September 11, 2007
Editor's Note: This is Part IIIof a three-part series on blanking developments. Part I, which appeared in the July issue, focused on fineblanking and tool design. Part II, which ran in the August issue, covered fineblanking part and process design guidelines.
This column was prepared by Serhat Kaya, of the Center for Precision Forming (CPF, formerly ERC for Net Shape Manufacturing), The Ohio State University, Taylan Altan, professor and director.
During high-speed blanking, press stroking rate and punch speed are higher than in conventional blanking. As a result, the shear zone has a higher strain rate, which affects a workpiece's sheared surface temperature and the tool surface. The heat generated at the highly localized deformation zone doesn't dissipate easily into the workpiece material. This causes a high temperature concentration in the narrow shearing zone.
One of the benefits of high-speed blanking is that temperatures up to 1,800 degrees F (980 degrees C) may be generated in the shearing zone, creating a stress-release effect within the material. Strain hardening, which takes place because of large, localized deformation, can even be neutralized. Because of stress release, the material can withstand a large amount of deformation until shearing strength is exceeded and the material fractures. As a result, deformation-related internal stresses and fractures are dramatically reduced.
However, these advantages are material-dependent. For example, heat created from high-speed blanking dissipates faster when cutting copper and aluminum because of their higher thermal conductivity. Ideal high-speed blanking process conditions are achieved when strain hardening is reduced through stress-release effects. This means that the optimum shearing speed has to be large enough so that stress release occurs, but not so large that enough time remains for stress relieving.
These effects improve part edge quality by decreasing the burr height and rollover, and increasing shear zone length (within a certain speed range). Less distortion is created than in blanks produced at low speeds. Figure 1illustrates a blanked part's shear zone geometry characteristics.
High-speed blanking experiments were conducted at the CPF at The Ohio State University using low-carbon alloy steel, high-strength steel, copper, and aluminum sheet. These experiments were conducted with an electromagnetic-driven, high-speed Lourdes 100-OH press (seeFigure 2). This press is controlled through nine power levels that accelerate the tooling to speeds up to 12 feet per second (FPS) (3,655 millimeters per second).
0.50 to 12 FPS (152 to 3.655 mm/s) 9 power levels
0.5 to 1.5 inch
The tooling in Figure 3consists of a punch, die button, and polymer stripper. All punches and die buttons are made of M-2 high-speed steel and are mounted to the retainers with a ball lock for fast punch and die change. The polymer stripper is mounted directly to the punch and is used for stripping the sheet from the punch after blanking.
Figure 4illustrates burr height (shown in Figure 1) versus the punch-die clearance for different cutting speeds. In the tests, for a cutting speed of 0.5 FPS, the burr height is three to four times higher compared to the 4 FPS, 9 FPS, and 12 FPS cutting speeds. It was also observed that for low-carbon steel, increasing the cutting speed above 4 FPS had no influence on the percentage of shear zone length (ratio of the shear zone length to sheet thickness).Figure 5shows the shear zone length as a percentage of sheet thickness and burr height measurements for high-strength steel, aluminum, and copper.
|Burr Height, Shear Zone Length|
Cutting Speed (FPS)
Shear Zone Lenth (%)
Burr Height (0.001 in.)
|High-strength Steel (50-XF)|
(Thickness = 0.054")
|Aluminum (2011-T3) |
(Thickness = 0.041")
(Thickness = 0.016")
Process simulation using finite element analysis (FEA) offers a practical solution for design and process-related issues in high-speed blanking because the blanking process is fully modeled and deformation takes place virtually. For simulating the blanking process, the FEM-code DEFORM 2D was used. Simulations were performed with the following settings:
Figure 6compares simulation and experiment results for the fracture zone length. The maximum difference was about 7 percent of sheet thickness, which indicates that FEA predicted the deformation of the shear zone reasonably well.
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