Cold Water Turns To Cold Cash

Looking for a way to cut a piece of stainless steel a foot thick? Try a stream of cold water -- or water with some other natural ingredients.

The use of a stream of water to cut various materials has been around since at least the 1970's. In its early days, the process referred to as 'hydrodynamic machining' used a very thin, high pressure stream of water to cut through soft materials such as rubber or wood and paper products; materials that were either porous, fibrous, or granular. Oddly enough, even granular rock such as sandstone was cut very nicely with pure water.

During the early 1980's, someone came upon the idea of putting abrasives in the water. But it was a challenge to put that idea into practice. It was hard enough to pump water at a pressure high enough to cut materials. Adding abrasives just made it more difficult.

After various attempts, methods were finally developed to effectively entrain abrasives in the water stream. Some systems do it by pumping high-pressure water into an abrasive slurry, forcefully displacing the slurry out through a nozzle.

Another common approach is to form a long, coherent, high-speed stream of water by pumping it through a round orifice in a jewel such as a sapphire, diamond, or ruby. Jewels are used because of their extreme hardness, durability, and their ability to hold a sharp edge. As the stream goes through the orifice, it enters the beginning of a long mixing tube where the abrasives are added. When the water and abrasives exit from the other end of the tube, they are entrained -- mixed and moving together at the same high speed.

This high-pressure stream cuts a variety of conductive and non-conductive materials of surprising hardness and thickness with a fine kerf. The process has become known as abrasive waterjet machining, or simply, waterjet cutting.

In general, the higher the speed of the water stream, the thicker the material that can be cut. Water-stream speed is dependent on the pump pressure of the waterjet, which, in turn, is dependent on horsepower of the pump. One company that is cutting glass up to two feet thick is using a 1000 horsepower pump.

The most commonly used abrasive is garnet. It's hard enough to cut virtually any metal. Yet, it is soft enough to allow a reasonable life for the mixing tube. If a harder abrasive is used, such as aluminum oxide, harder materials can be cut. But the mixing tube can have a very short life.

Conversely, if the material to be cut is relatively soft, like aluminum, then a softer abrasive such as silica may be used. Silica is hard enough to cut aluminum well, but inadequate for cutting hardened steel. For some applications, water without abrasives can be used.

Process Advantages

Waterjet cutting has advantages over saw cutting, plasma cutting, and electrical discharge machining (EDM). For example, when the cold-cutting abrasive waterjet is used to cut heat-sensitive metals such as titanium and Inconel, heat-affected zones are eliminated and metallurgical changes are avoided.

Abrasive waterjet cutting also achieves a closer net cut than saw blades and plasma cutting. As a result, secondary finishing is often eliminated. Users save time and money by avoiding secondary operations. They also gain the freedom to use their milling machines for other operations.

The process typically cuts within a tolerance of 0.002 inch. Electrical discharge machining can cut materials accurately to within 0.0001 inch. However, EDM can only be used to cut conductive materials. Abrasive waterjet cutting can cut virtually any type of material, including non-conductive materials such as glass, plastic, rubber, and composites.

Improved materials utilization is another benefit of abrasive waterjet cutting. Abrasive waterjets are ideal for tight nest cutting, a significant advantage for mass production runs. Integrated with a CNC controller, an abrasive waterjet can tightly nest multiple parts on a single sheet, including parts sharing a common side. Also, omni-directional cutting capabilities inherent with abrasive waterjet cutting enable operators to cut intricate shapes and inside corners not possible with saw cutting.

Technical Advancements Improve Process

Waterjet cutting equipment makers in the United States include OMAX Corporation of Auburn, Washington; Flow International Corporation of Kent, Washington; Jet Edge of Minneapolis, Minnesota; Ingersoll Rand in Farmington Hills, Minnesota; and Aqua-Dyne in Houston, Texas.

Constant research and innovation by these companies have led to many improvements in the technology over the years.

For example, increases in the durability of the mixing tube have made abrasive waterjet cutting more cost effective than it was in its early stages. Mixing tube life, with abrasives, was initially about four to six hours. That life is now up over 100 hours, and continues to climb.

