Eric R. Marsh

Micro machining glass with polycrystalline diamond tools shaped by micro electro discharge machining

Brittle materials are difficult to mechanically micro machine due to damage resulting from material removal by brittle fracture, cutting force-induced tool deflection or breakage and tool wear. This paper demonstrates the feasibility of micro machining glass materials with polycrystalline diamond (PCD) micro tools that are prepared in a variety of shapes using non-contact micro electro discharge machining. The PCD tools contain randomly distributed sharp protrusions of diamond with dimensions around 1 µm that serve as cutting edges for micro machining grooves in soda-lime glass and pockets in ultra-low expansion glass. Results indicate that smooth surfaces are obtained with process conditions allowing material removal by ductile regime cutting instead of brittle fractures, and the PCD tools show very little wear. With further improvements in material removal rate, micro machining with PCD tools is a promising approach for producing micro molds and micro fluidic devices in glass materials.

Nanometer-Level Comparison of Three Spindle Error Motion Separation Techniques

This work demonstrates the state of the art capabilities of three error separation techniques for nanometer-level measurement of precision spindles and rotationally-symmetric artifacts. Donaldson reversal is compared to a multi-probe and a multi-step technique using a series of measurements carried out on a precision aerostatic spindle with a lapped spherical artifact. The results indicate that subnanometer features in both spindle error motion and artifact form are reliably resolved by all three techniques. Furthermore, the numerical error values agree to better than one nanometer. The paper discusses several issues that must be considered when planning spindle or artifact measurements at the nanometer level.

An integrated approach to structural damping

Abstract This paper considers a passive damping method that can be applied to a wide range of structural geometries including machine tool bases and components. The method uses viscoelastic materials to dissipate energy in the manner of classic constrained-layer damping, however, the layers are embedded within the structure as opposed to being applied externally. This provides a robust means of incorporating damping without encountering several of the common disadvantages associated with external damping treatments. An analytical solution to the amount of damping in bending modes is presented using a modal strain energy approach. The utility of this passive damping technique is demonstrated experimentally, and examples showing the accuracy of the modal strain energy solution are presented.

Micro-machining and micro-grinding with tools fabricated by micro electro-discharge machining

This paper provides an overview of several approaches to micro-machining by mechanical and electro-discharge means of material removal. Two steps are required in machining micro features. Firstly, a custom-shaped tool is created from suitable stock. In many cases, this is carried out using a small-scale version of wire Electro-Discharge Machining (EDM) in tool materials such as sintered PolyCrystalline Diamond (PCD) or tungsten carbide. Then the micro-tool can be used as a miniature end mill, drill or abrasive wheel. Each of these mechanical machining methods can be combined with EDM to achieve a customisable surface finish and feature accuracy. Trade-offs such as tool wear, Material Removal Rate (MRR) and machining time are discussed in this paper within the context of several examples.

Experiences with the master axis method for measuring spindle error motions

In this paper, the master axis method of machine tool spindle measurement is described. This method allows spindle measurements to be carried out at speed and under load. For example, a radial load representing the cutting force in a turning operation can be conveniently applied during characterization of a lathe spindle. The synchronous and asynchronous error motions have been observed to vary in both magnitude and shape with changes in load. Test results from both static and dynamic loads during testing are shown to demonstrate the utility of the method.

Effects of Spherical Targets on Capacitive Displacement Measurements

Capacitive displacement sensors are widely used in precision manufacturing and metrology because they measure displacements with nanometer resolution. Prior literature usually treats capacitive sensors consisting of electrodes arranged as parallel plates. In this work, the target electrode is spherical, which is common in machine tool metrology, spindle metrology, and the measurement of sphericity. The capacitance due to a gap between flat and spherical electrodes is less than that of two flat electrodes, which causes four effects. As the diameter of the target electrode is reduced, the sensitivity increases, the sensing range decreases, the sensing range shifts toward the target, and the sensor becomes nonlinear. This paper demonstrates and quantifies these effects for a representative capacitive sensor, using finite element analysis and experimental testing. For larger spheres, the effects are correctible with apparent sensitivities, but measurements with the smallest spheres become increasingly nonlinear and inaccurate.

Measurement and simulation of regenerative chatter in diamond turning

In this work, the stability of outer diameter turning is explored to extend previous results from the orthogonal turning geometry. The work begins with a numerical approach to the determination of the stability limit using a nonlinear chip area model. A complete experimental verification follows for turning of 6061-T aluminum with single crystal, synthetic diamond tools. Although diamond-turning operations are not particularly susceptible to chatter, the cutting process is well understood, and experimental tests may be conveniently carried out. Recent work to define the specific cutting energy better at small depths of cut is incorporated. The results show qualitative differences from the orthogonal cutting geometry. The role of machine parameters and tool geometry is explored using the verified model.

Micro-tool characterization using scanning white light interferometry

Micro-electro-discharge machining produces tools for micro-machining processes with diameters down to 3 ?m and lengths up to fifty times their diameter. This paper describes a metrology technique for characterizing the geometry of these tools by the standardized components of roughness, waviness and form. In this technique two-dimensional (2D) profiles are extracted from three-dimensional (3D) surface measurements acquired by scanning white light interferometry with height resolution below 1 nm. The profiles are transformed into the frequency domain and separated into form (wavelengths longer than 100 ?m), waviness (wavelengths between 10 ?m and 100 ?m) and roughness (wavelengths shorter than 10 ?m). Standard height and shape parameters are computed from the roughness, waviness and form profiles for 81 tools, enabling relative comparison of tool geometry in a quantitative and reproducible manner. This approach provides a more comprehensive quantification of geometry than alternatives such as measuring the diameter at a few points along the tool or scanning probe microscopy over small areas. The utility of the technique is demonstrated by characterizing the precision of three micro-tools produced by wire electro-discharge grinding (WEDG).

Interferometric measurement of workpiece flatness in ultra‐precision flycutting

Purpose – The purpose of this paper is to show how an ultra‐precision manufacturing process (flycutting) can be improved through interferometry.Design/methodology/approach – The paper presents a theoretical model of the machine tool cutting system and then uses interferometer measurements to validate the results. The model is then used to show some general findings relating process conditions to workpiece quality.Findings – A realistic cutting model can predict the workpiece flatness with excellent accuracy and closely match interferometer measurements. The process parameters in precision flycutting should be chosen such that the flycutting tool is in contact with the workpiece for an integer number of vibration cycles. The machine tool stiffness and structural damping will affect the workpiece quality, but the most significant improvements are made through thoughtful selection of the flycutter spindle speed as it relates to the machine dynamics.Originality/value – This paper presents a math model that ac...

Predicting surface figure in diamond turned calcium fluoride using in-process force measurement

Single crystal calcium fluoride (CaF2) shows significant variation in material properties as a function of crystallographic orientation. The surfaces generated by material removal processes such as diamond turning are influenced by this anisotropy and consequently show periodic undulations aligned with the crystal structure. This article explores the relationship between surface figure and cutting forces measured during the diamond turning of single crystal calcium fluoride. The cutting forces, when mapped to the physical geometry of CaF2 plano (flat) optics, show good correlation with surface figure measured by interferometry. A model is presented to predict the surface figure error from the experimentally measured normal component of the cutting force. The model also shows how the surface figure obtained under various machining parameters may be extrapolated from force measurements made during a single diamond turning operation.