The Art of Tube Bending
Sometimes referred to as a Bend Die. It is the main tool around which the tube is bent. It has a groove corresponding to the shape of the tube machined around the periphery to contain it during the bending process. The radius from the centre of the die to the centre of the tube usually but not necessarily the outer edge of the die is referred to as the CLR ( centre line radius ). A bend radius may also be referred to by the relationship of the tube diameter to the CLR. For example 50mm tube bent on a 150mm CLR would be referred to as a 3D bend. Similarly the same 50mm tube on a 50mm CLR would be an ID bend.
The straight section of the bendformer is called the former extension. This may be integrally machined or be a detachable component, which can be replaced with a different type of gripping surface or a “compound clamp”, containing the shape of a previous bend to allow the forming of another in the close proximity to it. The bend radius and the resistance of the tube to bending determine the length of the clamp section. As a generalisation the tighter a bend radius is, the longer the clamp area needs to be to overcome the additional resistance, although serrating or coating the gripping face will assist to keeping the length to a minimum.
The Loose clamp or clamp die is the moving block used to retain the tube to the bend die during bending. It has a machined form corresponding to the bend die but will be less than half dept to allow the clamp to close under load without touching the outer edge of the die. The amount of gap will depend on the thickness of the tube. It is permissible to have a slight pinch on this item to improve grip. This should never be permitted to close down the tube onto the mandrel especially the ball type. This practice will almost certainly result in damage to the mandrel.
Interlocking clamps may be used to remove the risk of vertical mis-alignment with the bendformer resulting in a mark around the top and bottom of the finished bend on thin – walled materials.
The mandrel is the tool positioned inside the tube and firmly anchored to the back of the machine with a stretch resistant rod during bending. The shank or body of the mandrel is the portion over which the tube is drawn and initially determines the quality and shape of the outside of the bend. On bends in tube likely to collapse due to a high relationship between the tube O/D to wall thickness it will be necessary to extend the support beyond the tangent point by use of ball segments mounted to the shank. The number of balls required is relative to the bend radius, the O/D to wall thickness ratio and the bend angle. The function of the balls is to internally support the shape of the tube and prevent flatting occurring after leaving the tangent point. If flatting is permitted to occur or if the support is inadequate, wrinkles will probably result on the inside of the bend.
The mandrel retraction unit, if fitted to your machine, is usually hydraulically operated and sequenced to retract the mandrel balls from inside the tube at or just prior to the end of the bending cycle. The latter is referred to as anticipated mandrel retraction. It is a common misconception that a function of the mandrel withdrawal stroke is to smooth out any wrinkles. This is not so. Wrinkles should be prevented by use of the correct tool selection and set – up. Serious damage to the mandrel may occur if it is used in this way.
The pressure die is the piece of the tooling set designed to provide containment of the tube together with a reactionary force across the tangent point which will oppose the radial force produced during bending. This function may be performed by the use of free – running rollers on machines designed for bending small tubes or bending application needing minimum levels of support. A moving pressure die is preferable to rollers because the roll is distributed over a greater area, thus reducing the problems of point contact and indentation. The roller method is not suitable for thin wall tubes and tight radius bends.
Pressure dies may be static on radius bends in excess of 5xD CLR, but may cause an unacceptable level of scratching and wall thinning of the tube. Generally pressure are allowed free linear movement to travel with the tube during bending. To bend thin wall tubes, typically to aerospace specifications and as a generally designed feature, a power follower slide may be fitted to the machine to provide hydraulic assistance to the linear travel of the pressure die. This should be variable in pressure and feed – rate and will help considerably to overcome drag, which, on materials such as aluminium will result in excessive wall thinning. Clamp slippage may also be a result of drag.
Interlocking pressure dies may be used to remove the risk of vertical mis – alignment with the die, resulting with a mark around the top and bottom of the finished bend on thin – walled material.
- The Machine – To fit the tools onto the machine, first ensure that the mounting faces are clean and free from debris. Back off the loose clamp and pressure die holders so that even at full stroke they would not reach the bendformer. This is for your own safety and protection of the tooling.
