1 Introduction
Many products are not described by CAD models during product development and manufacturing. The design and the manufacturer are faced with physical samples, and sometimes even the reference drawings are not available, and the customer also requires optimization and improvement of the samples. In order to adapt to the development of advanced manufacturing technology. These objects need to be converted into CAD models. It can be processed or managed using advanced technologies such as CAD/CAM, RPM, CIMS and PDM. Reverse engineering is to reverse the geometric shape of the original physical model through various measurement methods and three-dimensional geometric modeling methods, and re-establish the three-dimensional digital model of the physical object. Thereby achieving the design and manufacture of product molds.
2 Traditional mold design and manufacturing methods
The traditional manufacturing method of the mold is generally to produce a proportional mold using a three-dimensional engraving machine or a hydraulic three-dimensional model milling machine, and then to produce. This method cannot create a product size image, nor can it be modified, and such a method has a large manual modification and uneven gap. It is necessary to repeatedly test the mold, the quality is unstable, and the processing cycle is long, which brings great difficulties to the later CNC machining and inspection. Due to the shrinkage and deviation of the physical casting model itself, the samples obtained by such molds are inevitably defective. Sometimes, the physical sample is directly used for profiling, and the profiling will copy all the defects such as damage or wear on the sample to the mold, and the final product inherits all the shortcomings of the sample, resulting in the appearance of the product. Poor performance, dimensional accuracy and performance are not up to standard. It is especially important that the mold cavities generated by conventional methods do not have the ability to be modified and redesigned, so such methods have gradually been replaced by new digital reverse engineering systems.
3 CAD design and manufacturing method based on CAD/CAM system
The CAD/CAM-body technology can effectively improve the shortcomings of traditional methods, and the product model established by CAD can directly generate CNC commands. The data communication between the machine and the machine tool is realized through the DNC interface, so that the use of the shape simulation transmission is changed to the data transmission in the production, so that the design and the manufacturing process are directly communicated. Moreover, it can perform appearance analysis, product assembly, inspection and interference of components in the CAD system, simulate the CNC machining process, check the machining process and interference, and realize the design and modification of the product. It can greatly reduce the amount of manual repair, shorten the development cycle of new products, and improve product quality.
Reverse engineering technology mainly includes two aspects: digital technology and surface reconstruction technology. The digital technology uses a three-dimensional scanning measuring instrument to collect physical or model surface data; the surface reconstruction technology is based on a series of point data of the geometric surface obtained by the measurement, constructing a shape curve and a surface, and finally reconstructing the three-dimensional model.
4 Reverse design method and process
4.1 CAD/CAM software selection
At present, there are many commercial CAD/CAM integrated system softwares with more complete functions. Now, Imageware's Surfacer and PTC's Pro/E are selected for reverse design. The specific contents include: 1 digitized point input and processing, including data input and digitization. The transformation and processing of the point data; 2 the area of ​​the quadric surface is divided. Construct model features; 3Create 3D modeling of parts for processed point cloud data; 4 3D modeling of parts and detection of measured point clouds; 5Formation of die casting mold solid model; 6Mold manufacturing and optimization
4.2 3D data acquisition
In order to facilitate the scanning of parts and ensure the accuracy of scanning, it is necessary to make necessary preparations for the parts, such as the reference point, the surface of the object, the imaging agent and the instrument and software calibration. Here, the three-dimensional optical scanning system is selected, and the data is automatically combined by the marking features on the object to be measured, and the three-dimensional measurement data is directly obtained from the scanning head. The system adopts a new self-positioning technology, the scanning mode is cross-type laser scanning, the measurement accuracy is 0.1mm/0.5m, the data processing is completed in real time, the triangle face is directly generated, and the data is output in .stl format to ensure no redundant data and data overlap. . A three-dimensional laser scanning system is used to scan different orientations of multiple angles of the entire part of the part. After the scan is completed, the point cloud file is obtained by point cloud alignment, triangulation, smoothing and thinning. Next, output the *.stl file for Imageware software to post-order the point cloud.
4.3 3D geometric modeling
Geometric modeling is the key to reverse engineering, and building surfaces is the key to geometric modeling. The process of modeling is also the process of processing the data. The purpose of data processing is to obtain the basic feature information of the input data. The common means are data adjustment, region trimming, data density modification, data smoothing, noise removal, sharp corner retention, etc., to obtain the required graphics.
