Regarding the angle, arc and spatial dimensions of CNC machined housing parts, the advantages and disadvantages of various measurement methods are presented, and the measurement accuracy is considered between simple prototypes, die-casting machines and measure three-dimensional, optical projection measurements or a profiler. measurement methods. In order to improve efficiency and economy, a new measurement method based on traditional test samples and modified measurement tools was selected to improve efficiency and reduce costs while maintaining measurement accuracy, and provide a theoretical basis for lightweight measurement.
1 Preface
In actual production and processing, the angular dimensions, arc dimensions, circle centers and spatial dimensions of some parts are difficult to measure accurately by simple methods if operators and quality inspectors only use traditional hard tools for dimensional inspection. accuracy often cannot meet the design requirements.[1]. In the modern manufacturing industry, the advantages of using CNC machine tools to process complex surfaces such as cones and arcuate surfaces are obvious. The dimensional accuracy and shape accuracy of the processed parts are much higher than that of ordinary machine tools. In actual production, both precision and fineness of measurements must be taken into account. Production progress cannot be limited due to detection methods, which place higher demands on measurement methods. If high-precision measuring instruments are used to facilitate measurement, such as profilometers, coordinate measuring machines, optical image measuring instruments or CT industrial measuring machines, the measurement accuracy can meet the requirements design drawings, but equipment procurement and maintenance costs are very high. The measurement is time-consuming and requires professionals to operate. These practical issues may be difficult for ordinary businesses to accept. If you only use traditional hard tools for measurement, they may not be able to meet the requirements of the design drawing, or it may not be possible to measure at all. In actual production, traditional measuring tools, modified measuring tools, CNC measuring machines and drawing
The effective combination of drawing software can solve most dimensional measurement problems. Aiming at the problems of measuring angle, arc size and spatial size of casing parts, this article proposes a simple measurement method based on traditional inspection models and modified measuring tools, which can not only meet higher measurement accuracy requirements, but also meet the needs of general businesses. Economic requirements.
2 Measurement methods
1) Example of measurement method
Early in part design, to ensure structural integrity, coordination, and strength of parts, designers will add features such as arcs, fillets, and spatial position relationships to parts. Designers often don’t think much about part processing technology and dimensional inspection methods, which adds difficulties to production and manufacturing. In actual production and processing, for fillet and arc detection, sample measurement is generally preferred, also known as R gauge comparative measurement method. This fillet and arc measurement method is very simple and quick, but it also has disadvantages. Since the arc jig can only measure arcs of specific sizes, such as R5mm, R6mm and other fixed sizes, it cannot accurately measure the size of arcs. and the non-standard arc size cannot be accurately measured with the R gauge. Therefore, for some parts with high arc size accuracy requirements, the sample measurement method cannot meet to the requirements. For measuring non-standard arcs, a wire cutting machine can also be used to cut the required arc size on a thin steel plate. After successful measurement and calibration, it can be used as an appearance model for actual partial arc measurement. production. Likewise, angle models can measure certain standard angles. In actual production, lathes often use angle jigs when setting tools. In some cases where the angles and relative positions of certain parts are good, it is more scientific and accurate to use an angle ruler to measure. However, in many real productions, the angle ruler cannot accurately measure internal angles and small angles that interfere with the angle ruler. In this case, an angle gauge should be used in conjunction with a feeler gauge, which can measure most angles.
