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The Choice of Press Type
The equipment used for stamping processing mainly includes a general press, a special press, and hydraulic press, etc. The choice of press type mainly depends on the process requirements of stamping parts and production batch, generally should follow the following principles.
- For small and medium shape blanking parts, drawing parts and bending parts, the main choice is open general press.
The body stiffness of open press is poor, and it is easy to produce deformation under the action of punching pressure, which affects the quality of punching load, but as long as the punching capacity is selected appropriately, it is competent. And more convenient to use, easy to operate, easy to achieve mechanization and automatic production. Therefore, the small and medium shape and precision of the stamping parts are not high use of this type of open press.
- For large and medium shape and high precision stamping parts, the straight-side general purpose press with better stiffness should be selected.
According to the size of the stamping parts, you can choose double point, four-point press.
- For the shape of more complex small and medium shape of mass production of stamping parts, should choose high efficiency press or automatic press machine.
- For the production of large plate stamping parts with small batches, hydraulic press should be selected.
Although the production speed of the hydraulic press is slow, the efficiency is low, the dimensional accuracy of parts is affected by the operation, but the pressure is large, and it will not be overloaded due to the thickness of the plate, which is suitable for thick plate production.
- For precision blanking parts, it is best to do it on a special fine blanking press.
When the production batch is not large, the pressing device and backpressure device can be added to the general press or hydraulic press to carry out precision blanking.
- For large drawing parts, to choose the upper drive of the closed double action deep drawing press; For small and medium-sized deep drawing parts, the lower drive double action deep drawing press is often used; Three-action press can be used for large complex drawing parts.
- For the correction of bending, shaping, stamping and other processes, the press is required to have enough stiffness, should be preferred to select fine press, in order to get a higher stamping part dimensional accuracy; When the production batch is not large, friction press can be used.
- For sheet blanking, the press with high precision and stiffness should be selected.
See Table 1-1 for the mapping between stamping type and stamping equipment selection.
|Stamping type||blanking||bending||A simple drawing||Complex deep drawing||Plastic leveling||Three-dimensional molding|
|Small stroke general-purpose press||√||Ο||×||×||×||×|
|Medium stroke general-purpose press||√||Ο||√||Ο||Ο||×|
|Large stroke general-purpose press||√||Ο||√||Ο||Ο||√|
|Double action deep drawing press||×||×||Ο||√||×||×|
|High-speed automatic press||√||×||×||×||×||×|
Table 1-1 Mapping table of stamping type and stamping equipment selection
Note: in the table, √ means applicable, Ο means acceptable, and × means not applicable.
Power Press Selection of Specifications
The Power Press Selection should be based on the size of deformation force, stamping part size, and die size to determine the equipment specifications. Specifically, the following main technical parameters must be considered when selecting a press machine to complete a certain process.
Determination of Nominal Pressure
The impact force during the slide of the press is the pressure of the press. The pressure varies with the sliding position of the slider, that is, with the rotation angle of the crank, as shown in Curve 1 of Fig. 1-1.
Nominal Pressure of The Press
Nominal pressure of the press refers to the maximum allowable working pressure on the slider when the crank of the press is turned to a certain Angle (called the nominal pressure Angle, approximately equal to 20° ~ 30°) before BDC. The size of the nominal pressure indicates the size of the impact that the press itself can withstand. The strength and rigidity of the press are designed for nominal pressure.
The relationship between the nominal pressure of the press and the actual required punching pressure
The stamping process in the size of the impact pressure is also with the punch (or press slider) travel and change. In Fig. 1-1, curves 2 and 3 respectively represent the actual punching pressure curves of blanking and drawing. It can be seen from the figure that the two actual impact pressure curves are not in sync with the allowable pressure curve of the press.
In the stamping process, the punch at any position required by the punch should be less than the press at that position. As shown in Fig. 1-1, although the maximum drawing force is less than the nominal maximum pressure of the press, it is greater than the impact pressure produced by the press when the crank is rotated to the position of the maximum drawing force, that is, the drawing pressure curve is not within the allowable pressure curve of the press. Therefore, the press with a greater tonnage of pressure than that shown in Curve 1 in Fig. 1-1 should be selected. Therefore, in order to ensure that the punching pressure is sufficient, the tonnage of the press should be 30% greater than the calculated punching pressure in general blanking and bending. When drawing, the tonnage of the press should be 60% ~ 100% greater than the calculated drawing force.
Power Press Selection of Slider Stroke Length
The slider stroke length refers to the distance that the crank rotates around the slider, and its value is twice the radius of the crank. When choosing the press, the length of the slider stroke should ensure that the blank can be smoothly put into the die and the stamping parts can be smoothly taken out from the die. Especially when forming deep drawing parts and bending parts, the travel length of the slider should be greater than 2.5~3.0 times of the height of the workpiece.
Power Press Selection of Travel Times
The number of strokes is the number of impacts per minute of the slider. Consideration should be given to material deformation requirements and productivity.
Power Press Selection of Countertop Size
The length and width of the working table shall be greater than the size of the die seat under the die, and 60~100 mm shall be left on each side to facilitate the installation of bolts, pads, and pressing plates for fixing the die. When the workpiece or waste falls, the size of the table face must be greater than the size of the falling apart. For the mold with a spring-top device, the size of the workbench face should be greater than the external size of the lower spring-top device.
Determine The Hole Size of The Slider Die Handle
The diameter of the die handle hole should match the diameter of the die handle, and the depth of the die handle hole should be greater than the length of the die handle.
Selection of The Closing Height
The closing height of the press refers to the distance between the bottom surface of the slide block and the surface of the work table (i.e. the plane under the pad) when the slide block BDC.
The closing height of the press can be changed within a certain range by adjusting the length of the connecting rod. When the connecting rod is adjusted to the shortest (the stroke of the eccentric press should be adjusted to the minimum), the distance between the bottom surface of the slide block and the plane of the working table is the maximum closing height of the press; When the connecting rod is set to maximum (maximum for eccentric press stroke), the slider is in BDC and the distance between the bottom surface of the slider and the surface of the worktable is the minimum closing height of the press.
The height of a press is the difference between the closing height of the press and the thickness of the backing plate. For a press without a backing plate, the height of the mount is equal to the height of the press closure.
The closing height of the die is the distance between the upper plane of the die seat and the lower plane of the die seat when the die is in the lowest working position.
Fig. 1-2 shows the relationship between the height of the die and the height of the press.
Hmax – 5 mm ≥ H + T ≥ Hmin + 10 mm
In the formula
H — the closing height of the mold;
Hmax — the maximum closing height of the press;
Hmin — the minimum closing height of the press;
T – Thickness of the backing plate.
The size of the worktable should be 60~100 mm (one side) larger than the lower die seat of the mold to facilitate installation. The aperture of the backing plate should be larger than the projection size of the parts or waste to facilitate material leakage.
Motor Power Selection
It must be ensured that the power of the motor of the press is greater than the power required for stamping.
Technical parameters of common presses can be consulted in related manuals.