Sheet Metal Bending – Methods, Design

Tips & K Factor

Bending is one of the most common sheet metal fabrication operations. Also known

as press braking, flanging, die bending, folding and edging, this method is used to deform a
material to an angular shape.
This is done through the application of force on a workpiece. The force must exceed the
material’s yield strength to achieve a plastic deformation. Only then can you get a lasting
result in the form of a bend.
What are the most common bending methods? How does springback affect bending? What
is k factor? How to calculate bend allowance?
All those questions are discussed in this post along with some bending tips.
We have also written another important post about press brake tooling. Knowing the tools
helps you to engineer products that can be manufactured.
Table of Contents  hide 
I Bending Methods
II Bending Springback
III Bend Allowance and K Factor
IV Sheet Metal Design Tips for Bending
V Metal Bending Online
Bending Methods
There’s quite a few different bending methods available. Each has their own advantages.
The dilemma is usually between going for accuracy or simplicity, while the latter gets more
usage. Simpler methods are more flexible and most importantly, need less different tools for
getting a result.
V-Bending
V-bending is the most common bending method using a punch and die. It has three
subgroups – bottoming, air bending and coining. Air bending and bottoming account for
around 90% of all bending jobs.
The table below helps you identify the minimum flange length b (mm) and inside
radii ir (mm) according to material thickness t (mm). You can also see the die width V (mm)
that is needed for such specifications. Each operation needs a certain tonnage per meter.
This is also shown in the table. You can see that thicker materials and smaller inside radii
require more force, or tonnage. The highlighted options are recommended specifications for
metal bending.

Bending force chart

Let’s say I have a 2 mm thick sheet and I want to bend it. To keep it simple, I also use a 2
mm inside radius. I can now see that the minimum flange length is 8.5 mm for such a bend,
so I have to keep it in mind when designing. The required die width is 12 mm and tonnage
per meter is 22. The lowest common bench capacity is around 100 tonnes. My workpiece’s
bending line is 3 m, so the overall needed force is 3*22=66 tonnes. Therefore, even a simple
bench with enough room to bend 3 m pieces will do the job.
Still, there is one thing to keep in mind. This table applies to construction steels with a yield
strength around 400 MPa. When you want to bend aluminium, the tonnage value can be
divided by 2, as it needs less force. The opposite happens with stainless steel – the required
force is 1.7x higher than the ones displayed in this table.
Bottoming
Bottoming is also known as bottom pressing or bottom striking. As the name “bottom
pressing” suggests, the punch presses the metal sheet onto the surface of the die, so the
die’s angle determines the final angle of the workpiece. With bottoming, the inner radius of
the angled sheet depends on the die’s radius.
As the inner line gets compressed, it needs more and more force to further manipulate it.
Bottoming makes exerting this force possible, as the final angle is preset. The possibility to
use more force lessens the springback effect and provides good precision.

The angle difference accounts for the springback effect

When bottoming, an important step is calculating the V-die opening. 

  Opening Width V (mm)

Method/Thickness
0.5…2.6 2.7…8 8.1…10 Over 10
(mm)

Bottoming 6t 8t 10t 12t

Air bending 12…15t

 Coining 5t

The inner radius has been experimentally proven to be around 1/6 of the opening width,
meaning the equation looks like this: ir=V/6.
Air Bending
Partial bending, or air bending, derives its name from the fact that the working piece does
not actually touch the tooling parts entirely. In partial bending, the workpiece rests on 2
points and the punch pushes the bend. Is still usually done with a press brake but there is no
actual need for a sided die.

Air bending gives much flexibility. Let’s say you have a 90° die and punch. With this method,
you can get a result anywhere between 90 and 180 degrees. Though less accurate than
bottoming or coining, this kind of simplicity is the beauty of the method. In case the load is
released and the material’s springback results in a wrong angle, it is simple to adjust by just
applying some more pressure.
Of course, this is a result of lessened accuracy compared to bottoming. At the same time,
partial bending’s big advantage is that no retooling is necessary for different angle bends.
Coining
Coining used to be far more widely spread. It was pretty much the only way to get accurate
results. Today, machinery is so well controllable and precise, that such methods are not
widely used any more.

The metal is given the die’s exact shape by applying great tonnage
Coining derives it name from coins, as they have to be identical to make fake money
distinguishable from the real one. Coining, in bending, gives similarly precise results. For
instance, if you want to get a 45 degree angle, you need a punch and a die with the exact
same angle. There is no springback to worry about.

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Why? Because the die penetrates into the sheet, pressing a dent into the workpiece. This,
along with the high forces used (about 5-8 times as much as in partial bending), guarantee
high precision. The penetrating effect also ensures a very small inside radius for the bend.
U-Bending
U-Bending Die with Hydraulic Press -U-Bükme Kalıbı
U-bending is in principle very similar to V-bending. There is a die and a punch, this time they
are both U-shaped, resulting in a similar bend. This is a very straightforward way for bending
steel U-channels, for example, but not so common as such profiles can also be produced
using other more flexible methods.
Step Bending
Step bending is, in essence, repetitive V-bending. Also called bump bending, this method
uses many V-bends in succession to get a large radius for your workpiece. The final quality
depends on the number of bends and the step between them. The more you have them, the
smoother the outcome.

