Running flat out

. The basic principle behind levelling machines, which remove internal stresses from sheet steel, has changed little in 50 years. Now a radical new design has introduced higher levels of automation and accuracy – allowing it to process high strength steels more effectively. Bronx Manufacturing chairman Tim Brown, who designed and patented the Completely Automatic Levelling Machine (Calm), says its ability to handle high strength steels will make it attractive to the automotive industry – or those who supply sheet-steel parts to it. “The trend in automotive is for weight saving,” he says. “These steels allow you to make stronger, lighter parts – but you need to be able to process them.” Three years ago, Brown sat down and asked himself which features should appear on an ‘ideal’ levelling machine. He came up with a number of attributes: the ability to swap roller sizes at the touch of a button; automatic set-up procedure; and continuous adjustment, allowing the leveller to sense the steel as it passes through the rollers and apply the correct tensioning automatically. These are all features of Calm, and have been delivered through various pieces of clever design and new hardware, including: l a special algorithm, which calculates settings automatically; l a digital interface valve from Moog – which sets hydraulic pressure very precisely, for accurate positioning of the rollers; and;a set of load sensors, which introduce automatic feedback to levelling machines for the first time. “What we’ve done is to produce a levelling instrument, rather than a levelling machine,” says Brown. “We’ve patented the way that it adjusts.” On the level The process by which Calm removes stresses from steel is unchanged compared with traditional machines. “The rollers bend the material one way then the other in ever-decreasing amounts,” says Brown. “You induce plasticity then reverse it through each roller. You are trying to come back to a flat piece of metal with equalised internal stresses.” However, the way in which it achieves this is more sophisticated and intelligent than before, he says. The main improvement is in the use of feedback. A set of 15 load cells, embedded into stainless steel pillars on the machine, measure the stresses within the metal. “I wanted to make the leveller ‘feel’ what was going on as the steel passes between the rollers,” says Brown. “The algorithm then adjusts the machine to compensate.” The bottom set of rollers is active, and they can be positioned relative to one another. It is the adjustment of these rollers that is key to the machine’s performance. “We know exactly where each roll is in relation to those around it,” says Brown. The rollers are controlled hydraulically. A set of hydraulic rams are bored into the solid structure of the machine. Taking input signals from the load calls, the Moog valve can position a piston to an accuracy of 5 microns, ensuring pinpoint precision of the rollers themselves. “If all the rollers end up within 20 microns of each other, and from the target, that would be okay,” says Brown. The ability to adjust the rollers so precisely – by controlling hydraulic pressure using the digital valve – ensures that the correct torque and force is applied to the sheet. The role of the digital valve has simplified this operation – and would earlier have been achieved using hand-cranked jacks. The Calm algorithm has also simplified machine set-up. The operator simply enters details of the material’s width, thickness and grade (which gives yield strength). The algorithm calculates roller positioning automatically. “You also set the incoming and outgoing conditions,” says Brown. “At the start, you might program in a plastic factor of 80%. At the end, it’s usually set at zero. Then you know that all the rollers in between are doing work.” First in line The first customer, a local steel service centre, will use the machine for the ‘ideal’ purpose, to process high strength steel. “He’s taken our first machine for a good price,” says Brown. “The trade-off is that I will get reasonable access to it, in order to gather data and introduce extra features. This gives us an extended development period for the machine in use.” The customer has two ‘cassettes’ for the machine – a 17-roll, 50mm version and an 11-roll, 82.5mm version (which will handle thicker sheet). In future, any customer would be able to buy – or rent – a cassette. According to Brown, it is important to match roller size and spacing very accurately with high strength steel – something his machine is capable of doing. Most levellers can only handle metals with yield strengths up to 450N/mm2, he claims. “These new steels can be up to 1200N/mm2,” says Brown. “Traditional machines cannot flatten them consistently. We designed this machine with these new steels in mind.” The machine can process a wider variety of grades and thicknesses. Brown says that a traditional leveller has a 4:1 ratio in the range of thicknesses it can handle – so the ability to process steels of between 0.5 and 6mm might require two machines. “With our system, you would only need the one machine,” he says. Bronx also has a test rig in its facility: it has a single hydraulic cylinder like the one on the machine, as well as an identical valve and transducer. This has helped to prove the machine concepts such as the feedback mechanism. “If I can sell three of these machines worldwide each year I’ll be over the moon,” says Brown. On a roll Sheet steel is supplied to industry in the form of enormous rolls: these must be ‘unflattened’ – but anybody who has taken a poster out of a tube and tried straightening it will know how hard this can be. Levelling machines attempt to equalise the internal stresses of a piece of sheet steel. They do this by bending the material back and forth between two sets of rollers. But there has always been a ‘black art’ to this process – with skilled operators estimating how much roller pressure must be applied, and where. A common problem, according to Brown, is a lack of understanding between the movement of rollers and the extra load this will generate. “For standard steel, moving the rollers by 1mm can lead to a quadrupling of load,” he says. “This can cause the machine to break.” For this reason, the Bronx machine has an automatic safety mechanism that detects when a proposed operation is dangerous or impossible. That is not to say that it cannot apply large loads: Bronx had to develop its own drive shafts for the machine, as standard models were not strong enough to withstand some of the forces. The machine is driven by two 125kW motors from Reliance, using drives from Control Techniques. Pointers * Digital technology adds new levels of automation and accuracy to levelling machines – with the developer claiming it is now a ‘levelling instrument’ * Designed with new high strength steels in mind – as they are difficult to process – so automotive industry is likely to be a major customer * Algorithm calculates settings automatically, and load sensors ‘feel the steel’ as it passes through rollers. Digital valve positions hydraulic pistons – and rollers – to a claimed 5 micron accuracy