How Does Waterjet Cutting Work?

Waterjet cutting machines have gained significant popularity for two-dimensional cutting of flat sheets, especially in applications involving thick sheets, such as in steel service centers, among equipment manufacturers, and fabricators. Some of the key advantages of waterjet cutting include the straightness of the cut and the absence of a heat-affected zone. To understand how these advantages are achieved, let's delve into the workings of waterjet cutting.

Waterjet: A Non-Thermal Process

Unlike thermal cutting processes like plasma, oxyfuel, or laser cutting, waterjet cutting is a mechanical sawing process. It can be likened to a band saw, where a thin strip of metal with sharp teeth slices through the material. Similarly, waterjet cutting employs a thin stream of water containing sharp rock fragments to penetrate the material.

The rock or abrasive used in waterjet cutting is typically crushed garnet. While other materials may be utilized in specific applications, garnet is the most commonly used abrasive for cutting sheet metal. Garnet, which is also a common component in sandpaper, is a relatively hard rock. Each abrasive grain that passes through the material gradually wears away a tiny portion of it.

The Role of Speed

Simply combining sand and water and applying it to a sheet of metal won't result in a cut. The crucial factor that enables waterjet cutting is the speed at which the abrasive impacts the plate. According to the principle of momentum, which is calculated as the product of an object's mass and its speed, the mass of an abrasive particle is extremely small. Therefore, to have a significant effect on a steel plate, the abrasive must achieve a high velocity.

To accelerate the abrasive to a speed sufficient for cutting hard materials, water is pressurized to "ultra-high pressure" and then released through a very small orifice. This concept can be illustrated by a garden hose: as the nozzle opening becomes smaller, the volume of water decreases, but the water exits at a higher speed. The same principle applies within a waterjet cutting head.

Typical waterjet cutting pressures range from 50,000 to 60,000 PSI. Many modern systems can reach pressures as high as 90,000 PSI. Water at such high pressures is released through small holes made in diamond or sapphire components, with diameters typically ranging from 5 to 15 thousandths of an inch. The resulting water flow is supersonic, meaning it travels faster than the speed of sound. Fortunately, the flow rate is relatively low, approximately 1/2 gallon per minute.

The Cutting Process

The formation of the water stream through the jewel hole and the mixing of the abrasive into the water stream both occur within the waterjet cutting head. To incorporate the abrasive into the water, the water passes through a funnel-shaped chamber. Here, the abrasive is carried and accelerated by the water flow. The water stream, now laden with abrasive particles, then passes through a focus tube or waterjet nozzle. This ensures that all the abrasive particles are fully integrated into the stream and moving in the same direction.

Once inside the focus tube, a supersonic stream of water and abrasive is created. This powerful stream can rapidly abrade a wide variety of materials, including aluminum, steel, stainless steel, armor plate, granite, tile, laminate, and even glass (although tempered glass is too brittle and not suitable for this process).

The Final Results

Since each abrasive particle removes only a minute amount of material through erosion, very little heat is generated during the cutting process. Moreover, any heat that is produced is quickly dissipated by the water flow before it can affect the surrounding material. Thanks to this, the fast-moving abrasive particles can cut materials without creating a heat-affected zone. Additionally, the momentum of the water stream keeps the abrasive particles moving in a straight line long after they exit the focusing tube. This results in extremely straight and precise cutting edges on numerous different materials.