The role of inspection in automated manufacturing

Computers ind. Engng Vol. 17, Nos 1-4, pp. 327-332, 1989 Printed in Great Britain. All rights reserved 0360-8352/89 $3.00+0.00 Copyright © 1989 Pergamon Press plc THE ROLE OF INSPECHON IN AUTOMATED MANUFACIXJRING Ahmad K. Elshennawy Department of Industrial Engineering & Management Systems University of Central Florida Orlando, Florida 32816 ABSTRACT This paper discusses the role of inspection in automated manufacturing. Also presented is an introduction to computerized coordinate measuring machines and their performance, since these constitute the most advanced inspection equipment available to industry today. INTRODUCTION During the past decade, numerous advances have been made in automating manufacturing systems. Among such advances is the integration of computer numerically-controlled (CNC) machine tools into systems with central computer control, automated inspection station, and automated material handling systems. Such systems, called Flexible Manufacturing Systems (FMS), can demonstrate higher productivity and programming capability of producing large mix of similar products. Recent advances in automated manufacturing impose new responsibilities for the quality control (QC) functions in the manufacturing facility. Because of an ever increasing set of demands by customers, such responsibilities represent challenges on the quality control function. The following represent some of such responsibilities: o o o o o o o Design for quality assurance Automated inspection techniques In-process inspection and on-line sensing Flexible inspection systems (FIS) Integrated quality control systems Statistical process control Software error correction and accuracy enhancement of measuring machines THE ROLE OF INSPECHON IN AUTOMATED MANUFACTURING Inspection has been traditionally performed after the product leaves the production floor using conventional gauging techniques. In the automated manufacturing environment where a constant demand for increased productivity is coupled with a growing demand for higher quality, a proliferation of computerized and automated inspection techniques has been a requirement. The objective of the inspection as a quality assurance is to provide continuous information and feedback to keep the manufacturing process in control. Integrated quality control systems and flexible inspection systems (FIS) provide the essential ingredients for the integration of quality assurance in automated manufacturing. Such systems are capable of providing continuous monitoring and control of machines performance and provide feedback requests for process adjustments. Flexible inspection systems (FIS) can be defined in the same way we define flexible manufacturing system (FMS). An FIS uses a number of computer controlled measuring machines - usually called coordinate measuring machines (CMMs) - to automatically load, inspect, and unload parts. Robots and automated material handling systems are also used to perform functions such as loading, unloading, and storage. All components are controlled by a cemral computer. In automated manufacturing, inspection can be performed in many ways, such as: Deterministic metrology, in which different parameters of the process are monitored to provide a feedback to control the machine performance. Functions supporting deterministic metrology may include: a. b. c. d. e. Software error correction for geometric and kinematic errors. On-line sensing of surface finish. Tool wear sensing On-machine probing computation of fixturing and clamping forces. 327 328 Proceedings of the 1lth Annual Conference on Computers & Industrial Engineering 2. In-process inspection, in which the workpiece characteristics are measured and defined during actual cutting. 3. Post-process inspection, in which the workpiece characteristics are measured and defined after cutting but before the workpiece leave the machine tool. 4. Inspection performed in a separate inspection laboratory where automated equipment are located for that purpose. Statistical Process Control (SPC) is a task of gathering data about the process, analyzing such data with respect to preset standards, detect any variability present in the process parameters and provide feedback to keep the process in control. In automated manufacturing, such procedure can be performed on location and may be integrated into the system. The ideal integration of quality control into automated manufacturing systems can be achieved by combining the automation of the inspection process with computerized quality information and analysis system. The best candidate for such integration that is available to industry today is the computercontrolled coordinate measuring machines (CMMs). COORDINATE MEASURING MACHINES Over the last decade, coordinate measuring machines have become a primary means of dimensional quality control for manufactured parts of complex form where the volume of production does not warrant the development of functional gaging. The advent of increasingly inexpensive computing power and more fully integrated manufacturing systems will continue to