Axial Error Motion Research Articles

Investigation of the influence of the main error sources on the capacitive displacement measurements with cylindrical artefacts

Capacitive displacement sensors are usually used with flat-targets in dimensional metrology applications, which require measurements with nanometer-level uncertainties. The use of capacitive sensors has recently expanded to cover the measurement of cylindrical artefacts (roundness, straightness and cylindricity). The error produced by the curved shape leads to the increase of the nonlinearity, since the sensing range between the targets and the sensitive part shrinks. This phenomenon cannot be ignored for applications requiring a nanometer-level uncertainties. In the context of LNE's (French National Metrology Institute NMI) on-going development of a machine for form measurement with an uncertainty of a few nanometres (<5 nm) using capacitive sensors, an experiment has been developed to characterize the behaviour of two commercial capacitive sensors. The experiment enabled the evaluation of the major error sources (axial and radial error motions as well as the deviation/tilt of the capacitive sensors) which influence capacitive displacement measurements. The research was completed with a flat-target and cylindrical artefacts whose diameter values varied between 50 and 200 mm. For radial error motion, both experimental and theoretical results were compared and were found to agree within 2 nm, especially when the radial error motions were small (<50 μm). Finally, the polynomial fitting methods using terms up to 4 and 6 resulted in a deviation between measured displacements and fit of 0.005%FS (FS: Full Scale corresponds to the working range of 90 μm).

An LDV-based methodology for measuring axial and radial error motions when using miniature ultra-high-speed (UHS) micromachining spindles

Abstract This paper presents a laser Doppler vibrometry-based methodology for measurement of axial and radial error motions when using miniature ultra-high-speed (UHS) spindles used for micromachining applications. The new methodology measures three-dimensional displacements from the surface of a custom-fabricated sphere-on-stem precision artifact using three mutually orthogonal laser beams. A precision alignment technique is developed to configure the three laser beams mutually orthogonal to one another. An infra-red sensor is used to provide a reference for the rotational angle of the spindle. The axial and radial motion data measured at operational speeds is then post-processed to obtain the synchronous and asynchronous components of the error motions in both directions. The presented approach enables obtaining error motions along both fixed-sensitive and rotating-sensitive directions. The methodology is then demonstrated by measuring axial and radial error motions when using a miniature ultra-high-speed spindle at four different speeds. Analysis of the measured data indicated the significant effect of spindle speed on the error motions along both fixed-sensitive and the rotating-sensitive directions. Finally, an uncertainty analysis is presented to quantify the overall combined uncertainty on the error measurements when using the new methodology.

Measurement of Radial and Axial Error Motion in a High Precision Spindle

The performance of a spindle is critical for high precision machining. In this paper, the spindle error motion in a high precision milling center was measured. The spindle is driven by air turbine with rotation speed of 120,000rpm. The radial and axial error motion of the axis of rotation was measured. The capacitive displacement sensors with nanometer resolution were mounted against the master gauge pin through the dedicated setup. Tlusty method was adopted to synchronize angular position of the spindle and data sampling. The measured radial and axial error motion of the spindle were 2.73μm and 2.59μm respectively. Despite of motion errors, the better machining accuracy still can be achieved. It seems that cutting force may improve the rotation performance of a spindle with aerostatic bearing.