In 1989, Flow developed a mixing tube known as the Paser II cutting head that increased life up to about 100 hours. Flow did this by developing a new composite carbide material, in conjunction with Dow Chemical, specifically for abrasive waterjet cutting heads.

Flow recently released a new cutting head, called the Paser 3, that is expected to provide 150 hours of life. The new head only has five components, simplifying maintenance and lowering operating costs.

Another innovation has made some smaller cutting units more accurate and versatile than in the past. OMAX developed a controller for their comparatively small machine (40,000 psi) that models the behavior of the jet of water. This allows much greater precision because the unit properly slows down for radii and fillets. FLOW uses the same controller on their similarly sized machine known as the Badger.

In the past, when approaching a radius, the cutting speed of smaller units had to be slowed. This was because the jet of water arcs backwards when cutting fast. Thus, the exit point would not be directly below the entrance point. If cutting something thick, it might lag behind by as much as half an inch.

If the cutting stream rounded a corner with this half-inch lag-behind, bad cuts could occur. Slower cutting speeds minimized the error and made it tolerable. The appropriate slow-down was manually determined by trial and error, and controlled by an experienced operator who would dial the speed down when he came to a corner. By using the new built-in cutting process model, the first cut can be a perfect cut.

The new controller is not needed on larger waterjet cutting machines. They usually do not require the same slow-down at corners.

Aqua-Dyne recently introduced a waterjet system that cuts well at a relatively low pressure using inexpensive copper slag as the abrasive. The company credits its ShapeJet (mixing tube for the ability to cut materials such as metal, glass, concrete, or stone at only 20,000 psi.

Emergency Gaskets -- No Problem

Lamon's Gasket in Houston, Texas uses an Ingersoll Rand HS-1000 waterjet cutting machine. Engineering and Operating Manager, Baron Allami, and Plant Manager, Bill Alsop, are enthusiastic about the machine.

One of the services the job shop provides is emergency cutting work. "Sometimes a customer will call me and say 'my plant is down and I need a new gasket. I'm sending a driver and faxing you the specifications. Can you have it ready while he waits'?" said Allami. The waterjet, he says, enables them to cut a piece in five minutes that would take an hour or more using conventional die-cutters. This efficiency has given them an edge on such time-urgent work. "Customers are amazed to find how quickly we can get work done for them, and are impressed with the quality," said Allami.

Bill Alsop said that although Lamon's didn't do any cutting for industries that required an absence of heat-affected zones -- one of the most popular reasons for using waterjet technology -- the shop needed the waterjet for its other properties.

A big problem with laser-cutting certain materials such as brass and copper is reflectivity. An overly reflective surface will cause the laser beam to reflect back up into the head optics. Lamon's learned this lesson the hard way when one of their lasers blew out its set of optics on a cutting job. As Alsop said, "When that happens, it's an expensive mess."

Their main business is cutting gaskets for the petrochemical processing industry. The waterjet is primarily used to cut Garlock, copper, rubber, steel, and brass gaskets and seals. It is important to have a uniform product that will not vary in quality. Maintaining a constant cold temperature has enabled Lamon's to do that more efficiently and reliably. The waterjet has proven itself so useful that the shop already runs a full shift on it.

Much of Lamons' cutting of non-asbestos soft materials can be done with nothing more than a stream of water at 40,000 psi. As a result, production costs are reduced compared to other cutting methods. Garnet abrasives are only required when Lamon's cuts harder materials.

Multi-Axis Waterjet Cutting

Laser Applications Incorporated (LAI) of Westminster, Maryland is a job shop that has specialized in waterjet and laser processing for the last seventeen years.

The firm was approached by a medical company that was frustrated. The medical company had a proven design for a multi-million dollar diagnostic device, but could not manufacture it economically. The initial prototype was processed by hand and took six weeks to complete.

The part needed a continuous spiral pattern cut in 1/8 inch copper. To add to the challenge, the pattern was 2,400 inches long in a three-dimensional 'top-hat' shaped part. The flanged half cylinder weighs forty-seven pounds, has a 16 inch inner diameter, a flange diameter of three feet, and is eighteen inches long.

