2020 Volume 61 Issue 1 Pages 156-161
We experimentally investigated the effect of the mass of a drop hammer in impact punching with polyvinyl alcohol (PVA) gel as the working medium. There is an optimal mass of the drop hammer. For constant initial energy, if the mass of the drop hammer is small, the impulse (change in momentum) becomes small, and punching cannot be accomplished. On the other hand, if the mass of the drop hammer is large, the speed of the pressurizing ram will be small for the same kinetic energy, the flow pressure of the PVA gel will not be sufficiently raised, and again, punching cannot be accomplished.
This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 27–32. Equations (1) and (2) are newly assigned numbers, and accordingly the equations numbers are postponed from then.
For high-quality punching of metal sheets, it is necessary to maintain uniform clearance between the die and the punch. It is particularly difficult to manufacture, assemble, and hold punches and dies having complicated shapes. A way to overcome this issue is by using punchless punching, which is a method of punching using a medium softer than the material being processed. One method, the Guerin process, uses rubber, an elastic material, as the working medium.1) In addition to this, methods using a plastic sheet2) or water3–6) as the punching medium have been proposed and studied. However, each of these methods have disadvantages. The Guerin process is not suitable for punching complicated shapes. When a plastic material is used as the medium, it is consumed in the process and cannot be reused. When water is used as the medium, enhanced sealing of the mechanism is necessary, which can be a challenge.
In this experiment, we study impact punching using a viscoelastic material, polyvinyl alcohol (PVA) gel, as the reusable, working medium.7) Since the PVA gel is viscoelastic, it will form a seal without any special treatment between the processing equipment and the sample. Then, we use the PVA gel as the working medium. However, there is a lack of knowledge about the necessary processing conditions for punching using a viscoelastic material, such as PVA. Even when examining the research examples of the entire punchless impact punching, there are only examples in which the deformation behavior of the workpiece in punching with water as the medium,4,8) the required energy for punching with electromagnetic force without using the medium,9–13) and the required energy in punching with the flour sheet as the medium14) are examined. Therefore, in this study, we have evaluated the effects of drop hammer mass and initial height on punching which uses PVA as the medium.
A1050 (JIS) aluminum sheet with a nominal thickness of 0.3 mm (measured value 0.28 mm) was used as the sample material. The shear strength of the sheet was 79.8 MPa when a 5.0 mm diameter hole was punched out with a normal punch and die. Disks with a 40 mm diameter were cut from the same sheet as workpiece samples.
2.2 Experimental deviceThe punching die was made of cemented carbide with a diameter of 30 mm and a height of 17.0 mm, and had a 5 mm diameter hole in the center. An SKD11 (JIS) knockout with a diameter of 4.9 mm and a height of 16.0 mm was installed in the hole of the die. Based on these measurements, the die surface has a stepped shape of 1.0 mm.
The punching cell was made of SUS304 (JIS) stainless steel, and is composed of a die holder, a pressurizing container, and a pressure ram. The mass of the pressure ram was 0.247 kg. The die holder and container were structured to sandwich the sample and fasten it with eight M6 size (JIS) bolts. Figure 1 shows a schematic diagram of the punching cell.
Punching cell.
Figure 2 shows the drop hammer impact system used in the experiment, which uses a structure to lift a cylindrical carbon steel hammer by hand.
Drop-hammer impact press system.
The punching medium is a boride gel of PVA. This gel was prepared by adding a saturated aqueous solution of borax to a commercially available liquid washing paste of PVA (produced by OSAKASEIKO CO., LTD.). The dry weight of the aqueous solution of PVA was 6.0 mass% of the original aqueous solution.
2.4 Punching experimentThe masses of the drop hammers used in the punching experiments were 9.215, 4.917, 2.404, 1.440, and 0.763 kg. Figure 3 shows an image of the hammers.
Various hammers.
The experiment was carried out using the following procedure. At first, a sample of the workpiece was set in a punching cell, and a pressurized ram was inserted by placing 3.00 g of the PVA gel divided into 5 pieces and 0.5 ml of water in the container. Next, to expel the air and fill the gel, the 4.917 kg weight was placed and preloaded at 48.22 N for 60 s. Next, the hammer was placed on the top of the ram, the guide shaft was passed through the hole of the hammer, and the workpiece was punched out by lifting the hammer by hand at the initial height and releasing it. However, for the 9.215 kg hammer heavier than the preload weight, the hammer was set directly to the initial height after the installation work. We experimented with hammers of 9.215 kg, 4.917 kg, 2.404 kg, 1.440 kg, and 0.763 kg, with initial heights of 0.005 m, 0.01 m, 0.02 m, 0.04 m, and 0.08 m, respectively. The initial height was determined so that the initial position energy of the hammer was almost equally spaced. After the experiment, the samples were observed and classified into three states: fully blanked, partially blanked, and no fracture, as shown in Fig. 4. The partially blanked and no fracture samples were classified based on the presence or absence of transmitted light. Experiments were conducted three times for same condition. In the case where the results were different after three runs, a fourth test was conducted, and a judgment was made based on that test. For each hammer, when fully blanked occurred after three consecutive measurements, no experiments with larger hammer initial heights were conducted, and, this was judged as the stable punching condition.
