Cutting Speeds

At first glance the production time must be related to the cutting speed – and, in the past, the sales people have concentrated on a comparison of the highest speed at which the laser can cut any given material. But this figure isn’t that useful in a general engineering context – for the same reasons that the top speed of your car has very little impact on how fast you can drive from one side of town to the other.

So – fiber lasers are considerably faster than their CO2 counterparts when cutting thin section material in large simple shapes – like refrigerator doors for example. When assessing the purchase of a machine for this type of job it is important to remember that the overall job time includes the changeover time from sheet to sheet. If you are cutting a full sheet of steel into two refrigerator doors in 3 minutes, the speed of the sheet changeover mechanism might have a considerable effect on production costs.A 3 kW fiber laser can cut 1 mm thick stainless steel at about 30 m/min (20 ips) and a 5 kW CO2 machine will only achieve about one third of this speed. However – if you are cutting typical job shop components (like the ones in figure 1) the speed advantage of the fiber laser might only result in a 25% – 50% increase in productivity rather than the hoped for 300%. This is because the machines spend most of their time accelerating, decelerating and stopping to pierce the material. Videos are now available from TRUMPF and Bystronic which demonstrate this point very clearly and show that the speed differential gets progressively smaller as the complexity of the cut part increases. Other work by Bystronic also makes the point that machine acceleration rates are just as important as laser type if you need the fastest production times. This is particularly true when cutting thin section materials – where a high acceleration (but more expensive) cutting machine attached to a CO2 laser can beat a fiber laser attached to a lower acceleration machine.

As material thicknesses increase to 4 mm (0.16 in) the cutting speeds of both lasers start to converge and, because they are cutting slower, the maximum cutting speed is reached more often. At thicknesses above about 8 mm (0.32 in) the CO2 laser is the faster technology – and in this regime, a comparison of highest cutting speeds starts to be useful because the laser cutting process is rate determining rather than the acceleration characteristics of the machine.

One other point to be made here is the fact that piercing times have been much improved by the better manufacturers (like Bystronic and TRUMPF) over the past few years. So a new machine will out-perform an older machine in terms of acceleration and piercing times irrespective of the laser type it is attached to.

In summary – the fiber laser cuts metal faster at thicknesses below about 4 mm (0.16 in) but these speed differences are most advantageous when cutting large, simple shapes.

Running Costs

For an accurate comparison the running costs of our two machines should be compared per item rather than per hour. If machine ‘A’ costs 10 percent more to run than machine ‘B’ but produces 20 percent more products an hour, then it’s part production running costs are lower, not higher than B.

However, to work out the actual costs we need to start from running costs per hour. Running costs can be divided into several different categories, including;

The most important of these considerations is that of electricity costs. Although the exact figures vary from model to model we can assume that a 3 kW fiber laser cutting machine (including dust extraction etc) consumes approximately 20 kW whereas a 5 kW CO2 machine consumes about three times this much. In the UK electricity costs approximately £0.10 per kilowatt – so the CO2 laser will cost approximately £4.00 ($6) more per hour to run. If you are trying to reduce costs above all else then you might find that a fiber laser attached to a cheaper (lower acceleration) machine can produce parts more cheaply because the purchase and running costs are minimized. This point has been proved on typical parts by a number of trials carried out by Bystronic. Part production costs will be low even though your rate of production will also be low. This point is more probably appropriate to a manufacturer rather than a job shop. In a job shop situation there should (hopefully) be plenty of work waiting to go on the laser – so high productivity is very important and each cut product has a profit associated with it. A manufacturer might only need the laser to produce goods for a certain part of the week – so a cheaper to run (fiber), less expensive (lower acceleration) machine might be the optimum purchase.

On a day-to-day basis, fiber lasers cost less than CO2 lasers to maintain – but, although the maintenance costs of a CO2 laser over ten years (including large item failures) are well known, fiber lasers are not yet old enough for large item replacement costs to be known. A typical large item failure on a CO2 laser would be a turbo/blower, at a cost of about $20,000. The large items on a fiber laser could involve considerably higher costs but we don’t have enough data on long term usage yet.

Cut Quality

In the early days of fiber lasers the cut quality achievable above a thickness of about 6 mm (0.25 in) was demonstrably poorer than that of CO2 machines. However, the various R&D departments of the manufacturers have improved this situation and the cut quality differences between the two technologies are now less dramatic. Figure 2 shows a comparison of cut edge roughness between the two types of laser from work carried out at The Welding Institute (UK). Over the range of thicknesses shown here the laser cut edge retains its low roughness, but the fiber laser cut edge experiences a steady increase of edge roughness with material thickness – particularly towards the bottom of the cut edge.

For Oxygen-assisted cutting of mild steel the cut quality for fiber laser cutting at all thicknesses has been much improved over the past few years, and is nowadays comparable to CO2 laser cutting.From a general engineering point of view, below 4 mm (0.16 in) the cut edges are similar for both types of laser for fusion cutting – and I have seen equivalent quality CO2/fiber cuts up to 6 or 8 mm (0.25 or 0.32 in) – however, as the thickness exceeds these intermediate thicknesses, it is true that the cut edge quality of fiber laser cutting is inferior to the CO2 laser. Fiber laser salesmen will point out that it’s still pretty good – but some customers would be unhappy taking a reduced cut quality to the one they are used to. Typical cut edge quality for 10 mm (0.4 in) stainless steel for the two types of laser are shown in figures 3 (CO2) and 4 (fiber). There is a clear increase in roughness of the cut edge towards the bottom of the cut edge in the case of the fiber laser cut.

Range of Materials

Fiber lasers are better than CO2 machines at cutting copper and aluminium alloys but cannot cut most non-metals such as polymers (plastics) or wood based products.

Most job shops cut only a small amount of non-metals so this inability to cut plastics should have been only a minor concern. However – there is one area where plastics cutting is important to laser cutting. A great deal of the stainless steel which is cut by laser is supplied with a covering of protective plastic. The CO2 laser beam is readily absorbed by both the plastic and the steel beneath and so the two materials are cut in one pass. In the case of Fiber lasers the plastics used are usually transparent as far as the beam is concerned and, if this is the case, the cutting machine needs to carry out the cut in two operations;

This double process has two disadvantages – it wastes production time, and it leaves a residue of melted plastic on the top face of the cut component along the cut line (although this residue is fairly easy to remove). Recently the steel suppliers have made some new plastic coatings available which are absorptive of fiber laser light – and which can be cut in one pass at the same time as the steel – but these are not yet easily available.

Safety

Both CO2 and fiber laser machines are adequately enclosed to protect the operators and are classified as completely safe. The only difference between the two types of laser concerns the transparent panels which are used to view the cutting operation. In the case of CO2 lasers these panels are made of cheap, readily available polycarbonate, and it is standard practice for a job shop to cut their own replacement panels as the old ones get scratched and damaged. The panels used on fiber laser machines are much more high-tech and must not be replaced by polycarbonate – as polycarbonate and most other plastics are transparent to a fiber laser beam.

The Verdict

If you are a job shop boss with the usual wide spread of cutting requirements then you should buy CO2 machines until you have enough suitable work to fill the capacity of a fiber laser. This will usually mean that you will have approximately three CO2 machines for every fiber machine.

If you are the boss of a manufacturing firm making items from thin section metals or copper or aluminium alloys then your first choice should probably be a fiber laser.

But – in either case it’s a good idea to get the potential suppliers of the equipment carry out actual cutting trials on typical jobs (and don’t forget to include the sheet changeover times in your assessment)