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In the previous article we saw that there are 5 main types of sheet metal bending. We mentioned that the most common types of bending are coining, hemming and air bending. Air bending is divided into partial air bending and bottoming. The present article is going to focus on coining.
Coining.
When using this technique, the punch and die must be an equal match to the angle you want to achieve (usually 90°).
In coining, the whole punch tip (its two sides shaped to the same angle to be fabricated, usually 90°) penetrates into the sheet metal, pressing it against the internal surface of the die. A 90° bend is formed, where the internal bend radius is equal to the punch tip radius: this is why, in order to ease the penetration, it is recommended that the punch have the smallest possible radius (approximately 0,25 mm) and that the die have a V-width about 6 times the sheet metal thickness. They are also both required to have the same angle, in order to allow "coining" the sheet metal eliminating springback.
What is springback?
It is the amount of elasticity that any material has, namely the tendency of a material to resist deformation and to revert to its original form. This factor is always present in metal sheet bending, and it results in wider angle and radius once the part is released from the deforming forces.
Advantages of coining.
This technique, accurate and perfectly repeatable, is particularly suitable to form 90° angles on thin sheet metal.
Disadvantages of coining.
The limitation of this type of bending is having to use extremely high tonnage (a great amount of bending force) which tends to destroy the integrity of the material. For this, coining can only be performed on lower thickness sheet metal (usually no thicker than 1-1.2 mm). In addition, it is necessary to use different tools for each desired angle. As such limitations outweigh the advantages, this type of bending is only used in a few specific cases. The next article is going to focus on air bending.
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