Improvement of the Method for Fixing a Punch in the Punch Holder

Featured Application

Improvements of punching processes.

Abstract

The punching process allows large quantities of parts to be produced at very low cost. This paper studies how the technique used for fixing a punch can be improved in order to increase productivity before punch fracture, which results in large numbers of parts failing to be produced, thus creating a significant shortfall. In this context, the study deals with an industrial case, specifically the manufacturing (metal sheeting and metal forming) of a connector made of stainless steel. A broken tool is first analyzed in order to identify the source of the premature breakage. Then, the tool and the process are modeled using finite element analysis (FEA) to act as a reference. Then, the improvements in the geometry and fixing method, intended to increase the tool lifespan, are assessed and modeled using FEA. Finally, the modified profile with only one central hole proves to be very efficient.

1. Introduction

Part designers are eager for improvements in the performance of tools, particularly in terms of fatigue but also with regard to the finish of the part obtained. The production of any part requires cutting at some point and one of the most widely used manufacturing methods is punching, a process which is required in, for example, the production of flat springs necessary in many industrial sectors, such as the automotive, aeronautical, electronics, and micromechanical industries, etc. The punching tools that manufacture flat springs from stainless steel sheets bend and cut the material. Unfortunately, during production, the punches gradually wear out and sometimes break suddenly, resulting in a significant shortfall after about 3000 cycles, since changing the tool after a fracture requires at least two hours of work. The fracturing of the tool has been studied by many authors and many parameters that influence tool wear and fracture have been explored with the aim of finding a way to ensure sufficient tool lifespan. For instance, a study by Böhler Uddeholm [1] showed the effect of the clearance between the punch and die and pointed out that the optimal clearance is equal to 10% of the thickness of the metal sheet. Tekiner [2] and Gréban [3] claimed that, by increasing the contact pressure between the sheet and the tool, too little clearance increases the cutting force required and causes rapid tool wear. In addition, Picas et al. [4], Hambli [5], and Falconnet et al. [6] studied the effect of the punch corner radius in reducing the stress on the edges, thereby counteracting edge chipping. A study conducted by Gürün [7] also showed the effect of the punch angle on the shearing force. When a punch angle is used, the required penetration depth is increased for the punch-out operation to be fully completed and this increase in the required penetration depth may lead to accelerated wear on the punch tool.

Several factors linked to the cutting process have been discussed. Kurniawan et al. [8] studied the effect of the cutting speed of the punch, showing that the cutting force increases with the speed of the tool and, therefore, increases the punch wear. The influence of temperature on the behavior of the material used in cutting is also very important [9]. Zunkler, cited in Johnson et al. [10], presents the influence of temperature rise on the maximum cutting force. Balendra and Travis [11] studied the effect of the hardness of the steel. They performed a set of low-speed and high-speed cutting tests that showed that the cutting force increases with the hardness. Many experimental studies [12,13,14] showed the different factors affecting the punch wear and, in a study conducted by Won et al. [15], the spring back effect was elaborated on, showing that, without pressure on the holder, it is possible to have severe abrasive wear on the punching pin surface prior to stripping failure. He [16] and Haddadou and Aichoun [17] studied the different types of fracture that could occur and put forward potential solutions to minimize their effects. Finally, Mikich [18] studied the evolution of the burr during blanking. The appearance of the cut edges was examined by means of a topographic survey in order to monitor their evolution. The studies conducted so far do not, however, explore the influence of how the punch is fixed to the punch holder. The work presented below is based on an industrial case where a punch dedicated to the manufacturing of connectors made of stainless steel shows a premature break at the level of the pins holding it. The main issue that will be resolved is the determination of the optimal fixture, thereby avoiding the high constraints causing the tools to break. Different approaches are discussed and studied in order to achieve a significant reduction in the stresses on the tools. The punching tool is improved by changing the geometry of the cutting edge, changing the way the punch is fixed, and avoiding the use of eccentric pins. First, the classic fixture model is introduced, then, using a 2D finite element model (FEM), the optimal centered fixture is investigated, from the optimal pinhole diameter to the optimal pinhole shape.

