Herbert Schulz

High-speed machining

High-speed machining is an advanced production technology with great future potential. However, as has been in many other realizations of technological progress, the implementation of fundamental knowledge of highspeed machining into the manufacture of industrial products took a relatively long time. In this particular case, the period of approximately 60 years was not only due to a cautious attitude of the industry, but also to the production facilities existing at the time when the first findings became available from research not meeting the requirements of high-speed machining.

High-Speed Milling of Dies and Moulds — Cutting Conditions and Technology

Abstract Due to the high feed rates, high speed milling (hsm) provides a great potential of rationalization for machining free formed surfaces like dies and moulds. The knowledge of the cutting conditions is decisive for tool life and workpiece quality. By machining with ball end mills, unfavourable cutting conditions can be avoided by using a tilt angle. This paper gives a report on a geometrical model to describe the cutting process by using ball end mills with a tilt angle. Based on this theoretical model, wear process and optimized cutting conditions for high speed milling of free formed surfaces can be determined. Practical experiments confirm the model.

Wear Mechanism when Machining Compacted Graphite Iron

Abstract The industrial application of compacted graphite iron in the automotive industry is taking a rather long time due to its uneconomic machinability, because of a significant decrease in tool life. After six years of holistic research of the PTW in cooperation with foundries, manufactures and material scientists, the wear mechanism was understood and clarified: Sulphur in the microstructure of compacted graphite iron has direct influence on the formation of a manganese-sulphur layer on the cutting edge. For machining gray cast iron this layer protects the cutting edge against abrasive wear. Therefore the design of cutting tools for the machining of CGI must consider the absence of manganese-sulphur layer.

Material aspects of chip formation in HSC machining

Abstract Investigations in the past have shown, that chip formation changes by increasing cutting speed, because most of the experimental work had been carried out in the original material stage. In contrary to this new machining investigations were made on an aluminium alloy at different heat treatment states. This investigations confirm that precipitation hardening is the key in changing from continuous to segmented chip formation and not only the cutting parameters. By applying this new knowledge, the relationship between chip formation and cutting forces was examined.

Performance of oxide PVD-coatings in dry cutting operations

Abstract Dry cutting processes effect increased economic efficiency and at the same time fulfil ecological requirements. However, elimination of coolants involves absence of their positive influences on the metal cutting process such as lubrication, heat dissipation and chip evacuation. This leads to high adhesive, abrasive and tribo-chemical tool wear. Innovative oxide coatings meet the sophisticated demands of dry cutting applications such as high wear resistance and the reduction of friction, which minimises the thermal load. The oxide materials provide a high resistance against adhesive wear, due to the high binding energy. Furthermore, the friction coefficient can be decreasing with increasing temperature, and a high hardness even at elevated temperatures is provided. This paper shows the performance of the deposited Al2O3 and ZrO2 coatings in dry drilling of high strength spheroidal graphite cast iron (SGI70). Influences of the coating composition on the tool wear behaviour, the cutting forces and the chip flow are highlighted. Additionally the coatings are evaluated by determination of the absolute temperature on the cutting edge of the drilling tool. Therefore, a suitable measuring technique was developed.

Investigation of the wear mechanism of cubic boron nitride tools used for the machining of compacted graphite iron and grey cast iron

Various experiments were performed to investigate the wear mechanism of cubic boron nitride (cBN) tools used for the machining of compacted graphite iron (CGI). Comparative studies for tools used to machine grey cast iron (CI) were also performed in order to find out why in this case the tool lifetime is significantly higher. Two main effects were found that are responsible for tool wear, namely: (1) oxidation of the tool, and (2) interdiffusion of constituting elements between tool and CGI. These wear mechanisms are more or less the same for the machining of CGI and grey CI. The difference in tool lifetime can be explained by the formation of a MnS layer on the tool surface in the case of grey CI. This layer is missing in the case of CGI. The MnS layer acts as a lubricant and as a diffusion barrier and is the reason for the reduced wear in the case of grey CI.

Turn-Milling of Hardened Steel - an Alternative to Turning

Turn-milling with parallel axes is a relatively new method for precision machining of rotationally symmetrical workpieces. Here the kinematic conditions and their influence on the design of tools and the choice of technological parameters are shown. Using CBN while cutting hardened steel (> 62 HRC) a tool life is achievable which clearly exceeds that of the hard-turning process. Surface qualities beneath Rz< 2 urn can be achieved.

Optimization of precision machining by simulation of the cutting process

Abstract In order to produce high precision workpieces, sources of error are made apparent through preventive simulation of the planned cutting process. Machining optimization prior to the start of production is thereby made possible. The simulation program developed by the Institute for Production Engineering and Machine Tools (PTW) is based on a permanent work-proceeding simultaneous transfer of force caused by the cutting forces onto the present work piece geometry. In order to improve the precision of the workpiece the entire manufacturing system “machine tool - workpiece - tool - clamping device - cutting technology - sequence of operations” must be considered. The simulation of workpiece machining and the optimization processes required to achieve higher precision will be demonstrated. Utilization of this preventive simulation method can gradually reduce workpiece form failures.

Balancing Requirements for Fast Rotating Tools and Spindle Systems

Abstract Technical standards as they are defined in ISO 1940 are more and more insufficient for high-speed tools and spindle systems in order to guarantee a stable machining process. This paper shows the system-specific limitations of the component focused balancing. The effects of the unbalanced tool onto the machining process as well as the machines behaviour can be used as system focused criteria in order to evaluate the required balancing status. These will be discussed by means of analytical and numerical approaches and are verified by experimental results. The new aspect to set the balancing requirements by vibrational loads of the spindle bearings is confirmed by simulation models

Machining of Compacted Graphite Iron (CGI)

The industrial application of compacted graphite iron in the automotive industry is taking a rather long time due to its uneconomic machinability, because of a significant decrease in tool life. After six years of holistic research of the PTW in cooperation with foundries, manufactures and material scientists, the wear mechanism was understood and clarified: Sulphur in the microstructure of cast iron has direct influence on the formation of a manganese-sulphur layer on the cutting edge. For machining gray cast iron this layer protects the cutting edge against abrasive wear. In case of CGI no layer formation occurs. Against the abrasive wear new cutting materials and tools for the machining of CGI must be developed.