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What is a large workpiece
The current concept of large artifacts does not have a strict definition. It is generally believed that a large workpiece has a certain measurement accuracy requirement, and the body size is particularly large, and a workpiece that is measured by a suitable fixed coordinate measuring machine cannot be found.
However, from the point of view of the current extreme manufacturing of large-scale machinery, large workpieces can be roughly framed in the following two aspects: 1 The workpieces are generally formed by machining or machined parts. The measured contents include not only dimensions, but also geometric tolerances (commonly known as Geometric tolerances). The measurement requirements of measurement accuracy and geometric tolerance are the first characteristics of large workpieces, which restrict the application of measuring instruments such as theodolites, total stations and general optical (including laser) measurement methods to a considerable extent; 2 its shape and body The amount is currently unable to cope with conventional size coordinate measuring machines. The emphasis here is on regular dimensions, not including dedicated coordinate measuring systems. In this case, the mobile measurement becomes the second feature of large workpiece measurement.
Therefore, large parts should be the unity of precision and volume. If it is necessary to distinguish from the size, it may be possible to consider the mechanical parts required for the accuracy of organic processing to be large workpieces with a mass of 2m. It is typical of large injection molding machine housings (2000mm×2000mm×800mm), small wind turbine hubs (ball diameter of about 3000mm), etc., and its hole system has a very high positional requirement (about 0.03mm), which can handle its length and width. Conventional fixed coordinate measuring machines with high volume characteristics and measurement accuracy are few.
By analogy, the measurement of a workpiece such as a car body in white is not considered to be a large workpiece, because there is already a series of dedicated coordinate measuring machines, and because it is a sheet metal forming and splicing component, its overall accuracy and machined parts There is still a certain gap compared to this. The on-line measurement tools in the body-in-white shop, whose detection and adjustment are often considered to be large parts, because they generally use mobile measurement tools.
Common measurement methods for large workpieces
From the preliminary definition of a large workpiece, the measurement method should have three functions: 1 has enough accuracy, that is, according to the principles and requirements of the measurement accuracy distribution, and the actual measurement situation on the spot, when the 1/3 of the drawings specification 1/10;2 has mobility, and can ensure measurement accuracy in multiple positioning (commonly known as transfer station, frog jump, etc.); 3 has a coordinate measurement function, that is equipped with a geometric digital measurement software function, can carry out sampling measurement, Geometric element fitting, measurement coordinates and evaluation criteria construction, dimension calculation and tolerance evaluation, geometric tolerance calculation and evaluation, etc.
At present, the measurement methods that meet the above requirements on the market include: laser trackers, portable measurement arms (commonly known as articulated arms), and 3D photogrammetry systems.
In fact, from the measurement principle of these measuring instruments, the measurement work is performed by extracting, sampling, fitting, calculating, and evaluating the points on the measured geometric features, and all belong to the category of coordinate measuring machines.
However, various measuring instruments have their own technical characteristics and application scope. Of course, the prices are also different. Here's a simple comparison:
Laser tracker
This is the largest measurement accuracy of the workpiece geometry and geometric tolerance measurement tools, and the largest one-time measurement range (currently the largest ball up to 60m). In addition, there is a wide range of applications through the configuration of hidden points such as Leica's T-probe and scanning attachments such as Lecia's T-Scan. In addition, the automatic tracking function of the laser tracker makes it also has the function of dynamic measurement. With the cooperation of related accessories (such as Lecia's T-MAC), it can complete the 6D measurement and evaluation of objects.
2. Portable measuring arm
The accuracy of this type of measuring device is relatively low, and the measurement range of one-time positioning is also small (generally about 1.5m ball diameter). At the same time, because of its multi-joint structural characteristics, the accuracy is relatively poor, generally used in production sites, etc. Less accurate applications.
3. 3D Photogrammetric System
This type of system for measuring and splicing through a photographic and target system has a low measurement accuracy, but its measurement speed is much higher than that of the above two instruments using a single point measurement method. It can be used not only as a mobile measurement but also in a production line. The main function of the on-line measurement station is rapid surface measurement and evaluation. This type of instrument is also widely used in reverse engineering.
The application of large workpiece measurement technology mainly focuses on the following aspects: 1 Geometry Measurement and Evaluation (GD&T): The detection of geometric features that have been processed, including the measurement and evaluation of dimensional tolerances, geometric tolerances, etc.; 2 Measurement assisted processing (MAM: Measurement Aided Machining): It is mainly used for the measurement, installation adjustment and on-line inspection of large workpieces in the machining process; 3 Measurement Aided Assemble (MAA): It is mainly used for auxiliary measurement adjustment in the process of assembly of large parts.
