A flat part is easy to picture, and 3D tube bending is easiest to understand by comparison with one. UK and Australian drawings call the radius of each bend the centreline radius. Every bend lies in one plane, the tube goes in, turns through its angles and comes out. A three-dimensional part is different. After the first bend the tube is rolled about its axis so that the next bend goes in a new plane, and then rolled again. Exhaust runs, roll cages, handlebars, thermal lines and many frame members are bent this way, and they are where the control of the machine matters most.
This article explains what the axes of a CNC tube bender do, what each additional axis adds, how a drawing becomes a program, and what to check before a 3D part is run. The published axes, speeds and options of Telhoo’s CNC range are the worked examples. The comparison with semi-automatic control is in CNC vs NC pipe bender and how to choose.
The Three Core Axes of 3D Tube Bending
Every CNC tube bender has three axes that describe a bend, and the bend schedule of a part is a list of three values for each bend. They are usually written Y, B and C, or as the length, rotation and angle.
Feed, the Y axis. The feed carriage clamps the tube and moves it along its own axis, so that the next bend lands at the right place. The value is the length of straight tube advanced before the bend, and it is a distance to the tangent point and not to the apex of the bend. After the first bend the reference has already moved, so each feed is a difference and not an absolute length.
Bend, the B axis. The bend die turns through the bend angle, drawing the tube around it. The value is the angle of the bend, with the springback correction included, so that the programmed angle is larger than the finished one. The bend axis is also where the speed is set, and a heavy section is bent slowly.
Rotation, the C axis. The rotation axis rolls the tube about its own axis between bends, setting the plane of bend for the next bend. A planar part has a rotation of zero throughout. A three-dimensional part has a rotation at every plane change, and an error here swings the rest of the part out of position, just as an angle error does.
On Telhoo’s CNC range these three are servo-driven axes with published precision. The 38CNC lists bending at 30 to 100°/s, feeding at 25 to 900 mm/s and rotating at 30 to 200°/s, with ±0.1° on bending and rotation and ±0.1 mm on feed. The page says the whole sequence is stored and replayed and not judged by an operator. That is the difference that matters on a 3D part.
| Axis | What it does | Value in the bend schedule | Published on the 38CNC |
|---|---|---|---|
| Feed, Y | Moves the tube to the next bend | Straight length before the bend | 25 to 900 mm/s, ±0.1 mm |
| Bend, B | Turns the die through the angle | Bend angle with the springback correction | 30 to 100°/s, ±0.1° |
| Rotation, C | Rolls the tube between bends | Rotation about the tube axis | 30 to 200°/s, ±0.1° |
What Extra CNC Tube Bender Axes and Options Add
Beyond the three core axes, other axes and options extend what a machine can do. Each one answers a particular problem on 3D parts.
Stacked tooling. A stack is a complete die set at one height on the bend head. Two or more stacks let a part with several radii run without a tooling change, with a vertical move that indexes from one radius to the next. Telhoo’s 38CNC and 63CNC pages describe three-stack tooling carrying three radii at once, so that multi-radius parts stay in one setup, and the optional list includes double, triple, quadruple and multi-radius stacks.
Left and right bending. A single-direction bender can only bend the tube one way, and a part that turns right then left has to be turned over, which loses the datum. A left and right bending axis lets the machine bend in both directions, so that a part is completed without unclamping. The 50CNC page describes itself as a five-axis machine bought for left and right bending, because turning a tube over loses the datum and shows up as asymmetry at assembly.
Head rotation. On a double head machine each head can rotate 90° inward or outward to add a plane before the part is unloaded, as on the SW38A. It is a way of making 3D shapes from a symmetrical part without a CNC rotation axis, and its limits are set by the geometry. The method is in double head tube bender advantages and when it pays.
Mandrel and pressure die axes. The mandrel moves into the tube and out again, and the pressure die can follow the tube. Telhoo’s CNC range lists a hydraulic moving mandrel as standard. These axes are part of the cycle and are programmed with it, and they matter on thin wall and tight radii.
| Extra axis or option | Problem it solves | Where it shows |
|---|---|---|
| Second and further stacks | Several radii in one part | No tooling change, shorter cycle |
| Left and right bending | Parts that turn both ways | No turning over, datum kept |
| Head rotation | A second plane on a symmetrical part | Fewer operations |
| Moving mandrel | Support on thin wall | Ovality and wrinkling controlled |
| Robot loading | Handling time | Cycle time on repeat parts |
The CNC Tube Bender Axes Count Explained
Machines are often described by a number of axes, and the number is less informative than it looks. A basic CNC bender has three servo axes. Telhoo’s published Thailand case used a 63CNC-4A2S, which the page describes as a four-axis full servo machine with a two-stack multi-die design, so that it can switch between two bending dies without frequent disassembly. The 50CNC lists five axes.