The part is a coil for an advanced Magnetic Resonance Imaging (MRI) system. This revolutionary device will allow surgeons to simultaneously view regions of a patient on a monitor while performing the surgery. The image is processed through a computer that will allow panning zooming, and rotation of the view.

If the final part has one defect, the device is worthless. The solution was LAI's five-axis robotic waterjet station. LAI's five-axis capability enabled this client's design to become a marketable reality. The 15 x 15 x 6 foot robot is one of seven waterjet stations available at LAI. Cutting pressure is produced by four intensifier pumps with a total output of 450 horsepower.

An engineer, a programmer, and a waterjet technician from LAI met with the customer and determine a work approach. Importation, manipulation, and post-processing of the part's data file were critical. After the data file's accuracy was verified, waterjet cutting parameters such as pressure and abrasive flow rates were developed.

The next step was the waterjet cutting of an actual part. LAI soon discovered that, at a given point, the remaining part could not support its own weight. The problem was solved by strategically locating tabs to hold the part together and add rigidity to the sections.

The five-axis waterjet was the only viable solution due to the delicate nature of the part. Standard two and three dimension cutting would not give satisfactory results. And repositioning the work piece to match the CNC programs would be too time consuming.

Since the first part was processed, LAI has successfully waterjet cut forty-eight MRI coils.

Alternative Didn't Cut It

Roland Nasrey, President of Metal Service Center, Inc. in Brockton, Massachusetts noticed that many part makers were using saw blades or plasma cutting to achieve near net shape parts from plate or bar. Saw blades were slow, had difficulty cutting hard materials, and could only perform straight line cutting. Plasma cutting distorted the metal and left a heat-affected zone. Both processes forced the part manufacturer to leave excess edge material for secondary finishing.

Nasrey believed Metal Service Center could help its customers cut costs by producing parts in less time using waterjet technology. So he bought a high-pressure abrasive waterjet cutting system from FLOW. Capable of 55,000 psi, this unit is larger than the OMAX 2652 described above.

One recent project demonstrated the benefits Nasrey was able to pass on to his customers. The project required machining parts from a plate of Inconel 600 Series nickel alloy measuring 1.5 inches thick. Metal Service Center needed to cut 21 horseshoe-shaped parts from the plate for its customer. Like titanium, nickel alloys are very difficult to mill or saw, consuming tools and teeth with its hard, tough properties.

Metal Service Center gave its customer two options. The first involved sawing, rough milling, and finish milling; the second used the abrasive waterjet to cut the parts to near net shape and then finish mill them.

Nasrey calculated that machining the parts with a milling machine required a stock plate measuring 10 x 22 inches, weighing 99 pounds and costing $1,188. Metal Service Center would initiate the process by saw cutting rectangles from the plate to a blank size of two inches by four inches. At a rate of $25 an hour, Nasrey estimated the labor cost for sawing the plate totaled $200, plus an additional $200 for saw blade replacements.

The company would then forward the blanks to the manufacturer for milling and finishing. Rough machining the parts with a mill would consume an estimated 20 hours at a rate of $65 an hour for a total of $1,300. Next, the manufacturer would finish-machine the parts at a cost of $1,200, with an additional charge of $280 for the carbide inserts. The total cost of the project would have amounted to $4,368, with an estimated delivery time of four weeks.

The second option of using abrasive waterjet cutting reduced the amount of stock plate necessary to fabricate the blanks to 8 inches x 11 inches weighing 39 pounds at a price of $468 (a savings of $650). Cutting the parts with the abrasive waterjet took less than five hours to complete at a cost of $378.

After the cutting, Metal Service Center sent the near-net shaped parts to the manufacturer for finishing ($1,200 plus $80 for replacement carbide inserts). The parts required minimal finishing since abrasive waterjet produces a sand-blast quality finish. The reduction in scrap materials, combined with the savings from eliminating the sawing and milling processes, resulted in a savings of $2,242 for the project.

That's turning cold water into cold cash.

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