July 24, 2003
A blank, stamped in the first station of a progressive stamping operation, usually is subject to subsequent forming processes to form a designated part. If the blank is subject to straining, deformation, bending, stretching, or lateral expansion in later stations, its edge condition should be carefully examined.
Blanking—like piercing, parting, notching, and trimming—basically is a shearing process. Sheet metal that is sheared undergoes a predictable process and yields a predictable cross-sectional profile.
Typically during shearing, as the punch initially engages the sheet metal, it pulls the material downward, slightly drawing the material into the clearance, which creates rollover.
As the punch continues to penetrate and shear the upper portion of the material, the material becomes locked between the punch and the die, which creates a burnished area before the remaining material is fractured or separated completely.
The sheared edge exhibits some distinctive characteristics. They include burrs, fracture, burnishing, rollover, and work hardening.
The sheared edge exhibits some distinctive characteristics. They include fractures, burnishing, rollover, burrs, and work hardening in the adjacent area (see Figure 1).
Figure 2shows a cross-sectional profile of the sheared edge of an aluminum blank. Note the burnished depth, fractured depth, burrs, microcracks, and other edge conditions.
The sheared edge's defects can affect subsequent forming processes. What is not visible just by looking at the sheared edge is its work-hardened condition. Shearing is concentrated within a narrow band along the edge, and its consequent hardening should be concentrated around the immediately adjacent periphery to minimize the affected zone.
The work-hardened sheared edge and microcracks reduce the material's ductility and cause premature failure in subsequent forming. Figure 3shows the cracked edge of a neck after rollover in an assembly operation. During rollover, the edge expands circumferentially while it is being stretched. In this case, even though strain calculation showed that the amount of straining during rollover was within the allowable maximum material elongation, the edge split soon after the rollover.
Similarly, when the sheared edge undergoes hole extrusion, as shown in the introductory photo, or bending and stretching, as shown in Figure 4, the microcracks created during shearing can cause cracking and distortion in these subsequent forming stations.
Edge cracking or brittle fracturing can be caused by work hardening. In both cases, once the edge cracks, it can split instantly from the stress concentration around the cracks. In theory, the stress concentration can infinitely raise the opening stress at the crack tip. Microcracks are obvious within the fractured region along the edge, as shown in Figure 2.
During rollover, the edge expands circumferentially as it is being stretched. Edge defects can cause cracking, as shown in this edge of a neck after rollover.
A blanked edge condition can be improved by adjusting the punch and die clearance tolerance, shaving the area of the defective blanked edge, designing an appropriate contact profile of the tool and die, and understanding the mechanical properties of the sheet metal used.
Die Clearance Tolerance. The clearance tolerance between the shearing punch and the die greatly affects the edge condition. In fact, it determines the distribution of the edge characteristics, because as the clearance gets tighter or looser, the burnished or fractured region will dominate the profile. It also is fair to say that tool life is proportional to the clearance.
For example, if a tight clearance is maintained during blanking, the edge could yield a profile that is entirely burnished, which would reduce its tendency to fracture. A tight clearance is used when a straight edge is required. In this case, the tool life will be extremely short because of the severe contact between the tool and the blank.
Microcracks created during shearing can cause cracking and distortion in subsequent forming stations that involve stretching and bending.
It is recommended a sufficient burnished depth be maintained to reduce the fractured depth to eliminate or reduce the edge's tendency to split in subsequent forming. In other words, if the edge is subject to subsequent forming operations, work hardening is preferable to microcracking.
Conversely, if clearance is loose, the edge will have a large rollover radius and a large burr containing microcracks that create a sharp edge. Typically, a loose clearance is adopted when edge condition isn't a concern or if the part will end up as scrap. In this case, the tool life can be maximized.
Shaving. Shaving is another method used to eliminate the area of the defective blanked edge that exhibits burrs and microcracks created during blanking. It is done in a station prior to forming. A thin slice of the sheared edge—only a tenth of a thousandth of a millimeter thick—is cut out. However, shaving can lead to other problems, such as scratches, cold welding of chips to tools, and die damage.
Instead of shearing the blank by applying tools at a 90-degree angle, the shearing can be done gradually by using an inclining angle either to the die or to the punch. The use of concave and convex blanking dies can reduce the stress on both the material and the dies.
Tool and Die Design. Producing a high-quality edge often requires quantitative design revisions during die development to prevent premature failure by brittle fracturing or cracking. In other words, to achieve a desirable edge profile in a blanking station, a systematic approach is needed for optimal tool design that separates the shearing operation from the progressive die. Trial and error with different design parameters, such as clearance, tool geometry, contact profile, and lubrication on a single station, will likely achieve an optimal design that will save time and cost during die development.
There is a way to increase tool life without sacrificing edge quality too much. Instead of shearing the blank by applying tools at a 90-degree angle, the shearing can be done gradually by using an inclining angle either to the die or to the punch (see Figure 5).
The benefit of the angled tools is that they come in contact with the blank gradually, allowing the material to be sheared locally instead of applying the shearing to the entire tool periphery. Because less force is required, and consequently the concentrated contact pressure between the tool and blank from tool misalignment is minimized with a lower shearing force, angled tools can decrease tool wear during production.
Mechanical Properties of the Sheet Metal. The average circumferential strain on the metal can be calculated analytically using the following simple formula:
For example, the strain in the rollover shown in Figure 3 can be calculated as:
The engineering strain induced during the rollover is 0.341, or 34.1 percent, which is less than the total elongation, 55 percent, of the material used. Therefore, the cause of cracking can be traced to splitting from a microcrack rather than to excess stretch on the blanked edge.
Once it is obtained, the strain can be compared with the maximum allowable elongation of the material's tensile properties. Many material suppliers provide specs on the tensile properties of the material they supply.
If the strain exceeds the maximum allowable elongation, the sheared edge most likely will fail in the subsequent operation, not because of the edge condition, but because of excess strain. However, even when the calculated strain is below the maximum elongation, the edge could fail by cracking.
Most metal forming processes are case-sensitive, which means that there is no universal rule of thumb that applies to all situations and all materials, even in blanking. Other factors that affect the edge condition include lubrication, tool coatings, and punch speed and materials.
In other words, what works with low-carbon sheet metal or stainless steel might not work well with aluminum or brass. For instance, if the burnish depth in low-carbon steel is 50 percent at a certain clearance, in stainless steel with the same clearance the burnish depth will be only 30 percent, while in aluminum the burnish depth will be 60 percent.
Therefore, the clearance used for low-carbon steel needs to be adjusted when used with other sheet metals. Obviously, the clearance needs to be increased with aluminum and reduced with stainless steel when 50 percent burnish depth is desired in a shearing operation.
The best way to tackle problems that arise during blanking is to know exactly what went wrong by analyzing the incremental procedures with sophisticated techniques, such as finite element analysis (FEA), optimization, and reverse engineering, for a better understanding, and then approaching the solution with proper scientific reasoning.
Many analytical tools are available, but some have not been well-accepted by the stamping industry. Many stamping die designers rely heavily on their experiences through trial and error, but that is not too scientific. Those who are already familiar with the fundamental techniques are advised to invest in the more sophisticated analytical techniques and to practice their applications, because the fundamentals are only scaffoldings to the mastery.