2) Profile meter measurement method
The profilometer is mainly used to measure the shape and contour of the exterior and interior surfaces of machined parts. It detects the diameter, arc, center distance, chord length and angular dimensions of the inner and outer surfaces of the workpiece through contact sensor probes without damaging the workpiece surface, wait. The profilometer measurement method is that the contact surface profile sensor probe directly contacts the interior and exterior surfaces of the workpiece. Driven by the motor, it slides slowly along the measured surface of the part. During the sliding process, the sensor converts the received profile. information in an electrical signal. The electrical signal is stored in the computer as a digital quantity. The technical measurement software analyzes and processes the data at the same time, the contour information of the measured part is displayed on the computer screen. can be selected for calculation and measurement, including arc radius, chord length and radian, center distance, angle, linear position relationship and distance, etc. The tip radius and nonlinear motion path of the probe can be compensated by the post-processing program to improve the accuracy of workpiece measurement. At the same time, the profiler measurement software can save the measured data and configure the data output and print function. , and can also save the measurement data to hard disk and export it as a document.[2]. The advantages of the profilometer measurement method are mainly reflected in the fact that it can not only directly measure the internal and external contour dimensions of some difficult-to-detect parts, but also directly and accurately represent the internal surface contours and exterior of the room according to a specific standard. The measurement results can be directly output through the software. However, the profilometer measurement method also has certain disadvantages, that is, the measured surface is easily scratched by the probe, which requires high-quality inspection personnel. For closed angles on certain parts, the precision of the curves drawn after measurement is relatively. weak.
3) Measuring method of three-dimensional coordinate measuring machine
At present, as a measuring equipment with high versatility, high degree of automation and high detection accuracy, coordinate measuring machines are widely used in aerospace manufacturing and scientific and technological research. The three-dimensional coordinate measuring machine converts the measurement of each captured geometric element into measurement of the coordinate position of points, lines and sets of these geometric elements through the probe.[3]after measuring the coordinates of each geometric element, the shape, size, angle, relative position in space and other information of these geometric elements are calculated through special engineering software according to rules and standards specific integrated. In terms of measurement principles, all geometric elements of any workpiece can be detected by a three-dimensional coordinate measuring machine, and the coordinate measurement accuracy of a high-precision coordinate measuring machine can reach the level of micron. The advantages of the coordinate measuring machine are obvious, such as accurate, fast and convenient measurement; the disadvantage is that for ordinary enterprises, its use and maintenance costs are very high, resulting in wasted measurement resources.
4) Optical projection measurement method
The detection function of the optical projection measuring machine is similar to that of the profilometer, and the measurement accuracy is slightly lower than that of the profilometer. When measuring, place the measured workpiece horizontally on the measuring machine workbench, illuminate the workpiece with light, and form a difference between the light-transmitting part and the light-shielding part. The built-in program calculates according to the corresponding rules. , and displays the enlarged outline of the measured part on the screen. Engineering-specific software processing can calculate arc radius, arc length, angle between two straight lines, straight line segments or angular position relationships and distances and other parameters geometry of the selected part. outline. However, optical measuring machines have obvious disadvantages. When measuring the groove of the rotating body, the workpiece must be opened to measure the transverse position. The measurement accuracy error is relatively large, and each part cannot be inspected. contour formed by the optical shadow, the computer calculated selectable curves have relatively low precision.
5) Measuring method of image measuring instrument
Image measuring instrument is an emerging precision measuring instrument in the industry in recent years. With the rapid development of image analysis and post-processing technology, image measuring instrument has gradually become a tool for detecting part size, assembly relationship and part shape in aerospace parts. and assembly workshops. One of the commonly used measuring instruments for appearance.[4]. The image measuring instrument uses a digital camera to photograph the parts, and through the recognition and processing of the image sensor, the parts are displayed on the screen as images. It uses image processing technology through dedicated technical analysis software to identify and extract various complex shapes. on the surface of the parts. The feature points and coordinate points are then used to form various geometric elements in the measurement space through information processing technology and calculation of the coordinates of the feature points. Finally, the shape, size and position relationship of the measured workpiece are obtained. software calculation.
