Current situation and development direction of mea

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The current situation and development direction of large workpiece measurement

with the development of large-scale extreme manufacturing technology, the shape of large workpiece is becoming more and more complex, and the accuracy requirements are becoming higher and higher, which put forward requirements for measurement and quality assurance. Although there are a variety of high-precision and large-scale measurement technologies and instruments, due to the requirements of large workpiece itself and the constraints of the environment, there are many problems in the actual measurement and quality control process, Here is a preliminary discussion on some major issues to play a role in attracting jade

what is a big workpiece

at present, there is no strict definition of the concept of a big workpiece. It is generally believed that large workpieces are workpieces that have certain measurement accuracy requirements and are too large to be measured by a suitable fixed coordinate measuring machine. Its application scope is also expanding rapidly

but from the perspective of the extreme manufacturing of large machinery at present, large workpieces can be roughly framed from the following two aspects: ① workpieces are generally formed by machining or assembly of machined parts, and the measured content includes 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 measurement instruments such as theodolite, total station and general optical (including laser) measurement methods to a considerable extent; ② Its shape and volume are unmanageable by conventional coordinate measuring machines at present. The conventional dimension is emphasized here, excluding the special coordinate measurement system. In this case, moving measurement becomes the second feature of large workpiece measurement

therefore, large workpieces should be the unity of accuracy and volume. If it is necessary to distinguish from the size, perhaps the mechanical parts with organic machining accuracy requirements can be considered as large workpieces from the volume of 2m. Typical, such as large injection molding machine shell (2000mm × 2000mm × 800mm), small wind power hub (ball diameter about 3000mm), and its hole system has a very high position requirement (about 0.03mm). There are few conventional fixed coordinate measuring machines that can cope with its length, width, height, volume characteristics and measurement accuracy

by analogy, the measurement of workpieces such as car body in white is not considered as large workpieces, because there are already a series of special coordinate measuring machines. At the same time, because it is sheet metal forming and splicing parts, its overall accuracy still lags behind that of machined parts. The detection and adjustment of those measuring tools in the body in white workshop are often considered as large workpieces, because they generally use mobile measuring tools

common measurement means of large workpieces

from the preliminary definition of large workpieces, its measurement means should have three functions: ① it has sufficient accuracy, that is, according to the principles and requirements of measurement accuracy distribution and the actual measurement situation on site, when it is 1/3 ~ 1/10 of the drawing specification; ② It has mobility and can ensure the measurement accuracy in the case of multiple positioning (commonly known as transfer station, frog jump, etc.); ③ It has coordinate measurement function, that is, it is equipped with geometric digital measurement software function, which can carry out sampling measurement, geometric element fitting, construction of measurement coordinates and evaluation benchmark, dimension calculation and tolerance evaluation, geometric tolerance calculation and evaluation, etc

at present, the measurement methods that meet the above requirements in the market mainly include: laser tracker, portable measurement arm (commonly known as articulated arm), 3D photogrammetry system, etc

in fact, from the measurement principle of these measuring instruments, the measurement work is completed through the extraction, sampling, fitting, calculation and evaluation of points on the measured geometric features, which belong to the category of coordinate measuring machines

but various measuring instruments have their own technical characteristics and application scope. Of course, the price is also different. Here is a simple comparison:

1 Laser tracker

this is the geometric dimension and geometric tolerance measurement tool for large workpieces with the highest measurement accuracy at present, and the measurement range of one-time positioning is the largest (at present, the maximum ball diameter can reach 60m). In addition, through the configuration of hidden points (such as Leica's t-probe) and scanning accessories (such as lecia's T-scan), the application range is very wide. In addition, the automatic tracking function of the laser tracker makes it also have the function of dynamic measurement. With the cooperation of relevant accessories (such as lecia's T-Mac), it can complete the spatial 6D measurement and evaluation of objects

2. Portable measuring arm

this kind of measuring device has relatively low accuracy, and the measurement range of one-time positioning is also small (generally up to about 1.5m ball diameter). At the same time, due to its multi joint structural characteristics, the accuracy retention is relatively poor, which is generally used in production sites and other occasions with low accuracy requirements

