Zhifang Wei, Yechang Hu Wu Lyu and Jianzhong Gao(.College of Mechatronic Engineering, North University of China, Taiyuan 0005 China; .No.08 Research Institute of China Ordance Industries, Beijing 00, China; .Heilongjiang North Tool Co., Mudanjiang 57000, Heilongjiang, China)

The application and development of model-based technology and 3D process design technology have changed the working model and application system of bullet manufacturing industry, where information sharing between design and manufacturing based on the 3D model[1-2]is crucial. Designers develop reasonable manufacturing technical procedures by simulating the entire forming process based on a 3D digital model in the 3D process management system[3]. These technical procedures must include an intermediate three-dimensional process model corresponding to every technical status and characterizing the shape and size change of the workpiece after a specific manufacturing procedure[4]. The construction of an in-process model that includes dimensional and technical information is important to the 3D process. Numerous scholars have studied the establishment of a 3D process model. Jain P K and Kumars[5]established an in-process model by extracting a 2D model wireframe in a PRIZCAPP system. Babic B R[6-7]created a process model by identifying IGES data and extracting model characteristics with the developed CAPP system. On the basis of the mapping relationship between process information and model information, Zhang Hui[8]proposed that the intermediate in-process model can be derived by reversing the final design model to the rough model. Wang Zongyan[9]first solved the rough model based on the final product model, then restored the final design model from the rough model, and finally obtained the middle process models according to the modeling method of the manufacturing characteristics in the restoring process from the original model to the final model. Xi Wu[10]proposed that the 3D model is generated adversely from the process design based on the database. All these studies obtained each intermediate procedure model from reversing the final result models by calculating structural sizes according to correlative design theory. This process is complex and only applicable for machining parts. Few studies on the creation of a 3D process model for punching techniques are available, and punching techniques are widely used in the manufacturing of cartridge parts.

Fig.1 Punching and forming process of cartridge cases

Aimed at the punching and forming process of cartridges, the construction method of the intermediate three-dimensional process model is studied in this paper. The simulation model of each procedure can be obtained, and the process analysis results, such as stress contours and deformation curves, are achieved by performing continuous simulation on the forming process of cartridge cases with DEFORM software, as well as according to the volume invariability principle. Moreover, the simulation model cannot annotate the geometric and process information; thus, a 3D annotation module is developed based on the secondary development of UG NX software, with which the geometric features such as points, lines, and surfaces can be rebuilt to correspond with the simulation model exported as an .stl format file from DEFORM software. Then, the 3D annotation on the simulation model can be completed by applying the PMI marking function of UG software. Consequently, the practical in-process model’s annotated geometric information and process information can be obtained and applied in the 3D process design of the cartridge case.

1 Finite Element Simulation on the Punching and Forming Process of Cartridge Cases

Cold punching or cold forging techniques are mostly used in manufacturing bullets[11]. In China, there is a huge manufacturing capacity and high quality and interchangeability for bullets. The forming process of cartridge cases generally includes blanking, cupping, drawing, indenting, heading, piercing, and tapering (Fig.1)[12]. The 3D punching and forming process design of cartridge cases is completed using the 3D process management system (Fig.2). A 3D in-process model should correspond to each processing state to characterize the changes in product shapes and sizes. Thus, the in-process model must be constructed meticulously. The finite element simulation of the punching and forming process of cartridge cases is performed using DEFORM software. Then, the simulation models of each intermediate procedure are obtained.

DEFORM software is a professional analysis software for the metal forming process. The valuable process analysis data for the deformation of metal flow are provided, and the finite element analysis for forging, rolling, and other processes is integrated into the uniform operation interface in the multi-process operation integration system. Hence, the simulation of any process can be optionally achieved, and the card type management of each process parameter realized. The continuous analysis of the entire process, including the formation and heat treatment, can also be completed. The forming process of cartridge cases is simulated continuously using DEFORM software, that is, the procedures of blanking, cupping, drawing, indenting, heading, piercing, and tapering, are simulated in sequence. Consequently, the 3D simulation models of each intermediate procedure are obtained. The simulation flow chart of the process is shown in Fig.3.

