Several issues to consider when designing inspection
1. The rationality and necessity of using inspection fixtures
Before designing the inspection fixture, consider whether the quality of the machining can be guaranteed by the machine tool and the processing technology. Regular inspection by quality inspectors in inspection or workshop measurement room with universal measuring tools. For more complex shells, consideration should also be given to whether three-dimensional measuring machines can be used to regularly sample various shape and position tolerances of the workpiece to avoid the use of complex and high-precision inspection fixtures.
2. Clarify the technical purpose of inspecting the fixture First, it should be clear whether the inspection fixture is used for process inspection or final inspection. In order to determine the appropriate positioning reference. According to the principle of benchmark coincidence, the positioning benchmark of the inspection jig for the process should adopt the positioning benchmark or process benchmark of the corresponding processing procedure, and the positioning criterion of the final inspection jig should adopt the design benchmark or assembly benchmark of the inspected workpiece.
3. The inspection jigs for measuring the shape and position tolerances must comply with the corresponding tolerance principles. As for the principle of independence, since the shape and position tolerances and dimensional tolerances given by the drawings are independent of each other, the requirements should be met separately. As for related principles, since the form and position tolerances given on the drawings are related to dimensional tolerances, and the former can also be compensated from dimensional tolerances, it should be considered when designing inspection fixtures.
4. The measurement efficiency of the inspection fixture. If a 100% inspection of the workpiece is required, an inspection fixture with a higher measurement efficiency should be designed. When the processing technology is relatively stable or only the inspection jig is needed to adjust the machine tool, after the machine tool is adjusted, the workpiece processing accuracy can be guaranteed, the measurement efficiency of the inspection jig can be lower. However, the inspection jigs used in mass production, in order not to delay production due to inspection, the measurement efficiency is still better.
5. The measurement error of the inspection jig, the measurement error of the inspection jig is the measurement method used by the jig, the structure of the positioning device, the clamping device, the transmission device and the movable element, the manufacturing error of the measurement device and the calibration part used by the jig , And the sum of a series of errors caused by measuring force and measuring temperature. It should be noted that the total measurement error also includes errors caused by deviations in the size and geometry of the positioning surface of the inspected workpiece and the inspected surface within a specified tolerance range. Generally, the total error of the inspection fixture must not exceed 10% -20% of the tolerance of the measured parameters. If the accuracy of the measured elements is very high (ITO, ITI, IT2), it can be enlarged to 33%.