A comprehensive method for calibration of volumetric positioning accuracy of CNC-machines

A comprehensive method for measuring the systematic errors of CNC-machine tools has been studied. The method used for measurement and calibration of machine tool errors should be general and efficient. The objectives of this study include:

Fiber deflection probe for small hole metrology

This paper presents the development of a new probing method for coordinate measuring machines (CMM) to inspect the diameter and form of small holes. The technique, referred to as fiber deflection probing (FDP), can be used for holes of approximately 100 μm nominal diameter. The expanded uncertainty obtained using this method is 0.07 μm (k = 2) on diameter. The probing system consists of a transversely illuminated fiber (with a ball mounted on the end) whose shadows are imaged using a camera. We can infer the deflection of the probe from the motion of the image seen by the camera, and we infer the position of the measured surface by adding the fiber deflection along x- and y-directions to the machine scale readings. The advantage of this technique is the large aspect ratio attainable (5 mm deep for a 100 μm diameter hole). Also, by utilizing the fiber as a cylindrical lens, we obtain sharp crisp images of the fiber position, thus enabling high resolution for measured probe deflection. Another potential advantage of the probe is that it exerts an exceptionally low force (ranging from a few micronewtons down to hundreds of nanonewtons). Furthermore, the probe is relatively robust, capable of surviving more than 1 mm over-travel, and the probe should be inexpensive to replace if it is broken. In this paper, we describe the measurement principle and provide an analysis of the imaging process. Subsequently, we discuss data obtained from characterization and validation experiments. Finally, we demonstrate the utility of this technique for small hole metrology by measuring the internal geometry of a 129 μm diameter fiber ferrule and conclude with an uncertainty budget.     

Improved machine tool linear axis calibration through continuous motion data capture

Machine tool calibration is becoming recognised as an important part of the manufacturing process. The current international standards for machine tool linear axes calibration support the use of quasi-static calibration techniques. These techniques can be time consuming but more importantly a compromise in quality due to the practical restriction on the spatial resolution of target positions on the axis under test. Continuous motion calibration techniques have the potential to dramatically increase calibration quality. Through taking several measurement values per second while the axis under test is in motion, it is possible to measure in far greater detail. Furthermore, since machine tools normally operate in dynamic mode, the calibration data can be more representative if it is captured while the machine is in motion. The drawback to measuring the axis while in motion is the potential increase in measurement uncertainty. In the following paper, different methods of continuous motion calibration are discussed. A time-based continuous motion solution is proposed as well as a novel optimisation and correlation algorithm to accurately fuse the data taken from quasi-static and continuous motion measurements. The measurement method allows for minimal quasi-static measurements to be taken while using a continuous motion measurement to enhance the calibration process with virtually no additional time constraints. The proposed method does not require any additional machine interfacing, making it a more readily accessible solution for widespread machine tool use than other techniques which require hardware links to the CNC. The result of which means a shorter calibration routine and enhanced results. The quasi-static and continuous motion measurements showed correlation to within 1 μm at the quasi-static measurement targets. An error of 13 μm was detailed on the continuous motion, but was missed using the standard test. On a larger, less accurate machine, the quasi-static and continuous motion measurements were on average within 3 μm of each other however, showed a standard deviation of 4 μm which is less than 1% of the overall error. Finally, a high frequency cyclic error was detected in the continuous motion measurement but was missed in the quasi-static measurement.     

Surface profile measurement of a sinusoidal grid using an atomic force microscope on a diamond turning machine

This paper describes the surface profile measurement of a XY-grid workpiece with sinusoidal microstructures using an atomic force microscope (AFM) on a diamond turning machine. The sinusoidal micro-structures, which are fabricated on an aluminum plate by fast tool servo-assisted diamond turning, are a superposition of periodic sine-waves along the X- and Y-directions (wavelength (XY): 150 μm, amplitude (Z): 0.25 μm). A linear encoder with a resolution of 0.5 nm is integrated into the AFM-head for accurate measurement of the Z-directional profile height in the presence of noise associated with the diamond turning machine. The spindle and the X-slide of the machine are employed to spirally scan the AFM-head over the sinusoidal grid workpiece. Experiments fabricating and measuring the sinusoidal grid workpiece are carried out after accurate alignment of the AFM cantilever tip with the spindle centerline.     