Use the Imageware software to load the scanned point cloud file, and according to the characteristics of the point cloud. Make some auxiliary benchmarks to align the part point cloud to prepare for the extraction of the section line. The point cloud data contains many noise points, so it is necessary to filter out the noise points, and perform optimization processing such as data smoothing, data cleaning, and missing points on the point cloud data, delete unnecessary data points, and appropriately reduce the density of the point cloud. , can speed up the processing of computers, build a symmetrical datum of parts, and so on.
The extraction of feature lines is the key to the reconstruction of the entire model. According to the shape characteristics of the part, the area of ​​the quadric surface, such as the inner hole, the groove, the plane, the cylindrical surface, etc., is divided. The part point cloud is segmented, and the quadric surfaces are fitted to the inner hole, the groove, the plane or the cylindrical surface, and the feature can also be directly made. The plane can be determined by three or two intersecting lines, and the hole or cylinder is determined by the section line and vector. Generated by measuring the solid model and combining the scanned data. For freeform surfaces. The feature lines of the surface need to be constructed. First make the necessary section line for the part point cloud, then remove the noise points of the section point cloud to perform the necessary smoothing, and finally fit the section point cloud into a curve to construct the free surface.
Open Pro/E software, for the completed point cloud data, complete the surface by editing the surface and surface editing, and perform Boolean operation to complete the 3D solid modeling of the part under the Pro/PART module, as shown in Figure 4.
After completing the 3D modeling of the part, the 3D model reconstructed using Pro/E software is read into the Imageware software. Compare and analyze the deviation between the final reconstructed 3D shape and the scanned point cloud. By using the cloud image to display the difference and analyze the difference between the surface and the point cloud, the accuracy of the reverse scan measurement can be detected.
After the 3D model of the part is created, a machining allowance of about 1.5 m is added to the part to be machined on the part to seal the hole position. The die casting mold solid model also needs to consider the shrinkage rate, which is generally 5 ‰. Pro/E can automatically increase the shrinkage rate, or it can be achieved by changing the ratio. After the appearance trimming, such as rounding, chamfering, draft angle, etc., the solid model of the part die-casting mold is obtained, as shown in Fig. 5.
4.4 Die structure design and manufacturing
After the solid model of the die-casting mold is completed, the mold can be further divided into molds, and the mold grooves, side draws and inserts can be generated to design the mold: 1 push-out mechanism design; 2 overflow tank and exhaust port design; 3 gate, runner design; 4 core mechanism and position design; 5 mold conventional parts design. The Assmbly and Modei modules of Pro/E software are used for mold design to generate the core and cavity of the mold. Using interference detection, the simulation master starts the sequence. The mold cavity is processed by Pro/E processing module Pro/Manufacturing, and the die-casting mold solid model is imported into the Manufacturing module of Pro/E software to make the NC machining program, and the rough finishing program is generated. Since the machine tool should avoid collisions during the machining process of the CNC machine tool, it is also necessary to avoid overcutting or erroneous tool path. For this purpose, the tool position verification is required to judge whether the tool path is continuous, whether the tool path is forward or backward, and whether the path is reasonable. . The dynamic graphical simulation of the machining process uses solid modeling techniques to create a solid model of the blanks, fixtures and tools of the part during machining, using fast Boolean operations. Finally, using the realistic graphic display technology, the machining process is dynamically displayed, and the dynamic simulation shows the tool processing trajectory. The G code for generating NC machining is input into a three-axis (or five-axis) machining center to process the mold; the mold electrode is processed to generate an electrode machining program, and the mold electrode is processed.
4.5 Mold structure optimization and improvement
Using the ceramic mold precision casting method, the main model (wax molding mold) - wax-group tree-staining and sanding-dewaxing and firing, and then casting, by adding a 2.2% increase in size, to obtain a part casting blank . Import the 3D shape created in the Pro/E software into the Manufacturing module of Pro/E, create a new Pro/NC file, and process the blank. According to the processing results and the three-dimensional data of the parts, the relative feature parts of the mold are effectively improved and processed according to the comparison result, so that the modified products are more in line with the customer requirements.
5 Conclusion
Using 3D laser scanning and copying as a means of obtaining spatial 3D data, the combination of reverse engineering software for processing measurement data, 3D digital model construction, mold detection and modification have achieved good results. Digestion, absorption and secondary development work is accurate and fast. Reverse engineering combined with CAD/CAM technology and NC machine tools greatly promotes mold design and manufacture, shortens product development cycle, and realizes system integration of product prototype design and mold design and manufacturing, enabling future mold development. Play a bigger role.
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