Thomas A. Dow

Application of a fast tool servo for diamond turning of nonrotationally symmetric surfaces

Abstract The fabrication of nonrotationally symmetric surfaces by diamond turning requires tool actuation at a bandwidth significantly higher than the rotational frequency of the surfaces. This requirement cannot be met by standard slide drives due to their large mass and consequent low natural frequency. This articles describes the development of a laboratory-scale diamond-turning machine with piezoelectric-driven fast tool servo. The capability of this apparatus will be demonstrated for high-speed features such as sine wave, square wave, and ramp-shaped surfaces. Also described is the implementation of this fast tool servo on a commercial diamond-turning machine. Several nonrotationally symmetric surfaces have been machined, and their images are included.

Tool force and deflection compensation for small milling tools

A technique to compensate for deflection of small milling tools (diameter < 1 mm) has been demonstrated. This open-loop technique involves predicting the cutting and thrust forces, applying these forces to the tool, calculating the shape error due to tool deflection and creating a new tool path to eliminate this error. The tool force model has evolved from a decade of research to predict the forces in diamond turning. This model was modified to include the effects of tool rotation in milling as well as the changes in contact area and force direction using a ball end mill to create a free form surface. Experimental measurements were made to corroborate the components of the tool forces in the cutting and thrust directions. The force model was then combined with tool stiffness to calculate the deflection of the tool as a function of the depth of cut, the up-feed per revolution and the geometry of the part. Two experiments were used to demonstrate the effectiveness of this error compensation technique-a slot and a large circular groove. Each experiment reduced the error due to tool deflection by an order of magnitude from 20-50 μm to 2-5 μm.

Thermomechanical effects in high current density electrical slip rings

Abstract An experimental apparatus was constructed to study the contact of electrical brushes passing high currents. This apparatus consists of a 100 mm copper slip ring and a brush holder which can be loaded against it. The surface temperature of the brush, the extent of contact and the friction can be measured with this apparatus. Four brush materials were studied: one electrographitic brush and three silver-filled graphite brushes. The results of the experiments showed that above a particular threshold speed the brushes exhibited a sparking phenomenon which consisted of periodic motion of a cluster of small sparks on the brush surface. The peak temperature and the periodicity of the sparking were shown to be a function of the properties of the brush material. A theoretical model of the sparking phenomenon has been developed which shows good agreement with the sparking motion observed using the electrographitic brush.

Tool force model development for diamond turning

Abstract Accurate determination of forces in the three-dimensional turning process is important for the development of a model to describe diamond turning (DT). This paper describes a dynamometer system which measures force magnitude in steady-state cutting. To build a quantitative model, the forces are experimentally separated into components. The response of these components to a variation of cutting parameters is explained in part by a hardness gradient near the part surface. This gradient is due to work hardening by the tool during previous passes. The extent of plastic work, and thus the hardness gradient, is dependent on the tool edge sharpness ( ≈ 100 nm ). Therefore, the turning forces are strongly influenced by the condition of the tool edge. This paper illustrates the feasibility of finding the connection between edge sharpness and tool forces. It also demonstrates the ability to monitor tool forces over extended periods of time. These relationships are important in predicting the diamond tool edge condition from tool forces during a turning operation.

The sparking and wear of high current density electrical brushes

Abstract In the operation of high current density electrical brushes sparking is frequently observed to occur. The cause and effects of this sparking were investigated. Experiments were conducted to measure the instantaneous voltage drop and current flow, the relative motions of the brush and slip ring, the brush temperature and the resulting dimensional changes of the brush. The results of these experiments have shown that, above a threshold velocity, sparking was observed and was often periodic in motion across the brush face. The phenomenon is dependent on sliding speed, current density and brush composition. The instantaneous voltage drop is also related to the sparking. Experiments have indicated that there is a noticeable increase in voltage drop with the onset of sparking. During sparking operation it was also found that the wear rate markedly increased. The increased wear with sparking operation can be related to the material properties of the brush. High wear rates occur for materials having high electrical resistivity and low thermal conductivity. Brushes having such compositions are subject to extensive Joule heating and do not have the ability to remove heat from the contact region. Thus, materials which dissipate large amounts of power in the electrical contact are subject to extremely high temperatures and wear. As a result of improved understanding of the wear process, brush materials may be selected and operating conditions set to minimize the wear. p]A model for wear of electrographitic brushes was developed. This model requires only the material properties and operating conditions to predict brush wear. The results of wear predictions agree well with experimental measurements.

