What decides the assembly
A tube assembly is a tube bent or coiled to a defined centreline path, with an end form that has to seal or connect and often a bracket, ferrule or sleeve held in a fixed relationship to the bends. Four things decide whether it can be built as drawn: the bend radii against the tube outside diameter and wall, the straight tangent length between bends, the end form, and the datum the end positions are measured from. Bending runs on CNC mandrel benders so the bend sequence repeats, and the machine compensates for springback by overbending and rotation correction. Tube is bought as bar or tube stock, or drawn in-house; joints are confirmed against the drawing before a fixture is built. Bulk heat treatment, plating and coating are carried out by qualified partners and are stated on the quote.
What we make in this family
- Bent tube to a defined path – CNC mandrel bending, including multi-plane bends, with the bend sequence set from the drawing. See tube bending services.
- Coiled and helical tube – tube wound to a stated coil diameter, pitch and number of turns, with controlled ends. See tube coiling.
- Sub-assemblies – tube plus the brackets, sleeves or end pieces that belong with it, assembled and checked as one item. See assembly and sub-assembly.
- Brazed joints – where the drawing calls for a joined tube assembly rather than a single bent piece. See brazing.
What the drawing has to say
- Tube material and wall. Wall thickness sets the minimum bend radius; the thinner the wall, the larger the radius or the more support the mandrel needs.
- Bend radii and the straight length between them. A mandrel needs straight tube to grip on either side of a bend. Two bends that meet immediately below that length cannot be formed in that order.
- The end form. Flared, beaded, swaged, threaded or prepared for brazing – each needs its own tooling step, and several need a fixture to hold the datum while the end is formed.
- The datum for the end positions. Where an assembly has to land in a fixed place, the drawing should say which end is the datum and what the tolerance is from it.
- Whether the part is an assembly or a single piece. If it is joined, say so, and say how: brazing, welding or a mechanical joint.
End forms and joints: what each one needs on the drawing
The end form is the part most often under-specified, and it is the one that decides the tooling. Each form below needs its own step, and several need a fixture to hold the datum while the end is formed.
| End form or joint | What it does | What the drawing has to state |
|---|---|---|
| Flare | Opens the tube end into a sealing cone that a mating fitting presses against | Flare angle and standard, the flare diameter, and which end carries it |
| Bead | Raises a ring near the end so a hose or clamp cannot pull off | Bead outside diameter and its position measured from the end |
| Swage / reduction | Reduces the end so it enters a mating bore or takes a smaller fitting | Reduced diameter, the length over which the reduction happens, and whether the bore or the outside is the datum |
| Threaded end | Accepts a screwed fitting directly on the tube | Thread standard and class, thread length, and the wall left under the thread |
| Brazed joint | Joins two tube pieces or a tube to a fitting with filler metal | Joint type and gap, filler, and the cleanliness the joint needs before brazing |
| Welded joint | Joins tube to tube or tube to a bracket as a permanent connection | Weld type, whether it is continuous or tacked, and the inspection the customer expects |
| Mechanical / compression fitting | Seals by a ferrule or an olive compressed onto the tube | The fitting standard, and the tube hardness and surface condition the fitting is rated for |
Which route for your tube
The same finished geometry can be reached several ways, and the route decides the tooling and the cost. This is the order the route is chosen in.
| Question on the drawing | Route that fits | Why |
|---|---|---|
| Is the tube a standard stock size? | Bought tube, cut and bent to the path | A standard tube avoids drawing tooling entirely; see stock steel tubes |
| Is the wall thin, the diameter unusual, or the tube seamless by requirement? | Tube drawn in-house, then bent | The tube is formed on our own presses of 25–350 t; see deep drawing |
| Does the tube follow a 3D path with several planes of bend? | CNC mandrel bending with a set bend sequence | Mandrel support holds the section through each bend; see tube bending |
| Is the tube wound into a coil rather than bent into a path? | Coiling to a stated coil diameter, pitch and number of turns | Different machine and tooling from bending; see tube coiling |
| Are brackets, sleeves or a second tube part of the item? | Assembly and sub-assembly, delivered as one checked item | Position is controlled at assembly instead of being left to the customer; see assembly and sub-assembly |
What is inspected on a bent tube assembly
A bent tube is not inspected the way a flat part is, because the cross-section changes through the bend and the material springs back when the tooling releases. Four characteristics are controlled, and the drawing has to say which of them matter:
- Centreline geometry. Bend angles, centreline radii and the rotations between bends, checked against the coordinate table or the 3D model rather than against a flat print.
- End positions. The distance and orientation of each end from the stated datum. This is the characteristic an assembly actually feels, and it moves if the springback compensation is set for the wrong material lot.
- Cross-section. Flattening (ovality) and wall thinning through the bend. These are only called out where the application needs them, because holding them tightly costs tooling access and cycle time.
- Surface condition. Marks from the die and the clamp, and the cleanliness a brazed or welded joint needs before it is made.
The design rules behind these four – how radius relates to diameter, how much straight tangent the tooling needs, and why springback compensation has to be re-established when the material changes – are set out in the CNC tube bending design guide.
Worked examples from the case archive
- Stainless steel coiled tube, helical coils to drawing
- Tightly wound stainless helical coil with tube ends
- Copper coiled tube with formed straight legs
- Coiled tube for fluid, refrigerant and heat exchange lines
- Connecting pipe for an exhaust gas temperature sensor harness
Related capability pages
Written by the Balford technical team, Zhuji, Zhejiang. Last updated 8 October 2026.