Geometric modeling techniques have been used extensively in the development and manufacture of products. However, for a variety of reasons, there are still many products that are not described by CAD models, and the design and manufacturer are faced with physical samples. In order to adapt to the development of advanced manufacturing technology, these physical objects need to be transformed into CAD models through certain means, so that they can be processed by advanced technologies such as CAD and CAM. At present, the technology related to the mathematical model technology for obtaining products from physical samples has developed into a relatively independent category in CAD and CAM, called "Reverse Engineering". By reversing the CAD model of the real thing, it is possible to make full use of advanced technologies such as CAD and CAM in the process of design and manufacture. Since the implementation of reverse engineering can accurately and reliably replicate physical samples in a short period of time, reverse engineering has become one of the hot topics of current advanced manufacturing technology. Using some non-professional reverse design software (such as: UG, Pro/ENGINEER, CATIA, etc.) and some professional reverse design software (such as: Surfacer, CopyCAD, Trace, etc.) for reverse modeling is the current phase of reverse engineering in enterprise applications. typical example.
Due to the company's new product development needs, the author used UG software to carry out the reverse of the parts to achieve better application results in the reverse modeling design of complex automobile stamping parts. The measuring equipment we selected is a three-coordinate measuring machine from LK, UK, which can be used to measure the spatial coordinates, scanning profiles, measuring parting lines and contours of features. The amount of data acquired by this device is not as large as that scanned by a laser scanner, so it is appropriate to use some non-professional reverse design software. The reverse modeling of UG follows the general principle of point → line → face → body.
First, the measuring point
Plan how to manage before measuring points. The design of the surface is required by the designer. The general principle is that the location of the curvature change is relatively dense, and the smoother place can be thinner. Since the efficiency of the point of the general coordinate measuring machine is much lower than that of the laser scanner, it is necessary to be targeted when measuring the parts. It is worth noting that in addition to the scanning profile and the score line, it is also necessary to measure the contour lines, etc., which will bring convenience in the facet.
Second, the connection
(1) Finish the points before finishing the connection, including going to the point of error and obvious defects. The section points in the same direction are placed in the same layer, the parting line points and the hole points are placed separately, and the contour points are also placed separately for easy management. Usually this work is done during the measurement phase and can also be done in the UG software. The general measurement software can set the placement layer of the point in advance, and arrange it while measuring the point.
(2) The point connection is divided into line points as far as possible to minimize the error and smooth. Because in many cases the parting line is the assembly line of the product. For general parts in automobiles and motorcycles, the wiring error is generally controlled to be less than 0.5 mm. The connection should be targeted, and the facet method should be roughly determined according to the shape and characteristics of the sample, so as to determine which lines need to be connected, and which lines do not have to be connected. Lines, arcs, and splines can be used for the connection. The most common is the spline, using the "through point" method. Select the spacing as evenly as possible. The corners are ignored first, and the corners are made. After the surface is finished, the corners are rounded.
(3) Curve adjustment Due to the error of measurement and the surface roughness of the sample, the change of the radius of curvature of the spline often has a sudden change, which has an influence on the smoothness of the later facet. Therefore the curve must be adjusted to make it smooth. One of the most common methods of adjustment is Edit Spline. Select the Edit pole option and use the mouse to drag the control points. There are many options, such as limiting the control point to move in a plane, moving in a certain direction, coarse or fine tuning, and turning on the "comb" switch that displays spline. In addition, adjusting the spline often uses one point of the moving spline to another point, so that the curve of the constructed surface has intersections. However, it must be noted that no matter what command is used to adjust the curve, deviation will occur. The more the number of adjustments, the larger the cumulative error. The allowable error value is determined by the specific requirements of the sample.
Third, facet
Use a variety of facet methods to create surfaces, including Thick Curve Mesh, Thick Curves, Rule, Swept, From point cloud, and more. The choice of the facet method depends on the specific characteristics of the sample. The most common use of the author is the Thick Curve Mesh, which is used to weave the adjusted curve into a curved surface. The advantage of the Though mesh mesh surface is to ensure the continuity of the curvature of the surface boundary, because the Though curve mesh can control the curvature of the surrounding boundary (tangential), so the quality of the facet is higher. However, the Thick curves can only guarantee the curvature of both sides, and the error is also large in the facet. If the intersection of the two curved surfaces is to be rounded, because the boundary of the Thick curve mesh is the intersection of the two curved surfaces, it is obvious that this line is smoother than the intersection of the two Thick Curves surfaces, so that the quality of the rounded corners of Blend is different. of.
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