Brian S. Dutterer

Stability Prediction for Low Radial Immersion Milling

Traditional regenerative stability theory predicts a set of optimally stable spindle speeds at integer fractions of the natural frequency of the most flexible mode of the system. The assumptions of this theory become invalid for highly interrupted machining, where the ratio of time spent cutting to not cutting (denoted p) is small. This paper proposes a new stability theory for interrupted machining that predicts a doubling in the number of optimally stable speeds as the value of p becomes small. The results of the theory are supported by numerical simulation and experiment. It is anticipated that the theory will be relevant for choosing optimal machining parameters in high-speed peripheral milling operations where the radial depth of cut is only a small fraction of the tool diameter.

The Stability of Low Radial Immersion Milling

Abstract Traditional regenerative stability theory predicts a set of spindle speeds with locally optimum stability at integer fractions of the natural frequency of the most flexible mode of the system. The assumptions of this theory become invalid for highly interrupted machining, where the ratio of time spent cutting to not cutting (denoted ρ) is small. This paper proposes a new stability theory for interrupted machining that predicts a doubling in the number of optimally stable speeds as the value of ρ becomes small. The predictions are verified against experiment and numerical simulation.

On the dynamics of high-speed milling with long slender endmills

Abstract Tool deflections of high length-to-diameter ratio endmills are measured with capacitance probes during high-speed milling and compared with the predictions of regenerative chatter theory. Poincare sectioning (once per revolution sampling) techniques are introduced as a new means of characterizing and identifying chatter. The regenerative chatter theory seems to accurately predict the stability of high-immersion slotting cuts; however, undesirable vibrations observed in partial immersion cuts seem inconsistent with existing theory. The practical utility of in-depth knowledge of the stability behaviour of long endmills is demonstrated by the high-speed machining of an example component using a dynamically tuned tool.

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.

The application of high-speed CNC machining to prototype production

This paper describes the application of high-speed milling to the production of monolithic, metallic, functional prototypes, with special emphasis placed on the applicable process times. An example component is selected and the relevant process times are presented. Fundamental requirements for the use of high-speed milling to produce prototypes in a timely manner are identified. These requirements include: 1) high speed/high power spindles, 2) proper spindle speed selection based on the system dynamics, 3) machining parameter definition based on tool wear, 4) high feed/high acceleration machine drives, 5) intelligent path generation, and 6) pre-process verification of arbitrary three-dimensional CNC part paths. The implementation of the Simultaneous Trilateration Laser Ball Bar (STLBB) system to measure the CNC part paths is described and the device verification procedure is outlined. Example two and three-dimensional path measurements are also shown.

Tool Length-Dependent Stability Surfaces

Abstract This article describes the development of three-dimensional stability surfaces, or maps, that combine the traditional dependence of allowable (chatter-free) chip width on spindle speed with the inherent dependence on tool overhang length, due to the corresponding changes in the system dynamics with overhang. The tool point frequency response, which is required as input to existing stability lobe calculations, is determined analytically using Receptance Coupling Substructure Analysis (RCSA). In this method, a model of the tool, which includes overhang length as a variable, is coupled to an experimental measurement of the holder/spindle substructure through empirical connection parameters. The assembly frequency response at the tool point can then be predicted for variations in tool overhang length. Using the graphs developed in this study, the technique of tool tuning, described previously in the literature, can then be carried out to select a tool overhang length for maximized material removal rate. Experimental results for both frequency response predictions and milling stability are presented.

Design and characterization of an infrared Alvarez lens

While Alvarez lens prototypes have recently been manufactured and tested for visible wavelengths, there is little discussion of these types of components for infrared applications in the published literature. We present and characterize a germanium Alvarez lens for infrared imaging. Mathematical analysis for determining the required cubic surfaces is presented, and ray-based and wave-based optical simulations are performed to confirm and refine the expected variable-focus behavior. As part of the design study, we examine the effects of effective f-number of the Alvarez lens and gap between the freeform surfaces on image quality, modulation transfer function, and Strehl ratio. The germanium Alvarez lens pair is fabricated through freeform diamond micro-milling, and characterized using a custom-built imaging test station in the mid-infrared. The variable-focus and imaging capabilities of this lens are demonstrated experimentally and compared to predicted results with good agreement.

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.

Experimental characterization of variable output refractive beamshapers using freeform elements

We present experimental results from variable output refractive beam shapers based on freeform optical surfaces. Two freeform elements in close proximity comprise a beam shaper that maps a circular Gaussian input to a circular ‘flat-top’ output. Different lateral relative shifts between the elements result in a varying output diameter while maintaining the uniform irradiance distribution. We fabricated the beam shaping elements in PMMA using multi-axis milling on a Moore Nanotech 350FG diamond machining center and tested with a 632.8 nm Gaussian input. Initial optical testing confirmed both the predicted beam shaping and variable functionality, but with poor output uniformity. The effects of surface finish on optical performance were investigated using LightTrans VirtualLabTM to perform physical optics simulations of the milled freeform surfaces. These simulations provided an optimization path for determining machining parameters to improve the output uniformity of the beam shaping elements. A second variable beam shaper based on a super-Gaussian output was designed and fabricated using the newly determined machining parameters. Experimental test results from the second beam shaper showed outputs with significantly higher quality, but also suggest additional areas of study for further improvements in uniformity.

Diamond Machining of Freeform Infrared Optics

We discuss the application of diamond machining to the fabrication of freeform infrared optical components and structures. Fabrication approaches, challenges, and experimental results are presented for several novel optical designs.