模具专业英语Blanking Technique2

strength.
The metal is subjected to both tensile and compressive stresses (Figure 8-2 ); stretching beyond the elastic limit occurs , then plastic deformation, reduction in area, and finally, fracturing starts through cleavage planes in the reduced area and becomes complete. The fundamental steps in shearing or cutting are shown in Figure 8-3. The pressure applied by the punch on the metal tends to deform it into the die
Center of pressure
If the contour to be blanked is irregularly shaped, the summation of shearing forces on one side of the center of the ram may greatly exceed the forces on the other side Such irregularity results in a bending moment in the press ram, and undesirable deflections and misalignment. It is therefore necessary to find a point about which the summation of shearing forces will be symmetrical This point is called the center of pressure, and is the center of gravity of the line that is the perimeter of the blank. It is not the center of gravity of the area. The press tool will be designed so that the center of pressure will be on the axis of the press ram when the tool is mounted in the press.
psi (413.7N/mm2).
As the load is further increased, the punch will penetrate the metal to a certain depth and force an equal portion of metal thickness into the die, as indicated at Figure 8-3b. This penetration occurs before fracturing starts and reduces the cross-sectional area of metal through which the cut is being made. Fracture will start in the reduced area at both upper and lower cutting edges, as indicated at Figure 8-3c. If the clearance is suitable for the material being cut, these fractures will spread toward each other and eventually meet, causing complete separation. Further travel of the punch will carry the cut portion through the stock and into the die opening
Lesson 8 Blanking Technique
Cutting (shearing) Operations In the following discussion, certain die terminology will be used frequently, Figure 8-1 presents the terms most commonly encountered. Shear Action in Die Cutting Operations The cutting of metal between die components is a shearing process in which the metal is stressed in shear between two cutting edges to the point of fracture, or beyond ita total forces required to cut and form the part with a 30% safety factor added. In many cases, you will have to add stripping force if stripping is being done with a spring-loaded stripper, because the press has to compress the springs while cutting the material. Likewise, any spring pressure for forming, draw pads, and the like, will have to be added. The force needed to punch a 2 " (50.8 mm) dia, hole in 1/8 “ (3.18mm) thick SAE 1020 steel having a shear strength of 60000
clearances
Clearance is the space between the mating members of a die set. Proper Clearances between cutting edges enable the fractures to meet. The fractured portion of the sheared edge will have a clean appearance. For optimum finish of a cut edge, proper clearance is necessary and is a function of the type, thickness, and temper of the work material. Clearance, penetration, and fracture are shown schematically in Figure 8-4. In Figure 8-5, characteristics of the cut edge on stock and blank, with normal clearance , are shown schematically. The upper corner of the cut edge of the stock (indicated a ') and the lower corner of the blank (indicated by a'-1) will have a radius where the punch and die edges, respectively, make contact with the material.
opening.
When the elastic limit is exceeded by further loading, a portion of the metal will be forced into the die opening in the form of an embossed pad on the lower face of the material. A corresponding depression results on the upper face, as indicated at Figure 8-3a.
This radius is due to the plastic deformation taking place, and will be more pronounced when cutting soft metals. Excessive clearance will cause a large radius at these corners, as well as a burr on opposite corners.