The edge dimensions and tolerances of the punch and die directly affect the dimensional accuracy of the stamped parts. The reasonable clearance value of the mold is also ensured by the edge dimensions and tolerances of the punch and die. Therefore, correctly determining the edge dimensions and tolerances of the punch and die is an important task in stamping die design.
Calculation principles for die edge dimensions:
Due to the existence of clearance between the punch and die, the cross-section of the stamped part has a taper. However, in the measurement and use of the stamped part dimensions, the dimensions of the bright zone are taken as the reference.
The bright zone of the blank is in the larger end dimension, and the bright zone is formed by the extrusion of the material by the die edge. The larger end (bright surface) dimension of the blank is equal to the die dimension.
The bright zone of the punched part is in the smaller end dimension, and the bright zone is formed by the extrusion of the material by the punch edge. The smaller end (bright surface) dimension of the punched part is equal to the punch dimension.
During the stamping process, the punch and die come into frictional contact with the stamped part or scrap, causing the punch contour to wear smaller and the die contour to wear larger, resulting in an increase in clearance. Therefore, determining the edge dimensions of the punch and die should differentiate between the blanking and punching processes and follow the following principles:
1. When designing a blanking die, determine the die edge dimensions first. Use the die as the reference, and the clearance is obtained by reducing the punch edge dimension. When designing a punching die, determine the punch edge dimensions first. Use the punch as the reference, and the clearance is obtained by increasing the die edge dimension.
2. Based on the wear pattern of the die during use, when designing a blanking die, the basic die dimension should be close to or equal to the minimum limit dimension of the workpiece. When designing a punching die, the basic punch dimension should be close to or equal to the maximum limit dimension of the workpiece hole. This ensures that even when the punch and die are worn to a certain extent, qualified parts can still be produced. The amount of wear allowance in the mold is related to the manufacturing accuracy of the workpiece. It is represented by x and Δ, where Δ is the tolerance value of the workpiece and x is the wear coefficient, with a value between 0.5 and 1. The selection is based on the manufacturing accuracy of the workpiece: for workpiece accuracy above IT10, x = 1; for workpiece accuracy between IT11 and IT13, x = 0.75; for workpiece accuracy IT14, x = 0.5.
3. Whether it is blanking or punching, the stamping clearance generally uses the minimum reasonable clearance value (Zmin).
4. When selecting the manufacturing tolerance of the mold edge, the relationship between the workpiece accuracy and the mold accuracy should be considered, ensuring the accuracy requirements of the workpiece and having a reasonable clearance value. Generally, the precision of the die is 2 to 4 levels higher than the precision of the workpiece. For simple shape round or square edges, the manufacturing deviation value can be selected according to IT6 to IT7 levels. For complex shape edges, the manufacturing deviation value can be selected as 1/4 of the corresponding part tolerance value of the workpiece. For edge dimensions that do not change after wear, the manufacturing deviation value can be taken as 1/8 of the corresponding part tolerance value of the workpiece with the addition of (±).
5. The dimensional tolerances of the workpiece and the manufacturing deviations of the die edge should ideally be marked as unilateral tolerances according to the "fit in" principle. The "fit in" principle means that when indicating the dimensional tolerances of the workpiece, they should be marked unilaterally in the direction of the material. However, for dimensions that do not change after wear, bilateral deviations are generally marked.
Calculation methods for die edge dimensions:
Due to different machining methods for the dies, the calculation methods for edge dimensions also vary and can be broadly categorized into two types.
1. Method of Separate Processing for Punch and Die According to the Punch and Die Drawings
This method is mainly suitable for circular or simple regular-shaped workpieces. Since the manufacturing of the punch and die for such workpieces is relatively simple and accuracy is easy to ensure, separate processing is adopted. During the design process, the edge dimensions and manufacturing tolerances of the punch and die should be separately marked on the drawings. The calculation formulas for the distribution of the punch and die edge dimensions with respect to the workpiece dimensions and tolerances are as follows:
(1) For blanking, the workpiece dimension is D-Δ. According to the calculation principle, the design is based on the die. First, determine the die dimensions so that the basic die dimension is close to or equal to the minimum limit dimension of the workpiece contour. Then, reduce the die dimension by the minimum reasonable clearance value to obtain the punch dimension.
(2) For punching, the punching dimension is d+Δ. According to the calculation principle, the design is based on the punch. First, determine the punch dimensions so that the basic punch dimension is close to or equal to the maximum limit dimension of the workpiece hole. Then, increase the punch dimension by the minimum reasonable clearance value to obtain the die dimension.
