Roller dies are very specific product and moreover – they are quite expensive according to standard typical solid dies. So we thought that it will be good to make in one place the main article. Here is the information what you can get from it and why you need it. We don’t know the details so only you can decide why it is good investment and why it is not.
This article could help you to understand if you really need roller dies. Or probably it will be just additional cost that actually will not help to your production.
Working principle
The standard die has V-opening. The part stays on shoulders of the die and during the pressing force of the punch deformation goes down towards to bottom of V-groove. So there are 3 important points that effect the bending – punch tip with the point of force and 2 shoulders of the dies. The bend usually is made as “air bending” – metal will not touches the bottom of V-groove and stays “in air”. It is the most common and effective method of bending based on possibilities, forces and accuracy.
With roller dies 2 rollers replace the upper part of traditional die. There are no die shoulders anymore and the roller’s surface has the role of die shoulder now. During the bending and material pressing the rollers start to rotate and support the material like the shoulders of traditional die.
Rollers (also are “rotors” in some articles) inside the die are fixed with springs to make the rotation thanks for the force of the punch and return with the power of the spring. As for all products with internal springs – they are subjects to wear but could be replaced easily directly on site without necessary of special services.
For longer and effective work the maintenance of roller dies is more complicated than standard dies. Manufacturers recommend time to time (recommended once per week) to disassemble the springs and to clean all completely internally and to put lubrication.
For sure suppliers offer roller dies as the replacement of traditional dies with all main typical sizes (according to thickness of sheetmetal) and different clampings solutions like Amada-Promecam, Trumpf-WILA-Bystronic etc.
All roller dies do not require any special punches for use. So only bottom part is the subject for the replacement. All dies as other bending tools are for any material: aluminum alloys, mild steel, stainless steel etc. For special hard steels it is also possible to use but better to request additional consultancy support to verify all the details with the press brake tools supplier.
Short flanges
Short flanges are the main advantage of roller dies. There are much more but the flanges or the distance between bending zone / radius till the end of the part is the main reason for what roller dies should be purchased.
Market offer various possibilities to deal with this problem like additional tools for punching machine to make the groove and break it after the bending for example. Or possibility to make the parts with microjoints and also to break the rest of the part after the bending.
The point is that with traditional solid bending dies the size of the flange depends of the V-opening size. If the flange will be too small it cannot lay on the shoulders or there will be not enough size to support during the bending – the part will just fall off the shoulder.
In theory to solve this questions press brake operators take the dies with smaller V-groove. Smaller size of V -> smaller flange. So they do not respect the “rule of 8” but work with the “rule of 5-6”. It can be done without the problem but such work increase the necessary tonnage that you need for the operations and sometimes the application is out of the limits of the press brake and tonnage possibilities.
And if we are talking about thin materials it is possible to reduce the V-opening sometimes even twice. But with hard steels or big thicknesses you will easily go off the limits of your press brake machine.
Reduction of bending flange probably is necessary for the part because it is already designed like this but could be also subject for project engineers to make smaller flanges and save material costs as well.
We already encountered the projects where the part was not possible to make with traditional die, the flange designed in the drawing was too short for it. Roller die was unique solution to continue the study. One more project for example which we have was the Z bending with short flange. The solution was to make the first bend on the roller die and the second bend to make with standard die with milled side.
Small radius
Small radius is other advantage of the roller die. In standard solid die the size of the radius is based on the V-opening (if the radius of the punch is smaller enough) and calculated based on it. With the roller die it is really possible to achieve the radius near the radius of the punch (officially talks are around 1,5 times of thickness). The secret is the surface of rollers – they are much closer as the support surface than shoulders of traditional die so deformation has another curve.
Normally bending radius is not the important point of the part. Engineers usually make a bit tolerance according to standard bending process but also the increase of bending radius usually doesn’t change the geometry of the part. So itself it is not a big problem or importance.
The other point if we are talking about thick materials and part details near the bending radius. Surface near bending zone is the subject of the deformation and if any holes are positioned near the bending they will be deformed as well. So the roller die can save the part to reduce the bending zone and material deformations around it. It could be a big advantage for some type of parts.
There are also special solutions to use roller dies in Z-bending applications. Sometimes there the reduction of radius could change the part itself, big radiuses and small size between bends with traditional bending leave no straight line between but in practice will be like radius to be transferred in other radius.
No bending marks
The surface zone of rollers make better support for the part sides than shoulders of standard tradianal dies. And there are less micro-movements during the operation of bending. As the result there will be no marks or very light.
There are a lot of solutions to get no-marks bending on press brake like special tooling with polyurethane or nylon material or use of special bending film. Absolutely sure that these solutions are much more economic than purchase of roller die. So we consider that advantage to bend without marks is the additional advantage but not the primary one. Anyway if to compare with bending film it eleminates the additional action so – save the time.
Probably with standart workshop it could be not really important but if we will think about robotic cell there are no possibilities to replace the bending film in automatic way and the intervention of the operator could stop the continious cycle completely. So in the automatic production where all actions are really automatic with the use of robots or automatic devices and every second is in calculation – roller die could be also a serious advantage.
Radius bending
At the end – about radius bending. Because of the springback of the material radius bending is quite problematic application. And there is no any possibility to make the precise distantly solution without tests.
Roller dies can help in 2 ways. In standard theory the recommdended die V-opening for radius bending is R x 2 + S x 2. Let us say, that you have R40 and S=1. So the recommended opening for you is V=82. And for radius bending it is better to have smaller angle of die and this means that in standard production program (for us but for mainly all main suppliers) you will not find the typical bending die and you should request the special tool.
First, with roller die thanks for big support surface you can go “deeper” and catch the point that you really need.
Other, if special roller die is used for big sizes, big opening and possibility to adjust the opening it could be the solution to make precisely adjustment of opening and after some tests to find the most effective solution than any other possible to get your materials radius bending without any additional operations of secondary bends in one single stroke. Yes, such solution could be quite expensive but based on production program and company needs because as repeated, could be the best possible.
