One of the inevitable side effects of a bending machine is frame deflection. We bend stainless steel with mild steel, and for physical reasons, mild steel will deflect. In particular, the ram will bend in the centre. At this point, the punch will move away from the die and the resulting angle will be wider. As a result, the bent profile will have a boat-like shape.
We will see how even small differences can turn metal forming into a process full of pitfalls.
This deformation can be compensated by convex compensation: the table is pushed upwards to keep the die at a constant distance from the punch. There are two types of convex machining: the former uses a series of wedges to mechanically lift the die; the latter uses a series of wedges to mechanically lift the die. The latter uses a short-stroke hydraulic cylinder embedded in the table.
But how much crowning is appropriate? Most manufacturers rely on tables calculated from the foundry’s stated bending machine construction and sheet metal characteristics. Apparently everything is fine. In reality, this approach doesn’t work because sheet metal behaviour is unpredictable. It is highly variable and depends on a range of factors. It is vital to understand them and compensate for them where possible to avoid wasting time and material on tests and samples. Especially today, as batches become smaller, getting the angle right on the first attempt becomes critical.
1: Not all steels are created equal
What does A36 steel stand for? Sheet metal is labelled according to its yield strength. This value is highly variable and depends on impurities in the casting and defects in the production technology. To avoid the risk of building weak structures, regulations require a declaration of minimum strength. Therefore, any steel with a yield strength greater than 36,000 psi will be marked A36.
Therefore, the 41,000 psi strong steel will still be sold as A36, albeit with a 13% increase in hardness. The increased resistance will require more force to be applied to the bending machine, which in turn will cause the punch to bend more. For example, switching from 36,000 psi steel to 41,000 psi steel, we’ll get about 0.002 inch depth of deflection. However, this small curve results in a difference of almost 1° on a 0.3″ V-die.
V-Die Opening Depth Δ = 1°
1/4″ (6 mm) 0.016″ (0.04 mm)
(10 mm) 0.024″ (0.05 mm)
1/2 inch (12 mm) 0.027 inch (0.07 mm)
In other words, less thickness than a sheet of paper (about 0.04 inches) already makes a significant difference. Keep in mind that narrow openings like these are often used to bend thin stainless steel sheets for a very high quality product.
2: You Can’t Win Them Holes
The labelling data of the steel we buy (which may or may not be reliable) becomes completely worthless when we modify the plate. In the case of thermal cutting as well as drilling or milling, the holes change strength along the bend line. Predictions are even more useless if, on the same part, there is some bending on the complete part and some on the drilled part. On the other hand, punching also introduces internal stresses, which makes it more difficult to consider the bulge as a fixed amount of deformation.
3: Conditions matter
Steel is a living thing. The direction of rolling creates fibres in the structure, so bending laterally or along the grain will affect the required bending force and springback.
Even after only a few weeks, recently cured plate is harder than aged or oxidised plate. In addition, bending a hot sheet is not the same thing as bending an ice-cold sheet. Temperature has an effect on moulding conditions and can lead to different results.
Even the dimensional figures vary considerably, especially the thickness. For example, the EN 10051:1991+A1:1997 regulation divides sheet metal into 5 groups. Let’s consider mild steel between approximately 37,000 and 49,000 psi (260 ÷ 340 MPa), 0.1 inches (2 mm) maximum, and 4 to 5 feet wide (1201 ÷ 1500 mm):
|Tolerance (in)||± 0.0055″(0.14 mm)||± 0.0075″(0.19 mm)||± 0.0087″(0.22 mm)||± 0.010″(0.25 mm)||± 0.011″(0.27 mm)|
This means that a sheet with a 0.1″ (2 mm) nominal thickness can fall anywhere within a 14% bracket, and within a 31% in the worst-case scenario.
ASTM A 480 Table 2.17 provides the following standard thickness tolerances for stainless steel sheet (extract):
|Gauge / Decimal equivalent (in)||Tolerance (in)|
|26 / 0.0178||± 0.0015|
|24 / 0.0235||± 0.0015|
|22 / 0.0293||± 0.0020|
|20 / 0.0355||± 0.0020|
|18 / 0.0480||± 0.0030|
|16 / 0.0595||± 0.0030|
|14 / 0.0751||± 0.0040|
We can see that also here we have about a tolerance range from 5.30% to 8.40%. Do you really think this can be ignored or foreseen?
4: Measure twice, cut once
While shearing alters the grain of the sheet metal, plasma, laser, and oxy-fuel cutting all produce non-negligible localised thermal shocks at the edges of the sheet and around the perimeter of the hole.
For these reasons, a good bending machine cannot and must not rely on any type of database, but should be based on estimating or predictive software. For the simple reason that no algorithm can predict sheet metal reactions. The idea that steel is a perfect and immutable material is an illusion that can quickly lead to a bad awakening.
If sheet metal is not formed perfectly, welding becomes more difficult, painting becomes more difficult, and joining can become a daunting task, especially when tolerances are close. Material waste and work time increase dramatically.
Ignoring rather than confronting these four factors could significantly reduce a company’s profits. Operators and bending machines must know sheet metal inside out, and they must be able to react and adapt to changes to ensure the best possible results.
The only way to manage convexity is to use a system that measures the actual deformation and corrects it in real-time. Only with such technology can one be sure that the result will be what the job requires, regardless of material properties. Any other system will deteriorate the quality of the bend and affect the finished product.
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