On-machine Inspection Research Articles

An inspection path optimization of impeller for balancing efficiency and accuracy

On-machine inspection becomes a trend for obtaining the real shape of the workpiece in adaptive machining. Troubled by the overlapping structure of the impeller, and the positioning error from the rotary axes of the machine tool, it is a challenge to generate an on-machine inspection path to balance efficiency and accuracy. Reducing inspection path helps to reduce the occupied time of the machine tool. Thus, to let the stylus needn’t leave the impeller’s channel during the inspection, the accessible space of whole inspection path is addressed in path generation. In order to improve accuracy, the inspection error affected by the positioning errors of the rotary worktable is discussed. Furthermore, the error differences of the unidirectional and bidirectional rotations of the rotary axes are considered in path generation. By the stylus orientation optimization, the bidirectional rotations of rotary axes are prevented as much as possible. At last, the effectiveness of improving inspection efficiency and accuracy is verified by the impeller on-machine inspection. The inspection time can be reduced to less than 40% of traditional inspection, and the inspection error can be decreased 0.008 mm at least.

Inverse projected-fringe technique for measurement of dimensions and surface profile of axisymmetric objects

An inverse projected-fringe technique based on simulating the system of a projector-part-camera is proposed for an on-machine inspection of axisymmetric parts. The parts considered for this study are relatively large, have high specular surfaces and have sharp slopes. A computer simulation-based method and the CAD model of the part was used to produce the inverse fringes as well as to find an appropriate trace passing through the symmetrical axis of the part that offers the best signal to noise ratio (SNR). This technique measures the deviation from a master-part and provides an accuracy of better than 10 microns for the part with a radial dimension of about 10 cm. It is shown that the proposed technique improves the SNR and the repeatability of the system compared to the standard fringe projection technique, in particular for the areas with a steep slope.

Multi-parameter optimization of machining impeller surface based on the on-machine measuring technique

Selecting the optimal machining parameters for impeller surface is a challenging task in the automatic manufacturing industry, due to its free-form surface and deep-crooked flow channel. Existing experimental methods require lots of machining experiments and off-line tests, which may lead to high machining cost and low efficiency. This paper proposes a novel method of machining parameters optimization for an impeller based on the on-machine measuring technique. The absolute average error and standard deviation of the measured points are used to define the grey relational grade for reconstructing the objective function, and the complex problem of multi-objective optimization is simplified into a problem of single-objective optimization. Then, by comparing the values of the defined grey relational grade in a designed orthogonal experiment, the optimal combination of the machining parameters is obtained. The experiment-solving process of the objective function corresponds to the minimization of the used errors, which is advantageous to reducing the machining error. The proposed method is efficient and low-cost, since it does not require re-clamping the workpiece for off-line tests. Its effectiveness is verified by an on-machine inspection experiment of the impeller blade.

Path planning method for on-machine inspection of aerospace structures based on adjacent feature graph

Abstract On-Machine Inspection (OMI) system has been playing an important role in modern aeronautical manufacturing, owing to its high efficiency, great convenience, and low application cost. The inspection path planning of OMI, which aims to find an optimal path that traverses all inspection points without collisions and by costing as little time as possible, is a main bottleneck that limits the achievements of higher efficiency and less inspection time. Besides, the path planning for OMI in aerospace manufacturing faces new challenges, which are barely explored, because aerospace structures have unique characteristics of complex geometrical features, large-scale dimensions and high-precision processing requirements. Thus, this paper proposes a novel, easily-implemented and robust inspection path planning method to plan paths for OMI of aerospace structures based on the properties of aerospace structures. In order to lift the inspection efficiency, this method makes three improvements on the path planning. First, reorganize the inspection features based on the cluster technology. Second, construct the adjacent feature graph based on Voronoi Diagram to plan the path. Third, a search algorithm is designed to search the adjacent feature graph to decide the sequence of inspection features and a convex hull based algorithm is used to avoid collisions. The proposed method has been tested for several cases and solid experimental results have shown that these improvements take effects in path planning for OMI of aerospace structures and suited paths can be provided for the inspection.

