Along with blanking/shearing and welding, bending is one of the core metal fabrication processes. After being cut from sheet metal or plate, pieces that need bending go onto a press brake.
Having just added an American-made Accurpress Accell, we now have many different press brake machines in our shop. Visitors will sometimes ask, “Why do you have so many different machines for doing the same job?” That’s a good question, so let’s explain.
Press brakes are used for forming sheet metal enclosures, brackets, stiffening beams, and many other shapes. A press brake bends sheet metal by using a blade or edge tool mounted on a vertical ram to push the sheet into a “vee” block.
The edge and mating “vee” are a matched set of interchangeable tools. When the operator needs to form a different angle or is working with a different thickness sheet they’ll swap one pair for another pair the shape needed.
Behind the tooling is a backstop. By pushing the edge of the sheet up against this the operator ensures the bend is positioned exactly where needed.
Types of Press Brake Machines
Most press brake machines mount the top tool in a ram that moves down against the sheet. The “vee” block is held on the bed underneath. This design is known as a down-acting press brake.
The less common alternative is the up-acting machine. These are designed with the ram mounted below and raise the edge tool upwards to make the bend.
Proponents argue that up-acting machines are safer, for two reasons:
- When power is off gravity will open rather than close the tools.
- The operator, (who holds the sheet in position on the lower tool and against the backstop,) feels the movements and so knows the machine is in-cycle.
There are four ways of powering the ram:
- Flywheel. These mechanical machines are the oldest type.
- Hydro-mechanical. Here the flywheel is replaced by a hydraulic motor.
- Hydraulic cylinder. One or two hydraulic cylinders lower and raise the ram, typically generating 100 to 300 tons of force, although there are bigger and smaller machines around.
- Electric power. These machines are generally small and limited to around 50 tons.
Press brakes are specified in terms of tonnage. This is the maximum force the ram can bring to bear on the sheet. More tonnage lets the press brake bend thicker sheet, form a longer bend, or both. Our new Accurpress is a 500-ton machine that will let us tackle even ultra-high-strength materials like S1300 structural steel.
Types of Bending
Having said that the upper tool pushes the sheet into the lower “vee”, it’s not quite that simple. Our preferred technique is to have the sheet not hit the bottom of the “vee” This is known as air bending and it produces a consistent radius without overloading the press brake.
The alternative is to push the sheet right into the bottom of the “vee”. Although useful to form very tight radii it takes more force and risks damaging the tool. If the ram keeps pushing down after the sheet has bottomed it will thin the metal around the bend in a process known as coining.
The biggest challenge with every press brake is a phenomenon called “crowning.” This where the center of the bed and ram – the region furthest from the cylinders at each side of the frame – deflects.
On short bends, crowning is virtually undetectable. It can become a problem on longer bends though where it results in a bend that’s not uniform along its length. The solution is to slightly deform the center of the upper and lower tools to compensate.
On older machines, this deformation is achieved by shimming the tools. New machines, like our latest Accurpress, have CNC crowning technology. This means the machine adapts to the loads and deflection and it ensures consistent bend quality on long pieces as well as short.
A second, somewhat related challenge is off-center bending. This is when the piece being bent is positioned to one side of the bed. The reason we might set a press brake up like this is to leave room for a second setup at the other side. This would let an operator put more bends into a piece in one go, and it avoids transferring work to a second machine. (Lean Manufacturing enthusiasts will consider this a form of one-piece flow.)
The problem with off-center bending is that the loads on the cylinders are uneven. On older machines, this produced a twisting effect but today’s modern machines have ways of handling the differential through control of the two.
Press Brake Selection Criteria
As a fabrication shop, we do a lot of bending. We bend materials like stainless steel, aluminum, copper, and brass as well as regular old carbon sheet. We handle thicknesses from 22 gauge up to ¼”, sometimes even 3/8” depending on the material properties. And then there’s also the size of the parts we fabricate. While many are quite small – let’s say shoebox size or less – we also do a lot of large work like machine enclosures and guards.
We could just buy big machines because they can handle small and large jobs, but that would be like the Postal Service using only semi trucks for every delivery. It just wouldn’t make sense. So instead we try to match our range of machines to the range of bending work we do. The main criteria we consider when buying a press brake are:
- Width of the machine bed. It must be wide enough for the job but no wider than necessary. A wide machine needs a lot of floor space!
- Tonnage — the maximum force the press can bring to bear. This is related to the thickness and properties of the material being bent and the length of bend.
- Type of bending required. Coining and bottoming need a mechanical press while air bending is best performed on a hydraulic press brake machine.
- Ease and speed of setup. To achieve this our latest machine has the same control used on some of the others. This standardization means there’s no learning curve and operators can easily move between machines.
See for yourself
Bending is just one of the processes that goes on in our shop. If you’d like to see more, and you’re in the Marion, Indiana area, call us and ask if you can visit. If you’re further afield you might want to watch our drone fly-through instead. Trust us, it’s very cool!