2. Materials and Methods

In this study, some broken punches are first analyzed in order to identify the source of breakage. Then, an FEM is built as a reference to evaluate the efficiency of potential improvements. The initial method to attach the punch to the punch holder uses two eccentric pins, one on either side of the tool. This method is used to divide the force in two so that each pin supports half the effort, and the pins are unaligned, as shown in Figure 1. The initial punch had a width of 15.75 mm, a thickness of 0.9 mm, and a length of 81 mm. The lower part of the tool (cutting edge) had an angle of 93 degrees with the side and the two pinholes had a diameter of 5 mm.

The operation of fixing the punch to the press is divided into two steps; the first consists of attaching the punch inside the punch holder (Figure 2) as described earlier, and the second step is fixing the punch holder to the shock plate at the top of the punch holder using a screw. This fixing technique allows the mechanism of the punching process to function.

During the perforation, the shock plate pushes the punch down by applying pressure to the top surface of the punch holder. Equation (1) demonstrates the applied force.

F_cutting = (R_g/blank P_punch e_punch),

(1)

where:

• F_cutting is the cutting force in N;
• R_g/blank = 0.8 × R_t/blank is the sheet metal shear breaking strength in MPa;
• R_t/blank is the sheet metal tensile strength in MPa;
• P_punch is the perimeter of the cutting surface of the punch in mm;
• e_punch is the thickness of the metal sheet in mm.

The next step of the punching process is the extraction of the tool from the metal sheet. The two pins are responsible for this action, which they achieve by coming into contact with the punch pinholes. The extraction force usually represents 7% of the cutting force [18] (2).

F_extraction = 7 F_cutting/100,

(2)

The material used for the tool was non-cobalt grade for high performance cutting tools, cold work tools and rollers for cold rolling, perfect for the punching process. The chemical composition and the mechanical properties (from mile sheet) are given in

Table 1. Chemical composition of the punch.

C	Cr	Mo	W	Co	V
1.28	4.0	5.0	6.4	-	3.1

and

Table 2. Mechanical properties of the punch.

E (GPa)	ρ (g/cm3)	ν	Rc (MPa)
230	8	0.3	2500
Rmc (MPa)	Hardness (HRC)	Rt (MPa)	Rmt (MPa)
3333	61	2270	3026

, respectively.

The sheet metal had a thickness of 0.6 mm and the material used to produce the flat spring was a 1.4310 austenitic stainless steel strip. Such steels have a face-centered cubic structure at room temperature. This structure is non-magnetic and is stored at room temperature using suitable alloying elements, the best known of which is nickel. Austenitic steels contain enough chromium to provide corrosion resistance and enough nickel to provide an austenitic structure at room temperature.

3. Results

3.1. Observations

During the punching process, the punch wear led to a sudden breakage after 3000 cycles (Figure 3), which also led to major production stoppages. Currently, with the speed of the presses being high, a stop to change or sharpen the tooling results in a large number of parts not being produced and therefore a significant shortfall. Two types of fractures were distinguished in this case. The first type was a diagonal crack in the width of the tool; this fracture appeared at the pinhole level. The second type was at the end of the punch.

For both types, brinelling was observed on the pinholes of the punch. A Dino-Lite digital microscope (see Figure 4) was used to measure the diameter of the fixing holes after fracture and showed that it increased significantly compared to the measurement before fracture, which was initially 5 mm (see Figure 5).

In order to obtain a precise image of the brinelling on the outer surface of the hole for both fracture cases, an Alicona optical 3D profilometer was used (see Figure 6). As illustrated in Figure 7 and Figure 8, and for the microscopic observation, a silver-white shining reflection was seen on the outskirts of the upper part of the hole where the impact between the pins and the punch occurred, and a radially spreading crack was present as mentioned earlier. The crack could be a brittle fracture.

For the other type of fracture, where the crack was on the lower part of the punch, a silver-white shining reflection was seen on the outskirts of the upper part of the hole.

