Every time a new part needs to be bent, there always seems to be a small trace of doubt: “will we have enough tonnage to bend it?”
Bending sheet metal is all about breaking the molecular structure of the metal. The idea is to provide at least enough power to achieve “plastic deformation.” This type of deformation, unlike elastic deformation, is irreversible because the molecular structure has changed.
Our Bending Solution calculator can be of great help when trying to calculate the required tonnage to bend metal to achieve plastic deformation.
“Plastic Deformation” vs. “Elastic Deformation”
While ED (Elastic Deformation) allows material to return to it’s original shape, PD (Plastic Deformation) implies that some fibers of our material have changed their structure, therefore material will not fully return to it’s original shape.
PD is basically what we all want to achieve when bending on press brakes. With the naked eye, It can sometimes be hard to tell if PD is achieved.
Feel free to contact us if you have technical questions about metal bending.
Imagine our sheet metal is a bridge running over our die. We need to know how much load this bridge can hold…
Unlike a bridge, we then have to go over the maximum load amount! This is what bends the metal.
Required Tonnage Rate for your Press Brake
First thing to keep in mind when calculating the required tonnage to bend a certain material is that it is not the total value that matters, but the ton rate. Meaning the tons/foot or tons/meter.
The second most important aspect to keep in mind is that the length of the press brake is irrelevant to our calculations, what does matter is the length of the sheet metal we intend to bend.
- We need to bend 1.5 meters of ¼ mild steel.
- We will use a V opening of 50mm (about 2).
- We will apply 85 tons… which means about 56 tons/meter.
This rate will, in fact, bend the material, but let’s think about the following:
Q) What would happen if we change the length to 3 meters?
A) If we continue to apply 85 tons we will be applying about 28 tons/meter.. so our material will not bend.
Q) What would happen if we change the length to 0.5 meters?
A) If we continue to apply 85 tons we will be applying about 170 tons/meter.. which will probably damage our tooling and our press brake.
But how do we determine the required ton/meter rate?
Let’s go back to our bridge comparison. Mathematics and engineering have provided for us an equation that we can use to find the maximum load (or tonnage).
When designing a bridge the intention is to know the maximum load that the middle of the bridge can handle. We, on the other hand, want to exceed that maximum load.
Starting from a simple formula that engineers use to calculate the maximum load a bridge can stand, we can add a factor for the limit which we will intentionally exceed.
Tons per meter (t/m) = Thickness (in mm)² x 1.65 x UTS (in Kg/mm²) / V Opening (in mm)
The number 1.65 is determined by the friction of a certain material (which in our case is metal) to determine when the bridge falls down.
Without the 1.65 factor this same formula would tell the exact maximum load that our sheet metal bridge can support.