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Punch and Die Clearance: Secrets to Perfect Parts

Last updated:
March 26, 2024

Table Of Contents

The punching clearance refers to the gap between the punch and the die (see Figure 2-7). Unless otherwise specified, it generally denotes the total clearance on both sides.

The punching clearance not only significantly affects the quality of the punched parts but also impacts the die life, punching force, stripping force, and ejection force. Therefore, clearance is a crucial parameter in the design of punching dies.

Punching Clearance
Figure 2-7 Punching Clearance: 1 – Punch; 2 – Die

Punch and Die Clearance Values

Analysis of the stamping process reveals that there is currently no fixed clearance value that can satisfy all the requirements for optimal section quality, highest dimensional accuracy, minimal warping deformation, longest die life, and minimal punching force, unloading force, and part ejection force.

Therefore, in actual stamping production, a clearance range is primarily determined based on factors such as section quality, dimensional accuracy, anddie life. As long as the clearance is within this range, acceptable stamped parts and a longer die life can be achieved.

This range is known as the reasonable clearance, with the minimum value termed the minimum reasonable clearance and the maximum value as the maximum reasonable clearance. When designing and manufacturing, it should be taken into account that wear during use will increase the clearance between the punch and die; thus, it’s generally best to determine the die clearance based on the minimum reasonable clearance value.

The smaller the selected clearance during design, the higher the manufacturing and assembly precision required, which correspondingly increases costs. A smaller clearance also demands higher strength and precision from the die, resulting in a shorter lifespan and a higher likelihood of maintenance. Consequently, the level of punching clearance is typically chosen based on the following criteria:

(1) Material shear strength

Materials with lower shear strength have better stretch forming capabilities and are more prone to burring, so the clearance should be on the smaller side. For instance, the punching clearance for copper materials of medium hardness is 70% of that for steel materials of the same thickness, and 60% for aluminum materials.

(2) Workpiece precision requirements

Products for the electronics and household appliance industries require high precision, while those for outdoor products, electrical, and agricultural machinery industries have lower precision requirements. Accordingly, the die clearance can be larger.

(3) Workpiece area and edge length

Larger workpiece areas or those with long edges tend to accumulate greater errors during die manufacturing and assembly, so a larger die clearance should be chosen. (An exception is made for rectangular blanking dies with long edges because during stamping, the workpiece exerts a compressive force on the concave die, causing straight-line die edges to expand.)

(4) Position in the process flow

When the burrs produced by punching adversely affect subsequent operations or the final product, the die clearance should be smaller. If there is no adverse impact, a larger clearance is preferable.

(5) Die structure

When the die structure is constrained by the material shape and cannot ensure optimal punch and die clearance (such as with tube cutting or punching dies on slanted surfaces), a larger clearance should be selected.

Rapid Calculation of Clearance Values Between Punch and Die for Economy-Grade Workpieces

In everyday production, economy-grade workpieces account for more than 80% of the total workload. If the workpiece geometry is an external polygon, or has internal protrusions or slots with a width no less than 12 times the material thickness (12t), it can still be treated as an economy-grade workpiece. In such cases, the bilateral clearance (C) between the punch and die is calculated as 10% of the material thickness: C = 0.1t.

If the calculated clearance is less than 0.02mm, it should be processed as if there were no clearance, meaning the punch should be designed not to enter the die.

A method for clearance-free punching involves using multiple layers of material for stacking punches. This technique is suitable for blanking dies but should be approached with caution for punching dies.

Clearance-free punching requires high precision in the mechanical press movement. The clearance between the slide and the guide should be between 0.005mm and 0.015mm, and the axial clearance between the connecting rod and the spherical cap should not exceed 0.02mm; it must not be too loose. The work environment should be clean and tidy.

A small amount of lubricant should be added during punching, and care should be taken to prevent foreign particles or dust from contaminating the materials and lubricant.

Principles for Selecting Clearance Directions

1) For blanking, use the die size as the reference, with the punch size being the die size minus the clearance value.

2) For punching, use the punch size as the reference, with the die size being the punch size plus the clearance value.

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