BENDING WITH BALL MANDREL AND WIPER DIE

When the radius of the bend is smaller and/or the wall is thinner, it becomes necessary to use a
ball mandrel and wiper die. The wiper die is used to prevent wrinkles. The ball mandrel performs like a plug mandrel. The balls are used to keep the tube from collapsing after it leaves the mandrel shank.
Bending issues are enlarged when making tight bends or with thin wall tubing. It becomes more difficult to retain the material during compression.
The pressure is so intense the material is squeezed back past tangent and buckles. This area must be supported so that the material will compress rather than buckle; this is the prime purpose of the wiper die. Note, the wiper die cannot flatten wrinkles after they are formed; it can only prevent them.
Bending thin wall tubing – requirements for the bending of thin wall tubing with tight radius bends of centerline radius equaling the tube outside diameter have become more common in recent years.
To increase the problem, new alloys have been developed that are extremely difficult to bend. The proper bending machine, good tooling, and a trained operator can make all the difference.
To facilitate this type of tube bending, the material to be bent should receive special consideration. To help maintain the consistency of the tubing dimensions and characteristics, the entire material required for a given job should be acquired from one supplier, preferably even from the same lot or heat number. Premium-priced close-tolerance tubing should be considered. It often saves many more times its added cost. It may be necessary on occasion to size certain tubing before bending.
There are many pipe bending machines but only a few are capable of thin wall, 1 x D bending.
Even those machines best suited for this special bending must be in excellent condition and of a size large enough to assure rigidity. The machine must be capable of retracting and advancing the mandrel with the clamp and pressure dies closed. A hydraulically actuated pressure die is desirable. One feature of this system is that it provides identical pressure on the tube regardless of wall variation.
A pressure die boost counteracts the drag of the pressure die, mandrel and wiper die. It pushes the tube into the bending area, preventing excessive wall thin out.
Without a pressure die boost, the thinning that normally can be expected is about three quarters of the elongation of the outer wall. Companies specialized in steel bending automation for a broad array of fabricated products, includes CNC tube benders, tube measuring equipment and complete cellular integrated systems.