Step bending with Bystronic Brake Press full automatic

Bump bending is used in many cases. Some examples include conical hoppers and
snowploughs. It makes large radius bending possible with regular tools. The easier setup
makes for a cheaper price, especially with small batches.
Roll Bending
Roll bending is used for making tubes or cones in different shapes. Can also be used for
making large radius bends, if needed. Depending on the machine’s capacity and number of
rolls, one or more bends can be done simultaneously.
W11 mechanical 3-roller symmetrical plate rolling machine
In the process, there are two driving rolls and a third adjustable one. This one moves along
via frictional forces. If the part needs to be bent at both ends as well as the mid-section, an
extra operation is required. This is done on a hydraulical press or press brake. Otherwise,
the edges of the detail will end up flat.
Wipe Bending
Wipe bending, or edge bending, is another way to bend sheet metal edges. It is important to
make sure that the sheet is properly pushed onto the wipe die. As a result, the wipe die also
determines the bend’s inner radius. The slack between the wipe die and the punch plays an
important role in getting a good result.
Rotary Bending
Rolla-V bending
Another way to bend edges is through rotary bending. It has a big advantage over wipe
bending or V-bending – it does not scratch the material’s surface. Actually, there are special
polymer tools available to avoid any kind of tool marking, let alone scratches. Rotary
benders can also bend sharper corners than 90 degrees. This helps greatly with such
common angles, as springback is not a problem any more.
The most common method is with 2 rolls but there are also options with one roll. This
method is also suitable for producing U-channels with flanges that are close by, as it is more
flexible than other methods.
If you would also like to read about steel tube bending methods, we have it covered along
with tube bending machinery.
Bending Springback
When bending a workpiece, it will naturally spring back a little after the load is lifted.
Therefore, it has to be compensated for when bending. The workpiece is bent beyond the
required angle, so it takes the wanted shape after springback.

Another thing to keep in mind here is the bending radius. The larger the inside radius, the
bigger the springback effect. A sharp punch gives a small radius and relieves the
springback.
Why does springback occur? When bending parts, the bend is divided into two layers with a
line separating them – the neutral line. On each side, a different physical process is taking
place. On the “inside”, the material is compressed, on the “outside”, it is pulled. Each type of
metal has different values for the loads they can take when compressed or pulled. And the
compressive strength of a material is far superior than the tensile strength.
As a result, it is more difficult to reach permanent deformation on the inner side. This means
that the compressed layer will not get deformed permanently and tries to regain its former
shape after lifting the load.
Bend Allowance and K Factor
If you design your bent sheet metal parts in CAD software that has a special sheet metal
environment, use it. It exists for a reason. When making bends, it takes material
specifications into account. All this information is necessary when making a flat pattern for
laser cutting.
Unless you use our manufacturing service where CAD models are accepted for production,
you need to keep producing those flat pattern drawings.

The arc length of the neutral axis must be used for flat pattern calculation
If you make your flat pattern drawings yourself, here’s something you need to know.
Bending elongates the material. This means that the neutral line or axis, as we talked in the
springback section, is not really in the middle of the material. But the flat pattern must be
formed according to the neutral line. And finding its position requires k factor.
K factor is an empirical constant, meaning that its value was determined by testing. It varies
according to material, its thickness, bend radius and bending method. Basically, the k factor
offsets the neutral line to provide a flat pattern that reflects reality. By using it, you get the
bend allowance which is, in essence, the length of the curved neutral axis.
K factor formula:
k – k factor, constant; ir – inside radius (mm); t – sheet thickness (mm)
Bend allowance formulas:
For bends between 0 and 90 degrees the formula is as follows:

ß – bending angle (°)

For bends between 90 and 165 degrees the formula is:

For bends over 165°, there is no need to calculate bend allowances, as the neutral axis
stays pretty much in the middle of the detail.
Calculating Bend Allowance
Let’s say you have a similar part to the one on the image above – it has a straight leg of 20
mm and another of 70 mm. The bending angle is 90°, sheet thickness 5 mm and the inside
radius is 6 mm. We want to know the final length of the detail. First, we must start with the k
factor:

Another way to determine the k factor is by following the “rule of thumb”. Just select a k
factor according to your material from the table below. This gives results accurate enough
for most cases.
Now we can move on to the bend allowance:

For the final length we just add the two leg lengths to the bend allowance:

Sheet Metal Design Tips for Bending

So, I talked to our experienced sales engineer who knows his bit about sheet metal bending.
He lit up and decided to make the fullest of the opportunity to share his insights on sheet
metal bending. Thus, he brought out a list of common mistakes and the solutions to avoid
them.
Minimum Flange Length
There exists a minimum flange length, as stated already before. See the bending force chart
for guidance. According to thickness, the die width is selected. If you design a flange that is
too short, it will “fall” awkwardly into the crevice and you won’t get the result you’re looking
for.
Chamfered Sides
The chamfer must stop before the base of the detail
If you want to make a flange that has one or two ends chamfered, the previous rule of a
minimum flange length still applies. The chamfers have to leave enough room to accomplish
proper bends, otherwise it will just look deformed and nobody’s really satisfied.
Hole Distance from Bend

