Tube & Pipe Bending Glossary: Essential Terms and Definitions
Tube & Pipe – This glossary defines the most important terms and abbreviations used in the tube and pipe bending industry. Whether you’re working with manual, hydraulic, or CNC benders, these definitions will help you interpret setup sheets, tooling specs, and machine settings accurately.
Captive Lip
A tooling design where the groove in the bend die and wiper die is deeper than the tube’s outer diameter.
Used primarily with interlock tools in high-precision aerospace applications to minimize tool marks on ultra-thin wall tubing.
Carbide Coating
A surface treatment applied to the grip sections of the bend die and clamp die.
Its purpose is to increase surface friction and prevent the tube from slipping during bending.
CCW – Counter Clockwise
Refers to the rotation direction of the swing arm on a rotary draw tube bending machine.
Also known as left-hand rotation.
CLH – Center Line Height
The vertical distance from the base of the bending machine to the center of the tube groove on the bend die.
CLR – Center Line Radius
The radius measured along the tube’s centerline.
It determines the curvature of the bend and is a critical parameter in tooling and machine setup.
Compound Tooling
Specialized tooling for applications with very short DBB (Distance Between Bends).
The bend die grip section and clamp die are machined with the previous bend built in.
Although costly, this approach is more practical than short grip sections in high-production bending.
CW – Clockwise
Describes the right-hand rotation of the swing arm on a rotary draw tube bender.
D of Bend
Formula:
D=Center Line Radius (CLR)Tube Outside Diameter (OD)D = \frac{\text{Center Line Radius (CLR)}}{\text{Tube Outside Diameter (OD)}}D=Tube Outside Diameter (OD)Center Line Radius (CLR)
Defines the relationship between bend radius and tube diameter. Lower ratios indicate tighter bends.
DBB – Distance Between Bends
The linear distance between the tangent points of consecutive bends in a part.
DOB – Degree of Bend
The angle (in degrees) through which the tube is bent.
It determines the arc length and bend geometry.
Easy Way
Refers to bending the short side of a rectangular tube.
When bent the “easy way,” the tube stands vertically in the bend die.
Galling
A frictional transfer of material between surfaces caused by high pressure and inadequate lubrication.
Common when stainless steel is bent with steel mandrels or when aluminum contacts aluminum bronze tooling.
Can be minimized by using better lubricants, alternative materials, or slower rotation speeds.
Hard Way
Refers to bending the long side of a rectangular tube.
When bent the “hard way,” the tube lies horizontally in the bend die.
Integral Grip
The straight section of the bend die groove located between the grip area and the tangent point.
Found on bend dies with removable grip sections.
ISR – Inside Radius of Bend
The radius measured along the inner surface of the bend.
Commonly used when specifying bends on rectangular or square tubing.
Knurl
A fine or coarse patterned surface machined into the grip section of the bend die or clamp die.
It increases friction and prevents slippage without deforming the tube like serration might.
Mandrel
A support tool inserted inside the tube during bending to prevent collapse or wrinkling.
Mandrels are made from hardened steel, aluminum bronze, or chrome-plated materials, depending on wall thickness and material type.
Mandrel – Thin Wall
A mandrel designed for thin-walled tubing.
It uses smaller links and closer ball segments to provide tighter support along the tube’s interior wall.
Mandrel – Ultra Thin Wall
Used for ultra-thin-walled tubing, this mandrel uses links two sizes smaller than standard for maximum internal support.
Mandrel Extractor
A hydraulic mechanism that automatically retracts the mandrel from the tube after bending.
Required only when a mandrel is in use.
POB – Plane of Bend
The angular rotation of the tube between consecutive bends, measured in degrees.
Defines how bends are oriented in 3D space.
Pressure Die Assist (PDA)
A hydraulic system that pushes the pressure die forward during bending.
This action reduces collapse, wall thinning, and ovality by redistributing material where stretching occurs.
Common in CNC and hydraulic benders for precise control.
Radial Growth
The increase in bend radius that occurs due to springback after bending.
More prominent when bending harder materials or using a CLR greater than 4× the tube OD.
Reverse Interlock
A tooling design where the wiper die, clamp die, and pressure die interlock with the bend die.
Used in cosmetic, aerospace, and high-production bending for better alignment and surface finish.
Serration
A coarse surface pattern applied to the grip sections of dies to prevent slippage.
Typically used when grip sections are shorter than three times the tube OD.
Expect minor tube deformation when using serrated tooling.
Springback
The elastic recovery of a material after bending.
Every tube attempts to return to its original shape; the amount depends on material hardness, wall thickness, and CLR.
Springback must be compensated by overbending slightly during setup.
SurfAlloy™
A proprietary carbide coating process applied to bend die inserts and clamp dies.
It increases grip and prevents tube slippage during high-precision bending operations.
Tube ID
The inside diameter of a tube, used in flow calculations and mandrel sizing.
Tube OD
The outside diameter of the tube.
Primary reference dimension for tooling selection and machine setup.
Wall
The tube’s wall thickness (or gauge).
A key factor in determining minimum bend radius, mandrel requirements, and bend quality.
Wall Factor
Formula:
Wall Factor=Tube ODWall Thickness\text{Wall Factor} = \frac{\text{Tube OD}}{\text{Wall Thickness}}Wall Factor=Wall ThicknessTube OD
Indicates how easily a tube can be bent — higher ratios mean thinner walls and greater risk of distortion.
Wiper Die
A precision tool positioned at the inside radius of a bend.
Its thin tip prevents wrinkles and ensures smooth inner wall flow during bending, especially in tight-radius applications.
Pro Tip from UTB Michigan
When setting up a Used Tube / Pipe Bender for sale, always document these glossary terms in your setup sheets, tooling specs, and inspection reports.
Misunderstanding terms like CLR, POB, or springback can lead to rework, wasted material, and production downtime.
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