As with most things in the manufacturing world, there is more than one way to tackle a particular job. This is especially true when it comes to cutting raw material. Two of the most popular methods that you’ll find in shops throughout the world are water jets and plasma cutters. While both machines will are known for their ability to quickly and easily cut through certain materials, there is quite a bit that separates them in both their capabilities and limitations.

How Water Jet Cutting Works

The water jet is a cutting system that relies on a mixture of water and abrasive to quickly break down material along a given path. Think of it as high-speed erosion. The abrasive (usually garnet) is fed through a line into a mixing tube where it meets the water source and is sprayed out with immense pressure (more than 50,000psi) to form a fine cutting stream at the tip of a ceramic nozzle. The nozzle then moves along a path determined by the associated CNC software, at speeds that vary depending upon material type and thickness. 

How Plasma Cutting Works

Instead of water and abrasive, a plasma cutter uses gas and electricity. The gas is forced through a constricted opening at high pressure, at which point it contacts an electric arc that is sent through a nozzle. This process causes a reaction, and the gas (typically nitrogen, argon, oxygen, etc.) is heated to such a degree that it enters the fourth state of matter, plasma. The plasma is so hot that it can melt through metal at a high enough speed that the excess material is blown away resulting in a relatively clean cut. Different gases can be used depending on material type and thickness.

Differences Between Plasma and Water Jet Cutting

Quality

The plasma cutter can produce smooth cuts in many steels, with an accuracy of +/- .01” – .030” depending on the setup. As material thickness increases, the cut will be less clean as the plasma struggles to melt through all of the material without producing unwanted slag. 

The water jet is generally considered the more accurate machine and will produce higher quality cuts across a wider range of materials and thicknesses. The water jet can hold tolerances of +/- .005”, with additional accessories available than can further increase accuracy by minimizing taper. The speed can be adjusted to improve surface finish as well, often eliminating any need for secondary machining.

Upkeep/Maintenance

The water jet requires constant maintenance and uses a fair amount of consumables. Because water jet parts are under constant pressure, their failure rate is high. Replacement parts and rebuild kits are a necessity and do not come cheap. The water jet tank must also be pumped out regularly as it fills with abrasive over time. While some shops equip their machines with costly waste management systems, others rely on vactor trucks or simply shovel out the excess garnet themselves. 

The plasma demands far less upkeep. Aside from the gas that is required for operation, there are simply fewer consumables and components that could break down. Preventative maintenance becomes the key to uninterrupted plasma production. This essentially entails proper cleaning and alignment of all essential items, which can be checked/performed on a routine basis.

Materials

The plasma is limited in what materials it can cut. For best results, it should be used primarily for steel, though certain machines are able to cut stainless and aluminum with the proper setup. Conductive materials do not tend to cut well on the plasma. Thickness is also a factor in plasma’s cutting capabilities. The quality will begin to wain on anything above 1/2” in thickness. 

The water jet is known for cutting nearly any material type. From hardened steels to softer materials like wood, plastic and even food items like bread or cake, the water jet will have great success across the board. Because the water jet does not introduce heat into the material being cut, it is not limited by thickness and can cut up to 10” thick steel (though this would take a considerable amount of time to complete.)

Cost

Both the water jet and plasma are affordable and effective methods for cutting material. On thinner steels, the plasma will be much faster and thus would cost less than the other alternatives. The water jet becomes more cost-effective on thicker plates, and materials that cannot be cut well on the plasma. The water jet can also be more cost-effective in instances where secondary machining operations would otherwise be required