Basics of Tube Bending

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Bending Tube Without a Mandrel

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• When a tube is bent, two things happen to metal (Figure 1-a).
• The outside wall is reduced in thickness due to the stretching of the material and the
inside wall becomes thicker due to the compressing of the material (Figure 1).
• The material actually is formed approximately about the centerline of the tube.
• The material that forms the outside of the bend has further to travel and therefore is
stretched; the inside of the bend has less distance to travel and is compressed.
Stretching and Compression
Principles

4
• When the ratio of the tube diameter to wall
thickness is small enough, the tube can be
bent on a relatively small radius (Centerline
Radius or CLR = 4 x Tube O.D.).
• Excessive flattening or wrinkling of the bend
should not occur.
• The outside and inside of the bend tend to pull
towards the centerline of the tube (flattening).
• Two factors that help prevent this from
happening are a grooved bend die, which
supports the tube along the centerline and the
natural strength of the tube; round or square.
Basic Primary Tooling

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• A bend die, clamp die, and pressure die
are the minimum essentials for bending
tube.
• The bend die helps to prevent the tube
from flattening and forms a given radius of
bend.
Basic Primary Tooling

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• Springback is the term used to describe the tendancy of metal that has been
formed to return to its original shape.
• Springback is excessive when a mandrel is not used. This should be
considered when selecting a bend die.
• Springback will cause the tube to unbend from two to ten degrees depending
on the radius of bend, and may increase the bend radius of the tube.
• The smaller the radiuses of bend the smaller the springback.
Control of Springback

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• Kinking or buckling may be due to hard
material which will not compress on the inside
radius of the bend. The material, not being
able to compress, pushes in toward the
centerline of the tube.
• This condition can be corrected (provided the
tube is not too hard) by proper set up of the
tooling.
• A plug mandrel is indicated if the tube buckles
and is still within the wall factor and the
diameter of the bend.
Kinked or Buckled Bends

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Plug mandrels are inexpensive, easy to maintain and cause little drag.
Ball-type mandrels, however, should be employed for small radius
bends, thin wall tubes or where high quality is desired.
The clamp die should have a minimum length of three times the tube
diameter when using a plug mandrel.
Bending Tube with a Plug
Mandrel

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• The purpose of a plug mandrel is to prevent the
tube from flattening and to bend without wrinkles
or kinks.
• The mandrel is held in a fixed position while the
tube is pulled over it.
• The tube stretching process is localized on the
outer radius of the bend and the material is work-
hardened to retain its shape and not flatten.
• The material stretching is done on the forward tip
of the mandrel. This force, acting on the mandrel
tip, supports the inner radius of the bend, holding
it firmly into the bend die groove.
Balanced Pressures

1
0
• The purpose of the pressure die is to keep the tube against the bend die through the
duration of bending.
• The pressure die also keeps the mandrel from bending and maintains a straight tube
between tangent points of bends (the portion of tubing left on the mandrel after
bending).
• The location of the mandrel affects the amount of springback. The mandrel in a
forward position (toward tangent) will stretch the material on the outside of the
bend more than is necessary. This increases the length of material on the outside
beyond that which is required to make a bend.
• When the bent tube is removed from the bend die, it will conform to the die and
there will be little or no springback.
Unbalanced Pressures

1
1
In this example, the outside of the bend actually
is in compression with forces acting at points A
and B.
Counteracting forces occur at C and D.
Forces A and B tend to close the bend while
forces C and D act to open the bend.
Unbalanced Pressures Example

1
2
• The mandrel in a position away from tangent will not stretch the material on the
outside of the bend enough; consequently, there is not enough material to reach
from A to B, putting a tension in the material.
• The forces at A and B are now the reverse of those shown in the example, tending to
open up the bend. Thus, mandrel location can cause excessive springback, which
reduces the angel of bend and also may increase the radius.
• The mandrel should be brought forward (toward tangent) when the radius is
increased.
• There is no given formula for correct mandrel setting.
• One thing is clear; when the angle of springback is more than 3 degrees, the
mandrel is too retarded and the tube’ s radius of bend will be larger than the
bending die.
Unbalanced Pressure Example, Cont.