Erosive wear behaviour of weld hardfacing high chromium cast irons: effect of erodent particles

Five commercial hardfacing high chromium cast iron alloys were deposited by flux cored arc-welding method. The solid particle erosion studies were carried out using air blast type erosion test rig with 125–150 μm cement clinker, 125–150 μm blast furnace sinter, 100–150 μm silica sand and 125–150 μm alumina particles at a velocity of 50 m s⁻¹ and at impingement angles of 15–90°. The observed erosion rates were rationalised in terms of relative hardness of erodent particles and ability of erodent particle to cause gross fracture of the carbides. The dependence of erosion rate on impingement angle was found to be quite weak for hardfacing high chromium cast iron alloys. However, significant differences were observed in the ranking of the alloys when eroded with different erodent particles. The presence of large volume fraction of carbides proved to be beneficial to the erosion resistance when the erodent particle were softer than the carbides. With silica sand particles at normal impact and with alumina particles large volume fraction of carbides proved detrimental to the erosion resistance. The operating erosion mechanisms involved small-scale chipping, edge effect, indentation and fracture and fatigue.     

Precision surface finish of the mold steel PDS5 using an innovative ball burnishing tool embedded with a load cell

A load-cell-embedded burnishing tool has been newly developed and integrated with a machining center, to improve the surface roughness of the PDS5 plastic injection mold steel. Either the rolling-contact type or the sliding-contact type was possible for the developed ball burnishing tool. The characteristic curves of burnishing force vs. surface roughness for the PDS5 plastic injection mold steel using the developed burnishing tool for both the rolling-contact type and the sliding-contact type, have been investigated and constructed, based on the test results. The optimal plane surface burnishing force for the PDS5 plastic injection mold steel was about 420 N for the rolling-contact type and about 470 N for the sliding-contact type, based on the results of experiments. A force compensation strategy that results in the constant optimal normal force for burnishing an inclined surface or a curved surface, has also been proposed to improve the surface roughness of the test objects in this study. The surface roughness of a fine milled inclined surface of 60 degrees can be improved from R_a 3.0 μm on average to R_a 0.08 μm (R_max 0.79 μm) on average using force compensation, whereas the surface roughness was R_a 0.35 μm (R_max 4.56 μm) on average with no force compensation.     

Digital twin-based analysis of volumetric error mapping procedures

The evaluation of the volumetric accuracy of a machine tool is an open challenge in the industry, and a wide variety of technical solutions are available in the market and at research level. All solutions have advantages and disadvantages concerning which errors can be measured, the achievable uncertainty, the ease of implementation, possibility of machine integration and automation, the equipment cost and the machine occupation time, and it is not always straightforward which option to choose for each application. The need to ensure accuracy during the whole lifetime of the machine and the availability of monitoring systems developed following the Industry 4.0 trend are pushing the development of measurement systems that can be integrated in the machine to perform semi-automatic verification procedures that can be performed frequently by the machine user to monitor the condition of the machine. Calibrated artefact based calibration and verification solutions have an advantage in this field over laser based solutions in terms of cost and feasibility of machine integration, but they need to be optimized for each machine and customer requirements to achieve the required calibration uncertainty and minimize machine occupation time.This paper introduces a digital twin-based methodology to simulate all relevant effects in an artefact-based machine tool calibration procedure, from the machine itself with its expected error ranges, to the artefact geometry and uncertainty, artefact positions in the workspace, probe uncertainty, compensation model, etc. By parameterizing all relevant variables in the design of the calibration procedure, this simulation methodology can be used to analyse the effect of each design variable on the error mapping uncertainty, which is of great help in adapting the procedure to each specific machine and user requirements. The simulation methodology and the analysis possibilities are illustrated by applying it on a 3-axis milling machine tool.     