The connection of simulation models between the seven procedures is important to continuously simulate the forming process, where the roughpart is continuously deformed into the final formed part. Accordingly, the simulation result model after one procedure should be the blank model for the next procedure. DEFORM software has a self-contact condition definition and a multi-die coupling analysis function. Hence, numerical simulations of the whole punching process of cartridge cases can be completed. In addition, grid fining can be performed for the next procedure simulation based on the previous procedure simulation result model by using the remeshing function of DEFORM software. Thus, the problem of grid fracture involved in some procedures can be solved, and finite element models of the punching article can be transferred from one procedure to the next between the seven procedures. DEFORM software also adopts the solving methods of Lagrangian algorithm, such that grid convergence does not need to be high. Consequently, completion of the continuous simulation of the entire forming process of cartridge cases can be ensured even when abnormal grids are produced. In the finite element simulation model of the punching and forming process of cartridge cases, the correlative information of deformation history and strain-stress field after one procedure simulation is saved in the corresponding .Db file. Then, after adding the corresponding mold model, the next procedure simulation begins by automatically succeeding to the previous information of deformation history and strain-stress field through the .Db file.

Fig.2 3D process management system

Fig.3 Process simulation flow chart of punching cartridge cases

2 Simulation Model of the Typical Punching Procedure

The punching formation of cartridge cases includes thinning of the rough and the fracture of material, which are different in simulation modeling. These two steps are involved in the drawing procedure. Thus, the drawing procedure is considered the typical punching procedure, and its finite element simulation modeling method is studied.

The drawing procedure of cartridge cases is the further thinning process of the cupping article after the cupping procedure (Fig.3). After the drawing procedure, the body length and thickness of cartridge cases are already determined. Thus, the stress change and the final forming size of the article in the drawing procedure are the concerns of designers.

2.1 Creation of geometric models

The geometric models for drawing procedure simulation include the geometric models of the rough article and the drawing dies. The 1/4 3D solid models of the upper and lower dies of the drawing procedure are created with UG NX software given the axial symmetry of the punching dies. The assembled position of geometric models can be located with the absolute coordinate system in UG NX software and in DEFORM software. Thus, the assembled position relationship of the upper and lower dies is defined by the absolute coordinate system in UG NX software. The assembly model of the drawing dies is established and exported in .stl file format. The geometric model of the rough article in the drawing procedureis achieved from the simulation result model of the cupping procedure, as shown in Fig.1. After completing the cupping procedure simulation in DEFORM software, the cupping dies are deleted and the drawing dies are imported in .stl file format. Then, the simulation result model of the rough article, through the cupping procedure, is moved to the proper position along theZaxis direction. Consequently, the geometric model of the drawing procedure is created.

2.2 Material model definition

The plastic deformation and the stress distribution of the rough article are the main concerns during the punching and forming process of cartridge cases. Thus, the rigid-plastic finite element method is used to simulate the process. In the finite element simulation model of the drawing procedure of cartridge cases, the material property of the rough article succeed to the round blank in the blanking procedure, which is defined as copper-clad steel, where the base steel is 20#steel, and the thickness of the H68 copper layer (CuZn37) is less than 4%. The influence of the copper layer on the material model is ignored owing to the thin copper layer of the blank article[14]. Thus, in the blanking procedure simulation model, the material model of the round blank is a plastic body, in which 20#steel is selected to substitute for the copper-clad steel. The flow stress[15]equation is as

(1)

whereAandBare the material parameters,εeqis the equivalent strain, andnrepresents the hardening index. The relative coefficients of 20#steel at room temperature, as obtained by Fan Zhiqiang, are as follows:A=258 MPa,B=329 MPa, andn=0.235[16]. In addition, the forming dies are all defined as a rigid body in each simulating procedure.

2.3 Refinement of grids

Because of the material fracture near the port place of the forming article in the drawing procedure, grids inherited from the simulation result model of the cupping procedure are supposed to be partially refined. The weighted factor is 0.5, the other factors are set by default, and the size ratio is 5. After refinement, the partial grids decreased by five times that of the original grids, and the number of units reached 150 000, as shown in Fig.4.

Fig.4 Partially refined grid in deep drawing process

2.4 Contact model and material failure model

In the drawing procedure of cartridge cases, the shear stress arises when the dies and the metal surface of the rough article move relatively, and the friction force can be calculated based on the Coulomb friction criterion and the shear stress friction criterion. When defining the contact model between the rough article and dies in the simulation model, the contact surface of the rough article is set as the main plane, and the contact surfaces of the upper and lower dies are the target planes. The shear stress friction criterion is used to calculate the friction force caused by the strong contact surface pressure and the dramatic metal deformation as[17]

(2)

wheremis the friction factor, andYrepresents the flow stress of the material. The friction factormis 0.08 during the entire forming process simulation.

