How to Determine Punch and Die Clearance?

If you’re in the business of CNC punching, you know that precision and accuracy are key to producing quality parts. But did you know that the gap between the punch and die can make all the difference?

Punch and die clearance is essential to ensuring a clean, accurate cut, extending the life of your mold, and reducing repair times.

In this article, we’ll explore the optimum clearance for CNC punching machines and turret punching machines, and why it matters.

We’ll dive into the harm of too small a clearance, the benefits of using the correct clearance, and how to determine the perfect clearance value for your specific material and product.

Whether you’re a seasoned stamping die fitter or a newcomer to the industry, this article will provide valuable insights into the importance of punch and die clearance and how to achieve the perfect cut.

So, get ready to optimize your punching process and take your product quality to the next level with the right punch and die clearance.

Punch and Die Fundamentals

Punch and die sets are essential tools in the manufacturing industry. They are widely used for creating holes, shapes, and other features in various materials through the process of sheet metal punching. In this process, a punch – which is typically made from hardened steel or other durable materials – is pressed through the workpiece and into a mating die, forcing the material to conform to the desired shape.

Material selection is crucial in the design and manufacturing of punch and die sets. The materials used in these tooling components should be durable, heat-resistant, and able to withstand the rigors of the punching process. Some common materials used for punches and dies include high-speed steel, tungsten carbide, and various tool steels.

In the punch and die system, clearance is an essential factor that determines the quality of the pierced hole and the tooling life. Clearance is the distance between the punch and the die edges, and it influences the shearing action, the size and shape of the hole or feature, and the force required for the operation. Proper clearance minimizes distortion, burring, and any other unwanted defects in the finished product.

Choosing the correct clearance depends on various factors, such as the material thickness, type of materials, and desired edge quality. Typically, the clearance is specified as a percentage of the material thickness, with different percentages being used for different materials. For instance, softer materials may require larger clearances, while harder materials might necessitate tighter clearances.

Performance of punches and dies can be improved by applying various surface treatments and coatings. Common treatments include nitriding, which enhances wear resistance, and chromium plating, which improves corrosion resistance. Coatings such as titanium nitride are used to enhance tool performance further, increasing the life of the tools and reducing the need for frequent maintenance.

The proper maintenance of punch and die sets is also vital in ensuring their longevity and optimal performance. Lubrication and regular cleaning of the tools help minimize wear and prevent any issues related to material buildup or corrosion. Additionally, periodic inspection of the punch and die sets can help detect any signs of wear or damage early on, allowing for timely replacement or repair.

In summary, understanding the fundamental aspects of punch and die sets, such as material selection, clearance, surface treatments, and maintenance, is crucial for ensuring successful manufacturing processes. Careful consideration of these factors will lead to improved product quality, longer tool life, and more efficient operations.

Clearance Basics

Punch and die clearance is an essential aspect to consider in the manufacturing industry, as it plays a vital role in product quality and tool life. It refers to the gap between the punch edge and the die, commonly known as clearance. The ideal clearance depends on various factors, including material thickness and type.

Different materials require different clearances, and the general rule of thumb is to have clearance percentages depending on the material thickness. For instance, when working with a soft material like aluminum, a larger clearance might be suitable, while a harder material like stainless steel would require smaller clearances to reduce wear and tear on tools.

Material thickness also influences the required die clearance. As the thickness of the material increases, so should the clearance value. This is essential to maintain the balance between tool life and part quality. Adequate die clearance helps to reduce stripping force and prolong the tool’s life.

It is crucial to choose the appropriate clearance between the punch and die to achieve clean and precise cuts during the manufacturing process. In turn, this minimizes the potential of burring and enhances the overall efficiency of the production process.

In conclusion, punch and die clearance plays a significant role in determining product quality, tool life, and manufacturing efficiency. Selecting the right clearance values based on material thickness and type is essential for optimal results.

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.

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 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 Table
Unit: mm

* Also applicable to Copper and Brass

The Harm of Too Small Clearance

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

Determination of the Punch and Die Clearance Value

The determination 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.

Determine the punch and die clearance 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

Determine the punch and die clearance by 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

Theoretical calculation of punch and die clearance

The following is the theoretical knowledge in the book, the main objective of which is to ensure the proper alignment of the upper and lower cracks to achieve a desirable cross-section.

According to the relationship of the triangle ABC, the clearance value c can be obtained as:

c =( t – h0 ) tanβ = t (1-h0/t) tanβ

In the formula,

• h0 – Punch penetration depth;
• Β – The angle between the maximum shear stress direction and the vertical direction

From the equation above, the gap ‘c’ is connected to the material thickness ‘t’, the relative penetration depth ‘h0/t’ and the crack direction ‘β’.

The properties of the material determine ‘h0’ and ‘β’, with harder materials having a smaller ‘h0/t’.

As a result, the material properties and thickness are the primary factors that affect the clearance value.

The harder or thicker the material, the higher the die clearance value.

