- Category: Optics
- Created: 2017-10-03
For the flexibility to cut varying material types in a range of thicknesses, zoom focus is a fiber laser technology must
by Stefan Colle, laser product sales manager, LVD Strippit with editorial assistance and images provided by II-VI Highyag
The market for metal laser cutting equipment continues to be dominated by fiber laser as the technology becomes increasingly more capable and flexible. Just a few years ago, CO2 was the laser of choice for sheet metal cutting because of the availability of high-power units with high beam quality. Fiber laser has changed the game, however, gaining ground in sheet metal processing applications because of its wavelength and high beam quality, along with the process benefits it offers over CO2.
Since fiber’s introduction to the market, laser manufacturers have been developing more powerful fiber laser sources and optical systems, driven by the end user’s need to cut a broader range of materials in varying thicknesses. A recent advancement is in the cutting head: the machine controlled and automated independent adjustment of focus position and focus diameter. This “zoom focus” control can dramatically improve piercing times, cutting speeds and cut performance in all material types and thicknesses while simultaneously increasing throughput and reducing the need for operator intervention.
The Phoenix fiber laser cutting head from LVD.
The fiber lasers of yesterday could cut a respectable range of material types and thicknesses, but fell short when cutting thick materials (3/4 in. to 1 in.) simply because the laser beam was not wide enough to create the kerf width needed to evacuate the molten material. To create more kerf width, it was necessary to enlarge the focal point.
In response, today’s fiber laser cutting heads can move the focal point up and down and can now also make the focal spot wider or smaller. A smaller focal spot is advantageous when cutting lighter gauge materials because by making the spot smaller, the density of energy is increased, and, as a result, so is the cutting speed. In thicker plate, the focus point is adjusted wider to create enough kerf width to successfully evacuate molten material and thus effectively cut thicker stock.
Taking the next step in the fiber laser’s evolution, zoom focus was developed. To make zoom capability possible, a collimator was introduced in the area above the cutting head, using two additional optics. Both optics are motorized and can move independently, much like a zoom lens in a camera, allowing the focal point to be expanded or decreased (see Figures 1 and 2).
To change the focus position, the collimation lens is moved. As such, the focus diameter can be changed by varying the focal length of the collimation. By allowing independent/automatic control of the focus position and diameter, it’s possible to cut different sheet thicknesses without manual intervention.
LVD produces laser cutting systems for a range of material types and applications.
Cutting and piercing
Machine controlled adjustment of focus position and diameter will maximize productivity no matter the material type or thickness. However, zoom focus provides a clear advantage when cutting thick material (1 in.). While both conventional and zoom focus laser cutting heads can achieve fast cutting speeds in light-gauge materials, only a zoom head design is recommended for thick plate processing.
Piercing is also better handled by zoom technology. A significant challenge when cutting thick mild steel is to create a fast and stable piercing process. The machine-controlled focus adjustment (zoom focus) optimizes the piercing process by enhancing piercing stability and quality. 
Piercing times are significantly reduced in thick mild steel versus CO2 technology. In part, this is because the absorptivity of metal surfaces is higher for the shorter wavelength fiber laser radiation compared to the longer wavelength CO2 laser radiation. Another factor is the improved (zoom focus) technology in the fiber laser cutting head.
By increasing the magnification (spot size), the half angle divergence decreases. This enables a reduced amount of molten material and, hence, results in faster processing. It takes a high-powered CO2 laser 10 to 15 sec. to make a small pierce hole in 1-in.-thick mild steel. Using a zoom focus cutting head, we can now pierce in less than 3 sec. With an average of, say, 500 holes per plate, the resulting savings can be significant.
In addition to piercing time, other values are optimized by zoom focus, including hole diameter and amount of material spatter. A small hole diameter is the first precondition to cut filigree contours even for thick workpieces.
An ideal process is to pierce a small hole quickly without using up too much material. Long piercing times should be avoided to minimize the processing time. 
In that regard, the amount of spatter can directly affect the cutting process stability. For instance, spatter may contaminate the optical elements and influence the capacitive height sensing, which is key to monitoring the height of the cutting head. A minimized amount of spatter corresponds directly to higher machine uptimes. 
While zoom focus technology makes fiber laser cutting more efficient and flexible, there is still the variable of material quality. A big factor in producing a good quality laser cut is the quality of the material.
Fortunately, a range of steel grades is now available; each absorbs energy differently based on composition or chemistry. Energy absorption is critical, as material that is too hot will generate burning defects in the cutting process, regardless of cutting method. Burning defects can be expressed as explosions, strong striations, curve width variations, excessive burr formation and welded parts in the material skeleton.
To minimize the impact of variations in material quality, most cutting system manufacturers now equip their laser systems with a pre-defined database for various steels, which the programmer can access during off-line programming. This data includes important settings (parameters), such as laser power, assist gas pressure, nozzle size, focal position and, now, focal spot size.
Today’s fiber laser cuts a range of material types and thicknesses, from thin to thick.
As laser manufacturers collect data about different steels and steel mills, the database continues to expand. In a perfect world, users would no longer concern themselves with having to make adjustments to a machine’s existing technology tables for a new material they haven’t cut before. As power levels continue to ramp up, this information is critical to control the heat generated in the material, which impacts cut quality.
For fiber laser technology and those that leverage it, zoom focus serves as major progress. For the flexibility to cut varying material types in a range of thicknesses, zoom focus is a must. Today, some higher-end fiber laser cutting machines have this technology built in – and in the future, other manufacturers may follow suit. Therefore, if you’re in the market for a fiber laser and need the versatility to handle a range of applications up to 1 in. thick, this technology is for you.
 D. Wolff, J.-B. Karam: Cutting Head for High-Brightness Lasers, A cutting-edge solution for 1 μm metal cutting, Laser Technik Journal, 5/2016, p. 19-21