Blog by Alan Pickering.
Rollforming or Push Bending are relatively new developments in the tube bending world.
With the all-electric tube bending machine, which was patented 30 years ago by Unison Ltd, more complex shapes are coming to the fore, making repeatability a bigger challenge, and expectations need to be realistic.
The basic principle is to use the carriage that holds the tube to push past a series of rollers that are attached where the Clamp, Pressure Die and Bend Former are.
Some machines will use the bend arm position to control the Roll Form radius, which is prone to variability caused by the inherent backlash in the bend-head drive.
Some also use the control of the clamp roller position if it is servo controlled and some use a combination of both.
The carriage push is often driven by a single rack and pinion, this tends to apply an offset load, which can cause mechanical stress in the carriage and can’t apply as much force as a ballscrew drive down the centre of the bed. Although a more expensive solution, it often gives better control and a more even load distribution through the mechanics.
Part shapes can sometimes require 3D roll forming, which then require the collet on the carriage to rotate while driving forward, making the collet grip essential and bringing into play Torsional forces the further away from the drive rollers you are.
One subtlety in the programming of a part is that the blend from straight to roll form is not the same program in reverse when going from roll form to straight, so some additional mathematics is needed in this space.
On a Multi-Stack machine discussed in the previous Blog – here – it is possible to Draw Bend and Roll Form within the same part.
Complex shapes should not be undertaken without having your eyes wide open to the challenges, and you mustn’t forget you are forming metal and these parts cannot be held to the same high standards as machining metal – something that is often overlooked by junior designers without practical experience.
One important point is that parts will not be consistent unless you can control all your inputs: Diameter, Wall, Ovality, Yield etc and if the tube is rolled so that the bore is central along its length.
If you are not 100% in control of the above then you must consider taking up the forming tolerance in the next process, typically a weld fixture or press.
Got a question for our expert team? Contact us now – sales@unisonltd.com.
