Sheet metal nesting CAD/CAM software


Abstract: The CAD/CAM software system for sheet metal nesting is widely used in industry. Due to the continuous improvement of the scheduling algorithm and the continuous change of industrial demand, the complete development of this kind of soft transmission has also become a lasting topic of software development. The article introduces the design and development principle and some key technologies of a CAD/CAM software system for sheet metal nesting.

This software system mainly solves the problems of design calculation, optimization of layout, and numerical control technology in sheet metal cutting. It adopts new ideas and methods in geometry and cutting feature design, and nesting search strategy, especially successfully solving cutting lead and cutting Practical problems of path optimization. The software system has been successfully applied to some shipbuilding companies and cutting machine manufacturers.

1 Introduction
Ships, pressure vessels, ventilation equipment and other manufacturing industries often need to cut a large number of steel plates as materials for subsequent production. How to improve the utilization of raw materials in the cutting process to reduce production costs is one of the problems that companies have been seeking solutions to. The use of computer-aided nesting to achieve high-efficiency and low-consumption sheet metal cutting has become a consensus in the business community.

Domestic and foreign cutting equipment manufacturers and related software developers are also developing software and hardware systems that meet market needs. However, due to the difficulty of the layout problem and the weakness of the domestic software market, China has been unable to develop internationally competitive software products. As a result, domestic cutting machine manufacturers have to spend huge sums of money to purchase foreign software to support their products. The direct consequence of this is to increase the cost of the cutting machine and reduce the market competitiveness of the product.

In addition, due to the particularity of foreign products in terms of language habits, operation and use, end users often need to undergo professional training to master the general operation and use, and the product lacks due technical support in terms of maintenance. Therefore, the development of software products with independent intellectual property rights and international competitiveness can not only reduce the production cost of enterprises, but also the needs of the development of my country's informatization manufacturing technology. In view of this, a set of CAD/CAM integrated software system for sheet metal nesting has been developed, which mainly solves the problems of design calculation, optimized nesting, and numerical control technology in sheet metal cutting.

2 System function and overall structure design
According to the requirements of production practice, the overall function of the CAD/CAM software system for sheet metal nesting includes three parts: feature processing, nesting processing, and NC front and rear processing. The feature is a geometric entity composed of a closed outer contour and several closed inner contours with manufacturing significance. Since the feature contains two parts of information, geometry and manufacturing, the feature processing must have the functions of two-dimensional graphics editing, processing contour generation and two-dimensional manufacturing feature generation; the arranging sleeve mainly arranges features of different shapes on the blank and guarantees the arranged features It has the largest area utilization rate and the smallest possible waste rate for the blanks, so the nesting processing must have the nesting task management, automatic and manual nesting and sheet material, residual material, and waste management functions; NC front and rear processing is mainly To process the NC cutting data with arranged features, the NC process parameter setting, NC equipment selection (supporting multiple cutting equipment), cutting compensation calculation, cutting direction planning, cutting lead processing, NC command generation and cutting are required for NC front and rear processing. Simulation function. Based on the requirements of the above system functions, the overall structure of the CAD/CAM software system for designing sheet metal nesting is shown in Figure 1.

3 Several key technologies for system realization
Since most of the international commercial CAD/CAM uses Visual C++ as the development platform, considering the needs of international connection and interface, this system also chooses VC++ as the basic development tool. It can be seen from Figure 1 that the feature processor is the source data module of the entire system, the nest processor is the key module for realizing the kernel algorithm of the nest processor, and the NC processor is the key module of CNC machining. Graphic editing and feature generation, arranging core algorithms and NC process processing have become key technologies for the development of the entire system. The following are introduced separately.

(1) Graphic editing and feature generation
The sheet metal nesting CAD/CAM software system defines a feature as a geometric entity composed of a closed outer contour and a number of closed inner contours with manufacturing significance. Therefore, graphic editing is the basis of feature generation. Although international commercial CAD/CAM software has been very mature in graphics editing technology, it is also very challenging to use VC to develop graphics editors completely from the bottom. Considering the two-dimensional graphic structure of sheet metal features, the graphic editor should have the functions of generating, transforming and editing general two-dimensional geometric pixels.

