Most bent tube parts are made by one of two processes: rotary draw mandrel bending or roll bending (also called profile bending). Choosing the right one early affects tooling cost, lead time and whether the part comes out round, smooth and in tolerance. Here’s how we decide at TBC Metalworks.
The short answer
| If your part has… | Usually choose |
|---|---|
| Tight radii, typically under about 12″ centerline radius | Mandrel bending |
| Several bends in different planes | Mandrel bending |
| Thin wall relative to diameter, with a tight radius | Mandrel bending |
| Large sweeping arcs, hoops, rings or coils | Roll bending |
| A radius too large for any bend die | Roll bending |
| Channel, angle iron or other open profiles in an arc | Roll bending |
| Both tight bends and large sweeps | Both, on the same part |
How mandrel bending works
In rotary draw bending, the tube is clamped to a bend die and drawn around it. A pressure die holds the straight section, a mandrel inside the tube keeps it from collapsing, and a wiper die on the inside of the bend prevents wrinkles. Each centerline radius needs a matching bend die, so the radius is fixed by the tooling.
Strengths: tight radii, repeatable angles, multi-plane parts, and good roundness on thin-wall tube. CNC benders also hold rotation and position between bends, which matters on 3D parts like handles and frames.
Trade-offs: each radius needs its own tooling. Designing to a radius the shop already has is the single biggest way to keep cost and lead time down.
How roll bending works
Roll bending feeds the tube through three rolls. The top roll is lowered in steps while the tube passes back and forth until the curve reaches the target radius. The radius is set by roll position, not a die, so one setup can make many radii.
Strengths: large and variable radii, full circles, hoops, helixes and coils, and the ability to curve square tube, rectangle tube, channel and angle.
Trade-offs: short straight sections at the ends of a rolled piece are usually trimmed off, and very tight radii aren’t possible. It also isn’t the right tool for several tight bends close together.
Questions that decide it
- What’s the centerline radius compared with the tube diameter? Tight radii relative to OD point to mandrel bending. Very large radii point to roll bending.
- How thin is the wall? Thin wall on a tight bend needs mandrel support to stay round and wrinkle-free.
- Is the part 2D or 3D? Multiple bends in different planes are a CNC mandrel bender’s job.
- Do you need a full circle or coil? That’s roll bending, often followed by welding the ring closed.
- What profile is it? Open shapes like channel and angle are roll bent.
- How many per year? Volume affects whether new tooling is worth it, or whether we design around existing tooling.
When a part needs both
Plenty of parts combine a large rolled sweep with tight bends at the ends, like handrails, frames and outdoor structures. A shop that does both in-house can sequence the operations and hold the overall shape without passing parts between suppliers.
TBC Metalworks capabilities
At our Green Bay, Wisconsin shop we rotary draw and CNC mandrel bend round tube from 1/8″ to 3″ OD and pipe from 1/4″ to 2-1/2″ NPS, and roll bend tubing from 1/2″ to 4-1/2″ OD and pipe from 1/2″ to 4″. For more background, see types of tube bending. Send a drawing through our quote form and we’ll recommend the process and reply with a quote within one business day.