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Waterjet Cutting

In the battle to reduce costs, engineering and manufacturing departments are constantly on the lookout for an edge. The Waterjet process provides many unique capabilities and advantages that can prove very effective in the cost battle. Learning more about the Waterjet technology will give you an opportunity to put these cost-cutting capabilities to work.

Beyond cost cutting, the waterjet process is recognized as the most versatile and fastest growing process in the world (per Frost & Sullivan and the Market Intelligence Research Corporation). Waterjets are used in high production applications across the globe. They compliment other technologies such as milling, kser, EDM, plasma and routers. No noxious gases or liquids are used in waterjet cutting, and waterjets do not create hazardous materials or vapors. No heat effected zones or mechanical stresses are left on a waterjet cut surface. It is truly a versatile, productive, cold cutting process.

The waterjet has shown that it can do things that other technologies simply cannot. From cutting whisper thin details in stone, glass and metals; to rapid hole drilling of titanium; to cutting of food, to the killing of pathogens in beverages and dips, the waterjet has proven itself unique.

Dr. Norman Franz is regarded as the father of the waterjet. He was the first person who studied the use of ultrahigh-pressure (UHP) water as a cutting tool. The term UHP is defined as more than 30,000 pounds per square inch (psi). Dr. Franz, a forestry engineer, wanted to find new ways to slice thick trees into lumber. In the 1950's, Franz first dropped heavy weights onto columns of water, forcing that water through a tiny orifice. He obtained short bursts of very high pressures (often many times higher than are currently in use), and was able to cut wood and other materkls. His kter studies involved more continuous streams of water, but he found it difficult to obtain high pressures continually. Also, component life was measured in minutes, not weeks or months as it is today.

Dr. Franz never made a production lumber cutter. Ironically, today wood cutting is a very minor application for UHP technology. But Franz proved that a focused beam of water at very high velocity had enormous cutting power—a power that could be utilized in applications beyond Dr. Franz's wildest dreams

INTRODUCTION

The basic technology is both simple and extremely complex. At its most basic, water flows from a pump, through plumbing and out a cutting head. It is simple to explain, operate and maintain. The process, however, incorporates extremely complex materials technology and design. To generate and control water at pressures of 60,000 psi requires science and technology not taught in universities. At these pressures a slight leak can cause permanent erosion damage to components if not properly designed. Thankfully, the Waterjet manufacturers take care of the complex materials technology and cutting-edge engineering. The user need only be knowledgeable in the basic waterjet operation.

Essentially, there are two types of waterjets; (1) pure Waterjet and (2) abrasive Waterjet. Machines are designed to employ only Waterjet, only abrasive Waterjet, or both. With any type, the water must first be pressurized

The pump is the heart of the waterjet system. The pump pressurizes the water and delivers it continuously so that a cutting head can then turn that pressurized water into a supersonic waterjet stream. Two types of pump can be used for waterjet applications — an intensifier based pump and a direct drive based pump

Waterjet Cutting Process

Pure waterjet is the original water cutting method. Waterjet cutting uses only a pressurized stream of water to cut through material. This type of cutting is limited to material with naturally occurring small cracks or softer materials like disposable diapers, tissue paper, and automotive interiors. In the cases of tissue paper and disposable diapers the waterjet process creates less moisture on the material than touching or breathing on it. The figure shows the waterjet cutting process.

In this process water is increased in pressure by high-pressure pump to about 40000-60000 PSI and is forced through the orifice on to the target material. This high-pressure water on striking the surface performs the machining operation. The potential energy contained in the water is converted in the process to kinetic energy, i.e., into jet velocity, thus achieving its "cutting" effect.

The basic waterjet process involves water flowing from a pump, through plumbing, and out a cutting head.

In waterjet cutting, the material removal process can be described as a supersonic erosion process. It is not pressure, but stream velocity that tears away microscopic pieces or grains of material. Pressure and velocity are two distinct forms of energy. The pump’s water pressure is converted to the other form of energy, water velocity by a tiny jewel. A jewel is affixed to the end of the plumbing tubing. The jewel has a tiny hole in it. The pressurized water passes through this tiny opening changing the pressure to velocity. At approximately 40,000 psi the resulting stream that passes out of the orifice is traveling at Mach 2. And at 60,000 psi the speed is over Mach3

The advanced technology in the pump is found in the intensifier. As mentioned briefly in the description of the water circuit, the intensifier pressurizes the filtered tap water to up to 60,000 psi. Intensifier pumps utilize the "intensification principle."

Hydraulic oil is pressurized to a pressure of, say, 3,000 psi. The oil pushes against a piston biscuit. A plunger with a face area of 20 times less than the biscuit pushes against the water. Therefore, the 3,000-psi oil pressure is "intensified" twenty times, yielding 60,000-psi water pressure. The "intensification principle" varies the area component of the pressure equation to intensify, or increase, the pressure.

Pressure =Force /Area

If Force = 20, Area = 20, then Pressure = 1. If we hold the Force constant and greatly reduce the Area, the Pressure will go UP. For example, reduce the Area from 20 down to 1, the Pressure now goes up from 1 to 20. In the sketch below, the small arrows denote the 3,000 psi of oil pressure pushing against a biscuit face that has 20 times more area than the face of the plunger. The intensification ratio, therefore, is 20:1

 

Waterjet Cutting

 

High Pressure Plumbing

Once the high-pressure pump has created the water pressure, high-pressure plumbing delivers the water to the cutting head. In addition to transporting the high-pressure water, the plumbing also provides freedom of movement to the cutting head. The most common type of high-pressure plumbing is special stainless steel tubing. The tubing comes in different sizes for different purposes.

174 inch steel tubing - because of its' flexibility, this tubing is typically used to plumb the motion equipment. It is not used to bring high-pressure water over long distances (for example, from pump to base of motion equipment). Long lengths of 10 to 20 feet are used to provide X, Y and Z movement (called a high-pressure whip). It is easily bent. This tubing can be bent into a coil (coils provide greater flexibility over short distances).

3/8 inch steel tubing - typical this tubing is used to deliver water from the pump to the base of the motion equipment. Can be bent. Not normally used to plumb the motion equipment.

9/16" steel tubing — this tubing is typically used to transport high-pressure water over long distances. The large internal diameter reduces pressure loss. When very krge pumps are present, this tubing is especially beneficial (the larger the volume of high-pressure water needed to be transported, the larger the potential pressure loss). This tubing is not bent. Fittings are used to created corners.