Precision Spindle Metrology

Professor Marsh is at the Penn State University Mechanical Engineering faculty working in the field of precision engineering.His book Precision Spindle Metrology (161 pages) starts with a case study of a CNC lathe demonstrating effects of spindle error motions to a turned workpiece. This is followed by a brief history of spindle testing, starting with Schlesinger and Tlusty and ending with ASME B89.3.4 standard Axis of Rotation which is the basis for definitions and expressions used in the book.Therefore the book opens the world of axial, radial, tilt and face error motion (instead of runout), influence of the structural loop, fixed and rotating sensitive direction, average and asynchronous error motion, and thermal drift. Excellent figures help in understanding these error motions and differentiations as well as their effects on a workpiece.Two chapters are dedicated to testing spindles and data analysis, which is mainly based on Lion Precision's spindle tester (Lion holds the copyright of the book together with Precision Instruments). Environmental effects, aliasing effects, analogue-to-digital conversion and filtering are clearly explained, again supported with instructive figures.As precision spindles are targeted in this book, reversal techniques (Donaldson reversal, face error motion reversal, multi-probe error separation, multi-step error separation, Grejda reversal, master axis reversal) are described and explained in detail in order to separate spindle errors from artifact errors in the sub-micrometer range.Case studies show the application of all the theory on a surface grinder, a CNC turning centre, a CNC machining centre, and a CNC diamond turning machine. The last chapter applies the theory on tapered roller bearing spindles, giving information on instrumentation for rolling element bearings, and the repeatability of such bearings and of ball bearings. Furthermore air bearing stiffness and pressure, and gas compositions and asynchronous error are discussed.157 references, starting in 1873 (Maxwell) and ending in 2006, covering axis of rotation measurement, roundness measurement, error separation technique, influence of temperature, measurement uncertainty, and spindle design, give a good overview of work presented in this field.Designers of precision spindles and people testing spindles, who want to apply the concept of spindle error motion which is also overtaken by ISO 230-7:2006, Test code for machine tools—Part 7: Accuracy of axes of rotation, will be supported by this book. The book also shows how to move measurement uncertainty to new levels by applying error separation techniques correctly, and gives insight into design of precision spindles.

A method for evaluating spindle rotation errors of machine tools using a laser interferometer

This paper presents a method for assessing radial and axial error motions of spindles. It uses the Hewlett Packard 5529A laser interferometer. The measurement is made using reflection directly from a high-precision sphere. Such object is used as the optical reflector. The sphere is affixed at the end of a wobble device, which is clamped in the spindle. The principle of measurement is similar to that of a linear interferometer, except that the high-precision sphere is used in place of the usual retroreflector. A convergent lens is utilized to focus the laser beam to a small spot on the sphere surface. This minimizes the dispersion of the beam due to the reflection on the spherical surface. A software package has been developed for data acquisition and presentation of the error motion polar plots of the spindle. Application of this spindle error calibrator on a CNC machining centre is undertaken. The results are presented and discussed.

A cylindrical capacitive sensor (CCS) for both radial and axial motion measurements

Conventional cylindrical capacitive sensors (CCS) are used only to measure the radial error motion of rotating machinery. However, axial error motion is also very important in rotating machines and it is very difficult to measure the axial motion due to the limited space. This paper presents a new CCS for both radial and axial motion measurements. The idea behind the new sensor is that the unused axial area of the CCS is exposed to measure the axial motion of a target. A theoretical model of the proposed CCS was derived. Based on the derived theoretical model, compensation methods to decouple the radial and axial motion measurements were proposed. In addition, an error analysis of the CCS was performed and a design rule was developed to guarantee the same accuracy in measuring both radial and axial motion measurements. Finally, a test rig and electronics for the proposed CCS were built and the effectiveness of the proposed CCS was verified through experiments and simulations. The developed CCS is expected not only to reduce the system complexity but also to afford a good balance between the radial and axial motion measurement accuracies.

Roundness measurement of spherical artifacts at arbitrary latitude

Abstract This technical note examines an approach to measuring the roundness of spherical artifacts at arbitrary latitude. A roundness measurement includes the combined contributions from the artifact’ form error and the spindle’ error motion. We show that it is possible to separate the spindle error from artifact form error at any inclination with respect to the axis of rotation if the axial and radial error motion is independently measured using an error separation technique such as Donaldson reversal. Comparison of the predicted and measured results shows agreement to less than a nanometer.

Separation of scanning error motions for surface profile measurement of aspheric micro lens

In a scanning measurement system for aspheric micro lenses and micro lens moulds, a contact-stylus sensor, which is mounted on an air-bearing linear slide, is employed to spirally scan the surface of the aspheric micro lens mounted on an air-bearing spindle. To remove the influence of the scanning error motions of the spindle and the slide, a ring artefact is attached around the lens and two capacitance-type displacement sensors are set on the two sides of the contact-stylus sensor to scan the surface of the ring artefact. The surface profile of the ring artefact, which serves as the reference for separation of error motions, is measured to be approximately 55 nm by the reversal method. The tilt error motion and the axial error motion of scanning stage are measured to be approximately 0.5 µrad and 10 nm, respectively.

