Cutting forces, tool wear and surface finish in high speed diamond machining
Introduction
High speed cutting (HSC), patented by Salomon in 1931 [1] and realized at Lockheed and GE in the 1970s [2], has been adopted by industry in the 1980s and since found widespread application in the aircraft and electronics industries and in tool and die manufacturing. HSC offers a high material removal rate and hence a significant reduction of machining time. It is characterized by a decrease of the specific cutting force due to the thermal softening caused by adiabatic shearing [3], [4] which leads to reduced heat transfer into the workpiece, reduced mechanical stresses, and an improved surface finish [5]. Thus, machining of titanium alloys [6] and hardened steel [7], [8] became accessible. Since HSC usually implies high frequency excitation of structural loops and high thermal loads on the cutting edges, ongoing research and development focuses on tool retention, the structural design of machine tools and the dynamic stability of the cutting process [9], [10], [11].
Also in the 1980s began the commercialization of diamond machining which has become a mature technology for generating asymmetric and structured optical surfaces [12]. However, diamond machining has never been married with HSC, although diamond milling operations usually involve very long machining times which everybody would have liked to cut down. The likely reason is the long-time non-availability of high-frequency spindles suitable for ultra-precision HSC. Fortunately, this situation has changed in recent years. We have adopted a Professional Instruments ISO 2.25C air-bearing spindle with a maximum spindle speed of 60,000 rpm, intentionally designed for ball-end milling, for high speed raster milling by mounting a custom-made fly-cutter onto its face plate. Thus, cutting speeds up to 4000 m/min could be realized. Similar cutting speeds were obtained in off-axis diamond turning using a Professional Instruments 4R spindle with a maximum speed of 10,000 rpm.
In the diamond turning community there is common belief that in the machining of non-ferrous metals the sharpness of the cutting edge is the critical parameter for achieving a low surface roughness, while the cutting speed is regarded to be irrelevant. One would also expect that with high cutting speeds and material removal rates the wear rate of diamond tools would increase similar to the wear rate of carbide tools in conventional HSC. However, these assumptions have never been verified by experiment for cutting speeds larger than approximately 400 m/min where the transition to adiabatic shearing has not yet occurred. We have designed turning and milling experiments for testing the validity of these assumptions at cutting speeds beyond the transition to adiabatic shearing. In a first set of turning experiments (chapter 2.1.) the transition zone was identified by recording the cutting forces as a continuous function of the cutting speed. 2D and 3D FEM simulations of the cutting process (chapter 2.2.) were carried out for estimating the stress, strain and temperature distributions in the cutting zone. In a second set of fly-cut experiments (chapter 3.1.) the wear rates of the cutting edges at cutting speeds below and beyond the transition zone were measured and compared. Chemically induced tool wear, which impedes the diamond machining of steel, was measured as a function of tool engagement time per revolution in a third set of high speed milling experiments (chapter 3.2.). In a fourth set of milling experiments (chapter 3.3.) the step heights at grain boundaries of diamond turned OFHC copper and brass CuZn39Pb3, obtained with low and high cutting speed, were measured and compared for testing the assumption that the cutting speed does not affect the achievable surface roughness.
Section snippets
Measurement of cutting forces
Measurement of the cutting forces as a continuous function of the cutting speed was achieved by face turning experiments with constant material removal rate. The flat workpiece with a diameter of 150 mm and a central hole of 5 mm diameter was mounted on a Professional Instruments 4R air-bearing spindle attached to the x-slide of a Nanotech 500 FG ultraprecision machine, while the tool was mounted on a Kistler 9256C1 force transducer fixed to the machine’s clamped workpiece spindle (Fig. 1).
The 4R
Abrasive wear
Since in HSC the thermal load on the cutting tool is much higher than in the cutting at ordinary speeds, tool life poses a problem and special tool geometries, materials and coatings have been devised for reducing the tool wear [5]. However, if the wear rate is referred to the material removal rate, in HSC, even considering a shorter tool life, a larger total volume may be removed than in conventional machining.
The surface roughness obtained in a diamond turning or milling operation strongly
Surface finish
The surface finish obtained in a diamond machining process depends on a number of factors: the integrity of the cutting edge, machining parameters, vibrations caused by imbalance or chatter, and error movements of slides and spindles. An ultimate limit to the achievable surface roughness, however, is imposed by the grain structure of a metal substrate. Since different grains have different relative orientations and the elastic recovery of a grain after cutting depends on the grain’s
Conclusions
The aim of this work was to assess the applicability of HSC to the diamond machining of metal substrates and to identify peculiarities of high speed diamond machining which might arise from the high thermal conductivity and the extraordinary sharpness of single crystal diamond tools. Our investigation has been focused on three important aspects of HSC:
- (1)
Transition to adiabatic shearing. Both the face turning experiments and the FEM simulations of the cutting process yield a decrease of the
Acknowledgements
Many thanks to all those who helped to carry out the experiments and simulations: Dennis Beck, Arne Beinhauer, Dezheng Chen, Jannik Haats, Dennis Kistowski, Horst Kosenski, Andreas Menten, Robert Mertin, Inken Ohlsen, and Lars Schönemann. This work was funded by the DFG German Science Foundation as part of the Research Unit FOR 1845 “Ultra-precision High-Performance Cutting”.
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