Reverse engineer can not simply reconstruct the surface, but to grasp the design concept at the right, based on the step by step along the design ideas to produce part features. This paper discusses the design of automotive sheet metal parts reverse the general flow, summed up the impact of adverse quality problems. However, the structure of complex sheet metal parts, reverse many different ways, relying on designers to reverse when the design experience, a concrete analysis of specific issues, according to the characteristics of components to develop reverse way to ensure that reverse the work of simple and feasible.
1, reverse design process

Sheet metal parts reverse design process shown in Figure 1.

1.1 point cloud inspection

Sample point cloud must be able to fully reflect the essential characteristics of components, measurement precision can reach requirements, the point cloud point of non-repetition. If can not meet the above requirements. You need to tested additionally.

1.2 Check (corrected) coordinates

Point cloud coordinates the completion of calibration normally takes three steps: ① analysis to determine the positioning of the base components (key hole, key lines, the key surface). A baseline must be able to determine the elements of a coordinate system, that is able to limit the Cartesian coordinates of the six degrees of freedom. ② Analysis of baseline alignment of parts to determine whether it has been aligned point cloud. If the point clouds to meet the requirements, then the next working procedure; if the point cloud does not meet the requirements, you need to adjust the coordinates of point cloud. ③ point cloud coordinates adjustments: First, establish a position in front of the base to determine the alignment of the corresponding element, if positioning the base element within a reference point, then the establishment of a reference point; have a baseline, then the establishment of a baseline; there is a baseline surface, then the establishment of a datum. And then use the software alignment (ALIGN) function of the coordinates of point cloud alignment, and aligned the results to be checked. If failed, then repeat the above until you pass up.

1.3 Analysis of point clouds, remove miscellaneous points

If the sample pieces of miscellaneous points around too much and affect the part feature identification and extraction, then used to remove (Remove) function to remove redundant points. If the Sample point cloud is too dense, affecting the normal operation of the software, then used the filter (Filter) function of dilution of point cloud.

1.4 merged point cloud

    If the original measuring point cloud is put together from a single block point cloud together, it would take to make all these single piece of point cloud combined.

1.5 Sample characteristics of classification

    According to automotive sheet metal parts design experience, like pieces of features can be roughly divided into A, B, C 3: A grade with the surface, installation of base-level, functional surfaces, positioning holes, feature lines, and require a higher dimensional accuracy, the point , line and plane, the error precision of generally less than 0. 5 mm; B grade structural surface, transition surface, such as dimensional accuracy, surface craft require general point, line and plane, the error precision can be properly amplified generally not more than 1 mm is appropriate; C grade through-hole, craft hole, stiffener borders, flanging and other points of dimensional accuracy, less demanding line of surface error can be amplified to 2 mm or so.

1.6 features (feature points, feature lines, characteristic face) distinguish

    Feature recognition in accordance with the degree of difficulty, the characteristics of components can be divided into a clear identity and clear characteristics. Parts of some basic features, such as linear, planar, cylindrical surface, etc. can directly distinguish; right do not know the part feature, common to distinguish between the following methods: ① Function display with the grid point cloud, so that part of the characteristics show even more clearly; ② to remove all of the rounded corners, with sharp corners instead of assuming the elimination round lead ambiguity caused by the characteristics; ③ right combination of features, we must rely on its own experience in the design a detailed analysis to determine the characteristics of the composition of the sub-types of features and sub-characteristics of production order, if necessary, can refer to other similar characteristics of the design; ④ For really can not distinguish between characteristics, according to parts of the function, under the premise of ensuring the deviation produced directly.

1.7 Feature Extraction

   According to the identification characteristics of the situation, activate the (Activate Areas) features were extracted from the characteristics of components.

1.8 Characteristics of fitting

    Part feature geometry fitting fitting into fitting and free features. Geometrical features fit more easily and directly fitted into a straight line, plane, cylinder, cone and ball, etc.; free features fitting more difficult, generally using traditional production methods, that is, from point to a line and then to surface. Therefore, the freedom to build key features is to establish the key points, key lines and key surface. Only at the point, line, surface make an effort to make a good feature will be free.

1.9 Features Editor

Feature editing is divided into parameters of feature editing and non-parametric feature editing. Parameter editing features relatively simple, directly to the characteristic parameters can be modified to target parameters; non-parametric features can not be edited, generally only to re-create it again. Therefore, fitting parameters characterizing the best fit.

