Data collection system and online variable control chart for diameter measurement

Transcription

1 Data collection system and online variable control chart for diameter measurement Mohammad Iqbal and Nur Zulaikhah Nadzri Universiti Kuala Lumpur Campus UniKL_MFI, Section 14, Jalan Teras Jernang, Bandar Baru Bangi, Malaysia Mohammad Zahid Mohd. Syafiq Hilmi Abstract - The application of data collection system and variable control chart for diameter measurement are vital in the field of manufacturing and production system. They are used to monitor diameter value as one of product s quality characteristic. Traditional methods currently used in factories are not effective as they are time consuming. It requires workers to take diameter dimension from random parts of the product and the control chart would be produced manually based on data collected. Since time and quality are crucial in the manufacturing industry the methods need to be improved. This project was specifically developed in order to solve this problem. The diameter dimension of a particular product was measured using a digital dial indicator that was connected to a computer. Once reading was detected by the instrument, the reading would be displayed on the computer through a table, and the control chart would be generated simultaneously using Minitab software. Therefore the time taken to record and construct the chart will be shorter. Furthermore, the proposed method makes a possibility of providing a real time process control monitoring. This because the control chart will be updated according to any single data collected from the production floor. Keywords: Automatic, inspection, diameter, control-chart, process-capability. 1. INTRODUCTION Diameter is one of the basic circular dimensions of cylindrical machine components. In manufacturing processes, variation on diameter dimensions may occur due to imperfect rotation, erratic cutting action, inadequate lubrication, tool wear, defective machine parts, chatter, misalignment of chuck jaws, etc (Chang and Lim 1993). The out of diameter dimension of cylindrical parts can greatly affect the accuracy of assemblies. Therefore there is a requirement to develop an efficient inspection system that will satisfy the need of diameter inspection. Base on the observation during research and consultancy activities in past view years, the authors found various diameter measurement methods were applied in manufacturing industries in Malaysia. The methods were purely manual or fully automatic. Manual method will lead to high error and time consuming, at contrast, the fully automatic method (i.e. the using of Coordinate Measuring Machine) requires a high investment and maintenance cost. This situation requires a measurement system that can provide automatic data collection and statistical control analysis. Less involvement of human operator is required by the system, but the cost of the system is still in the economic level of investment and maintenance. Automatic inspection is desirable because human inspectors are not always consistent in their product assessments (Prieto et al. 2002). Furthermore, modern industries involve numerous situations that require simultaneous monitoring and control of inspection activity such as data collection and statistical analysis (Lyu and Chen 2009). The using of latest technology as well as mathematical modeling and computer program could be an alternative solution to fulfill the above requirement. 1

2 (a) (b) (c) Figure 1: Components of the system; (a) digital dial indicator, (b) dogmatic guage, (c) computer and Mini-Tab software This project aims to develop an automatic inspection system for the measurement of diameter dimension of cylindrical parts. The data will be collected by using a digital sensor and sent automatically to computer system. An algorithm is developed in computer environment for data analysis and related calculation. As the result, the system will produce variable control chart and process capability report. 2. SYSTEM DESIGN Components of the system are shown in Figure 1. It consists of a digital dial indicator as measurement tool, a digimatic gauge as data transfer device and a computer with Mini-Tab software as data processor. The measurement is provided by direct contact between the dial indicator s contact point with the surface of the object. The reading from the digital dial indicator then being sent to the computer through the digimatic gauge. Base on the input data, control charts are then developed by using Mini-Tab software. The measurement mechanism is shown is Figure 2. The dial indicator is held by a magnetic stand that will stay at a fix location during the measurement process. However, it can be removed to provide the measurement at other location of the object. A partition is built align with the measured object as reference surface of the measurement. The magnetic stand will be moved along this reference surface to provide different locations of measurement. The cylindrical object is supported by two Vee-blocks, one at each end. Holding the object in Vee-blocks for diameter inspection by mean of indicator instrument is widely used in general shop practice (Curtis and Farago, 2007). There are various functional reasons and aspects of convenience for the using this method, as follow: - From a functional point of view, it can be an effective method of inspection for the case of nonsymmetrical out of roundness - It offers a convenient way for holding round objects by gravity - The object s axis is confined to the bisecting plane of the Vee-block, thus producing automatically controlled staging locations. Vee-block Dial indicator dial indicator measured object magnetic stand reference surface Figure 2: The measurement mechanism 2