In the drawing procedure simulating model, the disappearance of fracture units is related to the material failure model, which is the Ayada ductile fracture criterion shown as

(3)

2.5 Movement relationship determination

In the drawing procedure, the lower die is fixed and the upper die drives the blank down. To simulate the actual situation in DEFORM software, the lower die model is added with fixed constraints and the upper die model is added with speed constraints which drives the rough article down. When the 1/4 model is adopted, the symmetry constraints are applied for the symmetry planes of the rough article.

2.6 Simulation results

After completing the finite element simulation modeling and solving the drawing procedure, the corresponding simulation results, including mainly the result model of the forming article shown in Fig.5 and the stress and strain contours, are obtained. When the die models of the next procedure is imported and the new simulation file is created, the next procedure can be simulated continuously and directly. In the meantime, the model of the forming article can be exported as the .stl format file from the DEFORM software, then imported into the UG software, annotated with the relative geometry information and process information, finally can be served as the final three-dimensional process model. The stress contour of the forming article when fractures come into existence is shown in Fig.6, where the fracture position and the maximum stress (678 MPa) can be observed. According to the relative position of the fracture, the rough size of the blanking procedure can be designed reasonably to save the material.

Fig.5 Simulation result model of deep drawing process

Fig.6 Stress nephogram at fracture

3 Three Dimensional Annotation Module

The in-process model can be displayed in both design and manufacturing by means of the collaboration platform of the 3D process associated with UG NX software. When designing the 3D punching and forming process of cartridge cases, the in-process model with information, such as size, tolerance, and manufacturing process, is the only production basis. The 3D model of each procedure can be obtained from DEFORM software simulation in an .stl format file, which cannot capture the modeling features to annotate in UG NX software and cannot satisfy the requirements for the in-process model. These challenges are encountered by the industry.

The .stl format file cannot be annotated in UG NX software because the geometric features, such as points, lines, and faces, cannot be identified and captured. These features are analyzed with the use of HOOPS library of C++ and outputted to a .txt format file. Then, the points can be rebuilt with the above geometric information through the function UF_CURVE_creat_point () of the UG/OPEN class library. The rebuilt points are associated with the model in the .stl format file using the functions of FindObject () and SetFirstObject () of the UG/OPEN class library. Similarly, lines, angles, faces, and other geometric features can be generated by the rebuilt points with the relevant functions. Finally, “MBD design support PMI tool” is developed to regenerate these geometric features based on the model in the .stl format file. Thus, the 3D annotation can be completed, and the required information on geometric features and process can be added by applying the PMI tool. The flow chart of the 3D annotation is shown in Fig.7.

The main steps include the following:

① The model simulated in DEFORM software is imported into UG NX software.

② The discrete points are generated on the model surface. Upon clicking “initialize STL file”, a number of points on the surface of the imported model in the .stl format file will be generated. When the model is greatly deformed, more points are produced.

③ The length can be marked directly in 3D by capturing the points. The angular dimensions can be marked through the “two points make a line” order, which generates the angle lines.

Fig.7 3D annotation flow chart

④After annotation, the size and tolerance are checked, and the in-process model is completed.

4 Case Study

The punching and forming process of 9 mm cartridge cases is considered an example. The seven simulation models for the 7 procedures such as cupping and drawing are obtained based on the finite element simulation of punching process, and the final 3D process models are obtained by adding dimensions, tolerances, simulation, process and other information through the 3D annotation module(Fig.8).

To verify the feasibility of the simulation models, which are considered as in-process models, they are compared with the physical models in size. The 3D model in the cupping process based on simulation is shown in Fig.9, and the physical cupping model is shown in Fig.10. The relative error between the models is less than 2% in terms of the inner diameterr, the outer diameterR, the body lengthH, the inner cone angle, and other important sizes in comparison (Tab.1). Therefore, the feasibility of constructing the in-process model for punching cartridge cases based on simulation is verified.

Fig.8 Final process models

Fig.9 3D simulation model of the cupping process

Fig.10 Physical cupping model

Tab.1 Comparison of key dimensions between simulation model and physical model

5 Conclusion

To construct in-process models in the design of the 3D punching and forming process of cartridge cases, the punching process is simulated with DEFORM software. To annotate the simulation models imported into UG NX software, the 3D annotation tool is developed with UG NX software secondary development technology. Finally, the in-process model, which satisfies the requirements, is obtained. In comparison with the physical model size, the validity and reliability of the method is verified. The construction efficiency of the in-process models can also be greatly improved using the method, and the reverse modeling course, which is widely used, is greatly simplified. Moreover, the stress and strain distributions of the forming article are highly significant for optimizing the process parameters and the design of dies.

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