expand the use of these machines into an even larger role in the overall quality assurance of manufactured parts. Coordinate Measuring Machines (CMMs) can most easily be defined as physical representations of a threedimensional rectilinear coordinate system. Coordinate measuring machines now represent a significant fraction of the measuring equipment used for defining the geometry of workpieees of different shapes. Most dimensional characteristics of many parts can be measured within minutes with these machines. Similar measurements would take hours using older measuring equipment and procedures. Besides flexibility and speed, coordinate measuring machines have several additional advantages: 1. Different features of a part can be measured in one setup. This eliminates errors introduced due to setup changes. 2. All CMM measurements are taken from one geometrically fixed measuring system, eliminating the accumulation of errors resulted from using functional gaging and transfer techniques. 3. The use of digital readouts eliminates the necessity for the interpretation of readings such as with the dial or vernier-type measuring scales. 4. Minimum operator influence with the use of automatic data recording available on most CMMs. 5. Part alignment and setup procedures are greatly simplified by using software supplied with computerassisted CMMs. This minimizes the setup time for measurement. Accuracy Enhancement To achieve optimum performance of a machine, there are two major approaches: 1. Eliminating the source of error. This has to be done during the planning, design, construction, and inspection stages of the machine. Better design, proper adjustments and environmental control are the basic requirements. In practice, these requirements can be met only up to a certain level. 2. Correcting the effect of design and construction errors on machine performance. To do this, cause and effect relationship must be carefully established and a complete understanding of the factors (effects) influencing the machine performance achieved before developing the appropriate techniques for error correction. Figure 1 shows a procedure for error compensation. Discussion of these approaches is outside the scope of this paper. Elshennawy: Inspection in automated manufacturing i Meleu¢lnoM a c h i n e 8tructure Define M a c h i n e R e p r e e a n t e t l o n ] 329 Mealuremenl I I Measuring Machine Data I Mathematic]al ! Measuring Standard (e.g. Laser) Data t Model 1 t Izcqulzltlon, i Computer 8 o | l w a r e for d a t a printing, etc. - I I Figure i. , '1 I E Ir°t I dlagnoetlca Procedure for Error Compensation Factors Affeetin~ Machine Performance The performance of measuring machines and machine tools can be defined as the ability of the machine to perform its functions effectively under a specified range of operating conditions. Machine performance is affected by different sources of error that include geometric errors, kinematic errors, thermal distortions, dynamic and static errors, workpiece errors, and probe-workpiece interaction. Evaluation of these errors is necessary for evaluating the overall machine performance. It is very important to define and understand each error before sensing or measuring its effects. Results of measurements can then be analyzed and interpreted to achieve a reasonably complete appraisal of the machine. Geometric accuracy of the machine can be affected by several factors, such as: 1. Errors in the form of shape of different machine components such as tables, guideways, etc. 2. Mechanical wear of linkages and joints which introduce undesirable effects such as lack of straightness and squareness, inadequate motion, etc. 3. Thermally induced errors due to variations in the operating environment which will cause structural changes. 4. Weight deformations caused by the weight of the machine structure or the part being machined or measured. 5. Errors in the control or measuring system which can be thermal, mechanical or electrical in nature. Thermal effects are critical. Any change in the machine thermal stability or its control system, environment, or other specified conditions will affect the machine performance. Bryan [2] provided a useful thermal effects diagram that is shown in Figure 2. The most troublesome are the thermal effects of heat generated within the machine itself and the effects of changes in environmental temperature. Kinematic behavior of the machine is concerned with the relative motion of different moving machine elements which are intended to move in accordance with specified functional requirements. Coordination of motions with respect to each other is important for defining the positioning accuracy of a machine moving element. Any disturbance to this coordination will affect the performance of the machine and create errors. Errors in coordinate measuring machines are not only caused by errors in the machine construction, thermal effects, and environmental conditions, but there are also errors associated with dynamic operation. The accuracy of measurement of a workpiece on a coordinate measuring machine is determined by the deviations of the probing point from the required position or path. In this case, as coordinate measuring machines are assemblies of individual machine elements, the behavior of these elements under dynamic operation can cause relative motions which will result in deviating the probing point from its required position. 