What matters is which axes are present and what each does, and not the total. A four-axis machine with a stack axis suits a part with two radii. A five-axis machine with left and right bending suits a symmetrical frame. A three-axis machine is enough for a part with one radius turning one way. Ask a supplier to list the axes by name and by function, and match each to a need on your part. Axes you do not need cost money, and axes you do need that are missing cost far more in turned-over parts and lost datums.
From XYZ Points to a Bend Schedule
Designers usually draw a tube as a path through points in space, given as X, Y and Z coordinates, and the machine needs feed, rotation and angle for each bend. The conversion is geometry, and Telhoo’s CNC pages list input as either Y, B, C or X, Y, Z. Most controllers convert the points for you, and it is worth understanding what they do so that the result can be checked.
Here is a worked example with assumed numbers. A path runs from P0 at 0, 0, 0 to P1 at 400, 0, 0, then to P2 at 400, 300, 0, then to P3 at 400, 300, 250. The three legs are 400, 300 and 250 mm long. At P1 the direction changes from the X axis to the Y axis, a bend of 90°. At P2 it changes from the Y axis to the Z axis, also 90°. The first bend lies in the XY plane and the second in the YZ plane, so the tube must be rolled by 90° between them.
With a centerline radius of 100 mm, the setback at each 90° bend is 100 mm. The feeds are then the leg lengths minus the setbacks. The first feed is 400 − 100, which is 300 mm. The feed between the bends is 300 − 100 − 100, which is 100 mm. The tail after the last bend is 250 − 100, which is 150 mm. The schedule is feed 300, bend 90°, rotate 90°, feed 100, bend 90°, and a tail of 150 mm. The arithmetic of setback and bend allowance is set out in pipe bending calculations.
| Step | Feed | Rotation | Bend |
|---|---|---|---|
| Bend 1 | 300 mm | 0° | 90°, plus springback |
| Bend 2 | 100 mm | 90° | 90°, plus springback |
| Tail | 150 mm | None | None |
The example is deliberately simple. Real parts have angles that are not 90°, rotations that are not 90°, and radii that vary, and the controller does the work. The check is to confirm that the leg lengths and angles it reports match the drawing, and that the rotation direction is correct, since a rotation of 90° the wrong way produces the mirror image of the part.
Rotation, Plane Errors and Why 3D Parts Are Harder
A rotation error acts like an angle error on a plane that is not the one you looked at. If the tube is rolled by 89° and not 90°, the next bend goes in a plane that is a degree off, and the whole of the rest of the part swings out of position by about the remaining length times the error. On a part with several rotations the errors stack, and they stack in three dimensions, where they are harder to see and to measure.
That is why published rotation precision matters. Telhoo lists ±0.1° rotation precision on its CNC range and rotating speeds of 30 to 200°/s on the 38CNC, 50CNC and 63CNC, 25 to 180°/s on the 75CNC and 20 to 150°/s on the 89CNC, and falling to 20°/s on the 168CNC. On a ten-bend part the rotation time is a meaningful share of the cycle. The effect of angle and rotation errors on the finished part is worked through in tube bending tolerances and what can be held.
Collisions, Clearance and Simulation
A 3D part has to be bent in an order the machine can reach. As the tube is bent, the finished portion swings through space and can hit the bender, the mandrel carriage or the support. A part that looks fine on screen may not be formable in the order it needs, and the tooling can foul an earlier bend. The cheapest time to find a collision is before any tube is cut.
Telhoo lists 3D bending simulation that can read CAD files among the options on its CNC models, together with an industrial PC that shows the tube in 3D with mirror and reversed images. The simulation runs the sequence on screen, shows the tube swinging through the machine, and flags clashes. Where a clash exists, the usual fixes are a change of bend order, a different starting end, a longer tail or a change to the design. The design side is in tube bending design guidelines for bendable parts.
Axis Speed and Cycle Time
The published speeds show how the cycle divides. On a small machine the bend itself is fast, up to 100°/s on the 38CNC, and the feed and rotation moves are fast too. On a part with ten bends and rotations, the sum of the moves between the bends is often longer than the bends, which is why rotation speed and feed speed matter as much as bend speed. A machine that rotates at 200°/s, which is the top of the published range on the 38CNC, turns a tube through 90° in about half a second, and one that rotates at 20°/s needs about four and a half seconds for the same move.
On heavy tube the picture changes. The bend is slow by design, the rotation is slow because the tube is heavy, and the cycle is dominated by handling. Telhoo’s 168CNC page says the cycle is dominated by handling and not by degrees per second. The size of the machine therefore decides which axis sets the pace, and the choice of axes should follow it. A part with many plane changes on small tube asks for fast rotation. A part on heavy pipe asks for rigidity and handling, as in large diameter tube bending process and machines.