1.10 Error Analysis

Error requirement is divided into the maximum error requirements, the average error requirement and minimum error requirements, the different functions of the various parts, choose different methods of analysis.

1.11 draw the boundary line, trimming the border

The boundary line, including parts and components on the boundary line of the boundary line of the hole. No matter what kind of borders, requires accurate, otherwise the wrong size parts.

1.12 lead fillet, editing a small feature

Edited guide to the rounded surface, so that part of the structure of reverse reproduction; As part structure and other reasons to make derivative can not be carried out smoothly rounded, generally only the part that can not lead to a small range of angle changes, edit, that is able to meet the requirements .

1.13 the remaining features by adding, editing,

The remaining features refers to the basis of other characteristics for the production of features, such a characteristic structure, relatively easy to identify, their production is sufficient to note the order of their identity can be built.

1.14 hole size holes round the whole

    By drawing the boundary line of holes in the sheet metal on the hole; right some of the features and benchmarks hole hole diameter and the relative distance between the holes round the whole, according to different requirements, generally the whole circle as an integer or 0.5 multiples of .

1.15 Overall Inspection

    Reverse in character at the end of digital-analog do want to reverse the overall inspection, to ensure that the features meet the requirements of reverse. Sheet metal parts of the body structure the number of mold, generally make the following checks: ① feature checks, check whether the missing features; ② bias check on the number of mold and point clouds to meet the requirements of the deviation value is; ③ holes check on the size of holes and round the whole situation; ④ Taper angle and fillet inspection process; ⑤ Sheet thickness inspection, digital-analog must be able to specify the direction of thickening along the specified value.

2 Reverse design of seat bracket

1) Stent point cloud shown in Figure 2, on the point cloud to grid processing.

2) Check the stand point cloud. Point cloud feature complete, as a whole (excluding holes) on the center plane symmetry.

3) stent-point cloud feature recognition. Stand the overall structure of the symmetry, only half of the features for recognition. Stent framework consists of the plane, lateral plane and the next plane composition; stent 12 ribs; on the plane, there is a groove, the hole in accordance with rebar processing; on the plane and lateral plane of the left edge and the next plane edge flanging; the next plane to install the left point there is a process convex sets; stent 14 holes.

4) To identify baseline characteristics, determine the characteristics associated with the relationship between. Bracket the upper and lower plane are installed flat, can be as a reference plane, select the features more clearly on the plane as a datum, the next plane parallel to the plane.

5) The characteristics of fit. First, fitting the base plane and make a center of symmetry plane, and then make the next plane and lateral plane, the last with features such as ribs and flanging, and features for editing, the results shown in Figure 3.

6) to reverse the structure of lead rounded out the results shown in Figure 4.

7) the structure along the symmetry face, said, and after the digital-analog on the symmetrical hole, right hole to be a complete bracket number after editing mode (see Figure 5).

8) to conduct various tests on the digital-analog to complete the reverse design of seat bracket.

3 reverse design should pay attention to the problem

    Reverse design should pay special attention to the following questions: ① give full consideration to the use of functional components; ② take full account of part of the manufacturing process; ③ take full account of part of the structural characteristics, as far as possible the geometric characteristics of the proposed combination rules; ④ give full consideration to the design of other parts hard spot ; ⑤ full account of parts and components manufacturing error distortion, should not have the parts manufacturing defects and biases lead people to reverse the design; ⑥ follow the horizontal, flat, vertical, straight principles, as the nature of a hole the size of the installation should be consistent rows the center of a hole through a straight line or an arc and so on.

4 Conclusion
    Reverse design as a method of product design, automotive design in China plays a very important role. This paper supports a car seat to be an example to illustrate the design of sheet metal parts reverse the general flow, summed up the reverse design should pay attention to the problem. Reverse design refers to the physical model into a CAD modeling method. Design and development of any product, whether it is innovation, improve, or imitation, all implies the inheritance of existing technologies and applications. Automotive design and development is no exception, particularly in China's automotive design, but also inseparable from the reverse design. China is a car-producing country, but most products are joint ventures or imported. Thus, through learning, imitation accumulated design experience, master design, in order to lay the foundation for product innovation, which is to enhance the level of China's automotive design the only way. In this paper, an example of a vehicle seat bracket, combined with CATIA software, the reverse function, auto sheet metal parts reverse design method were discussed.