A mandrel is the internal support in rotary draw bending. It sits inside the tube with its nose at the tangent point and keeps the bore round while the outer wall is pulled and the inner wall is pushed around the arc. Without it a thin tube ovalizes, and with the right mandrel the inner wall has nowhere to buckle.
Choosing among the types of mandrels for tube bending is not a matter of taste. The type, the number of balls, the diameter and the position are all set by the geometry of the part and by what the bender can carry. This article compares the types of mandrels for tube bending, shows how to choose between them from numbers on the drawing, and explains the mandrel length figure that buyers often overlook when they compare benders. The wider process is on the mandrel tube bending page, and the rest of the die set is covered in tube bending tooling and how to match a die set to the job.
The Three Types of Mandrels for Tube Bending
Plug mandrel
A plug mandrel is the simplest type. It is a solid nose on a rod, sized to the bore, and it supports the tube at the tangent point only. Once the tube has passed the tangent point the support ends, so the section can still ovalize a little further around the arc. That is acceptable on gentle bends and moderate wall, where the tube is stiff enough to hold its shape for the rest of the arc.
The advantages are low cost, little drag and easy extraction. The limits are that it cannot support a thin wall, and it does not help much once the radius gets tight. It is the first thing to try when a bend is near the edge of needing a mandrel at all.
Form mandrel
A form mandrel has a curved end that follows the radius of the bend, so it gives more support than a plug without any moving parts. The curved end sits further into the arc, and it holds the bore round for a longer distance. It suits medium bends where a plug is not enough and a full ball assembly is more than the job needs.
Because it is a single piece, it is simple and sturdy, and it has a fixed shape that suits one radius. That fixed shape is also its limit. It cannot follow a tighter arc, and it can drag on the inside of the bend if the fit is not right. For parts with several radii on one tube it is usually replaced by a ball mandrel.
Ball mandrel
A ball mandrel has linked segments, usually called balls, that articulate as they pass around the bend. The nose supports the tube at the tangent point, and each ball behind it extends the support further around the arc. The assembly follows the radius and keeps supporting the bore past the tangent point and not only up to it. It is the type used for tight radii and thin wall.
The number of balls follows the difficulty. A common starting point is two or three balls between 1.5D and 2D, and three or more below 1.5D or when the wall factor is above about 40. Every ball adds support and also adds drag, which is why mandrel jobs need longer clamp dies and more clamping force, and why a three-ball mandrel on a job that a single ball would handle is not free insurance.
| Type | Support through the arc | Best for | Main limit |
|---|---|---|---|
| No mandrel | None | Generous radius on heavy wall | The section ovalizes as the wall gets thinner |
| Plug mandrel | At the tangent point only | Gentle bends, moderate wall | Does not follow the arc |
| Form mandrel | Follows one fixed radius | Medium bends on one radius | Fixed shape, can drag |
| Ball mandrel | Follows the arc, set by ball count | Tight radius and thin wall | More drag, more lubrication, more wear |
Two further points apply to all three. A mandrel works together with the wiper die, because the mandrel supports the bore and the wiper stops the inner wall folding at the inside tangent. And every mandrel adds friction, so lubrication reaching the tube interior is part of the process and not an extra.
How to Choose Among the Types of Mandrels for Tube Bending
The choice comes from two ratios. The first is the wall factor, which is the outside diameter divided by the wall thickness. The second is the radius ratio, called the D of bend, which is the centerline radius divided by the outside diameter. UK and Australian drawings call the radius the centreline radius, and it is defined in centerline radius explained for tube bending.
| D of bend | Wall factor | Usual mandrel choice |
|---|---|---|
| 2.5D and above | Under 20 | No mandrel on heavy wall |
| About 2D to 2.5D | 20 to 30 | Plug or single ball |
| 1.5D to 2D | 30 to 40 | Two or three balls, with a wiper die |
| Below 1.5D | Above 40 | Three or more balls, wiper die and a following pressure die |
These are industry starting points and not guarantees. The boundaries move with the material and with how much ovality the drawing allows, and a part near a boundary should be sampled before tooling is cut. The worked calculation is short. A tube of 50.8 mm outside diameter and 1.5 mm wall has a wall factor of 33.9. On a 76.2 mm radius it is a 1.5D bend. Both numbers fall in the third row, so plan two or three balls and a wiper die, then confirm against the ovality limit on the drawing.
Material shifts the bands. Soft metals such as copper and aluminum are easy on force and hard on shape, so they tend to need support earlier. Stainless hardens as it forms and gets galled by bare tool steel, so the mandrel material matters as much as the ball count. The comparison is in aluminum tube bending and how tube metals compare and in bending stainless steel tubing and what changes.
Mandrel Diameter, Position and Material in Tube Bending
Choosing among the types of mandrels for tube bending is only the first step. Three settings decide whether the mandrel does its job.
Diameter. The mandrel is cut to the bore less a small working clearance. A loose fit lets the tube ovalize before the support arrives, and a tight one binds, drags and scores the bore. Bore varies with mill tolerance, so measure the real bore of each incoming lot.
