Checking Fixture
Checking fixtures are widely used in various manufacturing fields and play an important role in manufacturers and large mechanical equipment. Inspection is beneficial for improving the quality and accuracy of products during processing.
Key Points of Inspection Fixture
Many key points affect the accuracy of inspection fixture, some of which are due to errors during measurement. Solving these defects helps to ensure product quality and efficient production. The influencing key points of each type of checking fixture are different, and adjustments should be made according to the requirements of each type of fixture testing.
Key Points Affecting Measurement Errors of Inspection Fixture
The error of the inspection fixture refers to the error inherent in the equipment itself. For any detection method, in general, there will be detection errors in every device or detection method used to measure products, and no device is error-free. Due to errors in measuring products, the test data cannot fully determine the quality of the product, and this data can only approximate the true value of the product. To obtain good detection data and reduce detection errors, it is necessary to carefully analyze the reasons for measurement errors, find solutions to reduce the impact of these errors on the tested parts, and effectively improve measurement accuracy. Many key points can cause measurement errors in the actual use of inspection fixture by operators, generally including the following aspects.
1.Human Error
Human error refers to the error caused by the operator’s operation of the inspection fixture, which can result in measurement errors. There are many types of human errors, such as measurement personnel not being proficient in the use of inspection fixtures incorrectly using inspection fixtures, and incorrect installation steps, which can lead to measurement errors during positioning and clamping.
2.Inspection Fixture
Many key points affect the error of inspection fixture, including various errors in the design, testing, and use of inspection fixture, all of which are reflected in data errors and repeated measurements.
3.Measurement Method Error
Measurement method error refers to the inaccuracy of the measurement method of the inspection fixture, such as incorrect selection of calculation method, improper selection of measurement method, inaccurate installation and positioning of the workpiece, etc. Errors caused by unreasonable or unreasonable key points can result in measurement errors. For example, in the contact measurement of a coordinate measuring machine, errors caused by the deformation of the measured part and measuring device due to the influence of the measuring force on the measuring head.
4.Environmental Error
Environmental error refers to the error caused by measurement conditions that do not meet the standards when technicians use inspection fixture for measurement. It will generate measurement errors, such as pressure, humidity, and ambient temperature that do not meet the standard requirements. The biggest influencing factor is the ambient temperature. For example, when measuring the length using an inspection fixture, the specified ambient standard temperature is 20 ℃. However, in actual measurement, the temperature of the workpiece and inspection fixture will change with the standard temperature, which may be a large deviation or a small deviation. Due to the different materials of each tested workpiece and inspection fixture, there may also be different linear expansion coefficients, which will result in some measurement errors. To reduce the impact of temperature on measurement accuracy, technicians use inspection fixture to accurately adjust the surrounding environmental temperature according to the detection requirements when measuring the workpiece.
The inspection fixture or measuring bracket has measurement errors, making it impossible to determine whether the part is qualified. There are several situations where this error occurs. For example, a certain shape measured by a part’s three coordinate measurement report results in compliance with requirements, but the result detected by the inspection fixture is unqualified, or the workpiece state detected by the inspection fixture is qualified, but the result measured by the part’s three coordinate measurement is unqualified.
To address these issues, exclusion methods are used. Step 1: Confirm the measurement method of the fixture: When measuring the workpiece, whether the measurement requirements of the three coordinate selections are completely consistent with the positioning reference of the fixture selection. In most cases, considering this point can solve the problem. Step 2: If the benchmark is consistent, check whether the detection size of the inspection fixture meets the design requirements. Step 3: If all the above inspection fixtures meet the requirements, then it is necessary to re-measure the parts and check the repeatability of the measurements. If the data from the two tests are similar, it indicates that there is a problem with the inspection fixtures. For example, the inspection fixtures may have failed and the accuracy may have changed.
Key Points Affecting the Accuracy of Inspection Fixture
Operator error:When production personnel conduct online inspections, they lack proficiency in theoretical knowledge and measurement experience, and incorrect operations can affect the collection of measurement data. Incorrect operations, such as failure to control the probe, probe holder, controller, etc. promptly during the flexible inspection fixture operation process, can result in significant differences between the data obtained and the actual dimensions.
The original accuracy of the inspection fixture:The original accuracy of the inspection fixture refers to the design and manufacturing accuracy of the inspection fixture, which is the basis for ensuring that the inspection fixture has high accuracy. Only when the inspection fixture has sufficient original accuracy can it better and fully play its role, to improve its service life and maintain stable accuracy over a long period.