Further reading:

• How to Determine And Optimize The Blanking Clearance?

Material Considerations

Common Material Types

There are several common material types used in punch and die clearance, such as aluminum, mild steel, and hard materials. Each material comes with its own unique properties, which affect the clearance required for the punch and die process.

• Aluminum: Lightweight and with high ductility, aluminum is relatively easy to punch. However, due to its softness, it may require slightly larger clearances to avoid material sticking to the punch.
• Mild Steel: This is a common choice for workpiece material, as it balances affordability with good mechanical properties. Typically, clearances for mild steel fall in the middle of the spectrum.
• Hard Materials: These include materials like stainless steel or high-hardness steels. Hard materials can be more challenging to work with and often require smaller clearances to ensure clean cuts and reduced punch wear.

Material Hardness

Material hardness plays a crucial role in determining the appropriate punch and die clearance. Harder materials require tighter clearances because they tend to produce cleaner cuts and there is less distortion when punched. However, it is essential to note that hard materials are more susceptible to cracking and should be processed with care.

Material hardness can be defined by various scales, such as the Rockwell or Brinell scales, depending on the type of material and the desired level of precision.

Tensile Strength

Tensile strength refers to the maximum amount of stress a material can withstand before breaking. It has a significant impact on the clearance required during the punching process. When a material has high tensile strength, it is more resistant to stretching and deformation, requiring smaller clearances for clean cuts.

Keep in mind that harder materials tend to have higher tensile strength and may require adjustments to the punch and die components to avoid wear.

Die Clearance Calculator

A die clearance calculator can be a useful tool for quickly determining the appropriate clearance for various material types, thicknesses, and cutting operations. These calculators typically require input of the material type, thickness, punch-to-die clearance, and other application-specific factors.

To use a die clearance calculator, follow these steps:

1. Find a reliable online calculator or mobile app.
2. Enter the material type and thickness.
3. Specify the punch-to-die clearance, if required.
4. Choose the cutting operation or performance preference (e.g., standard, fine, burr-free).
5. Submit the information and review the result.

By utilizing these methods – die clearance formulas and calculators – users can achieve accurate and consistent results in their punching operations, ensuring a more efficient and cost-effective process.

Importance of Proper Clearance

Accuracy and Precision

Proper clearance between the punch and die is crucial for achieving accurate and precise results in any punching operation. Correct clearance ensures repeatability by minimizing material deformations that can occur during the punching process. A consistent clearance reduces the possibility of deviations in part dimensions, ensuring that the produced parts are within specified tolerances.

Quality and Burr Control

One of the primary consequences of improper clearance is edge quality degradation, which can result in excessive burr formation. Burr is an unwanted, raised edge on the material, and controlling it significantly affects the overall quality of the final product. High-quality punched products require minimal secondary operations, such as deburring, decreasing production time and costs. Proper clearance ensures not only a smoother, cleaner edge but also reduces stress on the material, preventing cracking and work-hardening during the punching process.

Tool Life and Wear

Appropriate punch and die clearance significantly contributes to extended tool life and reduced wear. Insufficient clearance can lead to increased friction, creating higher temperatures that accelerate wear and decrease the hardness of both the punch and the die. On the contrary, excessive clearance can lead to chipping or breakage of the cutting edge, as it imposes uneven loads on the punch and the die.

In conclusion, properly maintaining punch and die clearance is a critical aspect of any punching operation. It directly affects accuracy, precision, quality, burr control, and tool life. By considering these factors during the punching process, manufacturers can optimize their operations, reducing costs and increasing overall productivity.

Effect of Clearance on Stamped Parts

Rough Fracture Plane and Rollover

When stamping a product, the appropriate punch and die clearance is crucial for achieving a clean and accurate result. If the clearance is too small, shear cracks can occur, leading to an uneven and rough fracture plane. On the other hand, if the clearance is too large, excessive rollover can cause the stamped part to distort and lose its intended shape.

To minimize the rough fracture plane and rollover, the die size should be chosen in accordance with the material type and thickness. For example, thicker materials usually require larger clearance to limit shear cracks and ensure a clean cut. The selection of the right punch and die clearance for a particular application can greatly improve the product’s appearance, dimensional accuracy, and overall quality.

In the stamping process, both the punch and die play a critical role in defining the final outcome. During the shearing action, shear stress is exerted on the material causing it to separate into two parts – the stamped part and the slug or button. Due to the force applied to the material, a certain amount of deformation occurs at the edges of the cut, known as the rollover. Ideally, to maintain optimal quality, this rollover should be minimal and the material should be cut as cleanly as possible.

Slug Inspection and Analysis

Analyzing the slug (also known as the button) is an important method to assess the suitability of the punch and die clearance in the stamping process. By inspecting the slug, one can determine if the clearance between the punch and die has been correctly set and if any changes need to be made.

