Investigation on thread rolling processes of screws for intelligent thread rolling system

The traditional screw production equipment is very large, and many procedures need manual operation, which has a certain influence on the production of screws. In order to improvement the production efficiency, a new type of “Intelligent Thread Rolling System” is to be designed for the requirement. In this system, it is necessary to provide and transmit the relevant data for screw manufacturing, especially in the process of thread rolling. The thread plate design changes the traditional flat shape to become the curved shape. This paper has used the numerical model of the finite element method to calculate and simulate thread rolling process of thread plate of curved shape to understand the applicability. The results of relevant parameters on the thread rolling are obtained to compare with experimental data of Intelligent Thread Rolling System. Under certain test condition, the screw embryonal material can produce threads and cut off tails, but the created thread shape and depth are non-standard. The results are consistent with experimental data. The main reason is that the thread surface of thread plates of curved shape has not been redesigned. Due to the comparison and verification, the numerical parameters can be provided for the actual thread rolling operation of the ITRS. This is very helpful for the completion of the ITRS development.


Introduction
The screws are essential basic components in all industries, and have been widely used in fields such as machinery, electronics, automotive, aerospace, and civil engineering.With the progress of the times, the screws are required to improve the lightweight and forming accuracy.The production equipment for the screws is an important role.The production of screws consists of a complex process, including the supply of wire rod in the early stage, the head and thread rolling of screws in the middle stage, and the quenching and tempering, surface treatment in the later stage.Thread rolling processes of screws is a key factor during the overall manufacturing, so many researchers have been focused in the work.In the past, Lovell et al. [1] used the finite element method to calculate the effect between the parameters such as friction force, and approach distance with the stroke load about different materials during thread rolling.Ivanov et al. [2][3][4][5] had derived the rolling process with the contact condition between the thread forming surface and the thread plates, and other basic parameters.The results were supported the design of the rolling tool and the rolling of internal threads.Domblesky et al. [6][7] compared the formation of thread for two different size screws by DEFORM-3D software.The effects were analyzed by material flow, thread shape and size, friction factor, and plastic flow stress during thread rolling.Pater et al. [8] had established a new type of thread rolling method for a specific specification screw by using Finite Element Method and Finite Volume Method.The results showed that thread was to be formed at any position on the embryo shaft.Cheng et al. [9] simulated the thread rolling process of screws without adding boundary conditions by DEFORM-3D software.Based on simulation results, the constant shear friction coefficient should be at least 0.6 or higher to improve the phenomenon of slipping of thread rolling.Kuo et al. [10] used the CAD/CAE/CAM system to simulate the thread rolling process for screws.The analytical results were to be obtained generally the same as the experimental results.It was also known that the thread angle was closely related to the stress and strain.Hsia et al. [11] were verified in an actual forming process of screws by design software and forming analysis software.The numerical analysis process was to be utilized to reduce the cost of development and promoting the overall competitiveness of the company.The traditional screw production equipment is huge and many processes require manual operation.In order to improve productive efficiency and product performance, a new type of Intelligent Thread Rolling System is to be designed for the requirement.The thread plate design changes the traditional flat shape to become the curved shape.From the literature on the thread rolling process described above, the thread plate die is mainly a flat shape, and is not a curved shape.This paper will use the numerical model of the finite element method to calculate and simulate the thread rolling process for thread plates of curved shape to understand the applicability.

Formulation
The schematic diagram of Intelligent Thread Rolling System (ITRS) is shown in Figure 1.Based on the design of the new system, the thread rolling die of curve shape has become an important key.In order to verify the feasibility of the system and reduce development costs, the thread rolling plates of flat shape which is provided by the Chun Zu Machinery Industry company is directly modified the thread plates of curved shape.In this study, the thread rolling molds of curved shape is used to simulate and analyze the thread rolling process of screws.The concepts of the Finite Element Method (FEM) are variation principle and discretization.The FEM formulation is used for rigid plastic material, and its function is as follows.
The first order variation of Equation ( 1) is obtained Equation ( 2 Symbolic description of Equations 1 to 4 is that  is effective stress, ̅ is the effective strain rate,  is surface tractions, and V and S are volume and surface of the deforming work piece, K is the penalty constant, and  s the volumetric strain rate.respectively.When the velocity fields satisfying the basic equations of the finite element method are solved, the corresponding stress values can be calculated using the plastic flow rule and the known stress distribution.

Simulation software
Based on the modified Lagrange's theory and finite element method calculations, this study can use DEFORM-3D software [12] to simulate the thread rolling process of screws on thread plates of curved shape.The architecture of DEFORM-3D program includes pre-processor, simulation engine, postprocessor, and utility module.In pre-processing module, the workpiece is created a grid system, and set various simulation conditions and usage factors.In simulation engine, the simulation control can be established the iteration number of simulation process, main module object, convergence method, convergence error, and thermal action options.About numerical method, these equations are first discretized into their algebraic counterparts by the standard FEM.DEFORM-3D software supports different iterative computations, such as the Newton-Raphson, Direct, and Explicit computations, which can be selected based on real condition.After completing the simulation process, the post-processing module can be arranged and analyzed the numerical results to plot required relative figures.