Micro-structuring of glass with features less than 100 μm by electrochemical discharge machining

Micro-electrochemical discharge machining (ECDM) was studied in order to improve the machining of 3D micro-structures of glass. To minimize structures and obtain good surface microstructures, the effects of the electrolyte, the pulse on/off-time ratio, the voltage, the feedrate, the rotational speed, and the electrolyte concentration in the drilling and milling processes were studied.In ECDM, voltage is applied to generate a gas film and sparks on a tool electrode; however, high voltage produces poor machining resolution. To obtain a stable gas film over the whole surface of the tool at a low voltage, a new mechanical contact detector, based on a loadcell, was used; the immersion depth of the tool electrode in the electrolyte was reduced as much as possible. In this study, various micro-structures less than 100 μm in size, such as Ø 60 μm micro-holes, a 10 μm-thin wall, and a 3D micro-structure were fabricated to demonstrate the potential for micro-machining of glass by ECDM.     

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.     

Micromilling of metal alloys with focused ion beam–fabricated tools

This work combines focused ion beam sputtering and ultra-precision machining as a first step in fabricating metal alloy microcomponents. Micro-end mills having 25 μm diameters are made by sputtering cobalt M42 high-speed steel and C2 micrograin tungsten carbide tool blanks. A 20 keV focused gallium ion beam is used to define a number of cutting edges and tool end clearance. Cutting edge radii of curvature are less than or equal to 0.1 μm. Micro-end mill tools having 2, 4 and 5 cutting edges successfully machine millimeter long trenches in 6061-T4 aluminum, brass, 4340 steel and polymethyl methacrylate. Machined trench widths are approximately equal to the tool diameters, and surface roughnesses (R_a) at the bottom of micromachined features are 200 nm. Microtools are robust and operate for more than 6 h without fracture. Results from ultra-precision machining aluminum alloy at feed rates as high as 50 mm/minute and an axial depth of 1.0 μm are included.     

Uncertainty analysis of a laser calibration system for evaluating the positioning accuracy of a numerically controlled axis of coordinate measuring machines and machine tools

This paper presents an uncertainty analysis of a Positional Error Calibrator based on a laser interferometer system. This laser calibration system is capable of evaluating the positioning accuracy of a numerically controlled axis of machine tools and coordinate measuring machines (CMM) under dynamic conditions. In order to assess the measurement uncertainty of this calibrator, an analysis of the uncertainty components that make up the uncertainty budget of this calibrator has been carried out. These uncertainty components can be classified into three categories as follows: (1) uncertainties intrinsic to the laser system; (2) uncertainties due to environmental effects; (3) measuring uncertainties due to the installation. The procedure for evaluating the uncertainty of this calibrator follows GUM (“Guide to the Expression of Uncertainty in Measurement”). This uncertainty analysis was carried out when this calibrator was used to assess the positional errors of the “X” axis of a moving bridge type CMM.     

Electrical discharge machining of submicron holes using ultrasmall-diameter electrodes

We have carried out the electrical discharge machining (EDM) of submicron holes using ultrasmall-diameter electrodes. Two types of electrode were used: tungsten electrodes fabricated by the combination of wire electrodischarge grinding and electrochemical machining, and silicon electrodes originally designed as probes for scanning probe microscopes. The diameters of the former and latter were 1 μm or less, and less than 0.15 μm, respectively. Holes were drilled using a relaxation-type pulse generator at an open-circuit voltage of less than or equal to 20 V with the machine's stray capacitance as the only capacitance. Using tungsten electrodes, holes of less than 1 μm in diameter and more than 1 μm in depth were successfully drilled. A 1.3-μm-wide slot was also fabricated by drilling many holes with a small pitch. It was possible to drill holes of approximately 0.5 μm diameter using silicon electrodes because the electrode diameter was less than those of the tungsten electrodes. These holes have the smallest reported diameter for holes drilled by EDM, indicating the possibility of submicron- and nanoscale machining by EDM.     