A well-stamped slug should exhibit the following characteristics:

• Shear zone: A distinct smooth shear area, typically covering approximately one-third to one-half of the slug’s thickness.
• Fracture zone: A relatively small rough fracture plane or break line in the material, resulting from the force exerted by the die.
• Minimal burrs: Ideally, the slug should have little to no burrs, indicating a clean cut and proper clearance.

In contrast, if the slug displays excessive burrs, a large fracture plane, or significant rollover, it may be an indication that adjustments need to be made in the punch and die clearance or die size. Additionally, careful monitoring of the stamped parts and slugs can help identify potential issues, such as cracks in the punch or die, which could lead to substandard product quality and the need for tooling maintenance.

Optimizing Punch and Die Clearance

Material Type and Die Size Adjustments

To ensure quality parts and efficient productivity, it is crucial to optimize punch and die clearance according to the material type. Different materials require varying clearance values, and utilizing the appropriate clearance size can prevent issues like material chipping and excessive wear on the cutting edge.

For softer materials, narrow clearances are recommended as they provide cleaner cuts and reduce the risk of burrs forming. In contrast, harder materials benefit from wider clearances to minimize stress on the punch and die button, prolonging their lifespan.

Die size adjustments are also an essential aspect of optimization. Naturally, larger dies require greater clearance, while smaller dies need tighter clearance. By adjusting die size and clearance accordingly, consistent results can be achieved across various production runs.

Specification and Maintenance Best Practices

Implementing specification and maintenance best practices are key to maintaining the effectiveness of punch and die clearance. Monitoring and adhering to industry-standard specifications ensures the production of high-quality parts, while regular maintenance helps to:

• Maintain a sharp cutting edge
• Prolong the life of the tooling
• Reduce downtime and increase productivity

Here are some best practices for punch and die maintenance:

• Regular inspection: Check the punch and die button for signs of wear, chipping, or damage. By addressing minor issues promptly, more severe problems can be avoided.
• Proper cleaning: Conduct regular cleaning of the tooling, using appropriate cleaner solutions to remove debris or build-up. This can enhance the cutting edge and overall performance.
• Lubrication: Applying the recommended lubricant type and amount will help reduce friction, ensuring smooth operation and reducing the risk of tooling wear.
• Replacement: Monitor the lifespan of the punch and die components. Replacing worn or damaged parts when necessary allows for continued quality production and prevents potential delays or damages.

By optimizing punch and die clearance and adhering to best practices for specification and maintenance, manufacturers can ensure the consistent production of quality parts while maximizing efficiency and minimizing tooling wear.

Conclusion

Punch and die clearance plays a crucial role in the success and efficiency of metal fabrication processes. Selecting the appropriate clearance is essential to reduce tool wear, maintain workpiece quality, and optimize the production timeline.

Various factors, such as material type, thickness, and tool geometry, must be taken into account when determining the optimal clearance. It is crucial to apply industry standards and guidelines, while custom adjustments can be made based on specific needs.

In summary, careful consideration of punch and die clearance is a vital aspect of any metal fabrication process. By selecting the appropriate clearance values, manufacturers can ensure the quality and longevity of the produced components and improve the overall efficiency of their operations.

Frequently Asked Questions

What factors influence the selection of die clearance?

The selection of die clearance is influenced by various factors, such as material type, material thickness, desired edge quality, punch and die wear rate, and the specific requirements of the cutting application. Considering these factors ensures the proper clearance is chosen to achieve a successful and efficient cutting process.

How is die clearance calculated?

Die clearance is typically calculated as a percentage of the material thickness being cut. The desired edge quality, required precision, and material properties determine the optimal percentage used. Commonly, manufacturers and industry standards provide guidelines and recommendations for calculating die clearance based on their experiences and testing.

How do material properties affect punch and die clearance?

Material properties, such as hardness, ductility, and thickness, directly affect the required punch and die clearance. Harder materials generally require larger clearances due to the increased resistance to cutting forces, while ductile materials often require tighter clearances for better control and less distortion. Additionally, thicker materials tend to require larger clearances to account for increased cutting forces and wear on the punch and die.

What is the optimal clearance for blanking and piercing?

The optimal clearance for blanking and piercing depends on the specific material being processed, its thickness, and the desired edge quality. Typically, clearances for blanking operations are larger than those for piercing operations, as blanking requires the material to be sheared and separated completely. The optimal clearance maintains part quality and functionality while minimizing punch and die wear.

How is cutting clearance different for various materials?

Different materials have different properties that influence their requirements for cutting clearance. Hard materials, like stainless steel or tool steel, usually require larger clearances than soft materials, such as aluminum or copper, due to their greater resistance to cutting forces. Ultimately, the cutting clearance must be adjusted for each specific material to achieve optimal results.

What are common issues related to incorrect punch and die clearance?

Incorrect punch and die clearance can lead to several issues in the cutting process, including poor edge quality, excessive burrs, material distortion, and increased punch and die wear. In many cases, material wastage and higher maintenance costs are the result of using incorrect clearances. Ensuring the proper punch and die clearance is essential for maintaining efficiency and quality in a cutting operation.