CNC Freeform Tube Bending for Furniture Frames

Karud has been freeform bending tubes on CNC machines since 2006. A second, newer-generation machine has been running since 2021. Tooling range: Ø14, 16, 18, 20, 22 and 25 mm. The main application is metal chair and furniture frames — sled-base frames, runners, and spatial frames where several radii meet in a single part.

Plant: Karud, Kiełpino, Pomerania, Poland. Frame manufacturing since 1995. Bending, welding, and powder coating under one roof.

Key facts

• Start: 2006, second line: 2021
• Diameters: Ø14–Ø25 mm
• Materials: precision carbon steel, stainless steel 304 / 316
• Core product: metal chair frames
• Other methods in our fleet: five tube bending technologies

How freeform tube bending works

Three things follow from this that matter for frame production:

1. One tool set per diameter, not per radius.
2. Radii can flow into one another with no straight section between them.
3. A full 3D shape can be closed in a single cycle — typical for sled-base frames and frames with a curved backrest.

In rotary draw bending, each different bend radius usually needs its own die (roller) plus a full set of tooling for that radius. For a leg with several arcs, the tooling investment for rotary draw machines can be very high. Freeform bending doesn’t require this. That said, a tight radius on a thin wall is still better suited to rotary draw bending — which is why we keep both methods in-house.

Where the technology comes from

Freeform tube bending didn’t come from last season’s machine catalogue. In the late 1980s, researchers in Japan described how to form a tube by pushing it through a moving head instead of bending it around a fixed roller. In the 1990s the concept went into production under the Nissin brand. In Europe it was taken up and developed further by the German company J. Neu GmbH, which first distributed the Japanese-built machines and later began building its own. Both of our machines run on Mitsubishi controls — J. Neu GmbH has equipped its machines this way for decades.

These machines were first used to build furniture frames and chassis — precisely because a single tool can combine several arcs and a shape in three dimensions on one part. It wasn’t until the early 2000s that the automotive industry took an interest in the method, which is where the tighter dimensional requirements — and the impression that this is “an automotive technology” — come from. Its roots, however, are in furniture manufacturing.

The same story appears in trade press on both sides of the Atlantic. The German publication Blechonline covers the technology’s Japanese origins, its development at Neu since the 1990s, and its early use mainly for furniture. Meanwhile, a 2010 article in the US trade magazine The Fabricator describes it from a shop-floor perspective: a tube through a single head, several radii with no straight section between them, and a tool change only when the diameter changes — on a Nissin bending machine.

The same bending discipline is in use in Japan, Germany, and the US alike.

Our first machine was commissioned in 2006, the second in 2021. That’s twenty years of programming experience and hands-on knowledge of how a tube behaves during bending, what corrections need to be made, and what can and can’t be formed from it. We keep building on that knowledge with every new project — given the nature of our production, most projects are one-of-a-kind and require testing the specific combination of material, tooling, and process. A machine can be bought quickly. Twenty years of experience can’t.

Why it matters for frames

What happens to the design matters just as much. The shape isn’t locked into a set of forming tools. A radius, a twist, or a stacking kick-up can be adjusted in the program without ordering new tooling. With rotary draw dies, every such change means touching the tooling itself. When you’re running a series of variants, or fine-tuning a line with a designer, that difference shows up immediately.

When to use freeform tube bending — and when not to

The method makes sense wherever several radii meet on a single tube and the shape moves through three dimensions. Typical examples: sled-base frames, runner-type bases (double sled, skid frames), multi-arc frames, backrests, headrests. It also works well when moving from prototype to series production — without assembling tooling for every radius — and for geometry variants within the same diameter.

Use more caution, or avoid it, with very tight radii on thin walls. A mandrel, which supports the tube from the inside, performs better there. The same applies when the cross-section needs to stay as round as possible after a tight bend, or when the part is a single elbow with one radius. In those cases we use a different method from our fleet.

We work out the right approach together at the start of the project. A drawing, a STEP file, or a physical sample is enough to begin.

Tooling

Diameter	Typical use
Ø14	Footrests, headrests, cross-bars, armrests, backrests. Also runner-type frames (double sled, skid frames).
Ø16 / Ø18	Often visually light four-leg frames and stools. Also used as sub-assemblies within larger frames.
Ø20	Mainly four-leg frames. The middle of the range — neither delicate nor heavy-duty.
Ø22 / Ø25	Cantilever metal chair frames, main chair frames, and armchair bases.

Not just chairs

Furniture frames are our everyday work, and this bending method is well suited to them. The bending head doesn’t know whether the part it forms is heading to an upholsterer as a sub-assembly, or is a finished product for someone else entirely.

On these same machines we’ve produced, among other things, a prototype door frame for SAM, Poland’s first electric car (2008), yacht handrails, transport carts, and armchair headrests, along with many other parts. The common thread is simple: the tube needs several arcs, often in three dimensions, and we don’t want to build a separate roller for every radius.

How to place an order

2D documentation, a 3D model (SolidWorks / STEP), or a physical sample — we have a 3D laser scanner. We first check the design for bendability, then produce a prototype, and finally move to series production.

Contact: karud.pl/en · Kiełpino, Długa 61, Poland.

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