“Open-Loop” Tracking Interferometer Measurement Using Rotary Axes of a Five-Axis Machine Tool

The tracking interferometer, or the laser tracker, is a laser interferometer with a steering mechanism to change the laser beam direction to automatically follow a retroreflector. Many researchers have studied its application to the multilateration to measure the retroreflector's three-dimensional position. This paper shows that the multilateration measurement can be done by regulating the laser beam toward the command retroreflector position, assuming that the machine tool's positioning error is reasonably small. The machine's rotary axes are used to regulate the laser beam direction. The proposed scheme enables a user to perform the multilateration measurement by using a laser interferometer and the machine's rotary axes only, without requiring any specialized tracking mechanism. An experiment is presented to investigate its measurement performance. The paper's emphasis is on the assessment of its measurement uncertainty, introduced by the elimination of automated tracking mechanism

A machining test to identify rotary axis geometric errors on a five-axis machine tool with a swiveling rotary table for turning operations

Lately, a cylindrical workpiece of relatively large diameter is often machined by turning operations by a swiveling rotary table in a five-axis machining center. This paper presents a machining test containing features finished by a turning operation by a swiveling rotary table. Unlike conventional machining tests for turning operations described in ISO 13041-6:2009, the present machining test can identify a complete set of position and orientation errors of the axis average line of rotary axes from the geometry of the finished test piece. The radial and axial error motions of the rotary table can be also observed when the swiveling axis is positioned horizontal (A = 0°) and vertical (A = -90°). Experimental demonstration is presented. The rotary axis geometric errors identified from the finished test piece's geometry are compared with those estimated by a conventional error calibration test using a touch-triggered probe and a precision sphere. The uncertainty analysis for the present machining test is also presented

Non-contact R-test with laser displacement sensors for error calibration of five-axis machine tools

The R-test is an instrument to measure three-dimensional displacement of a precision sphere attached to a spindle relative to a work table by using three displacement sensors. Its application to error calibration for five-axis machine tools has been studied in both academia and industry. For the simplicity in calculating the sphere center displacement, all conventional R-test devices use contact-type displacement sensors with a flat-ended probe. Conventional contact-type R-test may be potentially subject to the influence of the friction or the dynamics of supporting spring in displacement sensors particularly in dynamic measurement. This paper proposes a non-contact R-test with laser displacement sensors. First, a new algorithm is proposed to calculate the three-dimensional displacement of sphere center by using non-contact displacement sensors. The compensation of measurement error of a laser displacement sensor due to the curvature of target sphere is incorporated. Then, the measurement uncertainty of four laser displacement sensors with different measuring principles is experimentally investigated in measuring the geometry of a sphere in order to select the laser displacement sensor most suitable for the application to a non-contact R-test. A prototype non-contact R-test device is developed for the verification of the proposed algorithm for non-contact R-test. Experimental case studies of error calibration of (1) static and (2) dynamic error motions of rotary axes in a five-axis machine tool with the developed non-contact R-test prototype are presented. Its measurement performance is compared to the conventional contact-type R-test device

A Framework for a Large-Scale Machine Tool With Long Coarse Linear Axes Under Closed-Loop Volumetric Error Compensation

A large-scale machine tool is typically very inefficient in size, cost, and energy consumption. Some large parts only have a set of machining features, each of which is within a small local region, and their location should meet position and orientation tolerances. In such a machining application, as a more cost- and energy-effective alternative, this paper presents the concept of a “portable” machine tool, where a small machining platform, with the capability to machine each local machining feature in the required accuracy, is moved by long coarse linear axes. The coarse axes only perform the point-to-point positioning to each machining feature and fixed by servo control during the machining. They do not have sufficient positioning repeatability. To ensure the position/orientation accuracy of each machining feature without having highly repeatable coarse axes, this paper proposes the application of a tracking interferometer to measure all the error motions of coarse axes, and then to perform their compensation. This can be seen as a closed-loop feedback control for coarse axes using the tracking interferometer in the loop. The proposed concept is demonstrated by the experiments with its prototype using a six-degrees of freedom robot moved by two coarse linear axes

