A flexible mandrel supports a tube internally as it bends. Correctly selected and adjusted, it helps control flattening, wrinkling and distortion. Incorrectly selected or positioned, it can create excessive drag, score the tube, damage its components or become stuck.
Successful mandrel bending requires more than choosing a tool that fits inside the tube. The assembly must suit the actual bore, material, wall thickness, bend radius and required bend quality.
Exact clearances, positions, adjustment increments and extraction timing must come from the machine and tooling supplier for the particular application. This guide explains selection principles; it does not replace the machine’s operating and safety instructions.
What is a flexible mandrel?
A flexible mandrel typically consists of a supporting body or nose followed by articulated balls, often called spheres, connected through a linking or cable arrangement. The articulated section follows the developing bend while providing internal support.
- Body or nose: provides support near the forming region.
- Spheres: extend support into the curved section.
- Links or cable: allow articulation and retain the spheres.
- Mounting connection: attaches the assembly to the machine’s mandrel rod or supporting system.
Linked-ball and cable-style designs may require different setup references. Do not apply one manufacturer’s nose-position formula to a different design without confirmation.
When should you use one, two or three spheres?
There is no reliable rule such as one sphere for steel, two for stainless and three for aluminum. Sphere count depends on the support required, the assembly geometry and its ability to articulate.
The supplier evaluates tube diameter and wall, centerline radius, bend angle, material condition, permitted ovality, sphere diameter and spacing, and the mandrel design.
One sphere
A one-sphere mandrel may suit a bend that needs articulated support beyond the nose but not a longer supported section. It uses fewer articulated components, but may provide insufficient support for a more demanding bend.
Two spheres
Two spheres provide a longer articulated support section than an otherwise comparable one-sphere assembly. This can be appropriate when one sphere cannot maintain the required section shape. It does not correct an undersized nose, unsuitable pitch or incorrect setup.
Three spheres
Three spheres may be required where a longer supported section is needed. Additional components also mean more contact and maintenance considerations. More spheres are not automatically better.
Some bends can use a plug mandrel; others need more than three spheres or specialized tooling. The objective is the appropriate support for the application, not the maximum sphere count.
Sphere spacing matters as much as count
Pitch describes the spacing between successive articulated elements. Two three-sphere assemblies can behave differently if their spacing, geometry or connections differ.
Closer-pitch arrangements can provide more closely spaced support and accommodate demanding geometry, but must be specified as complete assemblies. Ask for sphere count, diameter, pitch, minimum permitted assembly radius and compatible replacement components.
A geometric minimum radius does not guarantee that a particular tube can be bent successfully at that radius. Do not mix spheres, links or cables merely because they appear to fit.
Describe the application with wall factor and bend radius
Wall factor = tube outside diameter ÷ wall thickness.
Bend-radius ratio = centerline radius ÷ tube outside diameter.
A higher wall factor means a thinner wall relative to diameter; a lower bend-radius ratio means a tighter bend. A 2.000-inch OD tube with a 0.065-inch wall bent on a 4.000-inch centerline radius has a wall factor of approximately 30.8 and a bend-radius ratio of 2.0.
These values help a supplier use a selection chart. Charts have assumptions about material, bend angle, tooling design and setup. A recommendation for round mild steel cannot automatically be transferred to another material or mandrel system.
Choose a mandrel material compatible with the tube
Mild steel
Bronze-based mandrels are commonly used for mild steel. Suitable chrome-plated steel mandrels can also be appropriate in some applications. The internal seam, friction, wear, lubricant and bend severity all affect selection. Hardness alone does not determine performance.
Stainless steel
Bronze-based mandrels are commonly evaluated for stainless applications because friction and galling need careful control. Grade, condition and bore finish matter. A bronze tool still requires suitable clearance, lubrication and setup.
Aluminum
Aluminum is susceptible to pickup and galling. Suitable polished and coated steel working surfaces, including hard-chrome systems, are commonly evaluated. Aluminum-bronze is not automatically suitable for aluminum tube; that pairing can create galling problems. Alloy and temper also determine formability.
Ercolina’s published order form includes AMPCO bronze and steel/chrome-plated mandrels. Confirm the final material selection for the actual application.