Results and discussion
The thread surface of moving plate has to be convex about 14.01 degrees, and the thread surface of fixed plate has to be concave about 14.01 degrees for the overall design requirements (Figure 2).The thread surfaces of the thread rolling die are not redesigned, and are directly modified for the original CAD drawings provided by the Chun Zu Machinery Industry as shown Figure 3-4.To verify the accuracy of the simulation analysis, the experimental operation equipment is shown in Figure 5. DEFORM-3D software simulates the thread rolling process of screws, by using 3.5x35mm experimental screw embryonal material to set up a grid system.The relative position of the two thread plates of curved shape and the screw embryonal material is according to the experiment condition as adjusted possible as.These operating position is plotted in Figure 6, and the test conditions are shown in Table 1.The number of grid is 80000 for the screw embryonal material as shown in Figure 7.The distance between the two thread plates is fixed, with the friction coefficient between screw embryonal material and two thread plates of 0.4 for Case 1 and 0.8 for Case 2, respectively.The screw embryonal material can roll about 1/4 of its stroke length while the remaining part of stroke length moves.The rolling stroke length with a friction coefficient of 0.8 is slightly more than the friction coefficient of 0.4.The depth of thread lines produced by Case 1and Case 2 are similar with little difference and their tails are not cut off.The similar results are obtained in comparison with experimental operation (Figure 8).The depth of thread lines is caused by the large distance between the two thread plates.The tail of the screw embryonal material is not able to be cut off because of the inability to complete the rolling on the full stroke length.However, a larger friction coefficient can help the thread rolling process of screws.The fixed friction coefficient is 0.6, and the distance between the two thread plates is as shown in Case 3. The thread rolling process of the screw embryo material starts with the formation of thread lines, and gradually deepens with increasing time (Figure 9).As can be seen from Figure 9, due to the decrease in the distance between two thread plates, these thread lines have the obvious depth.The rolling of the screw embryo material is more 1/3 of the stroke length, it is still impossible to cut off the tail.The experimental operation has similar results, as shown in Figure 10.The distance between the two thread plates is reduced to Case 4. It is found that the screw embryo material which has more obvious and deeper thread lines are pinch-off within 1/2 of the stroke length (Figure 11).According to the test conditions, the distance between the two thread plates is sure between Case 3 and Case 4.  In the simulation process, the screw embryo material between the two thread plates wants to be increased the rolling stroke length by the condition of Case 5.According to the simulation results shown in Figure 12(a), the screw embryo material has rolled about 1/2 stroke length, and the tail has been cut trace.The tail cannot be cut off due to movement on the remaining stroke length.The experimental data shown in Figure 13 have similar results.In Case 6, due to the more proximity of the thread surfaces of two thread plates, the simulated results are obtained that the thread lines are obvious and has to be pinch-off.The rolling of the screw embryo material is about greater than 1/2 of the stroke length.There is a cut trace in the tail which is still unable to be cut off, as shown in Figure 12(b).
The distance between the two thread plates is adjusted to Case 7. In Figure 12(c), the rolling of the screw embryo material is greater than 3/4 of the stroke length, and its tail has been cut off.As shown in Figure 13, the experimental data also has similar results.During the tail cutting process of screw embryonal material in Figure 12(c), it is found that the shear strain at the cutting position is larger than that at other positions at the same time.The results have indicated that the location bears large shear stress, as shown in Figure 14.In Figure 15, By setting a point close to the cut-off position, from the analytical result, the shear strain at this point becomes large to increase with time.As the tail is cut off, the shear strain reaches the maximum.In order to achieve thread generation and tail cutting of the screw embryo material, various test conditions have used, which are not listed in Table 1.It has been found that the simulation results are similar to those of Cases 5 to 7, with the main differences being the depth of the thread lines and the pinch-off location of the screw embryo material.

Conclusions
By using DEFORM-3D software to simulate and analyze the thread rolling process of screws on the thread die of curved shape for the ITRS, the following conclusions can be summarized: (a) To simulate the thread rolling process, it is very important to set the relevant parameters of the DEFORM-3D software, especially the value of the friction coefficient, which clearly affects the rolling stroke length of the screw between the thread plates.(b) Due to the relative position of the two thread plates, there is a significant effect on the depth of thread lines, pinching, or cutting tail of the screws.(c) It is discovered that the screw embryo material can be cut off the tail because of completely rolls during stroke length on a specific distance between the two thread plates.The tail cutting of screw embryo material is not easily to achieve in all simulation process.The main reason may be that the thread surfaces of the two plates are directly curved from the surface of original thread plate of flat shape.(d) The screws cannot be achieved perfect thread shapes, regardless of software simulation or experimental operation.The dimension and surface of thread die of curved shape need to be re-designed to develop the ITRS.(e) These obtained results are in accord with the experiment data by the simulation of thread rolling process of DEFORM-3D software.The numerical parameters can be provided for the actual thread rolling operation of the ITRS.This is very helpful for the completion of the ITRS development.

Figure 1 .
Figure 1.The schematic diagram of intelligent thread rolling system.

Figure 6 .
Figure 6.The relative position of the two thread plates and the screw embryonal material.

Figure 9 .
The thread rolling process of the screw at different time in Case 3.

Figure 10 .Figure 11 .
Figure 10.The thread Shape of the screws on experiment in Case 3.

Figure 12 .
Figure 12.The thread shape of the screws in Case 5, Case 6, and Case 7.

Figure 13 .
Figure 13.The thread shape of the screws on experiment in Case 5, Case 6, and Case 7.

Figure 14 .
Figure 14.The shear strain at the cutting position.

Figure 15 .
Figure 15.The variation of shear strain at the position of a cutting point with time.

Table 1 .
Test conditions for the thread rolling process of the screw embryo material.