Stylus-laser surface calibration system

A stylus-laser surface calibration system was developed to calibrate the NIST sinusoidal roughness Standard Reference Materials (SRM) 2071-2075. Step height standards are used to calibrate the stylus instrument in the vertical direction, and a laser interferometer is mounted on the traversing unit of the stylus instrument to calibrate the instrument in the horizontal direction. The calibration uncertainty (±2δ) for SRM 2075 is ±1.2% for roughness calibrations ((R_a = 1 μm), and ±0.06% for spatial wavelength calibrations (S_m = 800 μm).     

On-machine measurement of a cylindrical surface with sinusoidal micro-structures by an optical slope sensor

This paper describes the measurement of a cylindrical surface with sinusoidal micro-structures over a large area on a diamond turning machine. The sinusoidal micro-structures, which are fabricated on the periphery surface of a cylinder by the fast tool servo-based diamond turning, are superposition of periodic sine-waves along the cylinder axis and the cylinder circumference with amplitudes of 100 nm and wavelengths of 100 μm, respectively. An optical two-dimensional (2D) slope sensor with a multi-spot light beam is developed for measurement of the 2D local slopes of the sinusoidal micro-structured surface. A cylindrical lens is employed in the sensor for removing the influence of the curvature of the cylinder surface. Experiments of fabrication and measurement of the sinusoidal micro-structured surface on an ultra-precision diamond turning machine are carried out.     

Design of a six-axis micro-scale nanopositioner—μHexFlex

This paper presents the design of a small-scale nanopositioner, the μHexFlex, which is comprised of a six-axis compliant mechanism and three pairs of two-axis thermo-mechanical micro-actuators. In this paper, we cover the modeling, design and fabrication of the μHexFlex. Specific attention is given to: (1) the use of constraint-based design in generating the compliant mechanism design, (2) the modeling of the actuators, and (3) the system model which links the actuator input and mechanism response. The measured, quasi-static performance of a 3 mm diameter prototype shows a maximum range of 8.4 μm × 12.8 μm × 8.8 μm and 19.2 mrad × 17.5 mrad × 33.2 mrad (1.1° × 1.0° × 1.9°). Experimental results indicate that a constant mechanical/electrical material property system model may be used to predict the position and orientation over a range of 3.0 μm × 4.4 μm × 3.0 μm and 6.3 mrad × 6.3 mrad × 8.7 mrad (0.36° × 0.36° × 0.5°). The dynamic characteristics of the device were investigated experimentally. Experimental results show a lowest natural frequency of 4 kHz. The resolution characteristics of the device have been measured at 1 Å/mV. The device was created using deep reactive ion etching (DRIE). Bulk fabrication costs are estimated at less than $ 2 per device.     

On the compressive behavior of sintered porous coppers with low-to-medium porosities—Part II: Preparation and microstructure

A polymer space-holder method was used in this study to prepare porous coppers with low-to-medium porosity within the range 5–50%. This provides the possibility to control the pore size, distribution and structure. Optical microscopy and scanning electron microscopy (SEM) with energy dispersion spectrum (EDS) were utilized to characterize the porous samples. Two different sizes of copper powders, 5 and 45 μm, were used to investigate the effect of raw materials powder size. Microstructure results have shown that there exist two different types of pore in the sintered samples: round-shaped macro-pores left over by the burnout of the space holder and irregular micro-pores or the intervals among metal powders. No matter which size powder was used, the size of the macro-pore falls into a range 200–500 μm, but the pore structures are different in the two cases, interconnected or open pores for the 45 μm raw powders and closed pore for the 5 μm powders. The sizes of the micro-pores among the copper powders in the two cases are also different, several microns for the 5 μm powders and 10–20 μm for the 45 μm powders, though all micro-pores are interconnected for both powder sizes. From the viewpoint of pore structure, it is concluded that the 45 μm powder is more appropriate for use to prepare the porous metal. In addition, the effect of the binder was also investigated. It is suggested that a binder that can be easily and completely removed should be used in order to induce the residue. This paper, as Part II of the publication, focuses on the fabrication of the porous samples where Part I [Lemons JE, editor. Quantitative characterization and performance of porous implants for hard tissue application. ASTM STP 953; 1985] has been published earlier for the mechanical properties of the material.     

