When configuring a plasma cutting system, people often struggle with the decision of choosing between a water cutting table and an air down-draft cutting table. While both styles of table are effective, each has its advantages and disadvantages. Before making your decision, you must take into consideration and evaluate the types of materials you will be cutting, size of parts, the specifics of your application, and the short- and long-term costs associated with each type of table.
PRICE: The first thing that most people look at when choosing their cutting table is the significant difference in the up-front cost between a water table and an air down-draft table. Water tables are simply much less expensive. If price is the only consideration, then the decision is an easy one, choose the water table. The more important and often overlooked factor when making this decision is the long-term cost of each system. Long-term costs include maintenance, waste disposal, operational efficiencies, and other factors that will be outlined later in this article.
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There are also cost issues to consider within these two categories of cutting tables. Water tables are available in fixed water level and adjustable water level configurations. Evaluation of your process will determine whether you need a fixed water level table or the ability to adjust the water level of the table. Likewise, air down-draft tables are available as a regular down-draft that draws air from the entire area of the table or as zoned down-draft tables that have dampers so that you are only drawing air from the area in which you are cutting. A zoned down-draft table is more expensive, but allows you to use a smaller, less expensive dust collector because the air volume requirements can be significantly less.
MATERIALS: The types of materials you will be cutting are important in this decision making process. If you are cutting primarily steel/stainless steel, either type of table will work well for you. However, if you are cutting aluminum, water tables can present a major safety hazard. When plasma cutting aluminum over a water table, the dross falling into the water produced by the cutting process will create a chemical reaction with the water. This chemical reaction will extract Hydrogen out of the water that will accumulate in pockets under the plates or in the bladder of adjustable level water tables, creating an explosion hazard.
The pockets of Hydrogen that accumulate under the plate will usually just produce "popping" sounds as they are ignited and are usually not large enough to cause a significant explosion. Some users have added water circulation systems to prevent/minimize this condition. The greater hazard is the accumulation of Hydrogen in the bladder of an adjustable water level cutting table. Adjustable level water tables basically have an inverted open-top box in them that is filled with air to raise the water level and emptied of air to lower the water level. If Hydrogen accumulates in the bladder and is ignited, it will produce a major explosion. These explosions have blown the gantries off of machines and severely injured and/or killed people in the vicinity of the machine. Manufacturers do not approve of and highly discourages any cutting of aluminum over a water table.
MAINTENANCE: The long-term cost of maintaining water tables versus air down-draft tables is a significant and often overlooked topic in the decision making process. All cutting tables accumulate scrap and slag and need to be cleaned out on a regular basis. Most users will have to do this at least once or twice per year, and heavy users may have to do this four or more times per year. In addition, water tables have a tendency to produce unpleasant odors over time as bacteria grows. With the ever increasing regulations and restrictions of waste water disposal, this cost is increasing every year. When you consider that a typical 8'x12' cutting table holds approximately 1,500 gallons of water, or a typical 10'x20' cutting table holds approximately 3,000 gallons of water, the water disposal costs can add up quickly.
An air down-draft system will require periodic emptying of particulate from the dust collection system, removal of slag/dross from the cutting table (slag pans can be added to make this process even easier), and occasional replacement of filters. There are no special waste disposal costs associated with this regular maintenance, and machine down-time is also typically less.
OPERATIONAL EFFICIENCIES: There are a number of factors to consider from an operational side when comparing water tables and air down-draft tables. These factors include the size and type of parts you are cutting, plasma performance, and cut quality.
The size and types of parts you are cutting may be a determining factor in this decision making process. Companies that cut many small parts that would have a tendency to fall through the slats and into the table should strongly consider an air down-draft system. This will allow your operators to quickly retrieve parts from inside the table, reducing the need to "fish around" for parts in murky water, and minimize "lost" parts. If you are cutting very long parts where straightness is an issue, a water table may be better suited for your application. By partially submersing or "floating" the plate you are cutting, the cooling effect of the water will help stabilize the temperature of the plate and minimize camber and/or warping.
For many years, the major plasma manufacturers have discouraged cutting over water. They claimed that cutting over water reduces their plasma capacity by as much as 20%, and reduced cut quality. In our customers' experience, setting the water level just below the level of the plate or partially submerging the plate does not have a noticeable impact on performance, cut quality, or consumable life. Cutting with the plate under water can cause problems with consumable life as the cutting torches are not designed to be submerged and coolant coming into contact with the electrodes can cause them to fail prematurely and possibly clog the air/gas nozzles creating problems with cut quality.
Cut quality between parts produced over either type of table has been fairly comparable. The biggest difference is that more dross adheres to the back of parts cut over a water table because the dross cools faster and sticks to the part as opposed to being blown down into the table. The "dross-free" rating by the plasma manufacturers is reduced when cutting over water. This dross/slag is not difficult to remove, but does add time/labor to the processing of parts.
SUMMARY: In order to select the best table for your application, it is important to take all of these factors into consideration. Spending the time to evaluate your processes and long-term costs now, will assure that you select the best, most cost effective solution to your particular application for now as well as for the future. Your plasma cutting system manufacturer can help you evaluate all of these factors and assist you in the decision making process.
Which Cutting Table is Right for Your CNC Plasma Cutting Application?
Water Table vs. Down-Draft Table
-by Oliver Osterhues
Water Cutting Table Vs Air Down-Draft Cutting Table for CNC Plasma Applications ONLINE APPLICATION