Technical information in this article was provided by MISUMI product engineer Kris Szewczyk.
Achieving clean and precise cuts while maintaining tooling requires an understanding of punch and die clearance.
This article explores what punch and die clearance (clearance) is, why it matters, and how to determine the optimal clearance for your specific application.
What is Punch and Die Clearance?
Clearance is the distance between the punch’s cutting edge and the die button’s cutting edge. This clearance represents the optimal space between the tooling required to puncture the material in shear and punch a clean hole through the workpiece.
As a reminder, the “punch” is the tooling piece that penetrates the material to create a hole, while the “die” is the solid tool on the back of the workpiece that supports the workpiece and provides the cutting edge for the punch.
It’s important to properly specify the clearance between the punch and die for each metal piercing application to produce high-quality parts.
The recommended clearance is a percentage per side, which refers to the clearance at each edge of the workpiece’s cut surface as a function of the workpiece’s thickness.
For example, a 10% recommended clearance means the clearance should be approximately 10% of the workpiece thickness. The “per side” means that this clearance must be present on each side of the die/punch of the workpiece. This clarification is important because the number represents the clearance that must be present on each side of the punch/die.
Why Punch and Die Clearance Matters
Clearance determines the impact of the quality of the cut. It is required to produce cut edges and features that reliably meet product design specifications, even for tight feature tolerances.
In short, proper clearance ensures clean, precise cuts with minimal material deformation and residual burrs on the cut edges. Burrs are small, raised residual material that remains on the cut surface after punching, and they are unwanted byproducts of the punch process that may interfere with the fit and function of parts within an assembly.
Many times, burrs must be removed through time-consuming, secondary hand-processing methods. Proper clearance minimizes part rework time and increases production efficiency.
Beyond part quality, proper clearance minimizes wear and tear on the punch and die tooling, lengthening the lifetime of expensive die tooling equipment while minimizing die tooling downtime and repair costs. Improperly specified punch and die clearances may even lead to outright fracture of punch and die tooling equipment, posing a safety hazard to manufacturing personnel.
Optimized clearance also helps to create energy-efficient production processes because less force is needed to shear the workpiece, reducing the power consumption of production presses.
Factors Affecting Clearance Selection
Proper clearance aims to align the grain boundary fracture planes of the workpiece at the top and bottom of the workpiece to create a clean cleavage site at the cut edge.
A few key manufacturing parameters specific to your workpiece will help you determine the optimal clearance required to achieve fracture in such a manner.
Workpiece Material
The workpiece material is the first and chief consideration when determining your punch and die clearance. The properties of the workpiece material under large loading before fracture greatly impact the optimal clearance.
Stronger, harder materials require increased clearance than a workpiece fabricated from a weaker, softer material.
Workpiece Thickness
Additionally, the workpiece’s thickness is the next consideration. While it may seem obvious, especially if you understand how clearance is specified (i.e., % per side), a thicker workpiece requires additional clearance.
Tooling Condition
Finally, your anticipated tooling life and permittable die and tooling wear will also impact the “fine-tuning” of your clearances for a particular process.
You should verify the tooling condition to understand the equipment’s accumulated wear and determine if your production tooling can accommodate a tighter clearance or whether it is better to redeploy newer equipment for a particular fabrication process.
Recommended Clearance for Common Materials
As stated, the recommended clearance values are cited as a percentage of the workpiece thickness per side. As you can see from the chart below, harder, stronger materials require increased clearances, while high-precision grades require decreased clearances.
How Die Clearance is Determined
Generally, the standard recommended per side clearance is 10% of the material’s stock thickness. However, modern manufacturing development focused on fine-tuning suggests that using a higher clearance value of 11-20% may considerably reduce the tooling strain during piercing and increase the tooling’s operational life.
In any case, when you have determined the Recommended Clearance Percentage (per side) using the chart above, you may then use this value to determine the actual, raw per-side clearance in length value using the following simple formula:
Adjusting Clearance for Special Applications
Further adjusting or fine-tuning the clearance for certain punch applications is necessary.
Fine Blanking
This specialized stamping technique produces parts with excellent dimensional accuracy and cut-edge finish. It creates parts with high dimensional accuracy where precision is paramount.
At a high level, fine blanking applies high pressures at the top and bottom of the workpiece via two specialized dies. Because the workpiece must be tightly restrained to prevent significant plastic deformation during forming, fine blanking requires very small punch/die clearances.
As such, it is a wear-intensive process on production tooling because of the extremely small clearances.
Hard and High Tensile Strength Materials
As alluded to in the chart, the hardness and tensile strength of material are vital characteristics of the stamping process that determine the required punch and die clearance.
Hardness measures the resistance of the material’s surface to indentation or scratching.
The reference table above shows types of material and various tensile strength. The workpiece materials with higher tensile strength require additional clearance.
Finally, you should note that small clearance will require additional alignment verification before every machine setup and operation. With all other factors held constant, reduced punch and die clearances will increase punch wear during long production runs compared to operations that require less clearance.
MISUMI: Your Supplier for Industrial Punch and Die Components
MISUMI is home to high-quality industrial punch and die components, including precision punches and dies, for all industrial applications.
