When deciding between a downdraft table and a water table, companies take many factors into consideration
CNC thermal cutting machines require a work table to hold a metal plate while shapes are being cut. The two most common designs are the water table and the downdraft table. Modern Metals discusses the advantages and disadvantages of water tables with Steve Zlotnicki, product manager, cutting, ESAB Welding and Cutting Products, Florence, S.C.
MM: What factors should a company consider when deciding on a work table?
SZ: Most customers already have some opinion on water tables versus downdraft tables—whether it’s positive or negative. Their decision is going to come down to what they think about the cleaning and maintenance of each design. Some people don’t like the idea of having water in their shop at all, and others aren’t bothered by it.
However, the water table does get dirty, and it can create a sloppy environment. You’ll get water on the parts, carry those parts out of the table and drip on the floor. It may be a little harder to keep the surrounding area and parts clean. Often, the operator will have a hose next to the machine, and he’ll spray the parts off as he takes them off the table. Or you can try different methods to keep the water clean so you’re not accumulating silt in the water that gets deposited on the parts. Depending on the table design, to clean the table you’ll need to remove grid sections from the table and take them to another area where you can remove the slats, scrape out the slag and replace the slats.
With a downdraft table, depending on its design, you also may need to remove the slats and lift out the slag buckets to empty them. In either case, customers will have to consider the maintenance and the upkeep to clean the table out, how much space it will require in the shop, where they will install it and how much labor will be involved. It will depend on the volume of cutting. Some shops clean out the table once a year, others maybe once a month.
MM: What are the benefits of plasma cutting underwater?
SZ: Underwater CNC plasma cutting submerges the plate in 2 inches to 4 inches of water, so the torch tip and the entire arc are submerged. The biggest benefit is the working environment. It keeps noise and brightness down, and it keeps parts cool so you don’t have to worry about your operator getting burned if he tries to touch or pick up a part that’s just been cut.
You also don’t have as many issues with the plasma arc flash burning somebody’s eyes. You’ll still have to wear safety glasses in the area, but working in and around a plasma table that’s cutting underwater is a much more pleasant environment than working around a dry table. The noise from dry plasma cutting can be as high as 120 decibels. When you’re cutting underwater, it’s typically below the 85 decibel limit so the operator’s exposure over an eight-hour day to noise above 85 decibels is reduced dramatically.
MM: Are there any concerns about parts rusting?
SZ: With thermal cutting, you’re heating up the part, it’s in water and you’re in a warm environment. Those factors mean mild steel parts rust pretty quickly. There are additives you can put in the water that will coat the parts and prevent rust. Often, that coating will stick on the side of a part and actually prevent it from rusting longer than parts that are cut on a dry table. So if you take parts off your dry cutting table and you’re in a humid environment, your parts could actually wind up rusting faster than if they had been cut on a water table and gotten coated with the rust preventative that’s in the water.
MM: Can you cut all types of materials on a water table?
SZ: Plasma cutting is well suited for carbon steel up to 2 inches or so. That’s really where it’s starting to get impractical and oxy-fuel is going to be better. Whether that’s on a dry table or a water table, the only issue with cutting on a water table is eventually you get to a point where the thickness is too much to cut underwater because the process slows down so much that water will start to impinge on the arcing and begin to degrade cut quality. So usually we say inch and a half to two inches is the maximum for cutting underwater.
When we cut underwater, we add an air curtain to the torch. There’s a device that bolts on the outside of the torch. We inject compressed air into that device and it creates a bubble on the front of the device. So basically, it’s cutting in a bubble underwater, which protects the cut quality as long as it’s not going too slow.
There’s no issue with cutting stainless steel on a water table; in fact we recommend that. When you’re cutting stainless steel on a dry table, the molten slag that sprays out under the plate is going to stick and adhere to other parts and anything else that’s underneath there. If you cut a small part and it falls into the table, it’s going to get sprayed with slag, and it’s going to be hard to get that off. You’re going to have to grind it off, whereas if you’re cutting on a water table and a small part falls in the water, the water quenches the slag enough so that it doesn’t stick.
Now, when you’re cutting aluminum on a water table, you have a hydrogen explosion hazard. You have to have an aeration system of some sort that breaks up those bubbles of hydrogen that accumulate in the table. When you dry cut aluminum, you have the issue of aluminum dust and a dust collector. There is a lot of concern that aluminum dust may be explosive. Evidence seems to point to the fact that it’s not because it’s already been oxidized, but some people don’t want to mix aluminum dust and other dust. In some situations, a customer with a dry table might be forced to put a lot of money into a complex dust collection system. They have to have two separate collectors, one for cutting aluminum, one for cutting other metals, and the one that’s cutting aluminum has to have explosion vents and it has to be outdoors. In that respect, a water table is a lot simpler and a lot less expensive. MM
Steve Zlotnicki has been working with ESAB cutting systems since 1989. After three years as a field service engineer and four years as a factory service engineer, he moved into the marketing department as a product manager. He has spent the last 17 years working with customers and suppliers to provide solutions to some of industry’s toughest cutting applications.