Two-Side Laser Processing Method for Producing High Aspect Ratio Microholes

Laser microprocessing is a very attractive option for a growing number of industrial applications due to its intrinsic characteristics, such as high flexibility and process control and also capabilities for noncontact processing of a wide range of materials. However, there are some constrains that limit the applications of this technology, i.e., taper angles on sidewalls, edge quality, geometrical accuracy, and achievable aspect ratios of produced structures. To address these process limitations, a new method for two-side laser processing is proposed in this research. The method is described with a special focus on key enabling technologies for achieving high accuracy and repeatability in two-side laser drilling. The pilot implementation of the proposed processing configuration and technologies is discussed together with an in situ, on-machine inspection procedure to verify the achievable positional and geometrical accuracy. It is demonstrated that alignment accuracy better than 10 μm is achievable using this pilot two-side laser processing platform. In addition, the morphology of holes with circular and square cross sections produced with one-side laser drilling and the proposed method was compared in regard to achievable aspect ratios and holes' dimensional and geometrical accuracy and thus to make conclusions about its capabilities.

Interference-Free Inspection Path Generation for Impeller Blades Using an On-Machine Probe

Inspecting an impeller is a challenging task in manufacturing automation due to its free-form surface and overlapping and twisted blades. Existing inspection method by a Coordinate Measuring Machine is complex and time consuming, because of offline reclamping and manual operation. Automatic on-machine inspection without transferring and reclamping workpiece is a promising technique, and has found an extensive application in recent years. This paper proposes a novel method of interference-free inspection path generation for impeller blades using an on-machine probe. The accessibility cone is defined to search for the approach direction vectors of a probe cylinder, whereby the sizes of both cylinder rod and cylinder disk are considered. Then, the idea of area sweeping minimization is applied to construct an objective function from which required direction vectors of the probe cylinder in a smooth path are easily solved. The generated path can avoid the interference and collision between a moving probe cylinder with the overlapping blades, and guarantee stable speed and acceleration of the rotary motions of the five-axis machine tool. Its effectiveness is verified by the on-machine inspection experiments.

Improving the machining accuracy of thin-walled parts by online measuring and allowance compensation

Thin-walled parts are important structural parts in the aerospace industry. Two common problems encountered in milling are workpiece deformation and tool wear, mainly due to the low rigidity of thin-walled structures and the poor machinability of difficult-to-cut materials. In this paper, we propose a novel online measuring and allowance compensation method that can be applied to thin-walled parts to improve machining accuracy. Automatic on-machine inspection is used to measure a workpiece after rough/semi-finishing without transferring and reclamping the workpiece. Then, a compensation value is calculated to adaptively adjust the depth of cutting during finishing. The method does not require complex cutting force modeling or nonlinear calculation of workpiece-tool deflection and avoids the uneconomical monitoring of the cutting force, power, and torque of the machine and cutting tools and can be extended to engineering applications easily. Its effectiveness and adaptability is demonstrated by an orthogonal experiment in which the structure, cutting depth, and material are considered.

Strategies to realize decentralized manufacture through hybrid manufacturing platforms

Centralized factory dominance has led to optimization and set-up for manufacture of a sole product or product-range. In-house efficacy of such systems has been greatly improved due to lean six-sigma methodologies; however, wider scale distribution and supply has undergone little to no transformation over the past century. Research pertaining to distributed manufacture, cloud-manufacture and reconfigurable systems have provided several decentralized models using traditional manufacturing capabilities. Due to practical limitations they have not been fully realized to date. This paper outlines the emerging concept and scope for hybrid manufacturing platforms – systems using a plurality of processes on a single motion platform, to manufacture products in a decentralized network. Necessity of on-machine inspection is highlighted, outlining the potential of generic artifact machine calibration and verification of parts made. A robust experimental method using a novel hybrid manufacturing approach is used to realize the post-process adaptation of subsequent parts and demonstrates significant improvement with adapted features being cut within 15 μm of nominal position, compared to the original 300 μm error. Social, economic and environmental ramifications of adopting such a system are highlighted. The combination of several key processes will allow users to manufacture parts with minimal intervention. A move to personal fabrication would allow greater customization and convenience for the end user; however, the issues of copyright and loss of economy of scale would inhibit mass uptake in the near-future. Growing interest by enthusiasts and early adopters will continue to make an impact on the way the populace views manufacture.

Calibration based on part set-up measurement for on-machine inspection using vision

Integrating inspection procedures in the machining process contributes to process optimization. To be highly efficient, inspection operations must be performed in the same frame as machining operations. Within the context of on-machine inspection using vision, a method of calibration based on part set-up measurement is proposed in this paper. The measurement frame thus coincides with the CAD/CAM frame in which the CAD model is built. The CAD-based calibration is performed by using 3D features of the part set-up, which are easily identifiable in the 2D picture of the scene. The robustness of the method with regard to 2D feature identification in real environment is assessed thanks to Monte-Carlo simulations.

