Search

We propose a technique to measure position-independent geometric errors (PIGEs) in the rotary and spindle axes of a hybrid parallel kinematic machine (PKM). The PKM investigated here includes one more rotary axis than an Exechon PKM, which is used to improve the productivity of hybrid processes, such as machining and direct-energy-deposition three-dimensional metal printing. Errors in the measured position and orientation of the rotary axis, and the orientation of the spindle axis produce volumetric errors in the processed workpiece. If accuracy is to be improved, the deviation of each axis must be measured and compensated. In our approach, errors are measured using three methodologies that require only control of the rotary axis: in the first, no offset is applied to account for positional deviation of the rotary axis; in the second, an offset is used to correct the orientation of the rotary axis; and in the third, a tool offset is used to correct the orientation of the spindle axis. We developed an algorithm that uses the three measured datasets to identify PIGEs. The proposed method was applied to a hybrid PKM and the PIGEs were measured and compensated. This technique uses simple measurement paths and sequential measurements to correct rotary and spindle axis errors, and could therefore be widely used in industry.

This study proposes an optimal on-machine measurement method to measure position-independent geometric errors of five-axis machines, including machine tools and coordinate measuring machines, with a universal head. This measurement requires only a calibrated three-dimensional (3-D) probe and a precision sphere, which are fundamental components of on-machine measurement systems in general, to minimize measurement costs, including operator effort and operating time. Eight position-independent geometric errors are used to describe the coordinate systems of the two rotary axes through a kinematic analysis. The center of the precision sphere, which is stationary during the measurement, is measured at various angles of the rotary axes. Then, the linearized relationship between the measured positions of the precision sphere and the geometric errors is derived using an error synthesis model under rigid-body and small-value assumptions. The proposed method is tested through a simulation for validation. The geometric errors of a five-axis coordinate measuring machine with a universal head are measured with measurement uncertainty by applying the proposed method. Then, the measurement results are confirmed by comparing the measured positions of the precision sphere and calculated positions using the measured geometric errors at the other (i.e., the arbitrary, or not measured) angles of rotary axes.

Circular tests to determine the circular/spherical deviation of machine tools based on double ball-bar measurements are proposed and applied to machine tools for accurate performance evaluation. In current circular tests, eccentricity is mathematically removed from the measured data to evaluate performance. However, eccentricity is affected not only by set-up error of the double ball-bar, which is trivial, but also by geometric error, which this test is intended to determine. The relationship between eccentricity and geometric error was investigated during circular deviation measurements using a simple example. The results demonstrated that the measured data must be evaluated as they are, without any mathematical modifications. Additionally, the circular test must be performed while ensuring negligible set-up error. The proposed approach was applied to three machine tools to investigate the effect of eccentricity when circular/spherical deviation appears.

In this study, the performance of five-DOF motion in ultra-precision linear stage is evaluated accurately by extending the application of the ISO 230-2 International Standard, which is focused only on the linear displacement motion of a linear stage. The bidirectional accuracy and bidirectional repeatability of positioning in five-DOF motion are calculated by measured geometric errors. Five geometric errors except for the linear displacement error of a linear stage are measured simultaneously using the optimal measurement system, which is designed to enhance the standard uncertainty of the estimated geometric errors. The geometric errors are in good agreement with those from the laser interferometer. In addition, the confidence intervals of the performances are determined by the uncertainties of the equipment used in the experiment.

In this study, we propose a robust and simple method using double ball-bar to measure position-independent geometric errors of a rotary axis involving single axis control during the measurement. The standard uncertainty for the proposed method is analyzed to quantify the confidence interval of the measurement result. Two measurement paths are planned to measure the position-independent geometric errors, including two offset errors and two squareness errors of a rotary axis. An error synthesis model using homogenous transform matrices and a ball-bar equation to represent the relation between the positions of two balls and the measured distance between them are used. Set-up errors, which are inevitable during the installation of the balls, are modeled as constants and added to the design position of the balls. Their effects on the measurement result are investigated in detail. Furthermore, a novel fixture consisting of flexure-hinges located at the tool nose is developed to minimize the set-up errors of the ball and to robustly keep the position of the ball during measurements. Finally, the proposed method is validated using simulation and is applied to the rotary axis located on a five-axis machine tool.

Joint kinematics and contact mechanics dictate the success of current total knee replacement (TKR) devices. Computational contact prediction is a feasible way of evaluating new TKR designs prior to physical testing and implementation. Previous finite element (FE) knee models have generally been used to predict stresses on contact areas and/or areas subjected to static or quasi-static loading. Explicit dynamic FE analyses have recently been used to effectively predict TKR kinematics and contact mechanics during dynamic loading conditions. In this study, we compared the functional load transmission and kinematic performance of two posterior-stabilized designs, standard and post-cam TKR versions, over a standardized loading cycle using three-dimensional contact finite element analysis. Our objective was to develop and experimentally validate an explicit FE TKR model that incorporates femoral-bearing articulations. Finite element-based computational contact pressure predictions were applied to gait cycles using both force-controlled and displacement-controlled inputs. A standard prosthesis showed a reduction in contact pressure compared with post-cam prosthesis components, as it redistributed the knee motion to two articulating interfaces with more linear motions at each interface. In this FE analysis, the wear of TKR bearings was dependent on kinematics at the articulating surfaces and on prosthesis design.

Geometric errors are one of the primary potential sources of error in a five-axis machine tool. There are two types of geometric errors: position-dependent geometric errors and position-independent geometric errors. A method is proposed to identify and measure the position-independent geometric errors of a five-axis machine tool with a tilting head by means of simultaneous multi-axis controlled movements using a double-ball bar (DBB). Techniques for identifying position-independent geometric errors have been proposed by other researchers. However, most of these are based on the assumption that position-dependent geometric errors (such as linear displacement, straightness, and angular errors) are eliminated by compensation, once the position-independent geometric errors have been identified. The approach suggested in this paper takes into account the effect of position-dependent geometric errors. The position-dependent geometric errors are first defined. Path generation for circle tests with two or three simultaneous control movements is then carried out to measure the position-independent geometric errors. Finally, simulations and experiments are conducted to confirm the validity of the proposed method. The simulation results show that the proposed method is sufficient to accurately identify position-independent geometric errors. The experimental results indicate that the technique can be used to identify the position-independent errors of a five-axis machine tool with a tilting head.