However, a feature that captured our attention was that, when the metrology of this fractured tool was studied, the width was observed to be too thin at the end, passing from 15.74 mm to 15.53 mm (see Figure 9) and revealing fast tool wear.

Considering the very small number of cycles before fracture, the indentations shown, and the increase in the diameter of the pinholes from 2.5 mm to 2.7 mm and 3.1 mm (

Table 3. Measurements of the two holes on the fractured punch.

Name	Length (mm)	Area (mm2)	Radius (mm)	Diameter (mm)	Angle (deg.)
AR0	5	7.844	3.138	6.276	91.292
AR1	5.013	6.856	2.735	5.471	104.993

), we can conclude that high plastification occurred during the punching process, resulting in a static failure.

3.2. Numerical Analysis

To resolve this issue, different characteristics of the tool were analyzed numerically to investigate how its lifespan could be improved and to decrease the risk of its failure. To interpret the results, ABAQUS v. 62.2 finite element (FE) software was used for the numerical modeling.

In this study, 2D simulations would be preferred to 3D simulations in order to reduce the calculation time and to enable subsequent improvements to be quickly tested. Thus, the 2D model is fully described and a 3D model is quickly exploited to validate the proposed approach.

3.2.1. Initial 2D Model

For the assembly, a punch, two pins, and a guiding system were modeled. The punch was a deformable part with the material properties introduced in Table 2. The two pins and the guiding system were rigid bodies.

The interactions between the punch and the pins, and the guiding system and the punch were:

• Tangential metal–metal contacts with a friction coefficient of 0.2, a classical coefficient for metal–metal contact;
• Hard contact.

The punching process was assimilated to two steps, compression and tension. Compression represented the cutting of the sheet with a magnitude of 20,000 N and tension represented the extraction of the tool from the metal sheet with a magnitude of 1400 N, found with Equation (2).

For the boundary conditions, the pins and the guiding system were fixed in all directions except the “y” direction (see Figure 10). The punch was fixed on the upper surface during the compression step to model the contact with the shock plate, and was free in the “y” direction during the traction step.

One of the important aspects of numerical modeling is the mesh used for the parts. During this operation, the deformations at the level of the punch pins and of its lower edge were significant. To correctly model the mechanical phenomena occurring in this area, and in the contact between the different parts, a regulated mesh of small quadrangular elements was used. In the partitions, the mesh was structured with an element size ranging from 0.2 to 0.5 mm (the smallest elements were used near the breakage areas to accurately model the strain gradients). For the rest of the tool, it was a quadrangular free mesh with an element size of 1.5 mm (see Figure 10).

The results of the numerical analysis in compression illustrated in Figure 11 show that a von Mises stress of 2832 MPa appears at the pinhole where the crack starts. We note that this value is greater than the elastic limit in compression of the material used for the tool, which implies that plastification does indeed occur during the first cycle, highlighting the existence of an irreversible deformation of the pinholes, which creates additional stress between the pin and the hole after several cycles.

During the tension phase, we can observe in Figure 12 a maximum stress of 2300 MPa, concentrated at the level of the pins. This stress is greater than the elastic tensile strength of the punch (2270 MPa). As such, there is plastic deformation of the tool from the first extraction of the punch from the steel sheet.

3.2.2. Associated 3D Model

Moreover, as this study mainly focuses on the fixing of the tool, a preliminary 3D model of the upper part of the punch has been built in order to evaluate the accuracy of the initial 2D simulations.

This model (see Figure 13a) utilizes a detailed design of the punch meshed with tetrahedral elements, the same material properties, boundary conditions, and load as the 2D model.

The 3D model gave similar results to the initial 2D model in the areas of interest (near the holes) in compression (Figure 13b) and in tension (Figure 13c), so 2D models were chosen to test some potential improvements.

4. Discussion

4.1. First Improvement of the Tool

In order to improve the lifespan of the punch, two issues were discussed. The first one was in relation to the shape of the cutting edge of the tool. A slight increase in the gap between the tool and the matrix was suggested with reference to previous studies [1,2,3], and a convex shape was proposed with reference to Singh et al.’s study [19], which showed that a convex cutting edge with an angle of 22.5° results in minimal radial deformation for punches compared to flat or inclined cutting edges.