Realization of Online Measurement Technology and its Application in the Intelligent Machining Cell

In view of the continuous unmanned machining of revolving workpiece, this paper has integrated the industrial robot with machine vision, mill-turning center and trigger online measurement systems into an intelligent machining cell. It has achieved the automatic loading and unloading material, automatic adjustment of the Machine Coordinate System (MCS) of workblank, automatic compensation of machining errors and workpiece diameter and roundness of the on-machine inspection. In order to ensure the accuracy of continuous unmanned machining of the intelligent machining cell, the main factors that affect the accuracy of online measurement system and the corresponding solutions are analyzed. On this basis, a machining instance is made to verify the validity of the trigger online measurement technique’s application in continuous unmanned machining of the revolving workpiece. The results of this study show that the intelligent machining cell can ensure stable unmanned machining with high precision for several hours.

In-situ Inspection Error Compensation for Machining Accuracy Improvement of Complex Components

In order to improve in-situ inspection accuracy of complex surface components, error factors influencing the inspection accuracy of the on-machine inspection system are discussed and error compensation methods are studied. The main errors of the inspection system are analyzed which contain geometric error of machine tools, pre-travel error of the inspection system and radius error of measuring probe. The geometric error model of a three-axis numerical control machine tool is established and each individual error value is identified and obtained through laser interferometer. For the pre-travel error of the inspection system, a radial basis function algorithm based prediction method is proposed and implemented. Based on the distribution of the pre-travel probe error obtained, an real-time compensation of the error can be performed during the actual inspection process. Considering the probe radius and its compensation of the contact-type measuring system4, a new algorithm is developed to improve the calculation accuracy of the normal vector which will affect the compensation accuracy. To verify the effectiveness of the proposed method, a complex part is exampled. Through the comparison of the in-situ inspection result with that of the CMM measurement, the proposed inspection system can achieve a great improvement of the inspection accuracy.

A Method for Measuring the Systematic Errors of CNC-Machine Tools

The method used for measurement and calibration of machine tool errors should be general and efficient. With this method, the machine tool status can be completely identified and its accuracy can be enhanced by software error compensation. The point compensation method can be used as a means for modifying the nominal tool path and on-machine inspection where the machine tool is used as a coordinate measuring machine. The validity of the error calibration method proposed in this' paper was shown using a vertical 3-axis CNC machine with a laser interferometer and a ball bar technique.

Calibration Method for On-Machine Composite Inspection of Machining Center

For overcoming the problems of traditional on-machine inspection only depended on the touch probe, an on-machine composite inspection plan of machining center is put forward, and its composition and work principle are introduced. Based on the non-contact and contact inspection principle, the composite measuring head is designed, by which the inspection for the workpiece with high efficiency can be achieved. Aim at inspecting the workpiece machined accurately, different inspection models must be built, and the calibration method for composite inspection is presented. Finally, experiment verification is realized for the calibration method.

A Method for Enhancing the Accuracy of On-Machine Inspection for Five-Axis CNC Machines

The efficiency of manufacturing structural parts can be improved by using on-machine inspection system. It is difficult to separate the valid information from hundreds of points and correct the errors by rotating and translating the workpiece coordinate system directly, especially for five-axis CNC machines. In this paper, a practical method is designed to correct errors easily. By using the least square method, an error reference frame is constructed, and the relation between the compensation values and coordinates of the theoretical points is derived. An example of aircraft structural parts is presented to validate the design.

Integration of machining and inspection

Fully automated machining would require automatic inspection process planning and real-time inspection result feedback. As inspection process planning is still based on G&M codes containing low level information or vendor-specific bespoke routines, inspection process planning is mostly isolated from machining process planning. With the development of new data model standards such as STEP and STEP-NC providing high level product information for the entire manufacturing chain, it is conceivable that both machining and inspection process planning are considered hand-in-hand to generate optimal machining and inspection sequences with real-time measurement result feedback. This paper introduces an integrated process planning system architecture for combined machining and inspection. On-machine inspection (OMI) is chosen to carry out inspection operations in-between machining operations and to provide real-time feedback to process planning. The system is able to automatically insert OMI operations for online measurement. A case study testified the feasibility of the proposed architecture.

