Selecting Force Tonnage for Simple Bending
When performing simple bends, we primarily make use of air bending and bottom bending, or coining.
Air bending takes place when the punch and die do not bottom out. This method can be utilized when the die is 6 times the thickness of the material, if the material is less than 3mm thick, and 12 times the thickness if it is above 10mm. Advantages of air bending are that specific dies are not necessary, and it uses the least press tonnage of any method. Disadvantages are that the bend angle is very susceptible to any material variation, as the material does not conform to the shape of the dies.
When looking at bottom bending, we encounter reduced spring back, and more accurate bends. The bends are less sensitive to material variation because they conform to the shape of the punch and die. However, this method does require more tonnage, and more specific tools for different bend angles and material thicknesses. While coining is technically different from bottom bending, if you just crank up the tonnage you are suddenly coining.
Properties of a Bend
Spring back depends on material yield and work hardening, as well as the bending process.
When you bend steel ideally half the thickness would stretch, and half would compress. Actually, it’s not that simple. It is more of a 60:40 ratio. The side that stretches, thins lengthwise as you would expect, but also pulls material in from 90 degrees to the bend. The opposite happens on the compression side, the material thickens and pushes material out at 90 degrees to the bend. The result is Edge Curl (see picture). Proper tooling can reduce it, but it never goes away entirely.
Hazards of bending
Overloading of tooling can occur when bending without enough die clearance. As a rule, die clearance should be 6-12x thickness of mild steel. Cracking when bending can occur when the material is not ductile enough. Back bending occurs when it is bent too quickly and the free end is whipped, or is too long to support its own weight.
Calculating bend allowance
The K factor in the standard bend equation BA = A (π/180) (R + (K * T)) should typically be 0.40. But it depends on the dies used and the steel temper and can vary within a single lot of steel.
Bend deduction is a much better method, since we use actual part measurements. The K factor is still an issue. For loose tolerance work, 0.4 works well. Precision bending will require that you bend a part using the actual material, press and tooling, and precisely measure and set up the bend. After bending, you will need to measure the result and calculate for K. In our shop we just subtract one metal thickness from the length, and that is close enough. The bend line is at one side length (2.0000 in sample shown) minus half the bend deduction (.293/2). (2.000-.1465 = 1.8535). A quick internet search will yield many charts and calculators to help with Bend Deduction.
Selecting Appropriate Tonnage
The internet has tons of Bending Tonnage Charts, so I will not duplicate that here. They are typically based on air bending mild steel. I find that when using a narrower than recommended die, it becomes harder to make the necessary bends. The tonnage is typically listed in tons per foot. So, a 4-foot bend in 16-gage will take about 15 tons and bending ½ x ½ will take just over one ton. Just a note of understanding, even though it takes just a ton to bend the ½ x ½ the dies must be rated at 25 tons per foot or greater, otherwise you will damage them. If you are going to do bottom bending, be certain that the reference chart specifically lists bottom bending. When estimating tonnage be sure to allow some extra tonnage just in case the part is extra hard, extra thick, or has excessive spring back which may require you to approach bottom bending tonnage to get a true 90-degree bend with 90-degree dies.
Even “simple” bending produces not so simple results. Phoenix has a wealth of experience in determining the best methods and most accurate tonnage for your application. Contact us for assistance!