Various worked samples.
The peak pressure was measured with a pressure sensitive sheet (PRESCALE made by Fuji Film Co. ltd.) installed at the position of the workpiece. In this measurement, a dedicated die integrating a frame portion of the processing jig and a flat die was used.
Figure 5 shows the cross-sectional shape photograph near the punched hole, and the SEM image of the fractured surface of the fully blanked sample. Compared with normal punching using a punch and die, it was normally observed that the shear drop is very high and almost no shear surface was recognized. In addition, almost no burrs are seen around the punched hole. Although the boundary between the shear drop and the shear plane is only a slight inflection point, in (a), but it appeared as a clear line due to the characteristics of the SEM, in (b).
Shape near the sheared edge (a) Cross section of punched sheet (b) SEM image of sheared edge.
Figure 6 shows the results of punching feasibility. Here, the horizontal axis is the mass of the drop hammer and the vertical axis is the initial potential energy of the hammer. The initial potential energy of the hammer, Ep, is calculated by:
| \begin{equation} E_{\text{p}} = mgh \end{equation} | (1) |
Fracture modes of specimen depending on initial potential energy and mass of drop hammer (preload: 48.22 N for 60 s).
If all of the kinetic energy of the drop hammer is converted to working energy, the dashed line indicating the stable punching condition should be horizontal in the figure. However, this was not observed, suggesting the existence of a loss depending on the mass of the hammer. A loss upon collision of the hammer and the pressure ram is considered as the first candidate of this loss. Generally, the collision between the hammer and pressure ram is typically somewhere between elastic and completely inelastic collisions. In this experiment, it was suggested that the collision of the hammer was almost completely inelastic collision because it did not bounce after the collision. It is with the complete inelastic collision that most energy losses occurred in the collision. Therefore, we examined energy efficiency under the assumption of a complete inelastic collision.
In the case of a complete inelastic collision, since the total mass of the ram and the hammer is integrated, eq. (2) is derived from the law of conservation of momentum.
| \begin{equation} m_{0}v_{0} = (m_{0} + m_{1})v_{1} \end{equation} | (2) |
Here, the mass ratio of ram and hammer, r, is defined as r = m1/m0.
| \begin{align} & m_{0}v_{0} = m_{0}(1 + r)v_{1} \\ & v_{0} = (1 + r)v_{1} \end{align} | (3) |
| \begin{align*} & E_{0} = m_{0}\mathrm{g}h = \frac{1}{2}m_{0}v_{0}{}^{2} \\ & 2\mathrm{g}h = v_{0}{}^{2} \end{align*} |
Here, substituting eq. (2) for v0
| \begin{align} & 2\mathrm{g}h = (1 + r)^{2}v_{1}{}^{2} \\ & v_{1}{}^{2} = \frac{2gh}{(1 + r)^{2}} \end{align} | (4) |
The total kinetic energy E of the ram and hammer after collision is shown to be:
| \begin{align*} E & = \frac{1}{2}(m_{0} + m_{1})v_{1}{}^{2}\\ & = \frac{m_{0}(1 + r)}{2}v_{1}{}^{2} \end{align*} |
Here, eq. (2) is substituted for v12 to yield:
| \begin{align*} E & = \frac{m_{0}(1 + r)}{2}\cdot \frac{2gh}{(1 + r)^{2}}\\ & = \frac{m_{0}gh}{1 + r} \end{align*} |
When energy efficiency k = E/E0 is defined:
| \begin{equation} \mathrm{k} = \frac{E}{E_{0}} = \frac{m_{0}gh}{1 + r}\cdot \frac{1}{m_{0}gh} = \frac{1}{1 + r} \end{equation} | (5) |
From eq. (5), the energy efficiency is influenced by the mass ratio of the ram and the hammer. If r = 0, then k = 1, so there is no energy loss at collision. At r = 1 where the mass of the hammer and the ram are equal, half of the energy is lost by collision. From this relationship, the energy of the stable punching condition increases as the mass of the hammer decreases.
Using the 2.404 kg hammer where the energy of the stable punching condition was the lowest, k = 0.907 and the potential energy was 6.6 J. Considering this effect, the stable punching condition with a 1.440 kg hammer should be about 7 J because k = 0.854. However, the experimental value was 15.2 J, which is much higher. It can be reasoned that a high initial position energy is required for a hammer with a small mass because of the loss of energy at collision. In this experiment, the surface of the guide is sufficiently smooth and the clearance from the hole of the hammer is sufficiently large. Moreover, the hammer and guide were dropped such that they do not come in contact with each other. Therefore, it can be assumed that the effect of friction between the guide and hammer is negligible.