Inspection path planning of on-machine vision inspection for CNC milling machines

Inspection path planning of on-machine vision inspection for CNC milling machines

On-Machine Inspection Method for Superduty Lathe Based on Improved Laser Triangulation

In the development and application of superduty lathe process, the size measurement of a large workpiece is a key issue of quality control because the cutting process of large workpiece is high linear speed and time-consuming . A novel surface position information acquisition method for rotational object is proposed. The method uses high speed linear CCD to acquire image sequences, adopts image restoration technique to resolve clear images of laser spot from the image sequences, identifies sub-pixel center point of laser spot with correlation information among the clear frame-to-frame images, then resolves surface position information of rotational object with triangulation principle. Therefore, defects of existing laser triangulation for high-speed rotational object measurement are overcome. According to the resolved surface position information of rotational workpiece, the stability of accuracy, tool wear and vibration characteristics of superduty lathe can be evaluated.

A study on automatic on-machine inspection system for 3D modeling and measurement of cutting tools

A 3D model of the maximum rotating envelope of a milling cutter with tool holder is required for Computer Aided Manufacturing (CAM) process design and machining simulation. The user may define the 3D model of the whole tool assembly in the tool library of CAM software. However, it is not convenient and reliable. Considering these problems, a new method based on single view 3D reconstruction algorithm has been proposed in previous research work, which is able to quickly reconstruct the 3D model of a cutter with tool holder while they are installed onto the spindle. As the extension of this work, this paper focuses on the recent progresses in order to improve the automation, accuracy, efficiency and reliability of tool modeling system. First, an improved flexible on-machine camera calibration procedure is proposed. The accurate motion of machine tool axis is used to calibrate the camera on machine tool instead of a physical calibration board. The whole procedure of calibration can be conducted automatically by running NC code. Therefore, the automation of vision system can be guaranteed. Second, the contour extraction module is improved by using a method of silhouette image composition. This method is applied to solve the problem of translucent and fuzzy cutter profile induced by motion blur. Third, the new algorithm for contour partitioning and classification are proposed, which is more reliable and robust. The reliability and accuracy of the vision system can be guaranteed. Finally, the vision system with an 8 mm lens and 1 mm extensions has been tested on different type of machine tool with smaller cutters. The average measurement accuracy is about 35 microns verified by comparison with a commercial tool setting system.

Compound Micro/Nano Machining – A Tool-Based Innovative and Integrated Approach

Compound micro-machining is the most promising technology for the production of miniaturized parts and this technology is becoming more and more important and popular because of growing demand for industrial products with not only increased number of functions but also of reduced dimensions, higher dimensional accuracy and better surface finish. In this paper, the development efforts in micro/nano-machining based on solid tools (tool-based micro/nano-machining) in NUS are introduced. In order to achieve meaningful implementation of micro-machining techniques, this research seeks to address four important areas; namely (a) development of machine tool capable to do both conventional micro-machining, (b) process control, (c) process development to achieve necessary accuracy and quality, and (d) on-machine measurement and inspection. An integrated effort in these areas has resulted in successful fabrication of micro-structures that is able to meet the miniaturization demands of the industry. In the area of nano-machining machine tool and process developments have also been carried out for electrolytic in-process dressing (ELID) grinding and ultra precision machining using single point and poly crystalline diamond tools to produce nano surface finish on hard and brittle materials. An ultra-precision diamond turning machine has been developed which incorporates a fast and fine tool servo system to produce nano-precision surfaces and features.

Study on On-Machine Inspection Technology of Machining Center Based

With the development of digitalized manufacturing (DM), on-machine inspection (OMI) has become the key link of integrated quality system in DM. In DM, The detection and monitoring system chiefly dealing with kinds of parameters in machining process, taking on-line detection, analyzing the process quality information and carrying out corresponding quality assurance, so the quality of product may be assured and DM can be run in good condition. Therefore, the OMI technology of machining center based is researched. The framework of OMI and the hardware structure of special driving card for probe or tool (SDCPT) are described. Computer aided inspection planning (CAIP) is the integration bridge between CAD/CAM and computer aided inspection. A feature-based CAIP system for OMI is proposed to inspect the complicated mechanical parts efficiently during machining or after machining. The CAIP system is designed to produce an inspection process planning directly from CAD model. The prototype CAIP system includes the tolerance feature analysis, accessibility analysis, feature identification, measurement planning, and collision detection. The CAIP system can minimize the number of part setups and probe orientations and the inspection feature sequence.