ESAB Knowledge center.
How Does A Plasma Water Table Work?
July 7, 2013
Why cut with plasma on a water table?
When CNC plasma cutting, there are a lot of good reasons for using a water table instead of a dry, or down draft table. A water table is less expensive to purchase, does not require a dust collector (in most areas), reduces noise, eliminates dangerous arc flash, reduces heat distortion and keeps parts cool, just to name a few. And even though it is an older technology, a significant number of large gantry CNC plasma and oxy-fuel cutting machines are still sold with water tables.
Water table basics
Some water tables are simply a tank, filled with water, and burning bars on top. But that won’t get you the full advantages of under water plasma cutting. So this discussion is about the type of water table that has an adjustable water level, allowing you to submerge the plate and the plasma torch under water.
Construction of the table is quite simple; a large chamber is built into the table, below the burning bars and a steel tray. The chamber only has openings inside the water table along its bottom edge, so air can be trapped inside, similar to submerging an upside down bowl. Then by putting air into or letting air out of the chamber, the water in the table is forced up or down.
At minimum, the water level controls have an air supply valve and an air release valve, which control the compressed air going into and coming out of the chamber. These valves can be simple ball valves operated by hand, or can be solenoid valves controlled by the CNC.
To raise the water in the table, the release valve is closed, and then the supply valve is opened. Compressed air pushes into the chamber, forcing the water out through openings along the bottom of the chamber, which results in the water level rising. Once the water has reached the desired level, the air supply valve is closed, and the air release valve remains closed, holding the air in the chamber so the water level stays up.To lower the water level, the release valve is opened while the supply valve remains closed, allowing the air to escape from the chamber. The weight of the water causes it to flow back into the chamber, pushing the air out through the exhaust.
Under water plasma cutting
The primary reason for using this type of table is for under water plasma cutting. Depending on the torch design and vintage, you may be able to submerge the torch and cut successfully without any additional hardware. However, most new Oxygen plasma cutting systems with precision plasma capability will require an air-curtain that clamps onto the torch. The air-curtain uses compressed air to create a bubble around the front end of the torch, so that the precision arc is protected from the water. Some plasma torches are not designed for use under water, but may still be used to cut a plate that is setting above water.
For larger tables, even if you don’t ever plan to cut under water, a water table with water level control is recommended. This allows the water level to be lowered for easier access to clean out small parts that drop between the burning bars.
Oxy-fuel cutting on a water table
The standard water table with water level control is designed to be used for both plasma and oxy-fuel cutting. When oxy-fuel cutting, the water cannot be above the plate surface, but it can be very close to the bottom of the plate. Also, there must be at lease an inch of water depth above the tray under the burning bars, to protect it from damage by the molten metal spray of the oxy-fuel cutting process.
Depending on material type, thickness, and quality requirements, some users prefer to cut with the water closer to the plate, so that the splashing water helps cool the part while cutting, reducing overall heat distortion. Other users want to avoid quenching of the slag that occurs, and therefore prefer to have the water level as far below the plate as possible.
So whether you are plasma or oxy-fuel cutting, a water table is a simple and affordable alternative to a down draft table.