1
3
• When the tube breaks repeatedly, it may
indicate that the material is too hard.
• Hard material does not have the ability to
stretch sufficiently.
• Working with recently fully annealed
material will rule out this likelihood.
• When the mandrel is set too far forward or
the tube slips in the clamp die, breakage
may occur also.
Tube Breaks Repeatedly

1
4
• Advancing the mandrel slightly forward, the
wrinkles may stop forming in front and begin
to form in back of tangent.
• The mandrel at this point is still not far enough
forward to generate the necessary pressure on
the inside of the bend and compress the
material.
• The bend may start out smooth, but as it
progresses past approximately 20 degrees, the
material pushes back, forming a ripple or wave
at point A. This ripple is forming and being
flattened continually between the mandrel
and the bend die.
Mandrel Too Far Back

1
5
• When the bent tube is removed from the
bend die, there is a large buckle at point A.
• It is necessary to continue to advance the
mandrel until the material can’t squeeze
back between the bend die and mandrel.
Mandrel Too Far Back, Cont.

1
6
• When the mandrel is too far forward bumps
appear on the outside of the bend at the
terminal tangent and a step on the inside of
the bend at the starting tangent.
• They will not always appear at the same time,
depending upon shape of the mandrel and
bend radius.
• The bump, obviously, is caused by the
mandrel. The end of the mandrel prying the
tube away from the bend die forms the step.
Mandrel Too Far Forward

1
7
When: The radius of the bend is smaller and/or the wall is thinner.
Why: The wiper die is used to prevent wrinkles.
How: The ball mandrel performs like a plug mandrel. The balls are used to keep
the tube from collapsing after it leaves the mandrel shank. Bending issues are
enlarged when making tight bends or with thin wall tubing. It becomes more
difficult to retain the material during compression. The pressure is so intense the
material is squeezed back past tangent and buckles. This area must be supported
so that the material will compress rather than buckle; this is the prime purpose of
the wiper die.
Note: The wiper die cannot flatten wrinkles after they are formed; it can only
prevent them.
Bending with Ball Mandrel and
Wiper Die

1
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The proper bending machine, good tooling, and a trained operator can make
all the difference when bending thin wall tubing.
To help maintain the consistency of the tubing dimensions and
characteristics, the entire material required for a given job should be
acquired from one supplier, preferably even from the same lot or heat
number.
Premium-priced close-tolerance tubing should be considered. It often saves
many more times its added cost. It may be necessary on occasion to size
certain tubing before bending.
Bending Thin Wall Tubing

1
9
• There are many pipe bending machines but only a few are capable of thin
wall, 1 x D bending. Even those machines best suited for this special
bending must be in excellent condition and of a size large enough to
assure rigidity.
• The machine must be capable of retracting and advancing the mandrel
with the clamp and pressure dies closed.
• A hydraulically actuated pressure die is desirable.
• One feature of this system is that it provides identical pressure on the
tube regardless of wall variation.
Bending Machine Considerations

2
0
• A pressure die boost counteracts the drag of the pressure die,
mandrel and wiper die. Without a pressure die boost, the
thinning that normally can be expected is about three quarters
of the elongation of the outer wall.
• To reduce the thinning it is necessary to move the neutral axis to
the outside of the bend. If the neutral axis coincides with the
outer wall there will be no thinning whatsoever, but heavy
thickening of the inside.
• One method of achieving this is to put the tube into
compression before bending commences; this will modify the
stress distribution in the section.
Pressure Die Boost

2
1
• A clamp plug should be used when the
wall is so thin that it is being distorted by
the clamp die or collapsing under the
clamp die pressure.
• A clamp plug helps eliminate slippage with
less clamping pressure.
• A clamp plug should be a press fit. It is
located in the clamping area prior to
closing the clamp die and removed before
the pressure die is opened.
Clamp Plug

2
2
• The quality and amount of lubricant used is
extremely important. One lubricant will not
work equally well on all materials.
• One lubricant for steel and another for
aluminum is recommended.
• A generous amount of lubricant can be applied
to the mandrel and inside the tube. The
lubricant must cover the entire inside of the
tube.
Lubricant

2
3
• A universal flexing ball mandrel must be employed having a
clearance no greater than 10% of the wall thickness of the tube to
be bent.
• It will be necessary to have enough balls on the mandrel to support
the tube around the entire bend.
• It is important that the spacing between balls (or pitch) be reduced.
• The close pitch universal flexing mandrel is recommended over the
standard.
• The ball spacing (pitch) is closer and the first ball off the mandrel
shank gives very close support both to the outside as well as the
inside of the bend.
• When possible mandrel balls should be undersized sufficiently to
eliminate the possibility of clamping interference.
Flexing Ball Mandrel

