In this article we will discuss about the meaning and quality requirements of integrated quality assurance.
Meaning of Integrated Quality Assurance (QA):
The success of computer-integrated manufacturing (CIM) to a great extent depends on integrated quality assurance (QA) into the process. It is being realised that QA has to be an intimate element of every step in the design/ manufacturing/distribution cycle of a product.
Only to increase the productivity by using CIM techniques would not serve much purpose till the output is defect free and meets the original design requirements. The overall quality assurance is now-a-days considered as the end result of a carefully thought out and controlled production process.
Fig. 34.19 shows how the quality can be achieved by building it into the system.
Requirements to Achieve Integrated QA:
Some of the essential requirements to achieve integrated QA are:
(i) Automated inspection at faster rates, preferably using non-contact methods, like machine vision systems.
(ii) In process and on-the-machine inspection.
(iii) Development of flexible inspection systems and designing for inspectability.
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(iv) Application of and reliance on statistical analysis techniques, including statistical process control.
(v) Effective, integrated quality-data management and reporting.
(vi) Inspection planning at the design stage and earlier QA involvement in the product cycle.
(vii) Computerised analysis and modelling techniques.
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The evolution of electronic gauging and digital techniques, the application of programmable controllers, the development of non-contact inspection and machine vision have all contributed to removing the inspection bottleneck through automation of the QA function.
The machine vision system is suitable for dimensional inspection and gauging, as well as for checking the flaws, location, recognition and identification etc. It provides information to take corrective actions before the process gets out of control. Machine vision systems, in addition to speed, are capable of collecting and processing a vast amount of disparate data from one instantaneous viewing.
The information about various dimensions and significant features is fed to a microprocessor-based system that stores all the data on a hard disk for analysis by automatic statistical process control software. Continuous trend analysis alerts the line operator about developing problems.
All fixture operations, including the gauging cycle, are controlled by a programmable controller that also provides all interfacing to the rest of the automated assembly line and advises both operator and main line controller of part status.
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In order to keep the manufacturing process under control, the automated techniques have to be integral part of CIM and provide timely and meaningful information.
In a factory the cost information and scheduling information can be correct only when on line (real-time) actual state of quality of a product and sub-assembly is determined correctly. Scheduling has to take into account both good and bad units and process them along their different parts. For achieving this, the quality of a component/sub-assembly should be determined right away and on-line inspection system should report it to the scheduling computer automatically immediately.
Thus the entire manufacturing process requires timely appropriate feedback to achieve a close-loop system to remain within specified control limits.
The corrective feedback starts at the machine tool. On-the-machine gauging and sensing techniques are concerned with determining the location and size or condition of both the work piece and the cutting separately, and also the condition of the machine and its functions.
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Touch probes are used to establish the location and size of work pieces and tools. Vibration and acoustic analysis techniques assist in predicting incipient tool failure and tool wear. Power and torque monitoring techniques provide overload protection and, with advanced electronic techniques, can provide each tool with a sense of touch and also help detect tool wear.
Signature analysis techniques can provide diagnostics to prevent catastrophic failure and pinpoint trouble areas.
Tool condition sensors have been developed which rely on three-axis force transducers mounted in the tool blocks of a turning centre to measure radial, feed and tangential forces during the cutting condition. The monitor uses a computerised “tool wear estimator” model to continuously analyse the relationship of these forces and automatically calculate the amount of wear on a tool, independent of the tool or material data.
When wear reaches a predetermined threshold, the sensor signals the need for a tool change. At that point, the tool offset sensor is used to qualify the new tool and make the necessary offset adjustments. In case of tool breakage, the sensor immediately signals the CNC to retract the tool before damage can be done, then it initiates a tool change.
Tool pre-setting systems have also been developed which feed complete information like tool identification number, tool length and diameters, type of tool etc. to the computer and accept the signals from computer to position the length and diameter slides to the programmed coordinates.
Multichannel data collectors with statistical computation capability are used to keep the process under control. These data collectors collect data from micrometres, callipers etc. equipped with appropriate interfaces and readings are stored and statistically information is displayed as and when desired.
The unit compares readings against specifications or subgroups against process control limits and displays an appropriate message, such as “In control”, or “Range out”. With the addition of a CRT, X̅ and R control charts or histograms can be produced instantly and the operators can use this information to keep the process under control.