High precision laser cutting of electrical steel
In the laser cutting market, it is common to assume that laser power is the dominant factor in determining throughput. And, when cutting straight lines in thick materials, especially at lower tolerances, higher power does deliver faster speed. However, there are many applications, especially cutting thinner materials at tighter tolerances, where very high power isn’t necessary, and may actually be a disadvantage. Furthermore, in precision applications, machine mechanical factors, and the ability to synchronize the laser with the mechanics, may be the ultimate rate limiting factors in terms of throughput. This article explores how one manufacturer, Stiefelmayer-Lasertechnik (Denkendorf, Germany), has addressed these issues to optimize the cutting of thin electrical steel.
Electrical Steel Cutting Requirements
Electrical steel is a thin (typically 0.1 mm to 1 mm), laminated material that is shaped and then stacked to make the stators and rotors of electric motors and generators, and the cores of transformers. It’s not uncommon for these stacks to consist of several hundred individual layers, each coated with a non-conductive outer lamination.
Typically, these parts are stamped in high quantity. Stamping achieves high repeatability, but limited absolute accuracy. Repeatability is important because it provides a smooth edge surface when a large number of these parts are stacked together. Also, stamping doesn’t produce edge burrs which would keep the parts from coming into complete contact when they’re stacked. Grinding to remove burrs isn’t desirable, because it could remove the material’s outer lamination. Finally, stamping doesn’t heat the part. Heating electrical steel can also remove its outer lamination or affect the magnetic characteristics of the bulk material, both of which are undesirable effects.
The main limitation of stamping is that it requires costly fixed tooling. This is acceptable in volume production, but can present a problem during prototyping, or for low quantity production runs.
Cutting using fiber lasers offers an alternative to stamping which avoids the cost of tooling, and generally delivers the speeds necessary to make it a cost effective option for small quantity runs. However, most fiber laser-based cutters have limitations which prevent them from being a viable choice for cutting thin electrical steel.
First, most commercial laser cutting machines have relatively large, non-constant tolerance deviations across their working area. Thus, a part cut at the front right from a large sheet of material may look different than a part cut from back left. This creates dimensional issues when these parts are subsequently stacked.
The other limitations of fiber-laser cutting are that it can evaporate the surface lamination from the part, produce dross on the edge, and create a heat affected zone (HAZ) in the bulk material where its magnetic properties have been altered. These issues typically get worse with higher cutting power. However, as mentioned at the outset, speed and laser power usually go together in traditional laser machines.
Lower Power, Higher Throughput
Stiefelmayer-Lasertechnik builds laser machine tools and performs contract manufacturing, with an emphasis on cutting thin sheet metal to tight tolerances. Their Effective series laser cutters are specifically optimized to produce high precision cuts with high throughput; their design philosophy favors finesse over brute force power. Company Managing Director Dieter Bulling explains this approach with a racing analogy. “We have tuned our machine for the tight city course in Monaco, not for the huge oval of Indianapolis. Our design aim was to achieve the highest possible average speed through all the tight curves, rather than maximum top speed on the straightaways. For the type of parts we typically produce, this yields greater throughput.”
There are two overall aspects that enable this performance in the Effective cutters – superior mechanics and precise synchronization of the laser to these mechanics. Stiefelmayer-Lasertechnik uses direct drive linear motors to provide both x and y motion of the cutting head. This eliminates the backlash associated the rack and pinion drives used in other systems. Also, the bridge which holds the beam delivery head is built from carbon fiber, rather than metal, to minimize its inertia without sacrificing rigidity. This lightweight bridge enables the machine to achieve a high jerk (the definition of jerk is the rate of change of acceleration). Dieter Bulling clarifies the importance of this, “No machine can instantly reach an acceleration of 4g or 6g from a standstill – that is physically impossible. The key to high productivity with tight contours is therefore how quickly the machine reaches the desired acceleration – in other words, the jerk.
Stiefelmayer's Effective Cutter Series
Bulling adds that a high degree of synchronization between the laser output and gantry movement is equally essential to both machine accuracy and throughput. All corners are cut with a small radius, because this eliminates the need to bring the beam to a complete stop at the vertex. This corner radius can be 40 µm in STIEFELMAYER Effective machines, or as big as 0.2 mm. Bulling explains, “There are two reasons we can achieve such a small edge radius. One is the high motion dynamics of the machine, the other is high synchronization of the laser output to gantry motion. Specifically, as the beam slows down, the laser power must drop in order to keep the total delivered power at a given spot constant. We address this by running the laser continuously (CW) in straight cuts, but then using a special pulsed output at the corners. So, the laser must be capable of rapidly switching between different operational modes. At the time we originally designed the Effective, Coherent | ROFIN had the only fiber laser in our target 2 kW to 3 kW power range that offered this critical functionality. Plus, their fiber laser used the same control software as their CO2 lasers, which we had already been using in other products, so this minimized our development time.”
Coherent | ROFIN HighLight Fiber Laser
In the company’s Effective cutters, the high quality output beam from a Coherent | ROFIN HighLight FL fiber laser is focused to a spot of around 60 µm to 65 µm at the workpiece, which is about half the spot diameter typical for machines of this size. This spot size is necessary in order to produce the small feature sizes required in many of their cutting applications. Furthermore, for cutting electrical steel, in particular, this combination of beam size and power minimizes the HAZ. It also reduces the amount of melt material (reducing the beam diameter by 2X decreases the amount of melt by 4X), which is blown off the substrate with gas, in order to yield a dross free edge.
Bulling summarizes, “The laser cutting market is becoming crowded with competitors. High cost machines usually emphasize laser power. But, as an example, moving from 6 kW to 12 kW does not reduce the cutting time per piece significantly. You could achieve more by going to a faster part loading/unloading system. Conversely, low price laser machines don’t have high precision mechanics; they might specify machine repeatability (although usually only in one axis), but don’t explain how that translates into the accuracy of the finished part. The discussion is going to more and more power, to be able cutting thicker material. Thick material needs less precision, thin material requires higher precision. These are completely different requirements for the machines. Thanks to better mechanics and improved laser synchronization, we differentiate Effective by offering a unique combination of precision, high overall throughput and guaranteed part repeatability (we specify that the Effective can cut 30 mm diameter holes over its entire 1.25x2.5m bed with ±25 µm repeatability). For cutting thin metals, we think this is the right direction for the future.”