A rolling die is a specialized press brake die which has two semicircular rotating pads set inside the top of the die. As the punch moves downwards the work piece begins to bend and the pads rotate with the work piece.
Rolling dies come by many names such as Rolla-V and Rotabend, some claim to be patented but I’ve yet to be able to locate the patent, if anyone out there is able to do so please post it!
There are certain advantages of a rolling die. It will support the majority of the outside mold line of the work piece compared to a regular die which only supports the work piece at a single point until it’s finished bending. This support prevents distortion of features and holes near the bend line, negating the two times material thickness distance rule for placing cutouts. This support also prevents slipping and can allow you to confidently fabricate very small flange lengths, while you should consult with the die manufacturer typically you can achieve 30% shorter flanges than air bending.
Further more the flat side of the die remains in contact with practically the same part of the work piece throughout the bend, preventing die contamination from galvanized steel and aluminum as well as part contamination on stainless steel work pieces.
A rolling die will also be able to support a wider variety of material thicknesses than a typical v die. Additionally adjustable versions can allow for a huge variety of fabricating options from a single die. This is great for the small job shop on a tight budget. By opening the gap on adjustable models you can punch through the work surface with a rounded straight punch and create u-bends.