October 10, 2006
This article is one of a 16-part series on the fundamentals of stamping. Descriptions of all the articles in this series, and links to them, can be found at the end of this article.
Cutting is the most severe metalworking process that takes place in a die and shouldn't be taken lightly.
Cutting metal requires great force. For example, it takes approximately 78,000 lbs. of pressure to cut a 10-in.-diameter blank from 0.100-in.-thick mild steel. Consequently, the punch, die, and press must absorb overwhelming shock.
Overshocking the press and die components usually is what causes them to fail prematurely. If you work in a shop that blanks heavy metals, you know what I mean. You can hear and feel the press shock. Doing everything you can to reduce the unnecessary loading and shocking is important. Factors such as cutting clearance and shear angles contribute significantly to the amount of force required. They also affect the amount of shock that is generated.
If you participated in a tool and die apprenticeship, you probably were taught the following rules for piercing punches:
These are good starting guidelines for cutting, but they aren't entirely true. Let's examine each misconception.
The punch determines the hole size. Although the punch produces a hole that is very close to its actual diameter, altering the clearance between the punch and the button (sometimes referred to as the matrix) also affects the hole size. The simple truth is that a hole can be made slightly larger or smaller than the punch diameter by increasing or decreasing the cutting clearance. This is because of the way that the metal deforms before the cutting actually takes place.
Think of the metal that you're cutting as Silly Putty® or a rubbery plastic. If the clearance between the cutting punch and the button is insufficient, it will cause the metal to compress or bulge out away from the punch before the cutting takes place. After the slug is created, the metal grips the punch sides. This increased friction between the sides of the punch and the metal raises the amount of force necessary to strip or pull the punch from the metal.
The insufficient clearance between the punch and the button means that a greater force is needed to create the hole. Inadequate clearance also increases the load on the edges of the punch and the matrix, which causes premature edge breakdown.
After the punch is removed, the metal that once was compressed decompresses and collapses around the void area (the hole). The result is a hole that is smaller than the punch's diameter (see Figure 1).
Keep in mind that changing the clearance does not affect the hole size to a great extent, about 0.001 in. to 0.002 in. Although it might seem small, this change can reduce the friction generated during punch withdrawal significantly and extend the punch life see Figure 2).
The cutting clearance always should be equal around your punches. Once again, unless you are piercing only round holes, this statement is not entirely true.
Cutting clearances should change around the punch perimeter with respect to the punch geometry. Let me explain using this example: If you are piercing a square hole, you may notice that the corners of the punches are the first areas to break down. Once the corners break down, the entire punch must be sharpened. Ever wonder why the corners break down first? It's because this is the area that is subjected to the highest cutting loads. Very simply, wherever there is a small radial feature in a cut (nothing is worse than a dead sharp corner), the compressive forces will be greater.
Excessive compression can be compensated for by increasing the cutting clearance in areas with small radial features or sharp corners. Increasing the clearance in these areas helps to increase punch and button life and reduce the probability of a large corner burr. A good rule of thumb is to increase the clearance in the corners to approximately 1.5 times the normal clearance. An even better scenario is to avoid dead sharp corners whenever possible (see Figure 3).
10 percent of the metal's thickness is a good cutting clearance for each side of the punch. Once again, this statement isn't always true. While 10 percent is by far the most commonly used cutting clearance, it most certainly is not always the ideal cutting clearance.
Cutting clearances can range from as little as 0.5 percent up to as much as 25 percent of the metal's thickness per side. Among the many factors that determine the best cutting clearance are the metal's thickness and hardness and the punch size and geometry. For example, the ideal cutting clearance for piercing a 0.500-in.-diameter round hole in a sheet of 0.100-in.-thick 300 series stainless steel is about 13 percent of the metal's thickness per side, or 0.013 in. per side. This calculates to a total clearance of 0.026 in.
However, changing from a 0.500-in.-diameter punch to a 0.100-in.-diameter punch requires more cutting clearance, from 13 percent to 20 percent per side. This is because the smaller punch has a smaller radius, and compressive forces congregate at the smallest radial feature of a cut (just as in the rectangular punch example noted above).
Metal type also affects cutting clearance selection. Harder, higher-strength materials require more cutting clearance, while softer metals, such as aluminum, require smaller cutting clearances.
As you can see, metal cutting is slightly more complicated than often perceived. Understanding the many variables and how they affect the cutting process are key.