Kate Medicus

Improving High-Speed Machining Material Removal Rates by Rapid Dynamic Analysis

Abstract Stability prediction and chatter avoidance in high-speed machining requires knowledge of the tool point dynamics. In this paper, three advances toward the rapid identification of the tool point frequency response and corresponding stable cutting parameters are described: 1) stable speeds determination using non-contact periodic impulsive excitation of the tool point (produced by spindle rotation and a stationary magnet) in conjunction with once-per-revolution sampling, 2) Receptance Coupling Substructure Analysis for the analytic prediction of the tool point response, and 3) once-per-revolution sampling of the audio signal during cutting to determine stability behavior.

Exploring once-per-revolution audio signal variance as a chatter indicator

The purpose of this study is an evaluation of the statistical variance in the once-per-revolution sampled audio signal during milling as a chatter indicator. It is shown that, due to the synchronous and asynchronous nature of stable and unstable cuts, respectively, once-per-revolution sampling leads to a tight distribution of values for stable cuts, with a corresponding low variance, and a wider sample distribution for unstable cuts, with an associated high variance. A comparison of stability maps developed using: 1) analytic techniques, and 2) the variance from once-per-revolution sampled time-domain simulations is provided and good agreement is shown. Experimental agreement between the well-known Fast Fourier Transform (FFT) chatter detection method, that analyzes the content of the FFT spectrum for chatter frequencies, and the new variance-based technique is also demonstrated.

Micro-optic reflection and transmission interferometer for complete microlens characterization

A combined Twyman–Green and Mach–Zehnder interferometer especially designed for the characterization of refractive microlenses is presented. This instrument allows for the quantitative characterization of the microlens form, the transmitted wavefront errors, the radius of curvature and the front focal length without removing the sample under test. All of these microlens properties are important when benchmarking different microlens fabrication technologies (Ottevaere et al 2006 J. Opt. A: Pure Appl. Opt. 8 S407–29). The interferometer was calibrated by the random ball test method. This paper describes the optical design and demonstrates the performance with the characterization of the instrument bias and measurements of a typical microlens. The performance is also compared with that of a semi-commercial instrument.

Improving optical bench radius measurements using stage error motion data

We describe the application of a vector-based radius approach to optical bench radius measurements in the presence of imperfect stage motions. In this approach, the radius is defined using a vector equation and homogeneous transformation matrix formulism. This is in contrast to the typical technique, where the displacement between the confocal and cats eye null positions alone is used to determine the test optic radius. An important aspect of the vector-based radius definition is the intrinsic correction for measurement biases, such as straightness errors in the stage motion and cosine misalignment between the stage and displacement gauge axis, which lead to an artificially small radius value if the traditional approach is employed. Measurement techniques and results are provided for the stage error motions, which are then combined with the setup geometry through the analysis to determine the radius of curvature for a spherical artifact. Comparisons are shown between the new vector-based radius calculation, traditional radius computation, and a low uncertainty mechanical measurement. Additionally, the measurement uncertainty for the vector-based approach is determined using Monte Carlo simulation and compared to experimental results.

Fabrication of freeform optics

Freeform surfaces on optical components have become an important design tool for optical designers. Non-rotationally symmetric optical surfaces have made solving complex optical problems easier. The manufacturing and testing of these surfaces has been the technical hurdle in freeform optic’s wide-spread use. Computer Numerically Controlled (CNC) optics manufacturing technology has made the fabrication of optical components more deterministic and streamlined for traditional optics and aspheres. Optimax has developed a robust freeform optical fabrication CNC process that includes generation, high speed VIBE polishing, sub-aperture figure correction, surface smoothing and testing of freeform surfaces. Metrology of freeform surface is currently achieved with coordinate measurement machines (CMM) for lower resolution and interferometry with computer generated holograms (CGH) for high resolution irregularity measurements.

Fabrication and metrology of high-precision freeform surfaces

Freeform applications are growing and include helmet-mounted displays, conformal optics (e.g. windows integrated into airplane wings), and those requiring the extreme precision of EUV. These non-rotationally symmetric surfaces pose challenges to optical fabrication, mostly in the areas of polishing and metrology. The varying curvature of freeform surfaces drives the need for smaller, more “conformal”, tools for polishing and reference beams for interferometry. In this paper, we present fabrication results of a high-precision freeform surface. We will discuss the total manufacturing process, including generation, pre-polishing, MRF®, and metrology, highlighting the capabilities available in today’s optical fabrication companies.

TOOL WEAR AND SURFACE FINISH IN HIGH SPEED MILLING OF ALUMINUM BRONZE

Aluminum bronze C95800 is used extensively for the manufacture of propellers because of its mechanical strength and corrosion resistance. Typically these components are machined from large castings and then hand ground and polished. In this work, we demonstrate the possibility of using high speed machining with tungsten carbide tooling to significantly reduce machining times and minimize or eliminate hand polishing/grinding. Tool wear rates for the high speed machining of aluminum bronze are assessed using three metrics: mean force, flank wear depth, and surface finish. Workpiece surface finish and tool flank wear depth are assessed using a new replica block technique. Wear rates in carbide tools remained low over a wide range of surface speeds such that material removal rates in aluminum bronze were increased more than tenfold over current machining practices. Our findings support the idea that high speed machining to produce fine surface finishes through ball end milling with very closely spaced tool paths will be cost effective.

Round-robin measurements of toroidal window

We have performed a round-robin study of surface irregularity measurements of a free-form toroidal window. The measurement tools were a Leitz scanning CMM at Optimax Systems, Inc., an UltraSurf, a non-contact measuring system at OptiPro Systems, a Zeiss scanning CMM at OptiPro Systems, a F25 micro-CMM at Carl Zeiss Industrial Metrology, and an ASI(Q)™ at QED Technologies. Each instrument resulted in a 2.5D surface error map. The measurements were compared with multiple analysis settings. The different analysis settings removed some low frequency height errors, which varied amongst the measurements. This highlights the need for more study to determine the reasons for the differences in the low frequency errors. With the low frequency errors removed, the measurements compared very well, to within 0.2 μm rms.

The need for fiducials on freeform optical surfaces

The evolution from spherical, to aspheric, to freeform optics is quickly progressing towards more complex freeform surfaces. Freeform surfaces typically have little to no symmetry making the alignment of the surfaces difficult. The alignment of such freeform surfaces relative to the other features on the optic has been little considered. A typical alignment specification like wedge (edge thickness difference) is not well defined for freeform optics, nor is the wedge measurement. We show that by using fiducials during the manufacturing of freeform surfaces, the alignment and locating of the freeform surface can be specified and measured.

Fabricating and Testing Freeform Optics: Current Capabilities, Lessons Learned and Future Opportunities

Freeform optics designed with non-symmetric features are gaining popularity with lens designers and system integrators. This paper overviews a freeform optical fabrication process that includes VIBE polishing, figure correction and testing of freeform surfaces.