Mandrel Position Relative to Tangent Line in Tube Bending

Flattening along the outside radius or humps near the bend transition usually occur when the mandrel nose is improperly positioned relative to the tangent line. In rotary draw bending, the mandrel supports the interior tube wall at the exact point where yield stress is reached. If the mandrel tip sits too far behind or too far ahead of the tangent point, internal support fails or creates excessive material drag.

When a tube is pulled around the bend die, tension acts on the outer wall while compression acts on the inner wall. The mandrel shank and nose must maintain the tube cross-section without obstructing the flow of metal. Determining the correct forward position or setback relative to the bend die tangent point is essential for achieving precise wall ovality and surface finish standards.

Mechanical Effects of Mandrel Placement Near Tangent

Positioning the mandrel nose slightly forward of tangent allows the tool to support the outer wall right as plastic deformation begins. In many thin-wall applications, setting the tip past tangent prevents the outer profile from collapsing inward under tension. However, extending the nose too far into the bend zone increases contact friction and concentrates stress on the extrados.

Conversely, setting the mandrel behind tangent leaves the tube wall unsupported at the point of maximum deformation. This results in section flattening, structural ovality, or wave formation ahead of the bend die contact area. On hydraulic benders such as the Atlas B series, improper placement leads to erratic pressure distribution across the clamp die and pressure die assemblies.

Identifying Flattening and Wall Tearing from Nose Placement

Tooling setup errors produce distinct physical indications on the bent tube. Observing the pattern of marks on the inner and outer radii clarifies whether the mandrel requires advanced positioning or retraction toward the shank.

• Outer wall tearing or humps: Indicates the mandrel nose is set too far past the tangent point, pinching the metal against the bend die.
• Section flattening (ovality): Indicates the mandrel nose sits behind tangent, allowing the outer wall to sink inward.
• Transverse ridging near bend start: Occurs when the mandrel tip profile creates a high-pressure point right at the initial point of bend die contact.
• Excessive trailing drag marks: Indicates improper alignment or excessive forward extension, scraping material along the inner surface.

This is one of the first things we check when a customer reports outer wall tensile failure or necking at the start of a curve. Verifying the physical location of the nose relative to the die center line resolves most sectional distortion issues before altering lubrication rates or clamp pressure settings.

Setting Mandrel Extension in Control Systems and Setup

On manual and semi-automatic machines, mandrel placement relies on physical rod threading and mechanical stop nuts. On modern 3-to-6 axis CNC machines like the Griffin range running ETU software, initial mechanical alignment is established during setup and stored within component positioning tables.

When setting up the tooling, the physical tangent line is determined by aligning a straightedge along the straight insert of the bend die where the curved cavity begins. Mechanical setup typically involves these steps:

1. Mount the mandrel shank to the tie-rod and slide the assembly into the straight tube section.
2. Advance the mandrel forward until the leading edge of the nose aligned visually with the bend die tangent line.
3. Adjust the position in small increments, moving forward past tangent for thin-wall materials or backward behind tangent for heavy-wall tubing.
4. Lock the rod jam-nuts or record the reference position in the control unit prior to running test samples.

Material yield strength variations mean that soft copper, aluminum, stainless steel, and carbon steel respond differently to nose extension. Soft materials generally require the mandrel to sit closer to tangent to prevent dragging, while high-strength alloys need precise positioning to prevent surface scoring.

Field Verification and Tooling Wear Checks

Over extended production runs, mechanical slop in the mandrel rod mounting, wear on the nose radius, or thermal expansion can shift the effective contact zone. Regular inspection prevents minor tool wear from causing out-of-spec tube geometry on the shop floor.

Check the mandrel nose for uneven wear patterns, localized material pickup, or ridging along its circumference. If wear is concentrated exclusively on the front edge of the nose, the tool has likely been operating too far past tangent. If wear appears only along the rear shank, the mandrel sits too far behind the active forming area.

To evaluate a new tooling setup or request customized tooling configurations, send us a STEP file or scaled drawing along with material specifications, wall thickness, center line radius, and yearly production volumes.