Punching with a slant angle-cutting surface quality

Abstract. For economic or process-related reasons, punching of structural sheet metal components often has to be used for car bodies. The difference in angle of attack between punch and sheet metal component is referred to as “slant angle”. However, at the current state of the art, no precise information is available on the characteristics of cutting surfaces in relation to the slant angles. For this reason, cost-intensive slider units are used for comparatively small slant angles of around 10° in order to ensure series suitability of corresponding punching processes. In this respect, recent studies performed by the authors have shown that good cutting surface qualities can also be achieved for slant angles distinctly beyond 10°. This contribution presents an empirical test series for the characterization of cutting surface parameters when punching with a slant angle. Here, the experimental cutting surface analysis showed an asymmetric characteristic of the cutting surface along the hole circumference. Furthermore, the investigated sheet metal materials HC340LA, DP600 and DP800 revealed recurring tendencies regarding the parameters “edge draw-in”, “clean cut”, “fracture surface” and “burr height”, which had been combined to corresponding three-dimensional regression models. With these regression models, cutting simulations could be calibrated, allowing a quality prognosis of cutting surfaces achievable when punching at specific slant angles.


Intr Introduction and Stat oduction and State of the Art e of the Art
The final part-contour of deep drawn sheet metal components is usually produced by shear cutting operations. Due to the geometry of these components, however, cutting operations often have to be performed in a non-perpendicular state (s. Fig. 1). Such processes are referred to as punching with slant angle, if angle β between the sheet surface and the horizontal is greater than 0° [2]. According to the state of the art, maximum slant angles in stamping technology are usually conservatively estimated.
Since generally valid tool design criteria do not exist for punching processes with a slant angle, expensive sliders are used today for most punching operations with slant angles. As experimental and numerical investigations of the research project [3] have shown, even the high-strength sheet metal material DP1000 can be reliably punched (no punch breakage) with a punch diameter of 5 mm for a sheet metal thickness of 1 mm up to a slant angle of 17.5°. In contrast, according to today´s conservative process design, a slider would already have been used at a slant angle of 5° [ 4]. In order to reduce the use of expensive sliders and thus to achieve cost-saving potentials in production, the cutting surface characteristics achievable at punching with a slant angle must additionally be predictable. According to the current state of the art, however, the cutting surface characteristics such as edge draw-in, clean cut, fracture surface and burr formation are almost unknown for punching with a slant angle.
Due to the inclined position of the sheet metal component during punching with a slant angle, an asymmetrical characteristic of the cutting surface parameters such as edge draw-in, clean cut, fracture surface and burr occurs along the hole circumference. In this respect, Fig. 2 shows the result of a numerical 3D punching simulation to illustrate this effect.
Punching with a slant angle -cutting surface quality 455/2     Table 2 shows a summary of these experimentally determined material data.
T Table 2. Sheet metal mat able 2. Sheet metal materials HC340LA, DP600 and DP800 -mat erials HC340LA, DP600 and DP800 -material data erial data Punching with a slant angle -cutting surface quality 455/4 In addition to the tensile tests, conventional shear cutting experiments (β=0°) were carried out to experimentally determine the sheet metal material specific shear resistance by equation (1).
The size of clearance was determined using the analytical method proposed by Dietrich [5]. According to equation 2, this results in a suitable clearance size for the sheet materials HC340LA, DP600 and DP800 ranging between 12.0% to 15.5% of the sheet thickness.
Due to the slight differences between the analytically calculated clearance sizes, a constant clearance of u = 15 % was chosen for the subsequent experimental process analysis. for all investigated sheet metal materials. In contrast, the clean cut height and the fracture surface height exhibit a more complex nonlinear behavior with increasing and decreasing tendencies. Table 3 also shows, that the burr height decreases with an increasing size of slant angle at both measuring positions.
Punching with a slant angle -cutting surface quality 455/6 T Table 3. Experimentall able 3. Experimentally det y determined cutting surf ermined cutting surface cont ace contours f ours for the sheet metal mat or the sheet metal materials HC340LA, DP600 and erials HC340LA, DP600 and DP800 DP800 In order to quantify these tendencies, a regression analysis was performed based on the experimentally determined cutting surface data (black points).

R Regr egression models f ession models for punching with a slant ang or punching with a slant angle le
The evaluation of the measured cutting surface parameters was performed using the response surface method. For this purpose, the black points, which define the typical cutting surface characteristics (s. Table 3  With the regression models in Fig. 6 and Fig. 7  simulation model is required for a corresponding numerical process analysis [7], the precise numerical calculation of the cutting surface parameters remains still as an unsolved problem for punching with a slant angle. As Fig. 8

Summary and further r Summary and further resear esearch ch
For economic or process-related reasons, punching of structural sheet metal components often has to be performed with a deviation from the optimum angle of 90°between sheet metal and punch. This angle difference is referred to as "slant angle". However, at the current state of the art, no precise information was available on the characteristics of cutting surfaces when punching with a slant angle. This paper contains an empirical test series for the characterization of cutting surface parameters when punching with a slant angle. The research results showed an asymmetric characteristic of the cutting surface contour along the hole circumference of punching line. However, the sheet metal materials HC340LA, DP600 and DP800 showed recurring tendencies with regard to the cutting surface contour. These tendencies could be quantified by 3D-regression models. With these regression models, stamping process planners can make prognostic statements regarding the cutting surface contour to be expected when punching with a slant angle. Another advantage of the determined regression models is, that they can be used for the calibration of complex numerical punching simulation. Future research at IFU Stuttgart will focus on further investigations regarding the transferability of the regression models determined so far. For example, the punching parameters sheet metal material, sheet metal thickness, punch diameter, punch guidance or punch wear are to be varied experimentally and numerically in order to extend the regression models presented here.
Punching with a slant angle -cutting surface quality 455/12