When LVD developed its Electra fibre laser machine, it decided to start with a blank piece of paper and with the objective of creating a machine that took maximum advantage of the fibre laser’s capabilities.

In material up to around 4 mm thick, 1 micron wavelength of the fibre laser beam is absorbed much better than the longer wavelength of CO2 laser. This opens up the possibility of much higher cutting speeds.

Working with Rofin Sinar, the manufacturer of fibre laser, the sheet metal machine tool builder sets out to explore this possibility.

Thierry De Vleeschouwer, LVD’s chief laser engineer, says: “The really big advantage of the fibre laser is the speed at which you can cut thin materials. You can easily achieve cutting speeds of 50 m/min and more with a 3 kW laser on thin materials.”

Making A Stable Machine

It was critical to design a machine that could maintain high cutting speeds on small features, holes and sharp corners as well as long straight edges.  The machine axis mostly has to slow down to cut these features, so you can only maintain high cutting speeds if your machine axis can quickly accelerate to top speed.

Mr De Vleesschouwer says: “Many of our competitors simply took their existing CO2 machine and fitted it with a fibre laser. But a CO2 machine is designed to carry a heavy optical system and cutting head, so it is overdesigned and doesn’t have the dynamics to bring out the benefits of the fibre laser.”

So, how do you build a machine that does? The company looked at the fundamental requirements for such a machine and decided that it needed to be able to ‘cut’ at 2g on a typical nest that would allow the machine to work at top speed for around 90 percent of the time. It would also need to achieve that acceleration quickly from a standing start. Most machines with acceleration specifications of 3 or even 4g use this acceleration only for rapids but need to bring this down to 1g or below for accurate cutting.

Mr De Vleeschouwer says: “When you start to move the axis it takes time to get to its maximum acceleration, and this rate of increase in acceleration is called the ‘jerk’. There’s no point having a 2g machine, or even a 4g machine if your jerk is slow, because you will never achieve your maximum acceleration. A 4g machine with low jerk might perform worse — in other words more slowly — than a 1g machine with high jerk.”

The way to achieve these dynamic characteristics is to design a machine where the moving parts have a low mass and a high stiffness, and the fixed parts (the frame) provide mechanical damping to absorb the forces of the high jerk.

“We looked at materials such as lightweight alloy castings and composites and optimised the design of every component,” says Mr De Vleeschouwer. “And for the drives we did a mechatronic analysis with Siemens to determine the best systems. This include linear motors on the short axes and a compact rack and pinion drive on the long axis. This mean you can reduce inertia to the minimum and have a very dynamic drive system with high accuracy.

“The result is that the Electra is 20 percent faster than competitors’ machines that appear to have similar or even higher specifications on paper.”

Working Together

“The laser is a standard product, so we don’t adapt it to the customer, but what we did with LVD was to start by discussing the market the machine was aimed at and the philosophy behind it. On that basis we talked about the options they needed to consider, the types of laser, the control and so on,” says Dr Wolfram Rath, product manager of laser sources at Rofin.

“We shared ideas about the process and about process optimisation and carried out trials in our applications lab to help them choose the laser power, fibre size, cutting head, focal length and so on,” he adds.

For example, one of the features is the way the process fibre — that delivers the laser beam to the cutting head — is connected to the laser resonator.

The sheet metal machine tool maker uses Rofin’s fibre-to-fibre coupling system to join the process fibre to the feed fibre that comes from the active fibre that produces the beam.

On a lot of competitors’ machines, the feed fibre comes straight from the laser resonator to the cutting head. There is no separate process fibre, so it cannot be disconnected. If the fibre has to be replaced it has to be spliced on by a trained engineer using special equipment. With Rofin’s system it is a matter of plugging a new fibre into the coupler.

Where the laser OEM’s applications know-how really comes into play is in the way the laser and machine work together. One critical area is on the way the power of the laser is controlled.

Dr Rath explains: “If you have a very fast machine like the Electra you have to reduce the cutting speed when you cut sharp corners, small holes and so on in thick plates. When you go slower you have to turn the power down, so we use a special pulsed mode and change the frequency and duration of the pulses to control the laser power.

“Cutting at full speed the laser works in continuous wave mode, but as you decelerate it changes to a pulsed mode. You reduce the frequency and the duty cycle in proportion to the cutting speed — so you don’t turn down the power of the laser coming out of the resonator, you just use less of it. That gives you the optimum quality and you can optimise the parameters of the pulsing for any material and thickness.

“The laser pulsing control links directly to the machine’s CNC and LVD’s software. LVD has built all these parameters into the control software. The customer doesn’t have to worry about choosing the right parameters, they just enter the material and the thickness and the machine sets them automatically and communicates with the laser to deliver the required power.”

Protecting Both The User & The Laser

Safety is a critical issue with fibre lasers, they can blind instantly if they shine into someone’s eye. To prevent this, the laser OEM uses a power supply that can shut down the laser in 10 milliseconds if there is a safety alarm. So if the operator opens the door, the laser shuts down before it can do any harm. When they close the door, it starts again without any warm-up or waiting time.

On the Electra machine, the door can only be opened if the program and the laser is stopped, this creates even more safety for the user. This together with certified machine windows and fully closed covers creates a safe machine to work with.

It is important to protect the laser too. Back reflection can be a problem when cutting highly reflective materials such as aluminium, brass and copper. When the laser is working within normal parameters, the laser OEM’s design conducts away the heat created by these back reflections to maintain reliability, and if the intensity of the back reflections gets too high the laser is automatically shut off to protect it.

Whole Package

Dr Rath and Kurt Van Collie, LVD’s product manager for laser cutting machines both agree that working together is not just about the technology. The laser OEM’s vertically integrated approach was also an important factor.

When the laser OEM developed fibre laser technology, it wanted to be in control of all the elements that went into the laser — such as the active fibre, the diode lasers that activate the fibre, the fibres themselves and the technology to splice them together to create an ‘all-in-fibre’ system.

“Normally, we try to develop new machines through a process of evolution but, for us, developing this fibre laser machine was a revolution. Our goal was to make the best machine in the market for cutting thin material quickly, and we achieved that,” said Mr Van Collie.