Nose position. The nose is set against the line of tangency with a small forward advance. Set back, it leaves the start of the bend unsupported. Set too far forward, it presses the outer wall and humps or splits it. The usual method is to start conservative, move forward in small steps on a trial bend, and re-check after any tooling change.
Material. The surface of the mandrel slides against the bore under pressure. For steel the usual choices are aluminum bronze or chrome-plated steel. For stainless the same dissimilar-metal logic applies, to avoid galling. For aluminum a hardened and polished steel surface avoids pick-up, and for copper a polished close fit avoids marks. A worn or damaged mandrel scores every tube it touches, so its condition belongs in routine inspection.
Lubrication ties all three together. A mandrel running dry in thin wall produces heat, drag and a scored bore, and the lubricant has to reach the inside of the tube. On parts that are cleaned afterwards, such as refrigerant lines, the lubricant must also be compatible with the cleaning step.
Mandrel Length and the Mandrel Bender
The phrase mandrel bender is how many buyers search for a machine, and a mandrel bender is simply a rotary draw bender with a mandrel carriage that moves the support into the tube and out again. What separates one mandrel bender from another is, among other things, the mandrel length, which is the reach of the mandrel rod into the tube. It governs how far from the clamped end a supported bend can be placed.
That reach matters on long parts. A part whose last bend sits beyond the mandrel length is formed unsupported and shows ovality whatever else is right. A common workaround is to bend the part from the other end, but that depends on the part having bends that can be reached from both ends. Check the mandrel length against the part and not against the tube, because the distance that counts is the distance from the clamped end to the furthest bend.
Telhoo publishes the mandrel length for each model in its CNC automatic pipe bending machine and NC semi-automatic tube bender ranges. The figures grow with the size of the machine, from 1,850 mm on the 38NC to 4,700 mm on the 168NC, and from 3,000 mm on the 63CNC to 6,000 mm on the 168CNC.
| NC semi-automatic model | Max tube | Mandrel length |
|---|---|---|
| 38NC | Φ38.1 × 2 mm | 1,850 mm |
| 50NC | Φ50.8 × 2 mm | 2,250 mm |
| 63NC | Φ63.5 × 2.5 mm | 2,950 mm |
| 75NC | Φ76.2 × 3 mm | 3,400 mm |
| 89NC | Φ88.9 × 3 mm | 3,520 mm |
| 115NC | Φ115 × 6 mm | 3,750 mm |
| 130NC | Φ130 × 8 mm | 4,230 mm |
| 168NC | Φ168 × 14 mm | 4,700 mm |
| CNC automatic model | Max tube | Mandrel length |
|---|---|---|
| 63CNC | Φ63.5 × 2.5 mm | 3,000 mm |
| 75CNC | Φ76.2 × 3 mm | 3,800 mm |
| 89CNC | Φ88.9 × 3 mm | 3,800 mm |
| 115CNC | Φ115 × 6 mm | 5,000 mm |
| 130CNC | Φ130 × 8 mm | 5,000 mm |
| 168CNC | Φ168 × 15 mm | 6,000 mm |
Telhoo’s CNC range lists hydraulic clamping and a moving mandrel as standard, and the NC range lists a mandrel shift with hydraulic clamping. These are published specifications for the standard machines. Which mandrel type, ball count and diameter your part needs is set by the drawing, and tooling is built to the customer’s radius and tube.
Types of Mandrels for Tube Bending by Application
The same three types turn up across industries, and the application usually points to one of them. Thin-wall stainless exhaust tube is the classic ball mandrel job. The ratio of diameter to wall is high and the radii are tight, so the mandrel has to follow the arc and a wiper die has to work beside it. Telhoo’s 75NC page says exactly this for thin-wall stainless exhaust, that it wants a mandrel and a wiper die to stop wrinkling on the inside radius. The process is covered in mandrel exhaust tube bending and what the part demands.
Roll cage and safety structure tube is another ball mandrel job, because the radii are tight and the members must be round and symmetrical. Telhoo’s roll cage page describes small and mid-size CNC benders with hydraulic moving mandrel tooling for DOM and chrome-moly cage tube. The part-led view is on the roll cage tube bending page.
Refrigerant and heat exchanger lines in soft metals need a close, polished mandrel fit and careful lubrication, and often more than one radius in a part. Handrail and structural tube on generous radii often need no mandrel or a plug at most, which is why those parts run on simpler NC machines. Heavy wall pipe such as marine mains needs force more than support. The applications are compared in tube bending applications by industry and what each needs.
How Mandrels Work With the Wiper Die
A mandrel and a wiper die are often described as separate remedies, but on a tight bend they work as a pair. The mandrel supports the bore from inside, which controls ovality and the outer wall. The wiper die fills the small gap at the inside tangent, which controls wrinkling. If either is missing or poorly set the other cannot make up for it. A good mandrel with a worn wiper still wrinkles, and a good wiper with an unsupported bore still flattens.
Their positions are linked too. The mandrel nose sits slightly ahead of the tangent point, and the wiper tip sits at the tangent with a near-zero gap. If the nose moves forward, the support reaches the wiper area differently, and the wiper may need resetting. Treat the two as one adjustment and change them one at a time, so that the effect of each is visible on the first article.
