Skip to content

2 Sheet Metal Bending Processes: Air Bending, and Bottoming

  • by
Bottom bending process
Can you easily rattle off the difference between air bending and bottom bending? 
If you’re a newbie to metal fabrication or are simply looking for a refresher on the essentials of bending, read on.

Air bending and bottom bending

Air bending and bottom bending are the three types of bending most often employed by precision metal fabricators. This article covers the basic definitions of each and the differences between the two.

Air bending and bottom bending are two of the most common ways of the sheet metal bending process.

Both can be achieved using a manual or CNC-controlled press brake.

Air bending

Air bending

Air bending is the most common type of bending used in sheet metal shops today. It is the process of forming materials by pressing a punch into the material, thereby forcing it into a bottom V-die to be mounted on the press. This enables the distance between the punch and the sidewall of the V to be greater than the material thickness.

The v opening is typically deeper than the angle which is sought in the workpiece. This allows for over bending to compensate for the Spring Back of the workpiece. Typically Acute Angle Tooling can be used to fully air bend and 90 ° or 88 ° tooling can be used to partially air bend. There has recently been the introduction of 75 ° tooling to allow for full Air Bending, without the tooling restrictions of acute punches.

This is the most preferred type of metal bending due to its benefits – such as less need for the punch tip to be pushed past the surface of the metal and less weight required for bending.

Advantages

  • No retooling is required to create different bend angles, saving time and expense with the process
  • Able to bend most material types and thicknesses
  • Faster than bottom bending
  • Less pressure is required to perform the bend
  • Less contact with tooling results in less surface damage

Disadvantages

  • Not well suited to parts requiring highly accurate bend angles
  • Material will have a spring back effect which needs to be considered in the bend angle
  • Older press brake equipment often results in inaccurate bends

Bottom bending

Bottom bending

Bottoming is a bending process in which sheet metal is pressed against a bottom die featuring a V shape. While other bending processes typically support the use of both U- and V-shaped dies, bottoming only uses a V-shaped die.

In bottoming, the sheet is forced against the V opening in the bottom tool. U-shaped openings cannot be used.

Space is left between the sheet and the bottom of the V opening. The optimum width of the V opening is 6 T (T stands for material thickness) for sheets about 3 mm thick, up to about 12 T for 12 mm thick sheets. The bending radius must be at least 0.8 T to 2 T for sheet steel. A larger bend radius requires about the same force as larger radii in air bending, however, smaller radii require greater force—up to five times as much—than air bending.

It’s often preferred over air bending because of its higher level of accuracy as well as less recoil with the finished sheet metal.

The advantages of bottoming include better accuracy and fewer spring-backs, while the disadvantages are that a different tool is usually needed for each bend angle, sheet thickness, and material.

Advantages

  • More accurate angles than air bending
  • Less spring back than air bending

Disadvantages

  • Unique tools are required for different bend angles, creating a more complicated process
  • Requires more pressure on the press brake than air bending
  • Contact with punch and die can leave tooling marks

Air bending vs Bottom bending

Air bendingBottom bending
Custom toolingNot requiredMay be required
 Single die can accommodate multiple angle radiiUnique die is required for each angle radius
Angle accuracy>± 45′±15-30′
Best used forParts where bend accuracy isn’t vitalParts where bend accuracy is important
PricingNo tooling investment / higher part priceTooling investment / lower part price