A controller architecture for integrating a fast tool servo into a diamond turning machine

Abstract Diamond turning has become an important fabrication technique for producing reflective optics. However, the generation of a general optical surface requires the capability to fabricate nonrotationally symmetric surfaces. Current commercial diamond turning machines cannot perform this task at production rates because of the limited bandwidth of their axis motions and the limited update speed of their controllers. Although a fast tool servo overcomes the bandwidth limitation, the problem of integrating a fast control process (for the servo axis) and a slower control process (for the slide axes) remains. This article describes the computer hardware and software required to integrate a high-speed, low-amplitude fast tool servo into a conventional T-based diamond turning machine. This system can machine θ-dependent features, synchronized to the radial and axial position of the tool, up to the displacement of range of the servo. A set of interface boards have been designed and built that pass the position feedback data from the laser interferometer to both a high-speed servo controller and a slower slide axes controller. This design allows the fast tool servo to be an independent add-on accessory to the diamond turning machine and successfully incorporates nonrotationally symmetric fabrication capability. As an example of the surfaces possible with this system, an off-axis segment of a parabolic mirror has been machined on-axis. The peak-to-valley figure error of this 125-mm optic is less than 1.1 waves (0.7 μm).

Design tools for freeform optics

Freeform Optical surfaces are defined as any non-rotationally symmetric surface or a symmetric surface that is rotated about any axis that is not its axis of symmetry. These surfaces offer added degrees of freedom that can lead to lower wavefront error and smaller system size as compared to rotationally symmetric surfaces. Unfortunately, freeform optics are viewed by many designers as more difficult and expensive to manufacture than rotationally symmetric optical surfaces. For some freeform surfaces this is true, but a designer has little or no feedback to quantify the degree of difficulty for manufacturing a surface. This paper describes a joint effort by Optical Research Associates (ORA) and the Precision Engineering Center (PEC) at North Carolina State University to integrate metrics related to the cost and difficulty of manufacturing a surface into the merit function that is used during the design of an optical system using Code V. By incorporating such information into the merit function, it is possible to balance optical performance and manufacturability early in the design process.

A new technique for studying the chip formation process in diamond turning

A new technique to examine the chip formation process has been developed. This experimental method involves cutting along the interface of two workpieces that have been joined, stopping the cut, retracting the tool and then separating the two halves of the workpiece. By viewing the cross section of the chip in the scanning electron microscope (SEM), the angle of shear and structure of the chip can be observed. The value of the technique is increased by measuring the tool forces while producing the chip. The shear and normal stresses on the shear plane and the forces attributable to elastic deformation of the workpiece can be calculated from this shear angle and the measured tool forces.

Development of a model for precision contour grinding of brittle materials

Abstract A graphical computer model of the chip geometry resulting from a three-dimensional grinding operation was developed for use in relating the critical depth data obtained from the one-dimensional plunge-grinding technique. This model predicts the resulting surface finish and calculates the theoretical roughness and the final chip geometry for a precision grinding operation. The model is based on euclidean geometry at the intersection of the surfaces of two solid objects. This model was programmed to calculate the remaining surface height as the wheel progresses across the part. The output of the surface profile for successive cuts can be subtracted to illustrate the shape of the chip removed for each revolution of the grinding wheel. Chip geometry as influenced by depth of cut, feed rate, and tool shape was shown to be an important parameter in diamond turning of brittle materials. Similar relationships are developed for the additional geometric complexities of a precision grinding operation. The theoretical surface features are then compared with the actual features generated by grinding brittle materials.

Design and performance of a small—scale diamond turning machine

Abstract Machining with single-point diamond tools is an area of increasing interest in the manufacture of optics, lens moulds, hard discs and other products Although many materials have been machined with impressive results, questions remain regarding the science of diamond turning. These questions concern the suitability of certain materials for turning, the properties of the tool and machine structure which limit the quality of the final product, and the modelling of the cutting process. To study these phenomena, a laboratory-scale diamond turning machine has been designed and built at the Precision Engineering Center. This machine, PAUL, although simple and compact, has produced excellent results on a variety of static and dynamic cutting experiments. The key points of its design, as well as an evaluation of PAULs performance, are given in this paper.