Page 40 - MetalForming September 2016

How Tonnage Impacts Press-Brake Bending
Find out how material choice affects tonnage, and how much tonnage is needed to make a bend.
Tonnage selection when press- brake bending can spell the dif- ference between a quality, eco- nomically produced part or a costly reject. Not only that, but incorrect ton- nage can spell danger for tooling, machinery and operators. Exceeding maximum tonnage on the press brake for any type of application can risk severe damage to the machine, cause tooling to break and dangerously com- promise operator safety. When press brakes experience over-tonnage, the ram and bed can suffer serious defor- mation, which often cannot be repaired. Learning how to select the appropriate tonnage for individual bending applications can help avoid irreparable damage to the press brake and tooling, and protect operators from injury.
Maximum Tonnage Defined
Maximum tonnage is the maximum amount of force that a press brake can
exert to form sheetmetal. This typically is expressed as tons/ft. or kN/m. Every machine has a different threshold or rating for the maximum amount of force produced. Tooling, holders and adapters also have tonnage limits. An operator must understand these limits in order to safely utilize the maximum range of applications for any given press brake.
How Much Do You Need?
How much tonnage is required? The following factors impact the tonnage required for any bending application:
• Material thickness
• Material type
• Material length
• Method of bending
• Tooling selection
In general, the thicker the material
the more tonnage or force required to form it. The type of material also plays a large role in this equation. Yield strength varies tremendously for dif-
ferent materials. For example, forming soft brass or copper requires much less tonnage than the same thickness of mild steel, and forming stainless steel requires much more tonnage than forming mild steel.
Keep in mind that tonnage ratings incorporate a length component. For example, a rating of 30 tons/ft. equates to 2.5 tons/in. If an application calls for a force of 50 tons across a 10-in.- long form, that equates to 5 tons/in. or 60 tons/ft. Some applications require that a large amount of tonnage be placed in a small area. The press brake may have the overall tonnage required to achieve the form, but if the tonnage must be concentrated in one location (known as isolated tonnage), then the tonnage rating of the tooling and hold- ers also must be considered.
The method of bending also plays a role. Air bending, bottom bending and coining all require various amounts of force. Air bending requires the least
38 MetalForming/September 2016
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