The Powers That Be

As technology improves over time, we are conditioned to expect that the latest and greatest is always better than what we had in the past. At this very moment, in fact, manufacturers are working hard to develop higher wattage machines to speed up the time it takes users to produce parts.

As a guy, it’s hard to make this next statement, but “power isn’t everything.” When it comes to laser cutting, power requirements are really dependent upon the type of material that’s being processed. As an example, mild steel melts between 2,500 and 2,800 degrees Fahrenheit, depending on the percentage of carbon, manganese or silicon elements present in the material.

But how do we get our Watts and temperature together to compare just how much power we really need to process mild steel? Well, we start with a few clarifications. Watts is a measure of power, and Fahrenheit is a measure of temperature. To make a long calculation short, a 1,000-Watt focused beam is equal to approximately 8 million Watts per centimeter squared, and steel vaporizes at 2 million Watts per centimeter squared.

This brings up the next question: Once we melt steel, how much more power do we really need? That, too, depends on several things, starting with what type of assist gas we will use to aid us in cutting.

Using oxygen, nitrogen, shop air
When oxygen is used as the cutting gas, the oxygen is blown through the kerf between 8 and 60 PSI, depending on material thickness. The oxygen interacts with the molten metal and begins to burn and oxidize. This reaction can release up to five times the laser energy while expelling molten material from the kerf. Once the material melts and expels, we need to move the drive system forward and continue the heating and expelling until we reach our maximum feed rate that is determined by the heat saturation point for that material thickness.

When nitrogen is used as the cutting gas, the nitrogen is blown through the kerf between 30 and 300 PSI, depending upon material thickness. Nitrogen is an inert gas so it does not react with the molten metal in the kerf. The gas is used specifically to expel molten material away from the material and simultaneously shield the cut edge from the air, giving it an oxide-free edge. However, the laser beam must supply all of the power needed for cutting. On thin material, we can break the melting point barrier restrictions and achieve higher cut speeds with the additional cost of the nitrogen gas.

To add to the mix, some lasers are capable of cutting with shop air. So, we really have a combination of approximately 78 percent nitrogen and 21 percent oxygen, and the balance is a few small traces of other gases. The key to cutting with shop air is supplying enough volume of clean, dry air otherwise the cutting lens could be destroyed.

But getting back to the original question: How much power do we need? To answer that question, let’s start with a basic example of a part and cut it using different power levels and with different drive systems and with different types of lasers, CO2 and fiber. Below is an example of a part used repeatedly for the comparison:

Dimensions = 22.675 X 18 inches

From the first two examples, you can see that the power level and the cut feed rate speed had no effect on the difference in cycle time. The ball screw versus the linear drive machine was really the deciding factor.

How fast a machine can accelerate and decelerate from start of cut through corners or from feature to feature has a tremendous impact on the final part cycle time. The next thing you’ll notice is that once we compare 2,000 Watts versus 2,500 Watts with the same drive system, the cycle time is almost half. However, when we than compare that to 5,000 Watts, we see no gain at all. At 2,500 Watts, we were able to break the barrier and use nitrogen as the assist gas, giving us the extra speed we desired, but again, we hit a limit of just how fast we can go with more power.
 
Comprehensive comparisons
Not all laser beams are created equally, nor do they perform equally. In the examples provided, the laser beam quality is specified as M2. M2=1 indicates the laser beam is diffraction limited and capable of being focused to the its smallest theoretical spot size. M2=1 is the holy grail of beam quality. A higher M2 number usually requires higher average power to accomplish the same processing capabilities as a lower M2 number.

Comparing the mode quality from a laser with laser beam mode quality of M2=01 to an M2=10 laser resonator with the same drive system and same power level, and we see yet another cycle time difference.

Finally, we see the difference between the fiber laser versus the CO2. First you will notice we used shop air, then you will notice that power once again had a factor in the overall cut speed.

But remember, power isn’t everything. The final cycle time of a part also depends on how fast we pierce material, how fast we accelerate and decelerate, and how fast the scan time is on the computer driving the laser.

There are, however, reasons why we need more power. Material thickness in heavy mild steel, aluminum and stainless steel all require more power. Or, with the fiber laser, we can add thicker copper or brass and a few other materials typically not cut with CO2 lasers.

So what makes a fiber laser different from a CO2 laser beside the obvious physical differences? For starters, fiber lasers have a shorter wavelength than CO2 by a factor of 10, which translates to more power per photon (E=hc/wavelength).

Fiber laser considerations
Comparing a fiber laser to a CO2 laser is one thing, but comparing fiber lasers to each other can be confusing. Many manufacturers use the same power source for the laser, but that’s where the similarities end.

Manufacturers use different size fiber cores, which influences the way the laser mode is delivered or more importantly, how it performs when you cut material. Honestly, we could write an entire article on just this subject alone. But for now, remember this is an important question to ask before purchasing.

Once the beam is through the cable, the cutting head design becomes the next major difference between the manufacturers’ equipment. How the manufacturer collimates or, for some, how they can program the change in beam diameter, which will determine how they can switch from thin material to thick material. How they protect the cutting lens from contamination and how you change a lens should also influence your decision.

Lastly, you should consider the user interface and how adjustments are made to cutting conditions or, more importantly, what tools are given to the operator to accommodate varying part geometry or material differences. Remember, keep things simple.

Service, warranty considerations
As you can see, many factors come into play when deciding which laser to buy or rather, which laser is best for your circumstance. The one factor we have not taken into consideration, however, is the service aspect.

It’s always great to get a good price or be the first person to own a particular brand, as long as you realize that every laser is just a machine and every machine will break eventually. Yes, every machine has a warranty, but eventually the warranty will expire and you will need to pay for the repairs.

Fiber lasers may be a lower maintenance option, but that is only one component of ownership. You need to consider how the manufacturer will support the machine. Ask questions, like: How many service technicians do they have? How many machines do those technicians support currently? Where are the technicians located in relationship to you? What is the typical response time for a service visit? What are the service rates, and do you pay for the travel? Does the manufacturer have a proven track record with lasers and cutting applications?

To answer those questions, each manufacturer will present you an Estimated Laser Hourly Operating Cost sheet. Unsurprisingly, the total hourly cost from one manufacturer to another can vary greatly.

For example: What one manufacturer considers as a consumable, another manufacturer may consider a repair cost, thus making a comparison sheet irrelevant – unless you understand the differences. Typically you would compare the consumables, optics, service labor and preventive maintenance costs. But don’t assume that what we do here in the United States translates to the same choices they make in another country.

Remember, with each laser power level comes an additional cost of burden to maintain the equipment – from the initial purchase difference to the difference in daily cost of consumables to the additional costs of optics from the power-induced stress. Ultimately, you need to decide what material you will process and what the additional costs are to have all the power that you could possibly use at your disposal.

Look at it this way, if you always drive 55 mph but you have a car capable of 200 mph, did you make a good purchase? If you are capable of using the extra power and speed, your production schedule will benefit. But in the end, sometimes keeping things simple and not over-designing a machine can come with great merit.

Laser Maintenance Group