Problem: Mandrel too far back form Tangenti
When the mandrel is advanced forward in small increments, wrinkles may begin to form in back of tangent. At this point, the mandrel is not far enough forward to generate enough pressure on the inside of the bend to compress the material. Initially, the bend may be smooth, but as it progresses past approximately 20°, the material pushes back forming a wrinkle or wave at point A. Continually formed and flattened between the mandrel and the bend die, the wrinkle never entirely disappears. When the bent tube is removed from the bend die and there is a large buckle or kink at point A, it is necessary to continue to advance the mandrel until the material can’t squeeze back between the bend die and mandrel.
Problem: Mandrel too far forward
Several problems occur when the mandrel is positioned too far forward. Bumps appear on the outside of the bend and are most evident at the end of bend. A step may begin to appear on the inside at the start of bend. Even though these malformations are shown on the same tube, they will not always appear at the same time depending on tube material, shape of the mandrel, and bend radius. The bump is obviously caused by the mandrel shank, and the step is formed by the end of the mandrel prying the tube away from the bend die.