Bent Aluminium Tubing & Mandrel Bends

What is bent aluminium tubing?

Bending turns straight tube into the elbows, offsets, loops and frames that carry air, fluids and loads around real equipment. Aluminium is chosen because it is light, corrosion-resistant and easy to finish, but it is less forgiving than mild steel: the outside of every bend is stretched and thinned, the inside is compressed and wants to wrinkle, and the round section tries to flatten. Good bent tubing comes from matching alloy, temper, wall thickness, radius and tooling so that none of these effects goes too far.

Three numbers describe any bend. The bend angle is how far the tube turns. The centreline radius (CLR) is the radius measured to the middle of the tube. The D of bend is CLR divided by outside diameter (OD): a 2 in OD tube bent on a 3 in CLR is a 1.5D bend. A fourth number, the wall factor (OD divided by wall thickness), tells the bender how thin-walled the tube is. Tight D and high wall factor together make the hardest bends.

Bent tubes are supplied either as ready-made mandrel bends (a short bend with straight legs that the customer cuts and welds into a system, common in intake, intercooler and exhaust work) or as fully formed parts bent to a CAD drawing with several bends in different planes, as used for frames, handrails, furniture and machine guards.

How radius controls stretching and wall thinning

When a tube bends, its outer fibre must lengthen. A simple geometric estimate of the strain at the outside of the bend is OD ÷ (2 × CLR + OD), or 1 ÷ (2D + 1). The table shows why tight bends are demanding: at 1.5D the outer surface is stretched by about a quarter, which is more than the tensile elongation of many T6 tubes. Mandrel bending succeeds anyway because the mandrel and pressure die apply compressive and supporting forces that move the neutral axis outward and spread the deformation, but the margin is small.

Wall thinning follows the same pattern. On the outside of the bend the wall typically thins by roughly the same proportion as the outer-fibre stretch, less the support given by tooling, while the inside wall thickens. For pressure tubes, structural members and parts that will be polished, the thinned outer wall must still meet the design minimum, so specify a minimum wall after bending where it matters.

Strain values are geometric estimates for guidance only. Achievable radii depend on the actual tube, tooling and machine, and should be confirmed by trial bends.

Aluminium alloys for bent tubing

6061 versus 6063 for bending: 6061-T6 is about 30 % stronger, which is why most performance mandrel bends are 6061. 6063 bends more easily, cracks less on tight radii and gives a smoother, brighter anodised surface, so it is preferred for visible architectural and furniture tubing where loads are modest.

Bend soft, then harden: heat-treatable alloys can be bent in the O or freshly solution-treated condition and then aged to T6. Bending immediately after solution treatment (the "W" condition) gives excellent formability, but parts must be bent within a short time or refrigerated to delay natural ageing. Full re-heat-treatment after bending can distort complex parts, so fixtures may be needed.

Typical strengths are Aluminum Association / ASM values for the stated tempers; tube specified minimums are set by ASTM B210, B221, B241 or B483 and EN 754/755.

Mandrel, compression, roll and stretch bending compared

For fluid systems such as intakes, intercoolers, coolant and fuel lines, mandrel bending is the standard because flattening and wrinkles disturb flow and make sealing harder. For frames, furniture and handrails, compression or rotary-draw bending without a mandrel is usually adequate and cheaper. Square and rectangular tube needs section-specific tooling and is more prone to inner-face buckling, so it is normally bent at larger radii than round tube of the same size.

Common 6061-T6 mandrel bend sizes

The table lists widely used 90° mandrel-bend sizes in 16 gauge (0.065 in / 1.65 mm) 6061-T6 tube. Arc length is the length of tube in the bend itself (CLR × angle in radians); add the straight legs to find the cut length. The same OD and CLR combinations are typically offered as 30°, 45°, 60°, 135° and 180° bends.

Many of these sizes are also made in 14 gauge (0.083 in / 2.11 mm) for boosted intake and charge-air systems, structural parts or where thinning must be limited. Metric tubes (for example 25, 32, 38, 50, 63, 76 and 101.6 mm OD) are bent on matching tooling.

