A circular tube becomes flat when it is bent with a conventional bending apparatus. Typically, a mandrel is used to prevent the tube from flattening. However, the mandrel cannot always be used because of its high cost or the tube design. The objective of this study is to present a valid method involving the application of the axial compressive force of the tube to prevent flatness. A prototype machine was fabricated to change the axial compressive force. The material of the tube was STK490. The bending die radius was threefold the tube-outer diameter. It was concluded that the flatness of the bent tube is directly related to the axial compressive force exerted upon the tube. To apply a satisfactory axial force for the reduction in flatness, the pressure die force is also needed for the prevention of buckling. Consequently, the flatness of the bent tube can be successfully controlled by applying both axial compressive and pressure die forces.
