Tube Bending
Also: pipe bending, rotary draw bending, mandrel bending, CNC tube bending, tube forming, compression bending, roll bending, bent tubing, tube fabrication
Tube bending forms tube and pipe around a die, rotary draw with a mandrel for tight clean bends; centerline radius, wall factor and ovality set the limits.
157 shops tagged Tube bending among 504 metal forming companies in the Noramark directory
Rotary draw bending is the job-shop standard: the tube is clamped to a bend die of a fixed centerline radius (CLR) and drawn around it, with a pressure die backing the straight, a wiper die holding the inside of the bend, and, for tight or thin-wall bends, a mandrel inside the tube to stop it collapsing. CNC benders index the tube in length and rotation between bends, so a multi-bend exhaust, handrail or hydraulic line is formed from a bend table in one setup.
The geometry is described by the CLR as a multiple of the outside diameter (a 2D bend on 25 mm tube has a 50 mm CLR) and the wall factor (OD divided by wall). Tight bends in thin tube flatten (ovality), wrinkle on the inside and thin on the outside by 15-25%; the mandrel and wiper control the first two, nothing changes the third. Bends of 1D with a mandrel, 1.5-2D as the practical minimum, and 3D or more without a mandrel on thicker walls are the usual working range. Tooling is specific to one OD and one CLR, so shops keep standard sets (1.5D, 2D, 3D) and charge for anything else.
Compression, press (ram) and three-roll bending cover the rest: press bending for cheap exhaust and structural bends where flattening is acceptable, roll bending for large-radius sweeps and rings, induction bending for heavy pipe. Ends are then cut, coped, flared, beaded, swaged or notched, and the assembly welded and finished.
At a glance
| Typical tolerances | Bend angle ±0.5° standard on a CNC bender, ±0.25° with care, ±1° on manual machines. Distance between bends ±0.8 mm (±1/32") standard, ±0.4 mm (±0.015") with care; plane of bend ±0.5°. CLR follows the die, ±0.8 mm (±1/32"). Ovality 8-10% standard, 3-5% with a mandrel and care. Overall envelope of a multi-bend part ±1.5 mm (±1/16"); expect wall thinning of 15-25% on the outside of 1.5-2D bends. |
|---|---|
| Size limits | Rotary draw benders in job shops handle 3-150 mm (1/8-6") OD and walls of 0.5-10 mm (0.020-0.375"); roll and induction benders take pipe to 600 mm (24") and larger. Minimum CLR about 1D with a mandrel and a wall factor under 20, 2-3D in practice. Minimum straight between bends and at the ends about 1-2 times the OD for the clamp die. Lengths to 3-6 m (10-20 ft) on typical machines. |
| Surface finish | As-received tube with light clamp-die and pressure-die marks, mandrel and wiper witness inside the bend, and faint wrinkles on the inside of tight bends. Polished stainless and aluminum need polished or urethane tooling and film to avoid marks. Ends are deburred; the part is then polished, powder coated, chem filmed or plated as required. |
| Lead time | Prototypes in 3-10 business days when the CLR tooling is on the shelf; add 2-4 weeks for a custom bend die set. Production runs in 2-4 weeks. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Mild steel DOM and ERW tube (1010, 1020, 1026), 4130 chromoly for roll cages and aircraft frames, 304 and 316 stainless for sanitary, exhaust and rails, aluminum 6061-T6 at 2D or larger (T4 or O for tighter, then age) and 6063-T52, copper and brass for plumbing and trim, Grade 2 titanium and Inconel 625 for exhaust and aerospace ducts. Square and rectangular tube bends on the same machines with matched dies. Cast, hardened and free-machining alloys are not bent.
What drives the cost
- Number of bends and the number of different CLRs and planes
- Tooling: a bend die, clamp, pressure die, wiper and mandrel per OD and CLR combination
- Tightness: mandrel bends run slower and cost more tooling than 3D-plus bends
- Material: stainless, titanium and Inconel bend slowly and spring back more
- End preparation: coping, flaring, beading, swaging, notching, threading
- Tolerances on angle, between-bend distance and ovality
- Fixtures and welding for assemblies
When to use it
- Handrails, frames, roll cages, exhausts, hydraulic and fuel lines, coils, furniture and conveyor guides
- Replacing welded elbows with one bent piece: fewer joints, fewer leaks, less cost
- Any tube geometry that keeps a constant diameter and a standard CLR
- Low to medium volumes; CNC bending needs no part-specific tooling beyond the bend die
When not to
- Radii tighter than about 1D: use a formed elbow, a cast or machined fitting
- Diameters that change along the part: hydroforming, swaging or a welded assembly
- Manifolds with many ports: machine or weld
- Thin-wall tube in very tight bends where ovality and thinning cannot be tolerated
- Parts that cannot leave clamp marks and cannot be film protected
Design tips
- Use one CLR for every bend on the part and pick a standard one (1.5D, 2D or 3D) the shop already has tooling for.
- Leave a straight length of at least 1.5-2 times the OD between bends and at each end for the clamp die.
- Specify tube by OD and wall (or pipe by NPS and schedule) and by spec and temper; wall factor decides whether a mandrel is needed.
- Tolerance angle and between-bend distance, and call out ovality only where flow or a fitting needs it.
- Supply a 3D model with the centerline, or a bend table (length, rotation, angle) and say which end is the datum.
- Note where the weld seam should sit (on the neutral axis of the bend) for ERW tube.
- Keep holes, slots and end features at least one OD away from the bend tangents.
- Ask for the shop's standard tooling list before finalizing the CLR; a custom die adds weeks and hundreds to thousands of dollars.
Frequently asked questions
- What is CLR in tube bending?
- The centerline radius: the radius of the bend measured to the center of the tube, usually expressed as a multiple of the OD (2D, 3D). Tooling is made per CLR, so using a standard one is the biggest cost saver in a bent-tube design.
- What is the minimum bend radius for tubing?
- About 1D (CLR equal to the OD) with a mandrel and a wall factor under 20, 1.5-2D as the practical minimum for good parts, and 3D or more without a mandrel on thicker walls. Thinner walls, harder alloys and square tube push the minimum up.
- What is mandrel bending?
- Rotary draw bending with a plug or a ball mandrel inside the tube at the point of bend so the wall cannot collapse or wrinkle. It keeps ovality under 3-5% and the inside of the bend smooth, at the price of more tooling and a slower cycle; it is the norm for thin-wall stainless, aluminum and exhaust tube.
- Can square tube be bent?
- Yes, on rotary draw and roll benders with matched dies and, for tight radii, a mandrel. Square and rectangular tube bend easiest about the weak axis and need larger radii than round tube of the same size; expect some concavity on the inside face and bulging on the outside.
- Tube vs pipe: what is the difference for bending?
- Tube is sized by outside diameter and wall; pipe by nominal pipe size and schedule, where the OD is larger than the nominal size. A bender needs the actual OD and wall to pick the tooling, so state which you have. DOM tube bends more consistently than welded pipe, whose seam should be placed on the neutral axis.
