It is now possible to look a little deeper into the limits of nonmandrel bending and expose some of the possible pitfalls.
One pitfall that may be difficult to diagnose is a concentricity problem in the material being bent. One bend may be good, and the next bend will experience a wrinkle. The problem occurs when the heavier wall is on the outside of the bend; this causes excessive flattening and possibly wrinkling on the inside of the bend.
Another common problem occurs when trying to bend too big of tube on too small of a machine. This could be referred to as a rigidity problem.
What can be done to exceed the limits of non-mandrel bending?
One possibility may be to reengineer for a harder material. However, this may not be an option.
Another option is to consider controlled wrinkles on the inside radius of the bend (see image to the right). These are effective and could raise the feasible difficulty factor to more than 40, but the wrinkles also need to be acceptable to the finished product.
Another possibility might be to employ a shaped tube groove with a properly-sized plug mandrel. This is clearly a compromise, but may allow the elimination of a ball mandrel in favor of a simpler operation.
The most important factor affecting the limits of nonmandrel bending is the user's persistence to find better and faster ways to bend tubes. Today's limits are in place because of a driving force to improve.