How to size a mandrel
For round tube, nominal ID equals OD minus twice the wall. A 2.000-inch tube with a nominal 0.065-inch wall has a nominal ID of 1.870 inches. That does not mean you should order a 1.870-inch mandrel.
The tool requires a specified running clearance, and actual bore dimensions may differ. Evaluate wall variation, ovality, internal weld-bead projection, burrs and tube-mouth distortion on representative production stock.
Have the supplier define the working diameter and its tolerance. Clarify whether clearance is diametral—the difference between bore and mandrel diameters—or radial, the gap on one side when centered. Confusing the two produces a factor-of-two error.
- Too large: loading interference, excessive drag, scoring or seizure.
- Too small: insufficient support and greater section distortion.
- Wrong sphere geometry or pitch: poor articulation, binding or inadequate support.
- Incorrect alignment: uneven contact and abnormal wear.
Do not grind down the tool or enlarge a tube mouth as an improvised correction. Ask the supplier to review the fit.
Adjust the mandrel systematically
- Confirm the tooling package. The mandrel, spheres, former, clamp, pressure die and wiper where required must match the approved application.
- Inspect safely. Follow the prescribed isolation procedure before physical access. Check wear, scoring, pickup, connections and lubrication passages.
- Establish the specified reference. Identify the tangent and the manufacturer’s mandrel reference. Linked-ball and cable designs may differ. There is no universal fixed distance past the tangent.
- Confirm alignment and loading. Use clean, deburred stock and specified lubrication. Investigate abnormal resistance rather than forcing an obstruction.
- Prove the setup. Use approved setup controls and guarding. Keep personnel clear and inspect trial bends for ovality, wrinkles, thinning, scoring and extraction resistance.
- Change one variable at a time. Use only approved adjustments within permitted ranges and record their effects.
A mandrel too far back may provide support too late, loading the spheres or links with work intended for the nose. Too far forward, depending on design, can increase drag, marking or extraction difficulty. These symptoms can also arise from other causes; they are not unique diagnoses.
How to prevent seizure
Prepare a clean, burr-free tube end
Deburr both edges and remove loose chips. An internal burr can catch or break off and become trapped. A clean saw cut helps, but does not replace inspection of the bore and weld seam.
Check incoming stock
New batches can have a tighter bore, more ovality or a heavier seam. Previously successful tooling does not remove the need to check material changes.
Use appropriate lubrication
Use an approved lubricant compatible with the tube, tooling and subsequent processes. Confirm delivery to the working surfaces and clear passages where through-mandrel lubrication is fitted. More lubricant cannot correct inadequate clearance, and gripping surfaces must not be lubricated indiscriminately.
Remove pickup using the approved process
Deposits alter the working surface and can progressively increase drag. Aggressive abrasive cleaning can damage coatings or change dimensions. Follow the tooling supplier’s cleaning or refurbishment instructions.
Maintain articulation and extraction settings
Damaged or contaminated joints may bind. Withdrawal must follow the approved machine sequence: removing support too early can harm the bend, while unsuitable timing can make extraction difficult. Do not assume every job uses the same settings.
If the mandrel starts sticking, stop
Rising loading or extraction resistance, new scratches, pickup, unusual noise or restricted sphere movement are reasons to investigate before continuing.
If the assembly is seized, stop the machine, follow its isolation and stored-energy procedures, keep personnel clear and obtain a recovery procedure from qualified maintenance or the supplier.
Do not increase pressure, repeatedly cycle the machine, hammer the assembly or pull against it with improvised equipment. A trapped assembly can release suddenly.
Request an application-specific recommendation
Provide the machine model, actual dimensions, material grade, seam information, radius, angle, drawing, quality requirements and production volume. For existing tooling, include diameter, material, sphere count, lubricant, setup information and photographs of defects.
The right flexible mandrel combines correct diameter, appropriate sphere count and pitch, compatible working material, proper positioning and effective lubrication. One, two and three spheres are configurations—not quality grades.
Ask Ercolina to review your mandrel application. For the complete tool set, see the mandrel tooling buyer’s guide.