Unit Two Die Design,模具与数控专业英语,2.4 Blanking Die,模具与数控专业英语,2.4.1 Shearing Action in Die Cutting Operations Before a sheet-metal part is made, a blank of suitable dimensions is first removed from a large sheet by shearing, that is, the sheet is cut by subjecting it to shear stresses, typically once developed between a punch and a die. In the following Figure 2-5 presents the cutting (shearing) operations. In Figure 2-5, (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. (b),(c) Characteristic features of a punched hole and the slug. Note that the scales of the two figures are different.,模具与数控专业英语,(a),模具与数控专业英语,(b),模具与数控专业英语,(c),Figure 2-5 Schematic illustration of shearing with a punch and die,模具与数控专业英语,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 its ultimate strengthThe metal is subjected to both tensile and compressive stresses, stretching beyond the elastic limit occurs，then plastic deformation，reduction in area, and finally，fracturing stars through cleavage planes in the reduced area and becomes complete,模具与数控专业英语,The fundamental steps in shearing or cutting are shown in Figure 2-6The pressure applied by the punch on the metal tends to deform it into the die 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 2-6a 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 2-6b,模具与数控专业英语,This penetration occurs before fracturing starts and reduces the cross-sectional area of metal through which the cut is being made Fracturing will start in the reduced area at both upper and lower cutting edges，as indicated at Figure 2-6c,d If the clearance is suitable for the material being cut, these fractures will spread toward each other and eventually meet，causing complete separation2. Further travel of the punch will carry the cut portion through the stock and into the die opening,模具与数控专业英语,Figure 2-6 Step in shearing metal,模具与数控专业英语,2.4.2 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.,模具与数控专业英语,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,模具与数控专业英语,2.4.3 Clearances Clearance is the space between the mating members of a die setProper 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 2-7,模具与数控专业英语,Figure 2-7 Punch-and-die clearance, punch penetration into and fracture of die-cut metal,模具与数控专业英语,2.4.4 Blanking Die Design The design of a small blanking die shown in Figure 2-8 is the same as that the piercing die of Figure 2-4 except that a die replaces the die bushings and the two piercing punches are replaced by one blanking punch A stock stop is incorporated instead of nest plates. Tins is a drop-through design since the finished blanks drop through the die, the lower shoe，and the press bolster,模具与数控专业英语,Figure 2-8 A simple blanking die,1. guide post, 2guide bushing, 3stock stop, 4upper shoe, 5lower shoe.,模具与数控专业英语,Large blanks are commonly produced by an inverted blanking die（Figure 2-9）in which the die is mounted to the upper shoe with the punch secured to the bottom shoe The passing of a large blank through the bolster is often impractical but its size may necessitate sectional die design（Figure 2-10）,模具与数控专业英语,Figure 2-9 A inverted blanking die Figure 2-10 Eight-section layout for die,模具与数控专业英语,Draft, or angular clearance in an inverted die is unnecessary because the blank does not pass through itFor ease of construction，regrinding，and strength the cutting edges of each section should not include points and intricate contour, with straight contour. Sections 1 and 2 of the die of Figure 2-10 were laid out to include the entire semicircular contour ，with straight contours included in the other six sections,