Mandrel Linkage Wear and Clearance in CNC Tube Bending

Surface scuffing and localized wall thinning along the inner bend extrados frequently stem from excessive mechanical play within the mandrel assembly. When the internal balls and links of a multi-ball mandrel develop radial slop, the support elements fail to track the neutral axis accurately as the tube enters the Griffin forming zone. Dural Machinery has built tube benders in Istanbul, Gungoren since 1991, and this is one of the first checks our service technicians perform when a production run exhibits irregular internal ripples.

Mechanism of Linkage Play and Internal Deflection

During rotary draw bending, the mandrel must sustain the internal hoop stresses generated as the material stretches on the outside radius and compresses on the inside radius. If the pins and mating holes connecting the mandrel balls experience wear, each successive ball segment shifts slightly out of alignment. This delay causes the nose and the subsequent balls to act as independent pressure points rather than a continuous internal contour. The resulting gaps permit the inner wall to drop inward prematurely, increasing the risk of structural collapse and tooling gouges.

Inspecting Pin Wear and Pitch Clearance

Verifying the mechanical integrity of the mandrel requires dismantling the assembly and measuring the clearance between the connecting pins and the internal bores of the ball segments. Elongated holes or scored pins reduce the tensile strength of the linkage and alter the effective pitch. Operators must check that each ball articulates smoothly without excessive side-to-side play. Even minor radial clearances compound over multi-axis cycles, leading to premature fatigue failure of the internal hardware and inconsistent bend geometries.

Lubrication Distribution and Friction Management

Internal friction accelerates mechanical wear between the mandrel links and the interior wall of the tube. Inadequate delivery of forming lubricant allows metal-to-metal contact, generating localized heat that causes galling on both the tooling and the workpiece. Ensuring proper fluid flow through the mandrel shank and maintaining a consistent film thickness across the ball surfaces minimizes frictional drag. When running high-volume components on a EOS system, lubricant pressure and viscosity settings must be calibrated to match the specific wall thickness and material alloy.

Verifying Assembly Tolerance During Setup

Before initiating a production batch, the axial position and free movement of the mandrel string must be verified relative to the bend die. Binding or restricted articulation indicates either pin distortion or incorrect shank mounting. Technicians should inspect the entire assembly for parallel alignment and ensure that the total free play remains within manufacturer tolerances. Correct mechanical setup prevents uneven internal loading and preserves tool life across extensive production runs.