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How does waterjet cutting work?

Metal cutting with no heat affect zone

CNC waterjet cutting machines have become very popular for 2D cutting of flat plate, even in heavy plate applications such as steel service centers, equipment manufacturers, and fabricators. Some of the great things about waterjet cutting are the straightness of the cut and the lack of a heat affect zone. To understand why it provides these advantages, let’s take a quick look at how waterjet cutting works.

Waterjet is a non-thermal process

Unlike a thermal cutting process, like plasma, oxy-fuel, or laser, the waterjet process is a mechanical sawing process. You can almost think of it like a band-saw, which is a thin strip of metal with sharp teeth which are pulled through the material that you are cutting. Similarly, the waterjet cutting process uses a thin stream of water with sharp pieces of rock which are pushed through the material.

The pieces of rock, or abrasive, are usually made from crushed garnet. In some applications other materials are used, but garnet is by far the most common for metal plate cutting. Garnet is also commonly used on sandpaper, and is a relatively hard rock. Each grain of abrasive that passes through will wear away a little bit of the material.

Speed makes it work

But just pouring sandy water onto a metal plate will not cut it. What makes it work is the speed at which the abrasive is hitting the plate. The momentum of an object is equal to it’s mass multiplied by its velocity. A grain of abrasive has very little mass, so to have any effect on a steel plate, it must have a lot of velocity. In order to accelerate the abrasive fast enough to cut through hard materials, the water is pressurized to “ultra high pressure”, and then released through a very small orifice. When you play with a garden hose, you know that as you make the nozzle opening smaller, you get less water volume but it comes out at a faster speed. That's what happens in the waterjet cutting head.

Typical waterjet cutting pressure is between 50,000 and 60,000 PSI. Many newer systems are pressurized up to 90,000 PSI. The water at this pressure is then released through a small orifice machined into a diamond or saphire, usually in the range of 5 to 15 thousandths of an inch diameter. The resulting stream of water is supersonic, which means it is going faster than the speed of sound. Fortunately, it is very low volume, only about 1/2 gallon per minute.

Waterjet cutting head - Click for larger viewAdding abrasive

Forming of the water stream by the jewel orifice and mixing of the abrasive into the stream all happen inside a waterjet cutting head, like the KMT Autoline™ II. To get the abrasive into the water, the stream passes through a chamber shaped like a funnel, where the abrasive is picked up by the water stream and accelerated. The water stream, now carrying abrasive particles, then goes through a focusing tube, or nozzle, which gets all of the abrasive fully involved in the water stream and moving in the same direction.

Once it exists the focusing tube, you have a supersonic stream of water and abrasive that will quickly wear its way through virtually any material, including aluminum, steel, stainless steel, armor plate, granite, tile, laminates, and even glass (though tempered glass is too brittle).

Results

Because each abrasive particle only removes a tiny amount of material by erosion, very little heat is generated. Also any heat that is produced is quickly quenched by the water stream before it has time to heat the surrounding material. Not only do these fast moving abrasive particles cut without any heat affect zone, but the momentum of the water stream keeps them moving in a straight line long after they exit the focusing tube, resulting in an extremely straight, accurate cut edge on many different materials. 

 

Posted in Cutting Systems , Tagged with Waterjet

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