3. 3D Photogrammetry System

such a system that measures and splices through photography and target system, its measurement accuracy is relatively low, but its measurement speed is much higher than the above two instruments using single point measurement method. It can not only be used as mobile measurement, but also be used in the measurement station in the production line, and its main function is rapid measurement and evaluation of shape. This kind of instrument is also widely used in reverse engineering

the application of large workpiece measurement technology mainly focuses on the following aspects: ① geometric measurement and evaluation (GD & T): the detection of machined geometric features, including the measurement and evaluation of dimensional tolerance and geometric tolerance; ② Mam: measurement aided machining: mainly used for the measurement, installation, adjustment and inspection of large workpieces in the processing process; ③ MAA: measurement aided assembly: it is mainly used for auxiliary measurement during the assembly of large components

with the development of large-scale extreme manufacturing technology and process, the latter two types of auxiliary needs in the production site are becoming more and more, especially in the fields with high precision requirements, such as nuclear power plants, ships, aviation, aerospace and so on

factors affecting large workpiece measurement

from the perspective of a complete measurement system, there are many problems in large workpiece measurement at present, and they are intertwined. It is mainly reflected in the following aspects

1. The accuracy of measuring instruments

at present, the accuracy verification of mobile measurement system has not been improved in terms of standards and specifications. Even if there are relevant standards and specifications, the conditions shown in Figure 7 are not consistent with the actual application. Therefore, the accuracy of actual use on site is an important issue worthy of our attention

2. The influence of the measured object

here includes the quality of the measured surface of the workpiece, which will directly affect the collection of measuring points. In addition, when multiple azimuth measurements are required, the impact of measurement transfer on the measurement results is inevitable, and the distribution of as many and scattered common points as possible will help to reduce the error caused by transfer

3 Influence of temperature on measurement

the change of temperature under field measurement conditions is the most easily ignored and fatal problem in large workpiece measurement at present. If the workpiece cannot be isothermal, the actual size of the workpiece with a large heat capacity is unstable during measurement, and the impact will be difficult to estimate, which will also be a problem that needs to be seriously considered in the whole measurement process

4. Influence of measurement site

the influence of on-site measurement environment is mainly reflected in the influence of ground vibration on the measurement process

5. The influence of measurement process specification

traditional measurement methods, especially the measurement of geometric tolerance, are mature, and there are relevant standards and specifications for reference, but the specifications of coordinate measurement have not been formed at present, so this will be the most fundamental factor affecting the accuracy, reproducibility and reproducibility of measurement results, and it is precisely an aspect that large workpieces and even the whole coordinate measurement field have not attracted enough attention at present. It will also seriously affect work efficiency

6. The influence of measuring personnel

because the measurement process of large workpieces is generally manual, the level of operators directly affects the collection quality of the original measurement data. In addition, in terms of the understanding of measurement requirements and measurement process, the perception of measurement site and measurement process, and the judgment of measurement results, the requirements for the skills and experience of large workpiece measurement personnel are much higher than those of general fixed CMM personnel, although theoretically speaking, they are all operating CMM

from the above, we can see that in order to control the measurement quality of large workpieces, the fundamental lies in the technical literacy of measurement personnel and the formulation of measurement process specifications, As well as the application of measurement system analysis (msa:measure production facilities are planned and built by the automation team of the technical capability center; the specialized technology of low-level packaging quality control and quality guidance policy comes from the expert system analysis of glass production and manufacturing in binder)

problems existing in the application of coordinate measurement technology in state-owned enterprises

from the current application of coordinate measuring machines in domestic manufacturing enterprises, there are many problems, and the problems exposed by large workpiece measurement are more comprehensive and serious because of its measurement accuracy, measurement volume and field measurement

first, enterprises' understanding and mastery of new technologies need to be improved. Quite a number of enterprises do not understand the digital coordinate measurement technology of large workpieces, which affects the improvement of industrial development level. When carrying out many high-precision geometric measurements, they still use the original methods, such as total station, theodolite, level, and general conventional optical methods, which not only affects the measurement accuracy, but also affects the measurement efficiency

secondly, the entire manufacturing industry lacks understanding of the technical requirements of coordinate measuring personnel. Large workpieces have very high technical requirements for the surveyors themselves. If measured by professional titles, at least a measurement engineer who can stand on his own side is needed, that is, a measurement engineer with theoretical knowledge, operating skills and practical experience

finally, the enterprise is obviously not aware of the requirements of measurement process management. Digital measurement is the source of tolerance process control of product geometric dimensions, but many enterprises take coordinate measurement equipment as a kind of decoration, which is actually not paying attention to product quality. (end)

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