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A tonnage limit tells you the range of material thicknesses that you can draw, punch, blank, or process. Some common capacities of gap-frame presses are 121 to 176 tons and 400 to 600 tons for straight-side models; however, the stamping industry produces such a variety of products that specific tonnages are hard to pin down. For example, for lighting products, the press bed size is large but the application requires less tonnage. For structural steel, the bed for producing connecting devices is much smaller, but the tonnage is high. Presses can have tonnage capacities as small as a single ton to more than thousands of tons. Understanding tonnage for progressive dies is very important because all stages must be added together to figure total required tonnage. Determining tonnage for progressive dies is further complicated by the number of stations and staggering of the punches. If you have three punching stations that all contact the material at the same time, the tonnage is simply the sum of the three stations. However, this is typically complicated by items such as stripper springs and nitrogen cylinders, for example. The formula for determining tonnage for a single-station blanking/perforating application is:

If you have three stations that contact the material at the same time and the material is the same size, tonnage required is 75 + 75 + 75 = 225 tons
If three stations contact the material at the same time and the material is the same size, tonnage required is 75 + 75 + 75 = 225 tons.
It’s a good idea to have at least a 10 percent higher tonnage margin to allow for tool wear, which increases tonnage requirements as the tool dulls. For example, punching a 1/4-in. hole through 10-ga. mild steel takes less than 3 tons, but as the punch and die dull, you can expect to see this tonnage increase. The decision to change or sharpen a punch is more of a quality issue since the part will start to leave a burr.

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