Wear of inspection fixture:During high-frequency use of the inspection fixture, friction and erosion may occur during the contact between the workpiece and the inspection fixture, which can have an impact on the inspection fixture. It directly leads to changes in some functional surface dimensions of the inspection fixture, and when the wear is too large, the accuracy of the inspection fixture will fail. Other parts of the inspection fixture can be damaged due to friction, such as positioning parts, guiding parts, and other parts with relative motion. The wear of these parts of the inspection fixture can lead to a decrease in its accuracy, which has an impact on the inspection.
Types and structures of inspection fixture:The type and design of inspection fixtures play an important role in the accuracy of inspection fixtures. For example, if a single-piece inspection fixture has a single structure, its accuracy is higher than that of a complex assembly inspection fixture. inspection fixture with guided sliding devices and those without guided sliding devices have lower accuracy than this one. The design of the inspection fixture structure has the characteristics of being firm and reliable, and its function is to reduce damage to the inspection fixture when subjected to impact.
Appearance of inspection fixture:If the stiffness and strength do not meet the standards, the result is that the stressed parts of the inspection fixture will undergo elastic or plastic deformation, reducing the accuracy of the inspection fixture. For example, when there is a phenomenon of stress concentration in the design structure of an inspection fixture, the inspection fixture will be impacted in case of accidental collision, resulting in inaccurate accuracy.
Requirements for the use of inspection fixture:The main component structure of the inspection fixture is made of steel parts. Due to the thermal expansion and contraction of steel parts, if placed in an environment that is too hot or too cold, it will affect the accuracy of the inspection fixture.
Measure the impact of the environment:Due to the influence of the coefficient of expansion of the parts, the inspection fixture is measured at a constant temperature of about 20 degrees Celsius. The temperature difference between winter and summer is relatively large, which can last for about 3 hours in summer. In winter, the inspection fixture generally needs to be kept at a constant temperature for more than 8 hours. It is necessary to have a constant temperature inspection fixture for measurement, otherwise, the accuracy and repeatability of the inspection fixture will be poor.
Failure Key Points of Inspection Fixture
There are various types and structures of inspection fixture, and the key points of failure are also different. However, there are mainly good types of normal inspection fixture wear and failure, such as wear failure, fatigue failure, etc.
1.Wear Failure
Because the inspection fixture is a type of testing machine, to reach the product’s testing position without changing the position of the parts, the functional parts on the inspection fixture need to be accurately moved and adjusted or effectively judged for quality with the help of other auxiliary machines. Mainly functional main models, opening inspection fixture, screw inspection fixture, with many detachable modules, each movement and handling will generate vibration and friction. When technicians use inspection fixture, mutual friction between workpieces, contact between handheld parts and products, etc. can all lead to adverse effects, such as causing wear between two workpieces. The wear of the inspection fixture will vary with usage time. If used for a longer period, the amount of wear will also increase, leading to increasingly severe wear.
There are many forms of wear, such as fatigue wear. The main criterion for determining whether an inspection fixture has failed due to wear is coordinate measurement. inspection fixture failure occurs when the measurement data shows that the inspection fixture size exceeds the tolerance range required by the standard. The structure with the highest frequency of wear and the greatest impact on the accuracy of the inspection fixture is the positioning pin, detection pin, etc. Testing personnel should regularly inspect these parts and focus on daily maintenance and upkeep of these areas.
2.Fatigue Failure
The general working mode of the inspection fixture is intermittent work, repeatedly loading and unloading, which puts the stressed parts of the inspection fixture under alternating stress. Some of the moving parts of the inspection fixture, such as flipping mechanisms, sliders, guide rails, etc., as well as some highly automated inspection fixtures equipped with electric motors, will carry a certain load during use, and the transfer of this impact force will be transmitted through other workpieces.
After a certain period of use, due to the effect of alternating stress, micro defects and stress concentration areas on the surface and interior of the parts will lead to many small cracks. As the inspection fixture continues to be used, these microcracks will increase. When the microcracks expand to a large extent, the load-bearing capacity of the inspection fixture parts will be severely reduced, ultimately leading to the phenomenon of inspection fixture damage.
The use of inspection fixture will generate different key points, and each type of inspection fixture has many different key points that can have a certain impact on the manufacturing of products, mainly affecting the accuracy of product quality.
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