Programming and Proving a Part
The route from drawing to production follows a short sequence. Enter the bend schedule, either as feed, bend and rotation or as XYZ points. Enter the tube size, the radius and the tooling set. Run the simulation to check the order and the clearance. Bend a trial piece, measure it in a fixture or with a measuring arm, and correct the springback and any feed or rotation errors. Bend a second piece to confirm, then store the program with the corrections against the part number and the material lot.
Proving a 3D part takes a little longer than a flat one, because the corrections interact. A change to the angle of the first bend moves everything after it, and a change to a rotation moves the plane of everything after it. Correct the bends in order, from the first, and recheck the whole part after each change. The procedure for the springback side is in springback compensation tube bending and how to prove it.
Checking 3D Parts in Fixtures and With Measuring Arms
A 3D part is checked in three dimensions, and the check must match how the part is used. A checking fixture holds the part by its datum features and shows whether the ends and fixing points fall inside their tolerance zones. It is fast and suited to production. A portable measuring arm or a coordinate measuring machine measures the centerline of the whole part and compares it with the model, which gives the numbers for the first article and for the record.
Use the fixture to monitor and the arm to establish. On the first article, measure the whole path and find which bends or rotations are off, and correct them. During the run, check a sample in the fixture at set intervals to catch drift. Keep both the fixture and the arm calibrated, and record the results against the part number and the lot.
Common Mistakes
The same few mistakes recur on 3D parts. The first is not simulating, and finding a collision with the machine only when a tube is already in it. The second is a rotation direction error, which gives the mirror image of the part. The third is correcting angles out of order, so that each fix moves what was fixed before. The fourth is ignoring the mandrel length, so that the furthest bend is formed unsupported. And the fifth is buying axes by count and not by function, which gives a machine with axes that the part does not use and none of the ones it needs.
A sixth mistake is treating 3D as a different kind of bending. It is the same process, with the same tooling, the same springback and the same wall and radius limits, and what changes is the number of moves and the care needed to keep them accurate. A machine that is good at flat parts is not automatically good at 3D, because the repeatability of rotation and the quality of the control decide the result. Ask for the rotation precision and for the simulation, and ask to see a sample part in three dimensions before ordering.
Mandrel, Wiper and Support on 3D Parts
The support of the tube does not stop at a plane change. A mandrel working in a thin-wall tube has to be withdrawn or kept in position as the tube rotates, and the way it is handled in the cycle affects the surface and the drag. Telhoo’s CNC range lists a hydraulic moving mandrel and an automatic lubrication system as standard, and an automatic mandrel lubrication system as an option. The wall factor and radius still decide the type, as in types of mandrels for tube bending and how to select one.
Mandrel length matters more on a 3D part, because the path of the tube may reach well into the machine. A part whose last bend sits beyond the mandrel length is formed unsupported, whatever the plane. The published lengths run from 3,000 mm on the 38CNC and 63CNC to 6,000 mm on the 168CNC and 219CNC.
Typical Parts and Their Demands
Before the individual industries, one general point. A 3D part is rarely difficult because of any one bend. It is difficult because many ordinary bends have to land in the right place in space, one after another, and each depends on the last.
Several industries live on 3D bending. Exhaust systems thread past the floor pan and axles, with several planes and tight radii on thin stainless. Roll cages and safety structures need symmetrical members with repeatable left and right bends, and Telhoo’s roll cage page describes small and mid-size CNC benders with hydraulic moving mandrel tooling for DOM and chrome-moly tube. Handlebars and motorcycle frames carry compound bends with rotation between each one. And thermal management and refrigerant lines route densely in three dimensions, as covered in automotive tube bending by part family and volume.
Each puts a different load on the axes. Exhaust asks for thin-wall support and positional repeatability. Roll cages ask for symmetry, and so for left and right bending and a stored program. Thermal lines ask for fast and accurate rotation. The part-led views are on the roll cage tube bending and mandrel exhaust tube bending pages.
What to Send for 3D Tube Bending
- The 3D model or the XYZ points of the centerline, or the bend schedule if you have it.
- Tube outside diameter, wall and material, with the tolerance you can hold in purchasing.
- Every centerline radius, and whether the design can use fewer.
- The tolerance on the end positions and the datum scheme of the checking fixture.
- The number of plane changes and whether the part has mirrored versions.
- Batch size and variants, which decide how much stored programs and extra axes are worth.
Send those and the reply on your 3D tube bending part can say which axes the part needs and which model carries them. Tell us about the part, and see the range on the CNC automatic pipe bending machine page. For the wider options see our tube bending solutions page.