Motion error compensation of a mechanical contact spindle using a multi-degree-of-freedom micropositioning stage

A three-degree-of-freedom micropositioning stage has been developed for controlling spindle runout. The stage consists of three wedge-shaped plates and PZT-stacked actuators. This stage can expand to 6 degrees of freedom and moves precisely without a stick-slip effect because there is no friction. Moreover, this stage has high loading capacity and stiffness because most of the load is placed on actuators having high compressive strength. In a trial, a PI controller actuated the stage to decrease the runout of a ball-bearing spindle weighing 400N. As a result, radial and axial motion errors of the spindle mounted on the stage were decreased from 0.2μmto0.04μm.

Measurements of Spindle Parameters on a Diamond Turning Machine to Nanometer Resolution(Precision positioning and control technology)

A single point diamond turning (SPDT) facility is utilized for the ultra-precision machining of planar surfaces, and aspheric mirror components for geo-orbital x-ray telescopes and x-ray microscopes. The spindle of the machine tool is encased in an ultra-precision hydraulic bearing, and the motion controller is programmable to nanometer resolution. The radii of diamond cutting tools, utilized in the SPDT process, approach nanometer accuracy. This combination of parameters produces planar and aspheric components with nanometer surface and shape quality. We examine the radial and axial motion of the spindle in the static and dynamic domains with an optical nanometer-resolution sensor. The sensing geometry is based on standard optical triangulation off the target surface, and the sensor can be calibrated in situ utilizing the position controller of the machine. We present results, which examine the parametric performance of the diamond turning operation. At 1000rpm we measured a minimum of approximately 35-nm rms radial and the axial spindle error motion.

Testing a low-influence spindle drive motor

Precision spindles used for diamond turning and other applications requiring low error motion generally require a drive system that ideally applies a pure torque to the rotating spindle. Frequently a frameless motor, that is, one without its own bearings, is directly coupled to the spindle to make a compact and simple system having high resonant frequencies. Although in addition to delivering drive torque, asymmetries in the motor cause it to generate disturbance loads (forces and moments) which influence the spindle error motion of the directly coupled system. This paper describes the tests and results for a particular frameless, brushless dc motor that was originally developed for military and space applications requiring very low torque ripple. Because the construction of the motor should also lead to very low disturbance loads, it was selected for use on a new diamond turning and grinding machine under developed at Lawrence Livermore National Laboratory. The level of influence for this motor-spindle combination is expected to be of order one nanometer for radial and axial error motion.

Implementation of the Estler face motion reversal technique

An implementation of the Estler face motion reversal technique [CIRP Annals 45 (2) (1996) 483] is presented that allows for the separation of the axial and tilt error motions of an axis of rotation from the circular flatness of a measured part. A single indicator method is used that has practical applications in testing spindle-type roundness measuring instruments. The method is presented along with experimental results that demonstrate the possibility of nanometer level accuracy.

多点法による工作機械回転主軸のアキシャルおよびアンギュラモーションエラーの精密測定に関する研究

This paper presents a new method for precision measurement of axial and angular motion errors in the main rotational axis of a manufacturing machine. Three displacement sensors and one angle sensor are used in this method. Applying a calculation technique, it is possible to separate axial motion error, two angular motion errors, and the circumference profile. The principle of this method is examined from the viewpoint of spatial frequency analysis. From this principle, it appears that this new method can measure all spatial frequencies, without omitting any frequency. A measurement system performed experiments. The measurement experiments confirmed the separate measurement of the profile and motion errors.

軸の回転精度に関する研究(第2報)

A new measuring method has been developed for measuring radial, angular and axial error motions of an axis with a rotating workpiece. The measuring and cutting tests were conducted with a lathe to verify this method. The measured radial and axial error motions of the axis were compared respectively to the out-of-roundness profile and face profile of the workpiece which was machined by a diamond tool. The following conclusions were obtained:(1) Radial, angular and axial error motions were measured exactly by detecting the fluctuations in the gaps between two master balls attached to the spindle and three pick-up devices attached to the tool post.(2) Radial and axial error motions measured by this method correspond closely and respectively to the out-of-roundness profiles and face profiles of the workpiece, machined by a diamond tool.