- The Bendformer – The bendformer should be a good fit on the center spindle or its location rings and should not be permitted to lift during bending. A good fit and alignment with the key ( if fitted ) will ensure that the former extension on the bendformer will also be parallel to the loose clamp to give maximum gripping performance. If a tie bar is fitted to your machine, we recommend its use. This will improve the chances of a good bend because of its stabilising effect on the center spindle and the avoidance of undue stresses on the machine head assembly.
- The Loose clamp – This part should be of adequate length to provide gripping ability without excessive force being applied. Care should be taken to ensure that the vertical alignment of the groove is correct. Placing a solid bar or sized tube in position at an early setting up stage before pressure loading is applied and before the clamp is locked into its vertical position.
- The Pressure die or Roller
The pressure die should be of adequate length to travel to the required angle of bend and still maintain a minimum 2 x D of support. The vertical alignment of this part is vital and can be achieved in the same way as the clamp die. Ensure that the alignment of the pressure die groove is parallel to the groove in the bend die. Whilst most pressure dies allow riding up at the back to align with the groove, very few can ride downwards!
- The Mandrel – Setting the mandrel in the correct position is regarded by many as almost an art form. Its position in theory is predictable, but many factors can influence it. For example – a badly fitting or loose mandrel will need to be set ahead of the true center line to compensate for the excessive clearance between its diameter and the tube I/D. Conversely, a tight mandrel may have to be slightly behind center to avoid tube breakage on some materials. As a general rule, the crown of the radius at the end of the mandrel shank should be on or slightly back of centerline by no more than 1mm initially. If a ball mandrel is being used, the stroke of the mandrel retraction cylinder should be sufficient to withdraw all of the balls from a completed bend. Beware of using a tight multi-ball mandrel with nipped, badly formed or worn clamps. The usual result is broken links or cables.
6. The Wiper Die –
To align the wiper die correctly, the bracket should have adjustment in all axis, i.e. forward and backward, up and down of the centerline and of the rake angle. The bracket face on which the wiper die is to be mounted should be 90deg to the vertical axis of the bendformer groove.
To fit a wiper die, position the die and bracket in an approximate position and secure them finger tight with core. Advance the wiper die towards center line of the bendformer, keeping it parallel to the pressure die in both axis using either the jacking screws on your bracket or gentle tapping with a soft mallet in a non – vulnerable area. The bottom of the groove should not rise above the level of the bottom of the clamp groove on the bend die. Using a straight – edge or rule, can check this. When the head of the wiper die is neatly fitted into the bendformer groove and the grooves are level, this is the optimum position. You will note that the wiper die is not quite at the true center for reasons mentioned in the wiper die section. A slight “negative rake” angle on the wiper die will assist in assuring that the back – end does not combine with the force of the pressure die to create a clamping effect. When all these factors are correct, fully tighten the bracket and wiper die in position, checking that the tightening action does not alter positions.
The function of a wiper die is to prevent wrinkles forming on the inside of the bend and to force the material to compress. The selection of a wiper die will always accompany a ball mandrel. Wiper dies may be made from Ampco bronze; toughened-steel not hardened or hard chrome plated steel. The material type will depend on the material to be bent. This item plays a vital role in the production of tight radius bends, and in the bends using thin – wall tubing. Great care should be taken to protect the tip from damage both on and off the machine.
The tube groove should be precisely manufactured to a true form with the “heel” form aligned accurately on centerline. The size and quality of the groove should permit the material to flow without resistance due to trapping. An oversize groove will allow the material to grow vertically and this induces wrinkles, whilst a tight groove will cause drag, followed by clamp slippage. Both should be avoided. The heel must fit the form of the bend die exactly in both form and radius and center line radius to achieve the desired results. A badly fitted wiper die will almost certainly result in failure to achieve a wrinkle free bend and probably damage the feather edge of the tip. Do not attempt to hand – grind the form to give clearance on a larger radius as this will only lead to damage and limit the potential for re-cutting.