330 Proceedings of the l lth Annual Conference on Computers & Industrial Engineering ROOM I I COOLANTS ENVIRONMENT I ]Cutting fluid / Lube oll IJ Framestabilizing J Hydraulic I lcu,.o PROCESS[ MACHINE Electronics Mechanical friction Hydraulic friction Motors A transducers Electrical Frame stabilization oli J heaters '~ I HEAT FLOW CONVECTION CONDUCTION ¢ ÷ I Uniform timp, other thin 68 F I RADIATION + >[ [ Temg. oradients| or .torte effect I l ) ¢,,....., / I ,rein ,reviou. / /I environment• I'Temp. variation 1 l or dynemio ettect~ I lI J Non-uniform L tempereture [ I Sir, J I Miatirl ! j' Meohi.elfrem. I I o.*'.',`° S''/ I O.o..tr,o Stz' 1 I ~..metr,o Size I t error erro¢~ I ~ e r r ° r error I | I TOTAL .THERMAL ERROR t error errorj i [ Figure 2. Thermal Effects Diagram [2] In most cases, coordinate measuring machines have similar dynamic errors as machine tools, particularly CNC machine tools or machining centers. The interaction between a coordinate measuring machine probe and the measured workpiece may have a considerable effect. Dynamic errors resulting from the motion of a machine tool table are similar to those resulting from the motion a fixed bridge coordinate measuring machine for example. Vibrations sLenificantly reduce the resulting accuracy of the machine. When these vibrations are transmitted to the machine, the overall accuracy and repeatability of the machine deteriorates by the effect of a relative movement between the probe and the workpiece. The part material can be a problem to both the machining of accurate parts and to their measurements. The reduced stresses resulting from pre-processing operations can actually be greater than the strength of the material itself. This significantly degrades the resultant accuracy. Mechanical characteristics of the workpiece material such as hardness, roughness, etc. may lead to errors in measurement. In contacting the workpiece under pressure condition using a hard probe on coordinate measuring machines, hard and soft workpieces will respond differently and this may lead to errors in measurement. Workpiece material stabd'"lty and impurities in its structure will also affect the resulting accuracy of measurement. Pefforman~ Evaluation of CMMs As coordinate measuring machines has evolved into a widely used substitute for conventional and gaging equipment, a uniform methodology has been developed for the accuracy specifications of such machines and for providing these specifications to buyers and sellers. An effort by ASME B89.1.12 subcommittee on coordinate measuri~ machines [3] has resulted in a standard whose primary purpose is to "clarify the performance evaluation of coordinate measuring machines." The standard establishes requirements and methods for specifying and testing the performance of coordinate measuring machines (CMMs) having three linear axes perpendicular to each other. According to the standard, three main objectives are sought: 1. Clarifying the performance evaluation of coordinate measuring machines. Elshennawy: Inspection in automated manufacturing 331 2. Facilitating performance comparisons among machines by unifying terminology, general machine classification, and the treatment of environmental effects. 3. Defining the simplest testing methods capable of yielding adequate results for the majority of CMMs. INTEGRATED QUALITY CONTROL SYSTEM The proposed system include three elements. Each element is related to the three stages that a machined part goes through in its manufacturing cycle. These elements are: incoming material inspection, in-process control, and finished part inspection or post-process control. In each element, certain characteristics of the workpiece or the machining process are measured and corrective actions are taken if necessary. Incoming Material Inspection This element should be capable of detecting material properties and its dimensions. The system should work on go/no-go basis where rejected material will not be permitted into the manufacturing area. The measured dimensions should be automatically compared to a model of the part which can be stored in the plant's CAD data base. In-Process Control Machined part accuracy is affected by many factors such as machine tool geometric errors, thermally induced errors, tooling quality, dynamic errors, and machining conditions. In-process control may include real-time sensing and process-intermittent gauging. Real-time sensing refers to those measurements that are made while the part is being machined. Quantities that may be sensed are the thermal state of the machine, the vibration level, and cutting forces. Thermal sensing is performed using arrays of thermocouples placed at strategic positions on each machine. The data may be combined with model of the machine distortions as a function of temperature to yield a