The Press brake Tonnage Calculator can be used to estimate the required tonnage to “air bend” a part. The actual tonnage may vary from this calculation due to several factors such as the friction of the material on the die, thickness variations of the material and actual tensile strength. It is recommended that the press have 20% excess capacity. If you are bottom bending, tonnage requirements are approximately four times the air bend tonnage and for coining, the requirements are approximately ten times the air bend tonnage.

Another factor that must be considered is the capacity of the tooling which is important when forming short parts. The total tonnage may not be high but the tonnage per foot of bend may exceed to capability of the tool or ram. Damage to the tooling or machine may result if the capacities are exceeded.

“The Material Strength Reference Table” provides tensile strength values to use with the press brake tonnage calculator. The “Max. Tensile” value should be used. This value is either the ASTM listed maximum tensile or an estimated maximum tensile strength of 15000 psi over the minimum amount. Also included in that table are any available recommendations for the die width ratio (die width divided by the material thickness) and minimum punch radius ratio (punch radius divided by the material thickness). These ratios are suggested by the steel manufacturers to avoid fracturing when bending to 90 degrees. For steels where nothing is suggested, an eight times die ratio is suggested. The punch radius for air bending should be less than the air bend radius.

The values calculated by the bending calculator include the die ratio, the inside radius of the part (the inside radius is determined by the die width NOT by the punch radius), the outside radius of the part, the minimum leg or distance between bends required by the die opening and most importantly the tonnage per foot required to make a 90 degree bend. Bends of less than 90 degrees may require less tonnage but not by a significant amount.