Tube Condition and the Mandrel
The tube itself changes the mandrel choice. Welded tube can carry an internal weld bead that the mandrel has to pass, and a bead that is large or uneven can score the mandrel and catch the balls. Drawn tube has a cleaner bore but a wider tolerance on wall thickness in some grades. In both cases the actual bore of the incoming lot, not the nominal, sets the mandrel diameter.
Cleanliness matters as well. Swarf, burrs and dried lubricant in the bore act like abrasive and score the mandrel. Square, deburred ends and a clean bore are part of mandrel care, and they cost far less than replacing a damaged ball assembly. On stainless and aluminum, any iron or debris carried in on the tube transfers to the mandrel and then to the next tube.
Cost, Lead Time and Spares for Mandrel Tooling
Mandrels are specific to a tube. Each combination of bore and radius needs its own, and a ball mandrel is a small assembly of parts that wear at different rates. That has planning consequences. Order spare balls and a spare nose with the first set, because a worn ball stops production and a replacement is not always on the shelf. Where several tube sizes run on one machine, quick-change mandrel mounting saves setup time, and a tooling list per part number keeps the right mandrel with the right die set.
The cost of a mandrel is small next to the cost of scrap. A part that comes out oval or wrinkled usually costs more in tube and time than the mandrel that would have prevented it. That is why the mandrel decision is better made at the quotation stage than on the first failed batch, and why the number of radii on a part matters. Each radius is a separate set of dies and may need a separate mandrel, as covered in tube bend radius standards and how to choose a radius.
Lead time deserves a line of its own. A new mandrel for a new bore and radius is a made-to-order item, and the first set is usually ordered with the die set. If a second tube size is likely within the year, ask for the mandrel quote for both sizes at once, and ask what the supplier holds as spares for common balls and noses. A mandrel that has to be made after the first bend fails costs days of idle machine time.
Three Parts, Three Choices
These examples are illustrations and are not customer drawings. They show how the numbers pick the type.
Part one is a handrail post in Φ42.4 × 2.6 mm tube on a 3D radius. The wall factor is about 16 and the bend is relaxed. A pressure die and clamp die may be enough, and a plug mandrel is the most it would need. A trial bend confirms it.
Part two is a seat frame member in Φ25.4 × 1.2 mm tube on a 2D radius. The wall factor is about 21, and the radius is 2D. A plug or single ball mandrel is the likely choice, with a wiper die if the surface shows early wrinkling.
Part three is an exhaust section in Φ60.3 × 1.5 mm stainless on a 1.5D radius. The wall factor is 40 and the bend is tight. This is the multi-ball case, with a wiper die and a following pressure die, and the mandrel material and lubricant matter as much as the count.
When a Mandrel Causes Its Own Problems
A mandrel is a cure for ovality and wrinkling, and it carries costs of its own. The most common are drag, scoring and extraction. Each ball adds friction, so thin wall tube can tear if the lubrication fails. A scored or galled mandrel marks the bore of every part. And a mandrel that is too far forward humps the outside of the bend and is hard to withdraw.
The warning signs are easy to read. Scoring in the bore points to lubrication, clearance or a damaged ball. Outside humping at the start of the arc points to a nose that is too far forward. Ovality that appears mid-batch with no tooling change points to wear. The full set of faults, with their causes and fixes, is in tube bending defects and what causes each one.
Wear is the quiet one. Mandrel balls and nose wear next after wipers, especially on stainless or with marginal lubrication. Keep spare balls and a spare nose from the first order, and measure them against the bore at set intervals. A mandrel that is measured stays a mandrel. One that is not measured becomes a source of scrap that is blamed on the machine.
Common Mistakes With Mandrel Selection
The same few mistakes recur. The first is choosing from the diameter alone. A mandrel is chosen from the wall factor and the radius, and two tubes of the same diameter can need different types. The second is using more balls than the job needs, which adds drag and wear and makes the bend harder to hold. The third is ignoring the mandrel length until a long part arrives and the last bend cannot be supported.
The fourth is treating the mandrel as a one-time purchase. It wears, it needs measuring, and it needs the right lubricant every shift. The fifth is changing the tube supplier or the lot without checking the bore, since a small change in bore moves the fit. And the sixth is leaving the wiper die out of the conversation, which is how a well chosen mandrel gets blamed for wrinkles that a worn wiper caused.
What to Send When Choosing a Mandrel
- Tube outside diameter and wall thickness, with the tolerance you can hold in purchasing, because the real bore sets the mandrel diameter.
- Material and condition, including whether the tube is welded, since an internal weld bead changes the mandrel choice.
- Every centerline radius and whether the design can standardize on fewer.
- The position of the furthest bend from the tube end, which tests the mandrel length.
- Ovality and thinning limits, since they decide whether a borderline bend needs a mandrel.
- Batch size and variants, which decide how much a quick-change mandrel setup is worth.
Send those and the type, the ball count and the machine can be named in one reply. Send us the part drawing, and for a wider view of where this sits among bending methods see tube bending methods compared and our tube bending solutions.