Following this suggestion, a numerical analysis was carried out using ABAQUS. As expected, the results (see Figure 14) showed a reduction in the stress on the cutting edges during compression.

4.2. Second Improvement of the Tool

The second issue discussed was the fixture position. In fact, most of the previous studies focused on the cutting part of the tool and the upper part is not described [8,9,12]. In many cases, the studies exploit a cylindrical punch with an external shoulder [2,7,18,19], but this geometry is not applicable in our case. The study from Cheung et al. [14] utilizes a flat punch without defining precisely how it is fixed to the system. Considering this context, we decided to perform a dedicated study to improve the design of the initial fixing. The improvements in the test consisted of replacing the pins with a single pin in the center of the punch to reduce the maximum stress (illustrated in Figure 15). This modification generated a larger contact surface between the pin and the tool, thus a smaller stress in tension.

To find the maximum permissible diameter d_max without exceeding the elastic limit in compression, the following Formula (3) was used:

R_{e_punch} = F_cutting/A_min,

(3)

where:

• A_min = e (𝑤 − d_max) is the minimum area of the punch;
• F_cutting = 20 000 is the cutting force calculated previously in N;
• R_{e_punch} = 2500 is the elastic limit of the punch in MPa;
• 𝑤 = 15.75 is the width of the punch in mm;
• e = 0.9 is the thickness of the punch in mm.

The maximum value of the diameter was therefore d_max = 7.03 mm.

To find the optimal value of the diameter, numerical analyses were conducted on diameters of 2, 3, 4, 5, 6 and 7 mm for compression and tension.

The results of

Table 4. Numerical results for compression and tension for different diameters.

Diameter (mm)	σcmax (MPa)	σtmax (MPa)
2	1639	2199
3	1798	1530
4	1793	1043
5	2037	976
6	2161	864

show that, as the diameter of the pinhole increased, there was a decrease in tensile stress and an increase in compressive stress. The goal was to find a compromise between the two constraints. First, and to ensure that our compressive stress did not reach 2500 MPa, we studied the diameters giving a compressive stress less than 2000 MPa, i.e., diameters ranging from 2 to 4 mm. Subsequently, since at this level the compressive stress varies little with respect to the tensile stress, the diameter with the minimum tensile stress, i.e., 4 mm, was chosen (see Figure 16 and Figure 17).

To expand the study, different pinhole shapes were tested in order to really optimize the tool performance. Other than the round shape, numerical analyses were performed in both compression and tension for two different shapes, a square hole (see Figure 18 and Figure 19) and an oblong hole (see Figure 20 and Figure 21). Each dedicated FE analysis was conducted with specific partitions in order to have only quadratic elements around the hole.

The results in

Table 5. Maximum stress for all shapes.

Shape	Max Stress in Compression (MPa)	Max Stress in Tension (MPa)
Round	1793	1043
Square	1879	2551
Oblong	1772	1318

show that the round configuration is optimal in terms of compression and traction. The round pinhole has a compressive stress slightly greater than that present in the oblong case, but its tensile stress is much lower than that of the oblong configuration, whereas the square configuration has a much higher stress in both compression and tension.

As a result of this study, the round shape has been selected as a new design for the punch and the punch holder, with satisfactory results in the production process. The first tests show a significant improvement, as the fatigue life of the punch is approximately ten times greater compared to the initial configuration.

5. Conclusions

This paper presents a common production problem encountered in industry, especially as far as the punching process is concerned in metal forming. It mainly discusses the effect of how the punch is fixed in the punch holder.

Based on a real industrial case, numerical analysis using ABAQUS showed that an improved fixing method could generate less stress on the pinhole and, therefore, increase the lifespan of the tool. A centered pin in a round pinhole, with an optimal diameter found by calculation and observations, could thus advantageously replace an eccentric fixture. The stress was reduced by 36%. In addition, a new shape for the cutting edge of the punch was introduced, helping to reduce the stress by 5.5%.

In future work, it would be interesting to investigate different types of materials for the tool.