With the development of large-scale extreme manufacturing technologies and processes, the latter two types of auxiliary requirements at the production site are becoming more and more, especially in fields such as nuclear power plants, ships, aviation, and aerospace, where high precision is required.
Factors that affect the measurement of large workpieces
From a complete measurement system, large workpiece measurement can be said to have many problems and they are intertwined. Mainly reflected in the following aspects.
1. The accuracy of the measuring instrument
At present, the accuracy test of mobile measurement systems has not been perfected in terms of standards and specifications. Even if relevant standards and specifications are already in place, the inspection conditions are not consistent with actual applications. Therefore, the accuracy of the actual use of the mobile measurement system is of concern. important question.
2. Influence of the measured object
This includes the quality of the surface of the workpiece being measured and it will directly affect the acquisition of the measurement points. In addition, when multiple azimuth measurements are required, the effect of the measurement station on the measurement results is unavoidable. The distribution of as many and scattered common points as possible will help reduce the errors caused by the transfer station.
3. Effect of temperature on measurement
The change of temperature under on-site measurement conditions is the most easily overlooked and most fatal problem in large workpiece measurement. If the workpiece can not be isothermal, then for a workpiece with a large heat capacity, the actual size of the workpiece during the measurement is unstable, and the influence of the workpiece will be difficult to estimate. This will also be a serious consideration in the entire measurement process. one question.
4. Measurement site impact
The influence of the on-site measurement environment is mainly reflected in the influence of ground vibration on the measurement process.
5. Measuring the impact of process specifications
Traditional measurement methods, especially the measurement of geometric tolerances, have matured, and relevant standards and specifications can be referenced. However, specifications for coordinate measurement are not yet formed, so this will affect the accuracy and reproducibility of measurement results. The most fundamental factor of reproducibility is precisely the aspect that the current large workpiece or even the entire coordinate measurement field does not attract enough attention. It will also seriously affect work efficiency.
6. Influence of measuring personnel
Because the measurement process of large workpieces is generally manual operation, the operator's level directly affects the quality of the entire measurement raw data collection. In addition, in terms of understanding the measurement requirements and the measurement process, the perception of the measurement site and the measurement process, and the judgment of the measurement results, the requirements for the skill and experience of the large workpiece measurement personnel are much higher than the normal fixed coordinates. Surveyors, although they are theoretically speaking operating coordinate measuring machines.
From the above, we can see that in order to control the measurement quality of large workpieces, it is fundamentally in the measurement of personnel's technical literacy and measurement process specification formulation, and the application of measurement system analysis (MSA: Measurement Systems Analysis) technology.
State-owned enterprises in the application of coordinate measurement technology
From the current situation of the domestic manufacturing enterprises in the application of coordinate measuring machines, there are many problems, and the large workpiece measurement due to its measurement accuracy, measurement volume and on-site measurement and other characteristics, its exposed problems are more comprehensive and serious.
First of all, the degree of understanding and mastery of new technologies by enterprises needs to be improved. A considerable number of companies do not understand the digital coordinate measurement technology of large workpieces. When carrying out many high-precision geometric measurements, they still use the original methods such as total stations, theodolites, level meters, and general optical methods. This not only affects The measurement accuracy also affects the efficiency of the measurement.
Second, the entire manufacturing industry lacks understanding of the technical requirements of coordinate measurement personnel. Large workpieces have very high technical requirements for the surveyors themselves. If they are measured by professional titles, they at least need to be independent of the measurement engineers, that is, they have theoretical knowledge, operational skills, and practical experience.
Finally, there is a clear lack of understanding of the requirements of companies in the measurement process management. Digital measurement is the source of tolerance process control for product geometries, but many companies use coordinate measuring equipment as a device, which is actually a devaluation of product quality.
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With the development of large-scale extreme manufacturing technologies, the shape of large workpieces has become more and more complex and the accuracy requirements have become higher and higher. These have imposed requirements on measurement and quality assurance. Although there are many kinds of high-precision, large-scale measurement technologies and Instruments, but due to the constraints of the large workpieces themselves and the environment, many problems have arisen in the actual measurement and quality control process. Here, some major issues are discussed in preliminary discussions so as to play a role in attracting attention.