Open Air vs. Underwater Plasma Cutting

At CMTS this past October, we showcased out 3000 Series CNC Plasma with a water table. Questions we received throughout the show were: Why do you have a water table? Why are you not cutting under the water? And why do some plasma systems have open-air tables with a fume extractor? The question of whether to use open air vs. underwater plasma cutting depends on a number of factors.  Are you concerned about smoke and noise in your factory when cutting?  Do you require True Hole Cutting or fine cuts?  What is your target price point? In addition, one would need to consider the different tradeoffs in cut depth and target quality. It is important to note that we are comparing open air vs. underwater plasma cutting, and not downdraft tables vs. water tables. It is possible to use open air cutting with a water table, so long as the water level is below the piece being cut.

Advantages of Open Air Plasma Cutting

Hypertherm’s True Hole Technology– Hypertherm’s True Hole system delivers bolt-ready holes without any secondary processing. However, this system requires above-water cutting, as do their fine-cut consumables. True Hole processes are fast becoming the industry standard and shops need the capability of producing perfect cuts without secondary processing. Cutting under water does not produce as clean a cut as open-air cutting. When water touches the cut face or if the cut face is submerged in water, the electrical process creates small amounts of hydrogen through electrolysis. This small amount of hydrogen immediately pops and nudges the arc, creating a rougher edge.

Ohmic contact cannot be used for underwater cutting– Ohmic contact or ohmic sensing is a method of initial height sensing (IHS) that is used to find the surface of the metal before each cut cycle. It is an electrical connection between the torch height control on a CNC cutting machine and the shield (front most part on a shielded plasma torch). All HyPerformance© plasma units, required for cutting mild steel thicker than ½”, require ohmic sensing. Ensure that your water level is at least 1/2″ below the piece or use a down draft table to guarantee the ohmic contact isn’t broken.

Ability to cut Aluminum– Aluminum is only safely cut with an open-air plasma table or if the water level on a water table has been significantly lowered. Cutting aluminum generates hydrogen gas, an extremely flammable and combustible gas. If this gas is not allowed to escape and safely evacuated, a buildup can cause an explosion.

 

Advantages of Underwater Plasma Cutting

Noise– One of the most obvious advantages of cutting under water is a reduction in noise. Open air plasma cutting can each noise levels of up to 120 decibels, requiring ear protection for the operator and anyone near the machine. By submerging the plate, you can effectively reduce the sound levels by 40 decibels, allowing continuous exposure without ear protection.

Arc-Glare– Open-air cutting produces an extremely bright electric arc. This arc can be so bright that looking directly at it can cause serious eye damage. Operators and spectators require dark shades or a welding helmet, in conjunction with an arc shield. However, submerging the torch head essentially eliminates the arc glare; one is only left with a glow under the water.

Smoke and Fumes– The main reason why a shop would choose to cut underwater is for the easy elimination of smoke and fumes caused by plasma cutting. Cutting below water can capture almost 100% of the smoke and dust. However, using a water table, regardless of cutting above or below the water level will capture a significant portion of the smoke and fumes. For example, when cutting with the water level about 1/2” below the bottom of the material to be cut, 98% of the smoke and dust are contained.

Heat Absorption– Obviously cutting below water reduces the amount of heat that is generated during cutting and absorbed by the material being cut. Reducing the heat helps prevent warping of the plate and allows safe handling of the part almost immediately after it’s been cut.

*Note: Water tables will accumulate slag and sludge and will require periodic cleaning and drainage. Depending on what you are cutting, the contents of your table may be classified as toxic and will require special drainage services. Depending on your location, environmental procedures will vary and you should consult your local environmental authorities when considering plasma water disposal.

In summary, understanding the limitations of either open-air or underwater plasma cutting will help you make informed choices about the best cutting process for your needs, saving time and potential frustration in the long run. For our demonstrations, cutting materials 1/2” above water provides the optimal combination of benefits. Noise and smoke are greatly reduced with the water being close enough to the cutting face however the ohmic contact is never interrupted and cut quality is maintained.