2
4
Tube Breaking
Tube Wrinkling
Scratches/Marking on Tube Centerline
Tube Collapses Either with or without Wrinkling Through Bend
Hump at the End of a Bend and Mandrel Ball Humps
Tool Marks/Scratches Excessive Springback
Possible Reasons for Link Failure
Final Tips
Troubleshooting

2
5
1. Material lacking proper ductility and elongation
2. Tube slipping in clamp die
3. Pressure die too tight causing excess drag
4. Material wrinkling and becoming locked between mandrel balls
5. Clamp die pressing on mandrel balls
6. Improper or insufficient lubrication
7. Mandrel too far forward
Tube Breaking

2
6
1. Tube slipping in clamp die
2. Mandrel not far enough forward
3. Wiper die not seated properly in bend die
4. Wiper die worn or of improper fit
5. Too much clearance between mandrel and tube
6. Not enough pressure on pressure die
7. Improper or excessive amount of lubrication
8. If mandrel & wiper die are in proper locations, check
inboard pressure on pressure die. You may need to
apply more pressure on pressure die to hold tube into
die. Adjust in slowly until you have no wrinkles. Lastly,
check mandrel fit.
Tube Wrinkling
Note: Materials offering less resistance to flow will have fewer tendencies to wrinkle. For instance, 3/8î OD x .020
wall Type 304 can be bent on as small as 2 x diameter radius without a wiper die. On the other hand, AM 350
CRES stainless will require a wiper die on any radius up to 3 x diameter. The wiper die must fit the bend die and
the tip as close to tangent as possible. The biggest problem in bending tubing on tight radii is making the material
compress without buckling. A worn or improperly fitted wiper is usually where the trouble occurs.

2
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1. Check alignment of all dies.
2. Ensure all tools are free of dirt and scale. If
not, clean tools.
3. Check tube is clean and free of oil and
grease.
4. Use interlock style tooling.
5. Check forces on clamp and pressure dies.
Scratches/Marking on Tube
Centerline
6. If deep scratches in bend and in wiper die areas, try to check wiper angle and increase a
little. Also, back end of wiper should not touch the tube; it must have a gap. Lastly, make
sure tube is getting plenty of lube along with lubrication on wiper die in bend area.

2
8
1. Check for clamp slip.
2. Check for sufficient push-assist.
3. Check mandrel positioning in relation
to tangent.
4. Check that number of mandrel balls is
sufficient for application.
5. Check for drag.
Tube Collapses Either with or
without Wrinkling Through Bend

2
9
1. Check mandrel positioning. Try to
move mandrel back from tangent until
hump goes away.
2. Check free movement of pressure die.
3. Check force on pressure die, pressure
die assist.
4. Check mandrel ball sizing.
5. Ensure material is suitable for
application.
Hump at the End of a Bend and
Mandrel Ball Humps

3
0
1. Check for alignment of clamp
2. Check location and pressure of all dies
3. Check clamp is clean and free of oil and
grease
4. If clamp force is excessive, check for
proper surface texture on bend die insert
5. Increase rake of wiper die
Tool Marks/Scratches
6. Check condition of wiper die. If worn, pitted or scored, re-cut or replace
7. Ensure sufficient lubrication
8. With wrinkles in bend area only and scratches in bend area make sure clamp die is
not slipping and recheck pressure die pressure. Clamp die may need serrations to
hold tube or it may also be worn out.

3
1
1. Check that pressure die is not holding
the tube back during bending.
2. Check ductile strength of tube and
program over bend to compensate.
Possible Reasons
for Link Failure
Excessive
Springback
1. Insufficient tool-to-tool clearance
2. Insufficient tool to part clearance
3. Geometry of bend
4. Mandrel positioning
5. Sub-standard tooling

3
2
1. Use complete sets of tooling, purchased together. Mixing and
matching tools may not match perfectly, causing inconsistent and
often poor quality bends.
2. Lubrication is a key factor in making good bends. Type of lubrication is
dependent upon the material of the tube to be bent. A generous
amount should be applied to the mandrel and interior of the tube.
Final Tips

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Basics of Tube Bending

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