Types Of Bending: Air Bending, Bottoming & Coining [PDF]

types of bending

The bending process is a widely used forming process for sheet metal parts. Parts like enclosures, computer cases, etc are made using the bending process. Many times plastic parts are converted to sheet metal parts and bending plays a big role in that conversion. Let’s learn more about bending, types of bending, and a detailed process related to air bending, bottoming, and coining.

What Is bending?

Bending is a sheet metal forming process to create V shape, U shape channels along a straight axis of sheet metal. The more ductile the sheet metal is, the easier to do the bending. Bending shape and size depends on the tools used, the thickness of the sheet metal, types of material used, etc.

You may like to read: Difference between HRCA and CRCA sheet.

How bending process works?

Before you dig into types of bending, we should understand how the bending process or bending operation works. We need to know the working principle of bending operations.

In bending there is usually a die which is fitted in the press brake. This is a stationary channel like part. The die has the outer shape of the bend that is going to be produced. There is a tool that is attached to the ram of the machine with the help of clamps and has the rounded edges which form the inner shape of the bend. This tool is called a punch. Punch is a moving part and creates the bending force.

The tool moves downward and puts pressure on the sheet metal. When the pressure is more than the sheet metal plastic limit, the sheet metal undergoes the plastic deformation stage and takes the shape of the die underneath. Once the operation is done, the punch again moves upwards for the next cycle. This is the whole process of bending.

types of bending

In bending, we need to understand a common phenomenon called Springback. When the tool is ejected, the sheet metal tries to go back to its unbend stage up to some extent. This happens due to the elastic property of sheet metal. That is the reason, always you will get a different angle than what is intended from bending. This is called Springback.

You always need to compensate for that extra bend angle during bending operation. For example, if you need a 90-degree bend, you may set the tool to produce an 89-degree bend. So that after spring back correction you get your 90-degree bend.

The spring back angle depends on the types of material used, the material composition, specially the carbon content. If the carbon content is more, you will have less springback. It also depends on the tool shape, sheet metal inner bend radius etc.

springback

Types Of Bending

There are basically three types of bending used extensively. However, there are some other types of bending process also. But discussing all those in a single article could be too much. I will post different articles on all those processes.

  • Air Bending
  • Bottoming
  • Coining

Air Bending

Air bending means the sheet metal part is in the air when the bend is performed. Please look into the below picture. It shows that only at two-point the part is touching the die. When the punch is coming in contact with the part, it is still in the air. That is why is called air bending. In case of air bending V shape and U, shape parts only can be fabricated

air bending
Air bending

Since the air bending is done on the air, and there is a very little surface point of contact between the part and die surface, it is very difficult to control the angle. Often in air bending, the angle variation is up to +/- 0.5 degree. But In air bending, you can choose a wide variety of angles from the same too set.

The bending pressure required is less in case of air bending. The reason being, the punch does not necessarily need to touch the die surface. The inner radius of the bend is not dependent on the die radius at all. However, it is dependent on the punch radius.

The spring back is more in air bending as the bend is done on the air. Due to the spring effect, the spring back is more in air bending. Below is a table that shows the ideal V width, Inner radius and angle precision of air bending , bottoming and coining

types of bending

Here T is the sheet metal thickness

Bending MethodV WidthInner RadiusAngle Accuracy
Air Bending12T-15T2T-2.5T+/- 0.45 degree
Bottoming6T-12T1T-2T+/- 0.15-0.30 degree
Coining4T-6T0-.5T+/- 0.10 degree

Advantages Of Air Bending

  • Wide range of bending angle selection possible
  • The less bending force is required
  • Comparatively cheaper tooling
  • Less maintenance
  • Wide variety of parts can have bends

Disadvantages Of Air Bending

  • More spring back
  • Difficult to control angle. More angle variations
  • Precision parts not possible

Bottomimg

Bottoming is more or less similar to air bending but the sheet metal parts get more contact surface than air bending. Due to this more precise bending is possible. The angle of spring back is also very less. The less bending force is required in bottoming compared to air bending.

In bottoming U shape bend is not possible. Only V shape bend can be created.

bottoming
Bottoming

Advantages Of Bottoming

  • More precise bend than air bending
  • Less spring back
  • Less bending force than air bending

Disadvantages of Bottoming

  • No U shape bend possible
  • Separate tooling required for separate angles
  • Not suitable for precise parts.

Coining

Coining is a bending process that gives the most precise bend. The bending force is almost 6-30 times more than the air bending. The finished part takes the shape of the die opening including the radius. The punch also goes all the way down to the die to create accurate bends.

The bend angle tolerance is coining process is about +/- .10 degree and the inner radius can go down all the way up to 0.5T.

coining
Coining

Advantages of Coining

  • Precise bend possible
  • Very fewer angle variations
  • Almost negligible spring back

Disadvantages of Coining

  • Higher tooling cost
  • More bending force
  • High maintenance

Wipe Bending or Wiping or Edge Bending

Wipe bending or wiping or edge bending requires a pressure pad to hold the sheet metal parts against the die. The punch then moves up and down to bend the edge of the sheet metal. Wipe bending is much faster than any other bending process but there is always a risk of scratches because the tool is moving over the surface directly.

If the tool radius is very less then it can even damage the sheet instead of bending it. Many times wipe bending is used to compensate the spring back of already bent part using air bending or bottoming or coining.

wipe bending
Wipe Bending

Rotary Bending

Rotary bending is similar to edge bending but instead of a punch rotary bending has freely moving cylinders with final formed shape cut into it. The rotary cylinder comes in contact with the sheet metal at two-point or edges of the cut and when it rotates further it creates the bend.

rotary bending
Rotary Bending

Conclusion

Those who work in the sheet metal fabrication industry might be well aware of these bending process. Types of bending that we discussed here are the basics one. There are numerous other bending types that industry uses. Covering all those types of bending in a single article will be too much to consume. Right? I will have a separate article to cover all those types of bending.

That’s all I have in this article. If you have any questions or queries, please do write to me in the comment section and I will be happy to reply back.

Frequently Asked Questions ( FAQ)

What are the different types of bending?

Air Bending
Bottoming
Coining

What is the difference between Air bending, bottoming, and coining?

All these three bending processes have the same type of tool. The only difference is the bending force required and how precisely the bend can be made. Bending forces increases when we move from air bending to bottoming or bottoming to coining.
In the case of air bending precision parts are not possible. But bottoming and coining gives more precise parts. Spring back decreases from air bending to bottoming or bottoming to coining.

What Is Springback?

Springback is the geometric change in a sheet metal part when the forming process is over and punch is withdrawn.

Here is one excellent video of how Amada Press brake works..

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