Weights calculated from nominal dimensions and a density of 2.70 g/cm³.

Calculating cut length and allowing for springback

The developed (cut) length of a bent tube is the sum of its straight sections plus the arc length of each bend, measured along the centreline. Arc length equals CLR × bend angle in radians, or CLR × angle ÷ 57.3 when the angle is in degrees.

Example: a 2 in (50.8 mm) OD 6061-T6 tube is bent 90° on a 3 in (76.2 mm) CLR with two 6 in (152.4 mm) legs. The arc is 76.2 × 1.571 = 119.7 mm, so the centreline length is 152.4 + 119.7 + 152.4 = 424.5 mm. The bender then adds a grip allowance at one or both ends, which is trimmed after bending, so the blank is cut longer than this figure.

Aluminium springs back after the bending force is removed, so the tube must be over-bent slightly. Springback grows with yield strength and with radius: a T6 tube bent on a large radius springs back more than an O-temper tube bent tightly. The radius also opens up a little. CNC benders store a correction for each alloy, size and radius, found by trial bends on the first pieces of each batch, which is why first-article inspection matters even for repeat parts.

Bending defects, causes and tolerances

Tolerances should be stated on the drawing. CNC rotary-draw benders typically hold bend angle to about ±0.5° and positions between bends to about ±0.5 – 1 mm on well-set-up jobs; ovality and wall-thinning limits depend on the use and are best agreed per part, with tighter limits for fluid fittings that receive hoses or couplings. Check parts with a bending fixture, coordinate measuring arm or a go/no-go gauge for repeat production.

Design rules for bent aluminium parts

• Use one CLR per part where possible: every different radius needs its own bend die, adding cost and set-up time.
• Leave straight length between bends: the clamp needs grip length, commonly at least one to two times OD, between consecutive bends.
• Allow straight legs at the ends: bends cannot start right at the tube end; extra length is added and trimmed after bending.
• Choose a standard OD and wall: standard sizes have existing tooling and stock tube, which shortens supply and lowers cost.
• Position seams carefully: on welded tube, orient the seam at the neutral axis (side of the bend), not on the outer or inner radius.
• Specify the measurement datum: state whether dimensions are to tube centreline or outside surface and give bend data as XYZ or LRA (length, rotation, angle).
• Plan heat treatment and finishing: bend before anodising or powder coating; if bending in O or T4, plan the ageing step and any distortion correction.

Surface finishes and secondary operations

Where bent aluminium tubing is used

• Automotive and motorsport: intake, intercooler and charge-air piping, coolant lines, roll cages for some classes, seat and chassis frames
• HVAC and refrigeration: 3003 and 1xxx return bends, coils and connecting lines
• Gas and fluid systems: burner tubes, compressed-air and vacuum lines, fuel and oil lines
• Transport: bus, rail and trailer handrails and grab bars, truck accessories
• Marine: boat rails, bimini and T-top frames, ladders (5052, 6061 or 6082, anodised)
• Medical and mobility: wheelchair, walker and bed frames, equipment handles
• Furniture, lighting and display: chair frames, lamp arms, shop-fitting and exhibition structures in 6063
• Sports and outdoor: bicycle frames, fitness equipment, tent and trekking poles
• Industrial equipment: cart and trolley frames, machine guards, conveyor and robot cable guides

How to order bent aluminium tubing

1. Define the tubeOD and wall (or gauge), alloy and temper, seamless or welded, and the tube standard (for example ASTM B210, B241 or B483, or EN 754/755).
2. Define the bendsFor single bends: angle, CLR and leg lengths. For multi-bend parts: a 3D CAD file (STEP/IGES) or XYZ / LRA bend table.
3. State tolerances and finishAngle and position tolerances, maximum ovality and minimum wall after bending, surface finish and any end forming.
4. Quotation and samplesWe confirm tooling, feasibility, first-article samples and delivery terms (EXW, FOB, CIF or DAP).
5. SupplyInspected parts with mill test certificates (EN 10204 3.1) for the tube, packed to protect finished surfaces, shipped worldwide.

Related tube products and guides

Bent aluminium (aluminum) tubing: frequently asked questions