BENDING RULE The bending rule is a very useful tool for anyone designing or realizing an object made of sheet metal bent with a press brake. The bending rule in a form of a table is usually stuck on the press brake or simply included in its instructions of use (see image below). Bending rules with an internal sliding table showing the same data have recently become more popular, since they are easier to be used (see image below). In this lesson we are going to explain the use of the bending rule with the sliding table and in the next lessons we ll investigate the formulas that lie behind this table.
The bending rule sets some parameters of air bending: Die vee opening Dimension of the shortest side that can be bent (minimum internal edge) Necessary tonnage per meter Internal radius of the bending The correct use of the bending rule can reduce trials and errors during the bending process. The best way to use it is starting from the upper part and going down analyzing every parameter given for the selected die. IMPORTANT: suppose to bend mild steel at 90 in air Step 1 Select the sheet metal thickness In the first row you can find a box showing the sheet metal thickness. If you slide the internal table you can select from 0.5 from 25 mm. Step 2 Choose the die vee (V) Once you ve selected the sheet metal thickness, the bending rule shows five numbers in the second row. They represent five possible die vee dimensions in millimeters that can be easily found in the market. These five vee dimensions guarantee the best ratio between internal radius of the bending and necessary tonnage without stressing the sheet metal too much. Among these five vee dimensions the best one is in the middle (i.e. in case you want to bend 3mm sheet metal the best die is with V 25mm). Now you have to check which die is available and choose accordingly (in case die with V 25mm is not available you can choose die with V 20mm). We always recommend designers are informed about the die availability in their production departments and indicate the selected die in the product drawing.
Step 3 Check the minimum internal edge (B) Once you ve chosen the die vee, check the minimum internal edge for it, which you can find in the third row. It s expressed in millimeters and represents the shortest side that can be bent at 90 in such vee because it can lay on the die before bending (for example, the minimum internal edge is 16.5mm for V 25mm). If the internal edge of the profile is shorter than this, the sheet metal will slip into the vee and subsequently the bending will be deformed. Product designers have to take this parameter into consideration and should avoid holes and other details in the area between the bending axis and the end of the minimum internal edge on both sides of the bending. If the internal edge of the profile is not long enough you have to choose a smaller die vee to enable the sheet metal to lay on the die before bending. Example: the internal edge of the profile on the left is 78 mm. Step 4 Check the internal radius Once you ve chosen the vee die and checked the minimum internal edge, you have to check the internal radius of the bending, which you can find on the fourth row. The internal radius depends on the die (for example die with V25 determines a 3.2mm internal radius) and not on the sheet metal thickness. For this reason the choice of the vee is fundamental to influence the profile features in terms of internal radius and, therefore, of sheet metal development. Thanks to the use of the bending rule, product designers can dimension drawings properly and calculate the right development. For example, if the drawings imposes 2mm internal radius, the operator has to choose a die with vee 16mm. Step 5 Necessary tonnage Once you ve chosen the vee die, checked the minimum internal edge and the internal radius, follow the arrow and find the necessary tonnage to bend 1 meter mild steel of the selected thickness. For example, you need 23 T to bend 3mm mild steel in a die with vee 25mm. In case your profile is 2m long, you will need 46 T (23 T x 2).
The same tonnage is required for bottoming. In case you want to calculate the necessary tonnage for other type of sheet metal, divide the tonnage you ve found in the bending rule by 420 (the average tensile strength of mild steel is 420 N/mm²) and multiply it by the tensile strength of the sheet metal you want to bend. Product designers have to calculate if the available press brakes are powerful enough to bend their profile and, in case they aren t, change the selected die. Consider that the bigger the die vee and the lower the necessary tonnage. Furthermore, product designers have to check tool capability to avoid any damage during bending. Correction factors in case you don t bend a 90 angle If you don t bend a 90 angle, you have to use correction factors listed on the bending rule, as the smaller the angle and the longer the minimum internal edge. This is caused by the fact that you have to push the sheet metal further down into the die vee. To find the right minimum internal edge, multiply the value (B) on the third row of the table by: B x 1,6 for a 30 angle B x 1,1 for a 60 angle B x 0,9 for a 120 angle B x 0,7 for a 150 angle Example: in case of a 30 bending with a die vee 25 mm, the minimum internal edge is = 26,4 mm (16,5 x 1,6 = 26,4 mm). Correction factors to bend stainless steel or aluminium In case you bend stainless steel or aluminium, the different springback determines different internal radii even if you use the same die vee. For this reason you have to use correction factors to find the right internal radius of the profile. R x 0,8 for aluminium R x 1,4 for stainless steel Example: in case you bend stainless steel in a die with vee 25 mm the internal radius is 4.48mm (3,2 x 1,4)
Differences between rule with sliding table and fixed table Although they show the same values and can be used in the same way, the fixed table has a different structure. In the fixed bending table the first column shows the sheet metal thickness (s) and the first three rows on the top the die vee (V), the minimum internal edge (B) and the internal radius (Ri). If you cross the sheet metal thickness with the selected die, you can find the necessary tonnage per meter. Knowing how the bending rule works is fundamental for designers to design their products properly and to avoid bending problems to press brake operators.