Among them, the most difficult part is graphics editing and stretching, trimming, extension, rounding, chamfering, etc., which involve the calculation of intersections, tangent points, and reparametric calculations between different graphics objects. Graphic editing also involves the necessary operations for interactive design such as graphic picking and dimensioning and measurement. In order to solve the above problems, the system strictly adopts the OOP programming idea, and designs Pub Tools, Ge-ometry Objects2D, Geometry Trans2D, Geometry Mappin92D, Screen Pick2D and other dozens of classes to complete these complex operations. Due to space limitations, this article cannot list the definitions and functions of each class. It only takes the two-line rounding operation process as an example for a brief description, as shown in Figure 2.

The transformation from geometry to features requires rational contouring. The so-called rational contouring is to find a closed contour in a geometric figure and remove the singular points caused by calculation in the contour. This system realizes the generation of a closed graph chain through interactive selection and automatic search, and then realizes rational contouring through segmented processing.

According to the system's description of the feature, a closed outer contour and several closed inner contours constitute the geometric data of the feature. Therefore, feature generation is also a search process to realize the search and representation of feature data.

(2) Comparison and selection of arrangement algorithms
The arranging algorithm of sheet metal cutting is called Cutting Stock Problem (CSP) in mathematics. The core work is how to arrange plane graphics of different shapes on one or more rectangles without overlapping to make the area utilization of these graphics. The highest? This problem is NP-complete and also NP-hard. It is very difficult to find the optimal solution. Approximate algorithms are mainly used to solve some problems. At present, common approximate algorithms include genetic algorithm, ant algorithm, simulated annealing algorithm, greedy algorithm and so on. These algorithms use different search strategies to give approximate solutions to CSP from different sides (constraints), and provide meaningful attempts to solve CSP in theory and application. These algorithms have different speed and efficiency in different fields. There are some residual rectangle algorithms, but they are not suitable for occasions where the shape changes greatly. This system adopts a new search strategy, adopts different methods for circle, rectangle, and polygon, and comprehensively handles it. There is a detailed analysis and introduction of this algorithm, here only the basic ideas are listed for readers' reference, as shown in Figure 3.

Since the calculations such as "circle set circle", "rectangle set rectangle", "convex sleeve convex", and "convex decomposition" all have corresponding practical algorithms, the realization of the above calculation is not difficult. It is especially pointed out that the nesting of sheet metal cutting needs to consider various cutting parameters such as compensation radius, lead length, machining allowance, contact distance, etc. before nesting. As for the issue of sheet material utilization, the author believes that with the further improvement of the CSP solution algorithm, the system will gradually improve.

(3) The key technology of NC front and rear processing
Although sheet metal cutting is only a simple two-axis machining, as a general-purpose software for processing mass cutting, there are still some problems that need to be dealt with carefully in the numerical control process. Especially the three aspects of cutting lead problem, cutting order problem and cutting path optimization.

Cutting lead is necessary pre-processing data for flame cutting. In order to ensure the smooth cutting contour, the cutting lead-in and lead-out lines should be added near the theoretical contour line (the actual production is usually 20 mm) during flame cutting. According to the cutting process requirements, the characteristic contours of different shapes have different requirements for the shape and position of the lead. For example, the arc profile generally needs to select the arc lead, and the arc direction must be judged. At the intersection of two clues, the judgment is more complicated. As shown in Figure 4(a), at the intersection of two outer contour lines, there are three situations that need to be judged. If these three cases are inner contours (as shown in Figure 4(b)), the situation is much more complicated!

This system adopts the method of taking the middle of the arc, taking the end of the straight line, and giving priority to the arc, which solves the problem of automatic lead generation.
Since the nested parts usually have size matching in the adjacent relationship, the cutting sequence should be handled in the pre-processing to avoid the change of the shape of the small parts caused by the thermal deformation of the plate. In principle, the strategy of first small and then large is adopted. . In order to meet the needs of different users, the system adopts the "recommend-choose" mode, that is, the system recommends several modes according to the setting of process parameters, and finally the user determines by himself.

Path optimization refers to an optimization relationship that avoids excessive idle travel after determining the "neighbor relationship", "lead relationship" and "compensation relationship". The path optimization algorithm belongs to the "knapsack problem", which is relatively difficult. The system adopts the path optimization method, which can basically solve the problem.