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The Art of Tube Bending
If you are purchasing or have purchased a tube
bending machine the following information will guide you on your way
to manufacturing quality components. The main ingredients to
manufacturing a good quality component are. Tooling, Setting and a
little splash of Lubricant.
Tooling
Bendformers:
Sometimes
referred to as a Bend Die. This is the main tool around which the tube
is bent. It has the shape of the tube machined around the periphery to
contain it during the bending process. The radius from the centre of
the former to the centre of the tube is referred to as the CLR ( centre
line radius ). Bend radius may also be referred to by the relationship
of the tube diameter to the CLR. For example 60mm Diameter tube bent on
a 180mm CLR would be referred to as a 3D bend (3 x tube diameter =
180). Similarly the same 60mm tube on a 60mm CLR would be a 1D bend.
The straight section of the bendformer is called the
former extension. This may be machined as one piece or be detachable, if
detachable it can be replaced with a different type of gripping surface.
The bend radius and the resistance of the tube to bending determine the
type extension selection. As a generalisation the tighter a bend radius
is, the longer the clamp area needs to be to overcome the additional
resistance, although serrations or coating the gripping face will assist
to keeping the length to a minimum.
Loose Clamp:
Sometimes
referred to as the Clamp
Die. This is the
moving block used to grip the tube with
the bendformer during bending. It has a
machined form corresponding to the bendformer.
The amount of gap between the former and clamp when in the closed
position will depend on the thickness of the tube. It
is permissible to have a slight pinch on this item to improve grip.
The clamp should never be permitted to
crush the tube onto the mandrel especially the ball
type. This practice will almost certainly result in damage to the
balls and mandrel linkage.
Pressure
Slide:
Sometimes
referred to as the
Pressure
Die.This
piece of the tooling
is
designed to
force the tube into the profile of the former.
Generally
the
pressure
slide is
allowed free linear movement to travel with the tube during bending.
Pressure dies may be static
depending on the materials and bend CLR.
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
adjustable
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.
Mandrels:
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.
There
should be a
mandrel retraction unit fitted to your machine,
this will
retract the mandrel balls from inside the tube at or just
before
the end of the bending cycle
usually 2-5 degrees depending on the type of mandrel in use and
lubricant.
this is
sometimes referred
to as
anticipated mandrel retraction.
Knowing this
we have manufactured a lubricant that has been proven to reduce friction
and lengthen tool life. To find
out more about this lubricant for your application contact us.
The Wiper Die:
The function of a wiper die is to prevent wrinkles forming on the inside
of the bend.
you will find This is common when bending a 1D bend.
The material wiper
dies
are
made
from
will depend on the material to be bent. This item
also
plays a vital
role in the production of
bending
thin – wall tubing.
The
profile of the tube
should be precisely manufactured to a true form
within the
wiper.
The size and quality of the
profile
should permit the material to flow without
any
resistance. An oversize
profile
will allow the material to grow vertically and this
may cause
wrinkles, whilst a tight
profile
will cause drag, followed by clamp slippage.
this
should be avoided. A badly fitted wiper die will
probably
result in failure to achieve a wrinkle free bend and damage the
wiper
tip.
Setting Your
Tube Bending Machine
Your
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 centre spindle or its location rings/key
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
you use it.
This will improve the chances of a good bend because of its stabilising
effect on the centre 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
profile
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
Slide
or Roller:
The pressure
slide
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
loose
clamp.
Ensure that the alignment of the pressure
slide
profile
is parallel to the
profile
in the
bendformer.
The Mandrel
and Lubrication:
Setting the mandrel in the correct position is regarded by many as
almost an art form. For example
a badly fitting or loose mandrel will need to be set
ahead of the true centre line to compensate for the excessive clearance
between its diameter and the tube I/D.
On the other
hand,
a tight mandrel may have to be slightly behind centre to avoid tube
breakage on some materials. As a general rule, the end of the mandrel
shank should be on or slightly back of centreline 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.
At this point
lubrication plays a large part in the setting of your machine. To reduce
the angle of mandrel retraction can depend on the mandrel lubricant you
use, this can make all the difference between a flat bend and a full
bend. if the wrong lubricant is used the
usual result is broken links or cables.
Knowing this we have manufactured a tube lubricant that has been proven
to reduce friction and lengthen tool life. To find out more about this lubricant for your application
contact us.
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 centreline 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
profile.
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 centre 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 centre 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.
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