Close-by holes may get warped

If the holes are too close to the bend, they may get deformed. Round holes are not as
problematic as other types but your bolts may still not fit through. Again, see the bending
force chart for minimum flange measurements and put the holes farther than the minimum.
Symmetry

To avoid confusion, the rectangular hole could be on both sides

There lies a great danger in making parts that are almost symmetric. If possible, make it
symmetric. If it is nearly symmetric, the bending press operator may get confused. The
result? Your part will be bent in the wrong direction.
The symmetry cannot be guaranteed in every instance, but then make sure that it is easily
understood how the manufacturing should be done.
Rivet Nuts
Rivet nut in the way of bending tools
If you use rivet nuts near the bending line, it’s known that inserting them before bending is
good for securing the applicability of it. After bending, the holes may be deformed. Still,
make sure that the nuts won’t be in the way of tools when bending.
Small Flanges on Big Parts
A small bend at the end of a large part may lead to difficulties
It is better to omit small flanges with big and heavy parts. It makes manufacturing very
difficult and manual labour may be needed. But it costs more than simple machining. As a
result, it is wiser to opt for alternative solution, if possible.
Bends Next to Each Other
Check the bending force chart for minimum flange length
If you want to include successive bends, check if it’s feasible. A problem arises when you
cannot fit the already-bent part onto the die. If your bends face the same direction – a U-
bend -, then a common rule is to make design the intermediate part as longer than the
flanges. 
Keep the Bends on the Same Line
This part needs numerous readjustments
It is best to keep the bends on the same line in case you have several flanges in succession.
With this in mind, you can keep the number of operations at minimum. Otherwise, the
operator needs to readjust the parts for every single bend, which means more time and
more money.
The Bending Line is Parallel to a Side
This kind of bending lines lead to inaccurate results
As the headline says. There has to be a parallel side to your bending line for positioning
purposes. If not, aligning the part is a real headache and you may end up with an
unsatisfactory result.
Bend Relief
Bend reliefs are necessary
To get the best outcome, it is advisable to not only make a small laser cut incision but an
actual cutout on the sides of the flange-to-be – a bend relief. The width of such a cut should
be above material thickness. This ensures that there are no tears or deformations to the final
bend. Another good practice here is to include small radii to the bend reliefs, as they also
relieve material stress.
Bending a Box
Small gaps guarantee a doable job
When bending a box, small gaps should be left between the flanges. Otherwise the last
bend can crash into the existing ones, breaking the whole structure. 
Check the Flat Pattern
One thing to keep in mind is switching your CAD view to flat pattern from time to time. There
are many upsides to that. Firstly, if you get carried away with your flanges, you may end up
with something that cannot exist in flat pattern. What cannot exist in flat pattern, cannot exist
in any other way.
Measure the layout. Maybe you can adjust the design for optimal fit. Try to avoid going for a
bigger sheet if the smaller size is in reach. Maybe you could fit 2 pieces onto the same
sheet, if you just shed a few millimetres off? It will reflect on the final price quotation.
Rule of Thumb for Minimum Bend Radius
Keep it simple. What could be simpler than choosing the inner radius (ir) just the same as
the material thickness. This avoids later troubles, overthinking and silly mistakes. Dropping
below that value can bring problems your way. Larger radius will just make some other
calculations a little more difficult.
Bending Direction

Bending perpendicular to rolling

You should not design your bends in the same direction as the material rolling was done.
This is especially important with aluminium and Hardox. Of course, we all know the
aluminium casing with 4 sides that means bending operations contrary to what we are
suggesting. Still, it is better to avoid it if possible. The result can be uneven surfaces or even
cracking.
Although the manufacturing engineers take care to notice these things, it is good to notice it
yourself. It helps to account for material usage.
Hemming

Leave an inside radius, if possible

If you want to strengthen the edges of your metal sheet, hemming is a great option. Still,
some advice applies. It is better to leave a small radius inside the hem. Completely crushing
the radius needs great power and tonnage. Also, it puts the material in danger of cracking.
Leaving a radius, on the other hand, relieves this danger.
Consider the Material
The regular thin 1…3 mm structural steel sheets can pretty much take anything. After that,
you need to do your research. Some materials are much more capricious about the way
they are handled. Getting a good result depends on your knowledge and on the help your
production engineer is able to provide.
Metal Bending Online
Fractory offers the aforementioned possibilities on a web-based platform. Getting an online
bending quote is very easy, you just have to upload your STEP files onto our platform and
we will contact you within 24 hours with the final offer.
Of course, you can provide a DXF drawing of the flat pattern to get an automated price for
the cutting procedure. This may help you with optimising the design and approximation of
the final cost.
Our capabilities:
Maximum force: 1000 tonnes
Maximum bending line length: 7200 mm

DIN ISO 2768 bending tolerances

If there are no extra requirements from the customer, ISO standard tolerances apply to our
products. Bending tolerances are shown in the table above.