Kinematic modeling and error sensitivity analysis for on-machine five-axis laser scanning measurement under machine geometric errors and workpiece setup errors

On-machine scanning measurement of workpiece geometry has a strong advantage in its efficiency, compared to conventional discrete measurement using a touch-trigger probe. When a workpiece is rotated and tilted, position and orientation errors of the workpiece with respect to the machine’s rotary axes can be a significant contributor to the measurement error. Rotary axis geometric errors also influence the measurement error. To establish the traceability of on-machine measurement with workpiece rotation, this paper kinematically formulates their contribution to measured profiles. As a practical application example, this paper presents the measurement error assessment for an axis-symmetric part. Based on the present kinematic model, this paper compares error contributors to the cases (1) where an axis-symmetric part is placed concentric to the rotary axis, and (2) where it is placed away from the rotary axis

On-machine identification of rotary axis location errors under thermal influence by spindle rotation

Position and orientation errors of rotary axis average lines are often among dominant error contributors in the five-axis kinematics. Although many error calibration schemes are available to identify them on -machine, they cannot be performed when a machine spindle is rotating. Rotary axis location errors are often influenced by the machine’s thermal deformation. This paper presents the application of a non-contact laser light barrier system, widely used in the industry for tool geometry measurement, to the identification of rotary axis location errors, when the spindle rotates in the same speed as in actual machining applications. The effectiveness of the proposed scheme is verified by experimental comparison with the R-Test and a machining test. The uncertainty analysis is also presented.This work was supported by JSPS KAKENHI Grant NumberJP15K05721

A Tool Path Modification Approach to Cutting Engagement Regulation for the Improvement of Machining Accuracy in 2D Milling With a Straight End Mill

In two-dimensional (2D) free-form contour machining by using a straight (flat

On the magnification of two-dimensional contouring errors by using contour-parallel offsets

The cross grid encoder is a diffraction grating type encoder to measure two-dimensional position of a optical head by using a grid plate, and is widely used in the industry to evaluate the two-dimensional contouring performance of a machine tool. In the graphical display of measured contouring error profiles, the error is often magnified to some given scale with respect to the reference trajectory. The conventional algorithm to compute the magnified contouring error profile, adopted in a commercial software to analyze an error profile measured by the cross grid encoder, makes the magnified trajectory discontinuous when the given reference trajectory is unsmooth, which makes it difficult to understand the magnified trajectory especially at corners. This paper proposes a new algorithm to compute the magnified trajectory of two-dimensional contouring error profiles such that the magnified trajectory becomes continuous even when the reference trajectory is unsmooth. Application examples are presented with error profiles obtained by using a cross grid encoder applied to a commercial machining center

A long-term control scheme of cutting forces to regulate tool life in end milling processes

Numerous researches on cutting force control in end milling processes have been reported in the literature. There have been, however, very few practical applications actually employed in commercial products. The paper presents a simple, practically feasible and effective scheme to regulate the tool life through a long-term control of cutting force. Cutting force changes quickly occurring due to tool path geometry are suppressed using model-based feedrate scheduling. Cutting force is monitored only at tool path check points, set typically at intervals of several dozen meters. Since it does not require continuous full-time monitoring of cutting forces, a “cheaper” estimation scheme of cutting forces can be potentially employed. Feedback control focuses only on a long-term point-to-point regulation of cutting force, targeting tool life providing cutting for the given desired distance. The effectiveness of the present approach is experimentally investigated by an application example to contour-parallel cutting of hardened steel

A machining test to calibrate rotary axis error motions of five-axis machine tools and its application to thermal deformation test

This paper proposes a machining test to parameterize error motions, or position-dependent geometric errors, of rotary axes in a five-axis machine tool. At the given set of angular positions of rotary axes, a square-shaped step is machined by a straight end mill. By measuring geometric errors of the finished test piece, the position and the orientation of rotary axis average lines (location errors), as well as position-dependent geometric errors of rotary axes, can be numerically identified based on the machine׳s kinematic model. Furthermore, by consequently performing the proposed machining test, one can quantitatively observe how error motions of rotary axes change due to thermal deformation induced mainly by spindle rotation. Experimental demonstration is presented