteria: -Accuracy maintained by applying and by improving the basic “Hembrug” design features such as: natural granite machine base, hydrostatic bearings and guide ways, linear measuring system having 0.01µm resolution and slant bed design, isolated from the environment by passive vibration dampers. -Sales price significant lower than the existing and comparable standard Hembrug Mikroturns. This criterion is important since it reduces the investment costs of potential customers and it establishes a solid market position in competition with “high end” standard turning- and grinding machines. -Small floorspace makes machines more competitive on the market. The interactions of new design elements such as T-design, main spindle with integrated motor and new service unit contributed to meet this criterion. -Automatic loading and unloading is implemented for turning of small and medium size of workpieces. An integrated loading/unloading system has been developed consisting of a universal robot and pallet station.
Investigations in the dependence of cutting edge micro-geometry on the surface and subsurface, which is generated by hard turning have been conducted. Tests on insert clamping have shown, that special clamping devices for the insert can decrease tooling errors. Measurements of temperatures in the work piece have given valuable input for FE-based calculations of the work piece expansion. Investigations in the state of residual stresses and lift off tests build the basis for the evaluation of the expected functional behaviour. The functional behaviour of hard turned hydraulic components was very positive. The findings from machining tests, modelling and use of the new system developments were taken into account for the improvement of the hard turning process. This involved also investigations in dry cooling strategies and the machining of air hardened steel. The findings from the tests give important input to the tool manufacturers and to the end-users in respect to a proper cutting edge preparation and tooling as well the use of the newly developed systems. Furthermore they allow a better assessment of the capability of hard turning.
FE-based simulations have been conducted in order to develop models which are able to predict the temperature distribution in the work piece during machining. Simulations have also been done for the tool. The prediction of the temperature distribution in work piece and tool makes it possible to predict the thermal expansion of these components, which is caused by the cutting energy. Based on the measurement in turning tests the models were verified. The models can be used to predict the thermal behaviour of the workpiece and the tool. The models can be used to set-up an error compensation strategy for the hard turning of components. However, the investigations also revealed that the thermal behaviour is sensitive already for slight changes in the state of the process (e.g. by tool wear, depth of cut etc.). The use of the models, which are in a basic research state at present, in manufacturing praxis will make an advanced process monitoring necessary.
In hard turning the clamping is essential since the roundness of the turned part very much corresponds to the clamping method used. The results from the test at the laboratory for metal cutting show very promising results on hard turned surfaces. Residual stresses from the heat treatment process may however affect the out of roundness on the turned ring, which can be reduced when using the new air hardening steel. The clamping principals tested and evaluated for bearing rings at the laboratory have a very positive effect on the work piece geometries but must be further developed to suite a wider dimension range before a complete implementation in sectors within bearing manufacture can take place. The results obtained in the project will be used to improve the current use of the hard turning technology and to define and develop new production lines in a very compact and agile design. A new chucking method for rings was developed in which the chuck is able to automatically adjust to the shape of the work piece in certain tolerances. This chucking method showed very good results when clamping on blank surfaces. This chucking method was tested in a prototype stage and has to be developed further for industrial use. Also in the field of clamping small components comprehensive investigations using mechanical chucking principles took place. By that the knowledge about the optimum design of the interface between the chuck and the work piece could be increased. This makes it possible to improve part quality considerably.
A feasibility study was conducted by the relevant partner. 10 different measuring principles were compared and evaluated in this study. A turret mounted analogue measuring contact probe was considered the best method to reach the overall project objectives. A prototype metrology system for dimensional accuracy has been developed and was manufactured by the relevant partner. The system comprises two PC/s, a probe, measuring and controlling software and the interface to the machine controller. The system has been installed on a high precision lathe for testing. The tests have shown that the system works as intended and reaches sub-micron accuracy. The system can be used for in-cycle measurement of the workpiece and allows automatic feedback of the measured deviations to the controller of the lathe. By that it is possible to use the system for error compensation.