Internal ripples and localized wall cracking near the apex of a rotary draw bend often originate from excessive mechanical play within the mandrel assembly. When the flexible links and pins holding the mandrel balls together wear down, the internal support shifts backward as the tube is drawn past the bend die. This momentary loss of positive internal reaction leaves the inner diameter unsupported against compressive forces.
Mechanics of Internal Linkage Wear
Multi-ball mandrels rely on tight pin-and-link tolerances to maintain smooth internal geometry through the forming zone. During high-friction draws, axial tension combined with radial compression induces shear stress on the pivot pins and mating holes. Over thousands of cycles, elongation of the pin holes introduces axial backlash. When the carriage advances or the Griffin executes a complex multi-axis sequence, this slack allows individual balls to tilt or lag behind the theoretical neutral axis.
Identifying Backlash and Ball Tilt
Visual inspection of bent components reveals intermittent wrinkling on the inner radius that does not respond to changes in pressure die assist force or clamp die pressure. Disassembling the mandrel train often shows ovalized pin bores, scored contact bands on the leading edges of the balls, and metal transfer between links. This is one of the first checks we perform on our EOS systems when a customer reports inconsistent internal wall support on tight centerline radii.
Pin Clearance and Lubrication Maintenance
Mitigating linkage degradation requires strict adherence to lubrication intervals and dimensional tolerance checks. If pin clearance exceeds original manufacturing limits by more than a tenth of a millimeter, the resulting impact loads during bending accelerate wear across the entire train. Operators must verify that internal lubricants reach every joint prior to heavy wall production runs, as dry operation creates rapid galling on the internal contact faces.
Verifying Assembly Integrity
Restoring dimensional stability requires measuring individual ball heights and link thicknesses using precision calipers. Replacement of worn pins and reaming of elongated link bores eliminates axial slack. Technicians should confirm that the assembled mandrel moves freely by hand through its entire articulation angle before mounting the tooling stack back onto the machine.
Frequently asked questions
What causes inner-radius ripples despite correct pressure die settings?
Excessive axial play in the mandrel link pins allows the internal balls to shift backward, leaving the tube wall unsupported during compression.
How often should mandrel linkage pins be inspected?
Inspect pins and pivot holes every five hundred production cycles when running high-strength or heavy-wall tubing.
What information is needed to quote a replacement mandrel?
Send a STEP file or scaled drawing, material specification, wall thickness, center line radius, and yearly production volume.