Boost Bending on Tight CLR Tube Bends: When to Apply It

Wall thinning and tensile tearing on the outside bend radius (extrados) increase rapidly when bending tubes at a center line radius (CLR) equal to or smaller than 1.5 times the tube outside diameter. Axial carriage boost pushes the tube from behind the bend zone during rotation, shifting the neutral axis outward to reduce tensile stress on the extrados. On CNC benders such as the CNC Boru Bükme Makinesi (Griffin), applying controlled carriage boost prevents tube necking, wall thinning beyond specification, and premature cracking on tight-radius jobs.

Deformation limits on tight center line radius bends

In standard rotary draw bending, the bend die rotates while pulling the clamped tube forward. As the bend angle increases, material on the outside radius elongates, while material on the inside radius compresses. When the CLR drops below 1.5D, tensile elongation on the outer wall frequently exceeds the ultimate tensile strength of the material, leading to wall thinning, ovality, or structural failure.

Without supplementary axial thrust, the tensile forces required to draw the tube around the radius originate entirely from the clamp die torque. This places the neutral axis deep inside the bend geometry, maximizing the stretch ratio on the outer wall. Thin-wall tubing compounded by tight radii accentuates this strain. This is one of the first things we check when a customer reports extrados necking or wall splitting on bends below 1.5D.

Distinguishing carriage boost from pressure die assist

Carriage boost feeds the tube forward via the main longitudinal carriage axis during the bend cycle. In contrast, pressure die assist pushes the pressure die body forward parallel to the tube tangency line. While pressure die assist reduces drag friction along the pressure die length, it cannot supply substantial compressive forces into the tube cross-section without causing surface marring or clamp slippage.

Carriage boost directly applies pushing force behind the clamp die, forcing material into the forming zone. This compressive force counters the high tensile stress generated by tight center line radii. On fully electric machines like the Tam Elektrikli Boru Bükme Makinesi (EOS), servo-driven axial boost allows exact synchronization between carriage advance and bend die rotation speed.

Evaluating CLR ratios and material elongation limits

Determining whether a part requires axial carriage boost depends primarily on three factors: CLR-to-diameter ratio (CLR/D), wall factor (outside diameter divided by wall thickness, D/t), and material ductility. Standard rotary draw bending without boost operates reliably down to approximately 1.5D CLR for standard carbon steels and stainless steels with wall factors below 20.

When CLR drops to 1.0D or smaller, or when thin-wall tubing pushes the D/t ratio above 30, conventional bending yields excessive wall thinning, often exceeding 25 percent of nominal wall thickness. In these applications, axial boost pushes additional material into the bend zone, allowing wall thinning to remain within standard engineering tolerances of 10 to 15 percent without causing inside-radius buckling.

Configuring boost force and synchronization in ETU

In DuralBend ETU software, carriage boost is programmed as a synchronized axis motion relative to the bend die angle rather than a simple timing delay. Over-boosting forces excess material into the bend, causing heavy wrinkles or wiper die binding. Under-boosting fails to relieve tensile strain, resulting in wall thinning.

Key parameter verification steps

• Verify that carriage advance speed matches the calculated centerline arc velocity prior to adjusting boost percentage.
• Increase boost force incrementally while monitoring extrados wall thickness using ultrasonic thickness gauging.
• Inspect the wiper die tip clearance to ensure boosted material does not wedge between the tube and the tool pocket.
• Check clamp die gripping length to ensure the tube does not slip under compressive axial loading.

Monitoring these variables ensures stable forming conditions on tight radii across production runs. For technical evaluation of your specific tooling or part geometry, submit your STEP file or scaled drawing, material specification, wall thickness, center line radius, and expected yearly volume to Dural Machinery in Istanbul, Gungoren.