Ultra and Advanced high-strength steels (UHSS/AHSS) are being used increasingly in the auto industry because they can help reduce the weight of a vehicle while making it more robust, saving fuel, and making it safer in a crash.
When making parts from sheet metal, shearing operations like blanking, piercing, and trimming are often used by metal stamping parts suppliers. In these cases, the shearing edges of the die press into the sheet material until a crack spreads between the upper and lower shearing edges.
Many things, like the cutting clearance, cutting angle, sharpness of the cutting blade, cutting speed, lubrication, material sheet parameters, tooling condition, tool wear, and strain rate, can change the shape of the sheared edge.
Let’s look at a few of these things in more depth.
Factors Affecting Sheared Edge Quality in Stamped Parts
Taking Away Clearance
When trimming, the clearance is the distance between the upper and lower shearing edges. When punching a hole, it is the distance between the round punch and the die. It is often given as a percentage of the thickness of the sheet metal. It can range from 5% to 12% of the sheet thickness for mild steel.
The amount of trimming clearance greatly affects the form and quality of the sheared edge. Finding the optimal clearance to reduce an uneven broken edge problem may require some experimentation.
Less pressure is put on the cutting tool when the clearance is increased, which helps the tool last longer. One limiter is the quality of the sheared edge and the maximum burr height that can be used. Reducing clearance helps improve the quality of the shear edge, but it also makes the sheet material deform more in the shear zone. It also speeds up work hardening, which makes the cutting forces and loads on the die’s cutting edges bigger.
Type of Material
The type of material being sheared also affects the quality of the edge of the part. Materials with low yield strength, high ductility, and homogeneity will have better-sheared edge quality, tighter dimensional tolerances, and longer tool life. On all of these sheet materials, the burr height gets higher as the clearance and pliability get better.
Material Anisotropy
Material anisotropy can also affect the quality of the part edge of stamping parts supplied by metal stamping parts suppliers. Different trim line orientations about the rolling direction give various sheared edge features for anisotropic materials.
Tool Use
Tool wear is an essential factor that directly affects the quality of the sheared edge. Tool wear is when shearing tools break down over time because they are used repeatedly. As wear on the trimming tool gets worse, the part’s edge profile and dimensional accuracy get much worse.
When AHSS/UHSS is trimmed, there is a lot of contact pressure and friction between the tools and the blank sheet. This causes galling, chipping, and abrasive and adhesive wear on the tools. In addition, the microstructure of UHSS/AHSS has a hard martensitic phase, which can accelerate tool wear.
Conclusion
In stamping operations, burr height is usually a meaningful way to judge the quality of a part supplied by a metal stamping parts supplier. The burr height indicates when the tool should be reground to restore a crisp die-and-punch radius. This means that the trimming die wear can be inspected offline by observing the profile of the cut edges. When die maintenance is needed, the burnished area and burr height can be used to tell.