3 Dimensional measurement of housing parts
The housing parts shown in Figure 1 constitute an important part of the aircraft fuel system and are characterized by large batch sizes and high dimensional accuracy. Parts have small dimensional tolerances, many spatial dimensions and geometric tolerances and high surface quality requirements. The measuring difficulties are grooves, angles, arcs and spatial dimensions. In order to guarantee the progress of production and the dimensional accuracy of parts, traditional measuring tools (R gauge, angle detection template, angle ruler, caliper and thickness gauge) and tools modified measurement (modified micrometer) are selected as measuring tools. Most fillet radii on parts are less than 1mm and are transfer radii. Tolerance values can be positive or negative. The dimensional accuracy is relatively wide. It can be compared and measured with a small R gauge to determine fillet accuracy. parts.[5]. Not all exterior circumferential angles of the annular groove on the workpiece can be measured using an angle ruler. The angle inside the groove can be measured using an angle detection jig and a feeler gauge to jointly measure the qualified range. The angle detection sample is shown in Figure 2. 88° in the sample is the theoretical angle of the part. When measuring, the longest side of the sample is positioned with the vertical edge of 4.1mm of the workpiece size, and the 88° hypotenuse of the sample is in contact with the hypotenuse from the bottom of the part groove. If there is a gap between the 88° beveled edge of the sample and the beveled edge of the groove bottom of the part, insert a feeler gauge to determine whether the angle of the part is qualified. The 45° in the sample is to avoid interference with R0.8mm and R1.2mm in the part groove during measurement. The use of the angle detection model is shown in Figure 3.
Figure 1 Housing Parts
Figure 2 Angle detection sample
Figure 3 Usage diagram of angle detection sample
There are also spatial intersection sizes such as φ72.3mm, φ72.4mm and φ67.1mm in parts, and the requirements for dimensional accuracy are high. Since these dimensions cannot be measured directly by hard tools at the production site, they can be measured by advanced CNC equipment such as expensive profilometers, coordinate measuring machines and industrial CT measuring machines. However, there are problems such as poor processing. efficiency and uneconomic measurement costs. It is therefore necessary to choose a measurement method that is precise, economical and rapid to ensure the normal operation of the production line. To this end, taking into account the accuracy and simplicity of measurement, a new measurement method based on traditional measuring tools and modified measuring tools is summarized to indirectly measure dimensions and reduce the need for equipment high-end measurement.
The dimensions of the space on the outer circle of the case are φ72.3mm and φ72.4mm. The distance between the two sides of the outer circle where the R0.2mm fillet of the housing groove intersects its bevel can be measured using an outer diameter. micrometer using indirect measurement method of zero case groove size; The size φ67.1mm can be converted indirectly by conversion from theoretical size to minimum outer diameter size where the groove thread R1.2mm intersects its slope. This size can be measured accurately by a modified outer diameter blade micrometer. The blade of the modified blade micrometer is shown in Figure 4.
Figure 4: Blade micrometer after modification
4 Conclusion
This article reviews common methods as well as the advantages and disadvantages of measuring part sizes in actual production, thereby providing a reference for technicians to choose appropriate measurement methods. Currently, the following two methods are mainly used to measure angles, arcs and spatial dimensions.
1) Use less expensive appearance models and angle models for comparative measurements. This method is suitable for parts with low dimensional accuracy requirements.
2) Use a coordinate measuring machine or profiler with higher maintenance costs for measurement. This method is suitable for parts with high dimensional accuracy requirements.
In actual production, the dimensional accuracy requirements of most parts are average. In the process of meeting the measurement accuracy, the detection method should also take into account the actual production efficiency and measurement cost. Practice has proven that the new measurement method is based on traditional measurement. measuring tools and tools modified in this article, it can meet the needs of specific housing part angles, spatial dimensions and general arc measurement, and has good practicality and economy.
Expert commentary
This article describes various methods for measuring arc, angle and spatial dimensions of housing parts and objectively analyzes the advantages and disadvantages of simple prototypes, coordinate measuring machines, optical projections and profiler measurements . A lightweight measurement method based on traditional test samples and modified measurement tools, which is relatively simple and ensures accuracy, is proposed and meets the economic requirements of general enterprises.
The highlight of the article is the lean measurement method, which takes into account measurement accuracy and economy. It can not only meet higher measurement accuracy requirements, but also reduce measurement costs and reduce dependence on high-end measurement equipment. .
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