Bend allowance is defined as the material required to add to the overall length of the sheet metal in order for it to get cut in the right size. During the design of the metal part, bend allowance based on the bending type and material thickness should be taken into consideration in order to get the accurate final dimensions.
Example:
For a 90° bend in a sheet metal with a thickness of 2mm and a bend radius of 4mm, you calculate the bend allowance using formulas like:
Bend Allowance=π×(Radius+Thickness/2)×(Angle in Degrees)/180
Practical Tip:
Always use bend tables or software to calculate allowances for various materials and bending angles.
Some material grades are likely to break or crack when bending is made from the force. Appropriate material grade for cold forming should be selected to prevent any cracks during bending.
Example:
- Not Suitable: High-carbon steel tends to crack when bent due to its brittleness.
- Suitable: Low-carbon steel (e.g., A36, S235 ) or aluminum alloys like 5052-H32, which have good ductility, bend smoothly without fracturing.
Practical Tip:
For challenging designs, perform a bend test on a sample piece to confirm the material’s behavior under stress.
Holes and slots are likely to deform when placed near a bend, therefore appropriate clearance should be considered in the part design.
Example:
If you place a 10mm diameter hole 5mm away from a bend line, the hole may elongate or deform when the metal is bent. Instead, maintain a minimum distance of at least twice the material thickness from the bend line to the hole edge.
Practical Tip:
Use guidelines such as:
Minimum Distance=2×Material Thickness
This ensures that features like holes and slots remain intact during forming.
The minimum radius of bends must be calculated in respect to the material thickness to prevent parts from cracking. The inside bend radius should be equal to the thickness of the material that you’re forming.
Example:
For a 1.5mm thick sheet of stainless steel, use an inside bend radius of at least 1.5mm. Tighter radii can cause the material to crack, especially with high-tensile-strength alloys.
Practical Tip:
Refer to the material’s bend radius chart or guidelines. Generally:
- Soft materials like aluminum can tolerate tighter bends.
- Harder materials like hardox steel require larger bend radii.
For shapes requiring multiple bends, it is essential to plan the bending sequence in order to not end up with a faulty product. It is recommended to try the sequence with a small test part and check the outcome before going with the real work piece.
Example:
Imagine a U-shaped bracket with three bends. If the two side bends are formed first, the middle bend may become inaccessible. Instead, bending the middle section first ensures smooth workflow.
Practical Tip:
Create a bending sequence plan or simulation using CAD software to verify feasibility before starting production.
