CNC Plasma vs Laser & Waterjet
Most of us who first contemplate getting into CNC plasma wonder about the relative differences of CNC plasma vs laser & waterjet. However, if you intend to cut metal, and cost is a major factor, plasma is the only way to go. We will not go into a technical, or scientific explanation of these processes here, but rather what they require in the way of equipment, and their relative capabilities.
Above: Plasma vs laser cut Vincent motorcycle
CNC Laser
The difference between laser and plasma cutting is similar to the difference between drawing an object with a magic marker and a ball point pen. The kerf, or gap that is produced by a laser is perhaps 20 times narrower than with plasma arc. As a result, substantially more detail can be included in the same sized piece. By the same token, the same detail can be included in a lot smaller piece. This is irrelevant if you do not require great intricacy or very small pieces.
Another difference is that a laser can cut a wide variety of materials, whereas plasma can only cut electrically conductive materials such as steel, aluminum, brass, etc. It should be noted that the faster a material absorbs heat, the worse job plasma will do on it. Laser doesn’t have this limitation. Plasma also produces a bevel in the cut face which is most noticeable in thicknesses above approximately 3/16″. The cut smoothness in steel is pretty much the same for both processes.
Now for the disadvantages of laser: Since the kerf width of a laser cut piece is much narrower than with plasma, any lack of rigidity or slop in the machine will be amplified in the finished cut. We mounted a 65-watt laser on a machine that had satisfactorily plasma cut steel at over 100 inches per minute. Of course, with 65 watts we were cutting posterboard, but we had to slow down to 5 inches per minute for a clean cut. Thus, a CNC laser machine’s rigidity requirements are greater than for plasma.
Any laser capable of cutting steel has significant cooling requirements. The 65 watt laser we used was air cooled, but a 1,500 to 3,000 watt unit meant for steel must be liquid cooled. This requires complicated plumbing that would be difficult on a home or small shop sized machine. Last, the 65 watt air cooled laser we used cost upwards of $20,000 over 10 years ago. Imagine what a 2,000 watt laser with all the installation paraphernalia would cost today. Then there’s the table, itself.
Waterjet
Most of what has been said about laser above is true of waterjet as well. It produces a narrower kerf than plasma, although not as narrow as laser. It can also cut a wider variety of materials than plasma. Unlike plasma, it doesn’t produce a bevel in the cut face. The cut smoothness is arguably superior to both plasma and laser.
The downside: While plasma arc is a messy process, waterjet is even more so. A waterjet shoots a narrow, high-pressure jet of water through the material. An abrasive called garnet is contained in the water jet and is used at a rate anywhere from a few ounces to a couple of pounds a minute. This accumulates in the tank below the cutting surface at a profound rate. The debris generated by cutting, while less airborne than plasma, is nonetheless problematic from a cleanup standpoint.
Like the CNC laser machine, the apparatus that is mounted on a waterjet unit is substantial. There must be plumbing to supply the water stream, as well as a high- pressure pump and a feeding device to mix the abrasive with the jet stream. Like laser, a waterjet installation is more costly and complicated than a simple CNC plasma system.
At right are photos of a waterjet set-up installed on a small CNC table. Note that the jet delivery nozzle is the most straight-forward part of the system. The high-pressure pump and abrasive mixing apparatus is far more complicated.
Above: 65-watt Synrad air cooled laser
Above: Eagle/flag shape cut with plasma & laser