calculation of the tool path error as a function of thermal state. Process-intermittent gauging refers to measurements performed on the workpiece while it is fixtured on the machine tool. This primarily means measurements taken between machining steps but also include measurements taken either during set-up or after machining, just before release of the part to the fixture. Measurement performed at this stage may include part dimension and form, part orientation, tool setting, and surface roughness [4]. Process-intermittent part measurement can be performed using touch probes on the machine tool. Such measurement enables the subsequent correction of tool path errors. Finished Part Inspection Finished part inspection, also called post-process gauging, can be accomplished with an error-corrected coordinate measuring machine (CMM) for the measurement of part dimension and form. A stylus type surface roughness measuring instrument can be used for surface texture measurement. The post-process gauging system will serve as a metrological anchor for the real-time sensing and processintermittent gauging procedures. Post-process gauging serves two main functions: certification and process monitoring. Both functions involve testing of the part against design tolerances. The data gathered for certification are attributes of the part itself whereas the process monitoring information is associated with the machining systems and with previous parts manufactured on them [4]. Important considerations for certification are the design tolerances, the measurement algorithm, and part sampling. The design tolerances must be accurately and realistically defined so that the part meets its functional purpose without excessive manufacturing expense. Measurement algorithm must be developed that adequately test those functional tolerances. Since measurements are taken by a CMM, the number of data points that must be taken for adequate testing against each tolerance is a crucial question. Part sampling refers to the question of whether to measure every part that is produced in a batch [4]. Process monitoring functions include monitoring for control and process diagnosis when measured quality characteristics appear to be in the state of out-of-control. These functions require the development of process control technique that include control charts and scatter plots. All of the process monitoring functions should be made accessible on nser-friendly terminal to serve as the quality monitor. The above describes the different technologies and procedures that have to be developed as part of a total quality system in automated manufacturing. Integration data supplied by each subsystem into a control system is necessary for presenting information and updating strategies as necessary. Data from each of these quality control subsystems go to a control system and provide user-friendly outputs for manufacturing managers. Outputs may include information about machine variability, process capability analysis for each process, and other useful statistical information. 332 Proceedings of the 1lth Annual Conference on Computers & Industrial Engineering CONCLUSIONS Because of the recent advances in automated manufacturing efforts, more responsibilities are imposed on the quality control function such as the development of flexible inspection systems and a built-in quality control system. A review of these issues is provided. Also presented is an introduction to coordinate measuring machines and their performance since they represent the most advanced inspection equipment for quality control that is available to industry today. BIBLIOGRAPHY 1. McKeown, P., High Precision Manufacturing in an Advanced Industrial Economy, 4th International Precision Engineering Seminar, Cranfield Institute of Technology, UK, 11-14 May 1987. 2. Bryan, J., Personal Communications, NBS, Gaithersburg, Maryland, 1986. 3. ANSI B89.1.12 Committee, "Proposed Standard for Performance Evaluation of Coordinate Measuring Machines," October 1983. 4. Vorburger, T.V., Private Communications, NIST (formerly NBS), 1988. 5. ANSI B46.1 - 1985, Surface Texture, Roughness, Waviness, and Lay, ASME, New York, 1985. 6. Hocken, R. J., '~Technology of Machine Tools, Vol. 5: Machine Tool Accuracy," Report of the Machine Tool Task Force, UCRL-52960-5, 1980. 7. Elshennawy, A.K., Performance Evaluation of Coordinate Measuring Machines, Ph.D. Dissertation, Pennsylvania State University, University Park, PA, 1987. 8. Stout, K., Quality Control in Automation, Prentice-Hall, 1985. 9. Donmez, M.A., et al., A General Methodology for Machine Tool Accuracy Enhancement by Error Compensation, Precision Engineering, 1986. AUTHOR BIOGRAPHY Dr. Abroad K. Elshennawy is an Assistant Professor in the Department of Industrial Engineering at UCF. He holds a BS and MS in Production Engineering from the University of Alexandria in Egypt and M.Eng. and Ph.D. in Industrial Engineering from Penn State University. His teaching and research interests are in the areas of manufacturing engineering, quality assurance, and reliability engineering. He is a member of ASQC, SME, IIE, AND ASEE.