It is necessary to lubricate stationary tools such as the mandrel and wiper die during bending and to have a film barrier between these tools and the tube. The tube material variable is both dimensional and metallurgical-the mechanical properties of the tube metal such as hardness and strength. The third variable is the setup of the tools relative to each other and the forces the machine creates during clamping and bending. Second, the tools themselves are a variable, in terms of the condition of the mounting surfaces and tube grooves. The five main variables are the bending machine, tools, setup, material, and lubrication.įirst, the bending machine comprises the devices creating the forces for bending, the control mechanism that positions the tube, and the devices for holding the tools during bending. In the dynamics of bending, several things are happening at the same time, and it helps to evaluate causes of defects in relationship with these variables. Correcting for defects requires some detective work on your part because there may be more than one cause for the defect, especially as more tools are added.īefore any discussion regarding correction for tube bending defects can take place, it is important that you understand the bending process. Part I discusses surface defects Part II covers other defects, such as wall thinning, ovality, buckling, and fractures.Īnalyzing tube bend defects and taking the proper corrective action require close observation skills and an understanding of all the variables in the bending process. The clamp continued to move forward, but the material did not, causing the slippage marks (horizontal lines) and wrinkles on the intrados.Įditor's note: This is the first part of a two-part article that examines tube bending defects, possible causes, and suggested remedies.

The clamp used to bend this 16-gauge, 3-in.-OD aluminized steel tube slipped about 2 in.

Groove size chart bending tube