Mandrel Retraction Timing in Rotary Draw Bending

A flattened profile, local wall collapse, or severe longitudinal scoring at the end of a bend arc usually indicates incorrect mandrel retraction timing. During rotary draw bending, the mandrel supports the internal profile of the tube as the material yields around the bend die center line radius. If the mandrel withdraws before the trailing edge of the bend reaches a stable geometry, the unsupported section collapses inward under remaining radial forces. Conversely, if the mandrel stays extended as the bend die completes its rotation or as unclamping begins, the internal surface rubs heavily against the mandrel nose or flexible ball segments.

Deformations Caused by Early Mandrel Extraction

When the mandrel retracts too early in the bending sequence, the inner and outer tube walls lose their internal backing while tension is still present in the deformation zone. This premature release allows the outer radius to sink inward and the inner radius to form an unwanted hump or localized flat spot near the tangent exit. In thin-wall applications, early extraction can produce a sharp transition ridge where the solid mandrel tip pulled away from the wall before plastic deformation ended.

This defect typically appears within the final 5 to 10 degrees of the bend angle. When a customer reports localized flattening only at the end of a bend, this timing sequence is one of the first parameters we check in field diagnostics. Advancing the retraction point closer to the true bend termination angle maintains internal support until the material stress stabilizes.

Drag and Surface Damage from Late Retraction

Holding the mandrel forward for too long creates mechanical drag as the bend die approaches its final stopping point. In severe cases, late retraction forces the mandrel shank and flex-balls to resist the natural rotation of the clamped tube. This friction increases torque demand on the bend axis and leaves heavy metallic pickup or galling marks on the inside surface of the tube.

Late extraction also complicates the unclamp cycle. If the clamp die and pressure die begin releasing while the mandrel is still pinned within the bent arc, the springback of the tube pinches the mandrel body. This pinching accelerates wear on bronze or chrome-plated mandrel components and increases the risk of bending or breaking the mandrel connecting rod over repeated production cycles.

Setting Retraction Parameters in Control Systems

Modern CNC machines do not trigger mandrel retraction using fixed time delays in milliseconds, because machine speed variations would cause inconsistent pull-out positions. Instead, retraction is commanded at an exact angular degree of the bending die rotation. On machines running the standard ETU software platform, such as the CNC Boru Bükme Makinesi (Griffin) or the fully electric Tam Elektrikli Boru Bükme Makinesi (EOS), the operator programs the mandrel sequence directly into the bend angle table.

The optimal extraction point depends on material wall thickness, yield strength, and center line radius. A typical baseline sets the retraction trigger slightly before the final bend angle is reached, allowing the hydraulic or servo cylinder to start retracting so the mandrel nose clears the tangent point exactly as the bend axis comes to rest. For tight radii or thin-wall stainless steel, fine-tuning this angle in 0.5-degree increments prevents both end-of-bend collapse and drag marks on the inner wall.

Verifying Support and Clearance Parameters

Establishing correct retraction timing requires checking the mechanical state of the machine alongside program values. Air in hydraulic retraction cylinders or play in mechanical linkages can cause physical delay even when the software signal fires accurately. Regular checks of cylinder seals, valve actuation speeds, and rod alignment ensure that the programmed position matches physical movement on the shop floor.

Dural Machinery has produced tube processing equipment in Istanbul, Gungoren since 1991. To evaluate tooling setups, mandrel configurations, or sequence timing for specific profiles, provide a STEP file or scaled drawing along with material grade, wall thickness, center line radius, and expected annual volume.