In punching, it is necessary to consider processing load in addition to processing energy. This is because no fracture occurs unless a load greater than the shear strength is applied to the shearing portion of the workpiece. In collision phenomenon, it is difficult to measure the time duration for which the force was applied. Therefore, it is common to think about impulse. Since impulse is force integrated over time, if the time required for the phenomenon is constant, the force will be directly proportional to the impulse, which is equivalent to momentum change. Since the hammer did not bounce immediately after collision, the absolute value of the momentum at the time of the collision can be regarded to be equal to the impulse.
The kinetic energy of the hammer at the collision, E0, is calculated as:
| \begin{equation*} E_{0} = \frac{1}{2}mv_{0}{}^{2} \end{equation*} |
| \begin{equation} v_{0} = \sqrt{\frac{2E_{0}}{m_{0}}} \end{equation} | (6) |
| \begin{equation*} p = m_{0}v_{0} \end{equation*} |
| \begin{align*} p & = m_{0}\sqrt{\frac{2E_{0}}{m_{0}}}\\ & = \sqrt{2m_{0}E_{0}} \end{align*} |
| \begin{equation} E_{0} = \frac{p^{2}}{2m_{0}} \end{equation} | (7) |
Fracture modes of specimen depending on momentum and mass of drop hammer (preload: 48.22 N for 60 s).
In this device, the pressure was maintained with the flow resistance of the PVA gel, so no other sealing devices were needed. Then, it is necessary to consider the behavior of the PVA gel entering the gap between the ram and the container, or between the workpiece and the container. Figure 8 shows the experimental results obtained by changing the preload time to confirm the effect of the amount of PVA gel entering the gap. In this experiment, the mass of the hammer was 1.440 kg and the preload times were 5 s and 60 s. The fracture start condition was higher when the preload time becomes shorter, and the stable punching condition becomes higher as the preload time becomes longer. It was considered fracture start when a load greater than the shear strength of the material was applied to the punched portion of the workpiece. In other words, it depends on the sealing property of the gap with the PVA gel. The results in this figure show that the sealing property was improved by increasing the amount of PVA gel entering the gap during longer preloading times. On the other hand, the effect of the stable punching condition suggests that there is an effect of inhibition as more PVA gel enters the gap.
Effect of preload time (mass of hammer = 1.440 kg).
Figure 9 shows the ram speed immediately after the collision under the fracture start condition at each mass of the drop hammers. As hammer mass increases, fracture starts at low ram speed, but converges to about 1.2 m/s. This is likely due to low ram speed, where the flow resistance of the PVA gel entering the gap decreases and the pressure for work does not rise sufficiently. Therefore, even if the kinetic energy and momentum at the time of collision are increased by increasing the mass of the hammer, work will not start because the entry resistance of the PVA gel into the gap will not increase enough. This means that there is a lower limit for the initial height of the hammer.
Initial speed of the ram after impact of the hammer under fracture start condition for various masses of drop hammer.
Figure 10 shows the average peak pressure of the 5 mm diameter circle at each mass of the hammer measured by the pressure sensitive sheet, in correspondence with the speed of the ram immediately after collision. For each hammer mass, as the ram speed increases, the peak pressure rises, and the gradient increases with the hammer mass increase.
Variation of peak pressure of PVA gel with initial speed of ram for various masses of drop hammer.
Figure 11 shows the effect of the mass of the hammer on the peak pressure at the lower limit of the stable punching condition and the fracture start condition. The fracture start condition is almost unaffected by the mass of the hammer and is found to be almost constant (∼13.5 MPa). It is thought that the fracture occurs when a stress higher than the tensile strength of the material is applied to the punched portion of the workpiece. Therefore, it is considered that equal peak pressure is measured without the influence of the mass of the hammer when measured under fracture start conditions.
Peak pressure of PVA gel under stable punching condition and fracture start for various masses of drop hammer.
However, the stable punching condition is the peak pressure when the entire circumference of the punched portion of the workpiece is fractured, that is, when certain work is performed. The results in Fig. 11 show that as the mass of the hammer decreases, a larger peak pressure is required for full blanking. Since the work done by the PVA gel on the workpiece is the product of the pressure and penetration volume, even with a small hammer mass and constant kinetic energy, the ram moving speed increases and the resistance increases. The work must be completed by volume of PVA, which would have required a large peak pressure.
Generally, shear resistance is generated in a fluid with a velocity gradient, which has a positive correlation regardless of Newtonian non-Newtonian fluid. In the experiment, PVA gel enters the clearance between the container and ram, which is expected to exhibit shear resistance according to the velocity gradient generated by the shear movement between the container and ram. When the total kinetic energies of the ram and hammer are equal, the collision speed increases as the mass of the hammer decreases. Therefore, it exhibits high shear resistance. Furthermore, the influence is large because the mass is small, and energy losses will be significant. In the case of punching with PVA gel as a medium, even if the initial potential energy is equal, different results will be obtained if the hammer masses are different. Therefore, an appropriate hammer mass must be chosen.
We observed the following in punching experiments using PVA gel as the working medium and drop hammers as the power source:
From the above, it was found that there is an optimal mass of the hammer for punching when PVA gel is used as the medium.
