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How to Determine Punch and Die Clearance?

Key takeaways:

1. The precision of punch and die clearance is crucial for the quality and longevity of both the stamped parts and the stamping dies; optimal clearance values vary depending on material thickness and type, with tighter clearances needed for higher accuracy requirements and wider clearances beneficial for reducing punching force and extending die life.

2. The selection of punch and die clearance is not arbitrary but is informed by empirical data and formulae, with experienced designers balancing the need for minimum reasonable values against the inevitability of wear during production, which necessitates adjustments to maintain quality and accuracy over time.

3. The article emphasizes the importance of using the correct die clearance to prevent issues such as secondary shearing, increased punching force, and reduced mold life, while achieving a high-quality punch with minimal burrs, accurate hole positioning, and optimal material separation.

What Is Punch and Die Clearance?

The punching and die clearance of CNC punching machine and CNC turret punching machine refers to the gap between the punch and the lower die when the punch is inserted into the die, and is commonly referred to as the total clearance.

Punch and Die Clearance

Optimum Clearance Between the Punch and Die

The CNC punching machine and the CNC turret punching machine can produce a sheared crack joint, maintain a balanced blanking force, ensure good punching quality, and extend the service life of the mold.

It is recommended to refer to the punch and die clearance table (as shown in Table 1) for optimal results.

Table 1. Punch & Clearance Chart
Unit: mm

ThicknessMild SteelStainlessAluminum*

* Also applicable to Copper and Brass

How to Choose the Right Punch and Die Clearance?

The choose of the die clearance value for a CNC punching machine or a CNC turret punching machine is generally done using empirical formulas and charts.

An experienced stamping die fitter and designer, with a good understanding of different products, including their material, size, and accuracy requirements, will know how to design the mold to produce qualified products and reduce repair times.

The gap between the punch and die has a significant impact on the quality of the stamped part and the life of the stamping die.

Therefore, when designing the stamping die, a reasonable clearance must be chosen to ensure the quality and accuracy of the stamped parts.

A smaller clearance value should be used for parts with high dimensional accuracy and perpendicularity requirements, while larger gap values can be used for parts with lower accuracy requirements to reduce punching force and improve the service life of the stamping die.

There are two methods for determining the punch and die clearance, by experience and by formula.

The minimum reasonable clearance value should be used when designing and manufacturing a new stamping die, taking into account the deviation in production and wear during use.

In the production process, the stamping die will continuously wear, increasing the gap, so it’s essential to keep the clearance within a reasonable range.

(1) By experience

The percentage of the die clearance to the material thickness:

◆ Min service life of dies: 15%

◆ Optimal clearance: 20~25%

◆ Long service life of dies: 30%

◆ Heavy duty die clearance: 30%

(Table 1) Recommended die clearance table

Mild Steel
Stainless Steel
Die Clearance Table

(2) By punch and die clearance formula

Its value can be calculated according to the following punch and die clearance formula:

Soft materials:

  • Material thickness t < 1 mm, blanking clearance c = (3% ~ 4%)t
  • t = 1 ~ 3mm, c = (5% ~8%)t
  • t = 3 ~ 5mm ,c = (8% ~10%)t

Hard materials:

  • t < 1mm,c = ( 4% ~5% )t
  • t = 1 ~ 3mm, c = ( 6% ~8% )t
  • t = 3 ~ 8mm, c = ( 8% ~ 13%)t

Further reading:

Why Use the Correct Die Clearance?

◆ Extend the service life of the mold

◆ Good material returning effect

◆ The generated burr is small

◆ Get a cleaner and tidy hole

◆ Reduce the possibility of sticking

◆ Workpiece leveling

◆ The position of the hole is more accurate

◆ The minimum punching force is required for blanking

The Harm of Too Small Clearance

  • Secondary shearing occurs
  • The punching force increases
  • The life of the mold is shortened

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