Rotary Draw Bending – Tooling
Basic Tooling Set-Up
A basic tooling setup on a rotary draw bender includes a bend die, which forms the bends radius; a clamp die, which clamps the tube to the bend die; a pressure die, which applies pressure to the outer surface; and a collet, which is used to control the flow of material during the bend, transport and rotate the tube.
Two additional tools for difficult bends are a wiper die, which prevents wrinkling on the inside surface, and a mandrel, which fills the tube to limit the amount of ovality.
The Bend Former
The bend former (also known as a bend die) is the part of the tooling system into which the bent tube is formed. Unlike a Compression bender, this former rotates and draws the tube around the circumference of it.
It is one of the primary tools as it determines the CLR (Centre line Radius) of the bend. It consists of a circumferential groove to suit the O/D (Outside Diameter) of the tube with a straight portion to which the tube is clamped. When the bending former rotates, the tube is wrapped around the former producing the bend.
There are several different types of bend former, which is dependent on the following criterion; OD, CLR, Max Bend Angle, Minimum straight between bends, machine size and mounting details.
The clamp die grips the tube to the straight portion of the bend former so that the tube is drawn through the machine as the bend former rotates. The clamp length is determined by the tube size, the tube diameter, the material, the degree of bend and the CLR (Centre Line Radius.
The Pressure Die
The Pressure Die holds the tube against the Bend Former and runs tangentially to the tool during bending. With pressure applied by the bending machine at the line of tangency during the bending process, it creates a point of bend.
The collet is the item of tooling which holds the tube throughout the bend process and rotates the tube to its correct orientation. There are two types of collet, the external and internal. The advantages of the internal collet are that you can get the end of the tube closer to the bend former during bending by reducing the collision area with pressure die and wiper die, thereby reducing cycle times and producing less waste material when roll forms or carriage boosts are necessary.
The purpose of the Mandrel is to support the inner part of the tube during a bend. The tool itself sits inside the tube to stop the tube from collapsing and to improve the quality of the bend.
Different types of Mandrels
Plug Mandrel: When the result from empty bending becomes unsatisfactory (too much collapse on the outside of bend, wrinkles/deformation on inside), the next step is to introduce a plug mandrel.
Ball Mandrel: As the bending process becomes more difficult (tighter CLR’s/thinner walls) the next step is to use a ball mandrel. Depending on the diameter to wall thickness ratio determines the number of mandrel balls required.
Ampco bronze frequently is preferred for stainless applications in order to reduce friction.
Hardened steel with chrome is recommended on non-ferrous materials such as aluminium, copper, etc.
The Wiper Die
The wiper die is fitted into the rear of the bend former and traps the tube between it and the pressure die. The wiper die helps to prevent wrinkles occurring on the inside of the bend. The die holds the tube wall steady when it would otherwise fail in compression against the mandrel.
The wiper die consists of a straight grooved die; one end of which is shaped to fit closely into the bending form. Proper setup of the wiper die is essential to a good bend and the life of the die.
Rotary Draw Bending Tooling App.
Unison has created a desktop app to simplify tooling calculations which can be found here.
In this app, you can simply input your tube data; Diameter of the Tube (OD), Radius of the bend (CLR), Wall thickness, Material and it will calculate out the toolset you require to bend it, the bending difficulty and even advise on mandrel setup!