Yin-Lin Shen

Sampling strategy design for dimensional measurement of geometric features using coordinate measuring machine

Dimensional measurement using a coordinate measuring machine (CMM) has been commonly adapted in advanced manufacturing environments to ensure that manufacturing products have high quality and reliability. To conduct dimensional inspection effectively in a computer-integrated manufacturing (CIM) environment, there is an urgent need to derive a sampling strategy which can be used to specify a set of measuring points that lead to accurate sampling while minimizing the sampling time and cost. Owing to the variations in characteristics of geometric features and manufacturing processes, different feature surfaces on a workpiece usually have different variations in their dimensional accuracy and surface finish. The variations may differ considerably from one surface to another, even though those surfaces may share the same feature. Therefore, the variation in dimensional accuracy and surface finish should be considered in determining the proper sampling size for each geometric feature generated by various processes with different production parameters. In this paper, a feature-based methodology which integrates the Hammersley sequence and a stratified sampling method are developed to derive the sampling strategy for various geometric features which have specified measuring points. Case studies are used to compare the effectiveness of Hammersley sequence sampling, uniform (systematic) sampling and random sampling. The results show that the derived sampling strategy based on the Hammersley sequence leads to a nearly quadratic reduction in the number of samples compared with the uniform sampling method, and hence units of time and cost, while maintaining the same level of accuracy. The derived sampling strategy also shows a better performance when compared with the random sampling method.

Error compensation for CMM touch trigger probes

Abstract We present the analysis of a simple mechanical model of a common type of kinematic seat touch trigger probe widely used on modern coordinate measuring machines (CMMs). The model provides a quantitative description of the pretravel variation or “probe-lobing” characteristics that limit the use of such probes for high-accuracy dimensional measurements. We include the effects of stylus bending and the frictional interaction between the stylus ball and the part surface. The model is restricted to probes with simple straight styli, and we demonstrate significant error reduction both for vertically and horizontally oriented styli. In the latter case, gravitational forces are shown to play an important role in probe triggering and pretravel variation. Extensions to arbitrary orientations are discussed.

Uncertainties in the acquisition and utilization of coordinate frames in manufacturing systems

Summary Accurate and consistent transformations between design and manufacturing coordinate frames are essential for precision part production. These coordinate transformations establish surface position and orientation relationships as manufactured pans pass from design through production processes to inspection and surface error analysis. Unfortunately, these coordinate transformations can be uncertain due to factors such as geometric form errors in workpiece reference surfaces, fixture locating point variations, and coordinate measurement errors. A method for representing and estimating these uncertainties and their propagation through multi-stage manufacturing processes is presented in this paper. The method is applicable to systems with combinations of fixturing and coordinate measurements.

A Finite Element Approach to Transient Thermal Analysis of Work Rolls in Rolling Process

The three-dimensional transient thermal problem of work rolls in the entire rolling process has been formulated. It includes the time-varying boundary conditions specified at the roll surface taking the schedule of both rolling and idling cycles into consideration. The corresponding finite element equations are derived and solved by the Runge-Kutta-Verner method. The finite element solutions indicate that the temperature variations in the circumferential direction are overwhelming. Case studies unveil the thermal characteristics of the work rolls in various kinds of mill operations. The effects of the specific heat and the angular velocity of the work rolls are presented. Numerical results are compared with Guos analytical solutions.

An Uncertainty Analysis Method for Coordinate Referencing in Manufacturing Systems

Accurate and consistent transformations between design and manufacturing coordinate systems are essential for high quality part production. Fixturing and coordinate measurements are common coordinate referencing techniques which are used to locate points or measurement points on workpiece reference surface to establish these coordinate transformations. However, uncertainty sources such as geometric form deviations in workpiece surfaces, tolerances on fixture locators, and errors in coordinate measurements exist. A result is that coordinate transformations established using the locating and measurement points are in herently uncertain. An uncertainty analysis method for coordinate referencing is presented in this paper. The uncertainty interval concept is used to describe essential characteristics of uncertainty sources in coordinate referencing and coordinate transformation relationhips. The method is applied to estimating uncertainties in simple and compound coordinate transformation obtained using coordinate referencing in an experimental mold manufacturing system. Results of Monte Carlo simulations are used to show that the uncertainty analysis method gives a consistent and high percentage of coverage in evaluating coordinate referencing in the examples studied

An analytical assessment of measurement uncertainty in precision inspection and machine calibration