Automatic alignment direct step-on-wafer projection system

This paper describes an automatic alignment direct step-on-wafer (dsw) projection system. Automatic alignment accuracy is better than ±0.25 μm and useful achievable resolution is 1.5 μm or better. Stepping of the stage is achieved using a record ruler (glass scale) and photoelectric microscope. Its precision is better than ±0.33 μm (3 σ). Also included in the system is a reticle box with automatic reticle detection and selection, high pressure pulsed mercury lamp and uv light integrating exposure controller as well as a closed-circuit tv viewing and display system. The fundamental design, some experimental testing data and error analysis of the machine are described briefly in this paper     

A novel technique for calibration of polygon angles with non-integer subdivision of indexing table

Polygons are basic angle standards for angle measurement, particularly used for calibration of rotary angular indexing, and for measuring equipment such as dividing heads and tables. A main application in daily life is in bar-code readers. Calibration of such angle standards is required for traceability and at the highest accuracy it is a responsibility of national metrology institutes. In order to investigate uncertainty parameters on polygon calibration and to establish the capabilities of national metrology institutes, intercomparision measurements in the name of EUROMET project 371 “angle calibration on precision polygons” between 12 European countries have been carried out. Two precise polygons with 7 and 24 faces have been calibrated by the participants. Difficulties arose for precise calibration of seven-sided polygon for those institutes, which do not have a high-resolution angle comparator or two autocollimators. UME, the National Metrology Institute of Turkey, has applied an alternative technique for precise calibration of seven-sided polygon without using high-resolution angle comparators (i.e., indexing tables or angle dividers) or two autocollimators. The technique is based on the circle closure principle. The pitch and cumulative angles of the polygon are extracted from the angle measurement between some polygon faces (such as one and four (1/4), analogous 2/5, 3/6, 4/7, 5/1, 6/2 and 7/3) the angle of which can be generated close enough by the indexing table. This means that the polygon can be regarded as unfolded in seven 3-pitch angle intervals of 3×360°/7≈154°17′, making up 1080° in total. The method gives the differences between these seven intervals; with the closure condition (the sum must be zero) this gives all absolute angles. A full uncertainty evaluation is given that is based on the model function which relates the measured values to the polygon angles. For the calibration actually carried out, this yielded an uncertainty of 0.24″. Within this uncertainty the measured polygon angles corresponded very well with the reference values of the intercomparison. The method is of use for laboratories which do not have a high-resolution angle comparator (i.e., an indexing table or angle divider) or two autocollimators for the calibration of such angle standards.     

Micro-drilling of monocrystalline silicon using a cutting tool

The micro-drilling of monocrystalline silicon using a cutting tool was tested with the aim of fabricating three-dimensional and high aspect ratio micro-shapes. Micro-tools with a D-shaped cross-section and cutting edge radius of 0.5 μm were fabricated by wire electrodischarge grinding (WEDG). The results showed that, with a depth of cut of 0.1 μm, ductile-regime cutting was realized, and that a tool clearance angle larger than 0° was necessary to prevent fractures at the hole entrance. The smallest machinable hole was of 6.7 μm diameter, which is the smallest not just in the present study, but of all holes drilled using a cutting tool so far. Furthermore, an aspect ratio of more than four was obtained in the drilling of a 22 μm diameter and 90 μm deep hole.