Carriage Boost on Tight CLR Bends: Application Guide

Excessive wall thinning and tensile necking on the outside radius occur when the center line radius (CLR) drops below 1.5 times the tube outside diameter. As the bend die rotates, tensile forces pull material around the outer arc while compressive forces compress material along the inner arc. When forming tight center line radii on thin-wall tubing, standard rotary draw action subjects the extrados to severe elongation. If tensile stresses exceed the ultimate tensile strength of the material, the tube wall thins beyond allowable engineering tolerances or tears completely along the bend apex.

Evaluating Tube Wall Thinning at Low Center Line Radii

In standard rotary draw bending, wall thinning on the extrados naturally scales with the tight radius ratio. On a 2D bend radius, wall thinning typically reaches 15 to 20 percent of nominal wall thickness under standard pressure die drag. When the radius drops to 1D or tighter, wall thinning can exceed 35 to 40 percent without active material assistance. Tube drawing specifications often limit wall thinning to 10 or 15 percent for fluid handling, structural load, or pressure vessel applications. Operating near the elongation limit of high-strength alloys or stainless steel increases the risk of micro-cracking and cross-sectional distortion.

Determining whether a job requires active boost depends on the wall factor, calculated by dividing the tube outside diameter by the wall thickness, alongside the CLR ratio. Thin-wall tubes with high wall factors subjected to low CLR ratios lack sufficient cross-sectional stiffness to withstand raw tensile tension. Applying axial thrust behind the tube during the bend cycle shifts the neutral axis outward toward the bend centerline. This action reduces tension on the extrados while transferring more deformation into controlled compression along the intrados.

Distinguishing Carriage Boost from Pressure Die Assist

Process engineers frequently contrast carriage boost with pressure die assist when attempting to eliminate extrados thinning. Pressure die assist relies on friction between the pressure die insert and the tube surface to reduce drag during forming. The pressure die moves forward at or near the bend tangent speed to eliminate back-pressure friction. However, pressure die assist cannot push material directly into the bend zone because force transmission remains limited by surface friction coefficients and clamping force.

Carriage boost utilizes the main carriage feeding axis to apply mechanical axial force directly to the rear tail of the tube. On advanced CNC tube benders, such as the CNC Boru Bükme Makinesi (Griffin) running the ETU control platform, carriage boost forces are programmed as synchronized mechanical vectors during the active bend sequence. By physically pushing the tube forward from the rear collet, axial thrust feeds extra wall volume into the forming zone, compensating for outer wall stretching far more effectively than surface friction alone.

Setting Carriage Boost Force and Synchronization

Improper carriage boost settings introduce structural defects on both sides of the bend. If the axial boost speed or feed force is set too low, tensile stress remains uncorrected, resulting in continued wall necking, excessive flattening, or tube rupture. Conversely, if boost force is excessively high or misaligned with bend die rotation speed, the excess material inserted into the bend zone causes severe inside radius wrinkling, localized wall buckling, or slippage inside the clamp die jaws.

Synchronizing the carriage boost speed with the rotation of the bend die requires precise servo control. Machines like the Tam Elektrikli Boru Bükme Makinesi (EOS) maintain exact axis velocity profiles throughout the full degree of bend. During a standard service check, field engineers verify that boost pressure builds immediately before bend die rotation begins and ramps down smoothly before the clamp die unclamps at final angle. Proper mechanical alignment ensures that axial thrust acts along the exact longitudinal centerline of the tube without introducing lateral bending moments into the machine spindle.

Verifying Tooling Setup and Material Elongation

Deploying carriage boost successfully requires complete system rigidity across all contact tooling. Because axial boost forces push material forward into the bend zone, resistance at the inside radius increases significantly. A fitted wiper die with correct tip clearance and rake angle is mandatory to guide the compressed inner wall smoothly into the wiper pocket without generating humps or heavy scoring.

Mandrel support must also be adjusted to account for shifted stress profiles. When boost is applied, the internal mandrel shank and balls sustain higher normal forces along the bend tangent. Inspecting mandrel nose position relative to tangent and ensuring continuous lubrication flow reduces mechanical friction during high-boost forming cycles. Evaluating raw tube heat numbers and material elongation certificates before production prevents unexpected wall failures caused by lot-to-lot yield strength variations. When requesting tooling or machine evaluation for tight radius applications, supplying a STEP model, exact material specification, wall thickness, center line radius, and yearly production volume allows engineers to determine exact axis feed requirements.