Dimensional inspection is commonly used to scrutinize the quality of manufactured products against the established standards and specifications. Meanwhile, dimensional measurement of an artifact is also commonly used as one of the methods for machine-performance calibration. However, the quality and reliability of many inspection and measurement processes are often contaminated by various uncertainties. Two prominent sources for measurement uncertainties are: (a) the imperfection of a measuring device, and (b) the dimensional deviation and geometric characteristics of a measured feature. Usually, the effects of both types of uncertainty are compounded by one another. To ensure the quality and reliability of any inspection process, measurement uncertainty needs to be addressed for all data acquisition activities. A method is also needed to identify and decouple the effect of compounded uncertainties. If this can be done, then the data collected can be properly adjusted and a more meaningful analysis result can be drawn. In this paper, the issues of uncertainty identification for machine calibration and dimension measurement using artifacts with various geometric features are discussed. Analytical models are derived first to identify and then decouple the compounded effect of both types of uncertainties. Finally, case studies are used to illustrate the procedures for both identifying and decoupling the compounded effect of the measurement uncertainties.

Comparison of Combinatorial Rules for Machine Error Budgets

Abstract An error budget is an analysis tool for the prediction and control of the total error of a machine system for which accuracy is an important measurement of performance. The error budget concept is commonly applied in designing precision machine tools and precision measuring machines and requires application of a combinatorial rule to assess total error which is contributed to by a number of individual error components. No single generally agreed upon combinatorial rule exists for predicting maximum told error that may result from many error components, and precision machine designers often apply empirical formula rules that have evolved from practical experience. Combinatorial rules based on the central limit theorem (CLT) in probability theory and uncertainty interval concept are described in this paper and compared to two empirical formula rules using two precision machine error source examples reported in the literature. It is shown using Monte Carlo simulations that the CLT combinatorial rules and the empirical formulas adequately predict maximum total errors. However, the CLT rules provides a more rigorous methodology than an empirical formula for predicting total error. Moreover, a confidence level associated with the estimated total errors can be specified for error budgeting in precision machine design.

Error compensation of coordinate measurements in computer-integrated manufacturing using neural networks

Abstract Coordinate measurements on computer numerically-controlled (CNC) machine tools and coordinate measuring machines are widely used in modern manufacturing settings. Switching probes, also called touch trigger probes, are the most widely used probe type in these dimensional measurement tasks. However, probe error exists and becomes one of the major error sources in todays coordinate measurement processes due to advances in machine accuracy and tighter part tolerances. In this paper, a three-hidden layer backpropagation neural network is designed and implemented to characterize probe pretravel pattern. The trained neural network can be used to perform on-line probe error compensation in coordinate measurements. Experimental data from a CMM are used to train the neural network and to show the prediction effectiveness of the approach.

Pretravel compensation for horizontally oriented touch trigger probes with straight styli

Abstract Touch trigger probes are devices through which coordinate measuring machines (CMMs) and machine tools collect dimensional measurement data in modern manufacturing environments. Advances in machine accuracy and tighter part tolerances leave probe error as one of the major error sources in these dimensional measurement processes. Pretravel accounts for the majority of touch trigger probe errors and is caused by bending deflection of the probe stylus shaft. A pretravel model for horizontally oriented touch trigger probes is described in this paper. A trigger force model based on the principle of mechanics is derived and used to develop the pretravel model, and the model accounts for bending deflection of the stylus shaft at the trigger instant. Pretravel is modeled as a function of probe design parameters (such as stylus length, tripod leg length, and stylus material) and probe operating parameters (such as spring force setting and probe approach direction). In applying the model to predict probe pretravel, only one model parameter needs to be calculated from 24 points of experimental probe data. In addition, the influence of probe approach speed on probe pretravel and model applicability has been investigated in this paper. The model is robust and can be implemented by means of a software correction module, which can be incorporated into machine systems.

Pretravel compensation for vertically oriented touch-trigger probes with straight styli

Abstract Touch-trigger probes are commonly used on coordinate measuring machines (CMMs) and machine tools for dimensional measurements in modern manufacturing environments. Pretravel is a highly repeatable error occurring in touch-trigger probe applications. Advances in machine accuracy and tighter part tolerances have resulted in pretravel being one of the major error sources in dimensional measurement processes on CMMs and machine tools. This paper presents a pretravel model for vertically oriented touch-trigger probes with straight styli. A triggering force model based on principles of mechanics was derived and used as the basis to develop the pretravel model. Experimental probing data were used to show that the model can effectively predict probe pretravel in various probe approach directions. In applying the model to predict probe pretravel, only one model parameter needs to be calculated from the probe calibration data. Furthermore, only 24 points of experimental probe calibration data are needed to identify the model parameter and to perform effectively the prediction. The pretravel model presented in this paper is robust and can be used to reduce probe pretravel in vertical probe applications. The pretravel model can be implemented by means of a software correction module which can be incorporated into machine systems.