3 limited number of samples gives a significant error in prediction of standard deviation of population. Process capability refers to uniformity of the process. The estimate of process capability may be in the form of probability distribution having a specified shape, center (mean), and spread (standard deviation). Alternatively, Process capability might be expressed as a percentage outside of specification. 3. EXPERIMENTAL STUDY A set of experimental study has been conducted to verify the capability of system. Round bar aluminium was cut by using 2 axis LS10-HAAS CNC lathe machines to produce cylindrical object. Material dimension was 40 mm diameter and 300 mm length. A single step rough cut is provided to achieve 38.1 mm diameter, continued by a single step finish cut of 0.1 mm to achieve 38 mm diameter of product dimension. Figure 3: Data of diameter measurement MiniTab software is used here to develop the system for data processing and related calculations. As the output, the system will produce variable control charts and process capability. Minitab is a statistic package. It was first developed at the Pennsylvania by researchers Barbara F. Ryan, Thomas A. Ryan Jr., and Brian L. Joiner in Minitab began as a light version of statistical analysis program namely Omnitab. Today, Minitab is often used in conjunction with the implementation of Six Sigma, Capability Maturity Model Integration, and other statisticsbased process improvement method. In this project, Mini-Tab was used to develop a system to produce variable control chart and process capability. Control chart for variables are used extensively for statistical control of single measurable quality characteristic, such as dimension, weight, or volume (D. C. Montgomery 2005). Statistical control of Variable quality characteristic requires for monitoring both mean value and variability. Control of process average or mean quality level is done with the control chart for mean, or the x chart. Process variability can be monitored with either a control chart for the standard deviation, called as s chart, or the control chart for the range, called as R chart. The R chart is more widely used since Fifteen products have been prepared as objects of the experiment. The object then put on the measurement system for data collection. Measurement was taken from five different locations of each object at the position of 50, 100, 150, 200 and 250 mm of the length respectively. Three measurement of different angle were taken from each position, and the maximum value of the measurement was taken as final data. Figure 4: x and R chart for diameter 3

4 could produce as high as 0.01 mm of precision (i.e. the target specification of the cutting process is from to mm). However, the result of process capability analysis (as shown in Figure 5) reported that the mean of the data is , which is mm higher than the target. The left tail of the distribution falls outside the lower specification limits and the right tail of the distribution falls outside the upper specification limits. It can be concluded that some objects are not meeting the lower specification of mm and upper specification of mm. These results indicate that the precision of the lathe machine somehow has been changed. 4. CONCLUSION Figure 5: Process capability of machining The reading from dial indicator would be stored automatically to Minitab file, as shown if Figure 3. The row of the table represents number of sample (15), and the column represents the sample size (5). Statistical analysis was then provided to produce variable control chart ( x and R) and process capability report. Result of the x and R chart is shown in Figure 4. It is shown that Upper Control Limit (UCL) is mm and Lower Control Limit (LCL) is mm. The chart also shows that all of the data fall inside UCL and LCL. It means that the manufacturing process of the object still in statistical control. If some of the data fall outside UCL and LCL, further investigation need to be provided to find cause of problems in manufacturing process. Process capability report is automatically produced by transforming the data follow normal distribution using Boxcox transformation in the Minitab software. The report consists of a capability histogram with two normal curves, and a table of capability statistics. The two normal curves are generated using the process mean and within standard deviation and the process mean and overall standard deviation. The cutting process was targeted to have 38 mm of product diameter, and it was believe that the lathe machine The development of automatic measurement system for cylindrical object has been reported. The connection among elements of the system has provided a proper integration, hence produce a smooth automatic data transfer from dial indicator to Minitab software. Finally the experimental study demonstrated the capability of the system to provide statistical analysis such as variable control chart and process capability report. Further study can be conducted for wider application of the system. The capability of the system in measuring diameter dimension cold be used as basic component to develop roundness measurement system. The algorithm developed by Gadelmawla (2010) can be adopted to develop the system. REFERENCES Curtis, M., and Farago, F. (2007) Dimensional Measurement, Industrial Press Inc., New York, NY. Montgomery, D. C. (2005) Statistical Quality Control, 5 th ed. John Wiley & Sons, Inc. Chang, H., and Lim, T. V. (1993) Evaluation of circularity tolerance using Monte Carlo simulation for coordinate measuring machine. International Journal of Production Research, Lyu, J. J., and Chen, M. N. (2009). Automated visual inspection expert system for multivariate statistical process control chart. Expert System with Applications, Prieto, F., Redarce, T., Lepage, R, and Boulanger, P. (2002). An Automated inspection system. International Journal of Advanced Manufacturing Technology,

5 Gadelmalwa, E. S. (2010). Simple and effective algorithms for roundness evaluation from the coordinate measurement data. Measurement,