(3) For the center-to-center distance of holes, it is a dimension that remains essentially unchanged after wear. In the same operation, when punching two holes on the workpiece with a distance of L±, the center-to-center distance of the die holes can be determined using the following formula.
From the above, it can be seen that the advantages of the method of separate processing for the punch and die are interchangeability between the punch and die, short manufacturing cycle, and ease of batch production. The disadvantage is that in order to ensure that the initial clearance is within a reasonable range, smaller manufacturing tolerances for the punch and die are required to meet the requirement of +≤, resulting in relatively higher mold manufacturing costs. The material of the part is Q235 steel with a thickness of t=0.5mm. Calculate the punch and die edge dimensions and tolerances for the stamping process. Solution: This part belongs to general punching and blanking without special requirements.
2. Method of Matching the Punch and Die
When using the method of separate processing for the punch and die, in order to ensure a certain gap between the punch and die, the manufacturing tolerances of the stamping die must be strictly limited, resulting in difficulties in die manufacturing. For stamping dies used for thin materials (where the difference between Zmax and Zmin is small), stamping dies used for complex-shaped workpieces, or stamping dies used for single-piece production, the method of matching the punch and die is often adopted. The matching method involves first manufacturing a reference component (either the punch or die) according to the design dimensions, and then manufacturing another component with a minimum reasonable clearance based on the actual dimensions of the reference component. The characteristic of this processing method is that the clearance of the mold is ensured by the matching, the process is relatively simple, and there is no need to check the +≤ condition. It is also possible to enlarge the manufacturing tolerances of the reference component, making manufacturing easier. During the design process, the edge dimensions and manufacturing tolerances of the reference component should be clearly marked, while the matching component should only indicate the nominal dimensions without tolerances. However, it should be noted on the drawing: "The punch (die) edge is matched based on the actual edge dimensions of the die (punch) to ensure a minimum reasonable clearance value of Zmin." When using the matching method, to calculate the edge dimensions of the punch or die, it is necessary to correctly determine whether each dimension of the mold edge will increase, decrease, or remain unchanged during the wear process based on the changes in the contour of the punch or die after wear. Then, calculate them separately using different formulas.
(1) Dimensions that will increase after wear for the punch or die - Category 1 dimension A: This refers to the dimensions that will increase after wear for the blanking die or punching punch, which is equivalent to the dimensions of a simple-shaped blanking die. Therefore, the determination method and formula for the basic dimension and manufacturing tolerance are the same as in section (2.4.1).
Category 1 Dimension:
(2) Dimensions that will decrease after wear for the punch or die - Category 2 dimension B: This refers to the dimensions that will decrease after wear for the punching punch or blanking die, which is equivalent to the dimensions of a simple-shaped punching punch. Therefore, the determination method and formula for the basic dimension and manufacturing tolerance are the same as in section (2.4.3).
Category 2 Dimension:
(3) Dimensions that will remain essentially unchanged after wear for the punch or die - Category 3 dimension C: This refers to the dimensions that will remain essentially unchanged for the punch or die after wear, without considering the effect of wear. It is equivalent to the dimensions of a simple-shaped center-to-center distance. Therefore, the determination method and formula for the basic dimension and manufacturing tolerance are calculated the same as in section (2.4.5).
Category 3 Dimension:
steel. Calculate the edge dimensions and manufacturing tolerances for the punch and die of the stamped part. Solution: This stamped part is a blanking part, and the design reference component is the blanking die. Only the edge dimensions and manufacturing tolerances of the blanking die need to be calculated. The edge dimensions of the punch are determined based on the actual dimensions of the blanking die according to the clearance requirements. According to Table 2.3.3, we find: . Referencing the tolerance table, we determine the tolerance grade for each dimension of the workpiece, and then determine the value of x. For the dimension of 80mm, we choose x=0.5; for the dimension of 15mm, we choose x=1; for the remaining dimensions, we choose x=0.75. The calculation of the basic dimensions for the blanking die is as follows:
Category 3 Dimension: Dimensions that will remain essentially unchanged after wear
The basic dimensions for the blanking punch are the same as the blanking die, which are 79.79mm, 39.75mm, 34.75mm, 22.07mm, 14.94mm. Tolerances do not need to be marked, but it should be noted in the technical specifications: "The actual edge dimensions of the punch are matched with the dimensions of the blanking die to ensure a minimum reasonable clearance value."
