Waterjet Frequently Asked Questions (FAQs)

How does a Waterjet work?

A waterjet uses a very focused stream of water at 40,000 to 60,000 psi to cut through materials. At the nozzle, the stream is focused to a very small diameter by a crystal orifice ("jewel"). The water is able to cut through the work-piece because the stress=force / area and the concentrated stress area is very high. Therefore, the water erodes away a fine kerf from the work-piece. There are two types of waterjet cutting: straight water and abrasive water.

Straight water cutting is used for softer materials like thin plastics, leather, foam, and foods. The high pressure water is focused to a pinpoint stream by a sapphire or diamond crystal. The focused stream passes through the material and removes a very thin section of kerf. Straight water orifices are available in .005 to .018 inch diameter in .001 increments. The most popular sizes are .007 and .010 inch.

Abrasive waterjet cutting is used for hard materials like thick plastic, metals, and stone. Water is focused to pinpoint stream by the same sapphire or diamond crystal used in straight waterjet cutting. However, abrasive is then injected into the high velocity stream at about 0.5 to 2 lbs. per minute. The slurry of water and abrasive is re-focused through a carbide focusing tube. It exits the nozzle and passes through the material. The sharp edges of the abrasive particles erode away a very thin section of kerf. Typical orifice/focusing combination sizes are .010/.030, .014/.043, and .018/.060. (The first number is the orifice diameter and the second number is the focusing tube diameter.)

How do you produce 60,000 psi water?

High pressure water is produced in an intensifier pump and piped to the cutting nozzle via autoclave tubing. A waterjet intensifier pump works like a syringe. High pressure is produced by the reciprocating action of a plunger. Relatively low pressure hydraulic oil pushes on a large piston. This piston is connected to a smaller diameter plunger cavity filled with water. The “mechanical advantage” of a large surface area acting on a small surface area results in ultra high pressure water. Pumps come in different sizes depending on the number and size of nozzles. KMT Watrerjet Systems pumps are available in 15, 30, 50, 75 and 100 horsepower. All KMT Waterjet Systems pumps are designed to run at 60,000 psi. By conservation of energy, increased horsepower produces increased water flow. More mass means higher momentum and allows you to cut thicker materials more quickly.

What makes a good application for a Waterjet?

Waterjets are ideal for any application that requires intricate cutting all the way through the material. They are exceptional for cutting a variety of materials ranging from very hard to very soft. There are few materials that a waterjet is not able to cut (such as diamond and carbide). You can think of waterjets as a liquid grinder that continuously erodes the material. The decision about cutting speed usually concerns the quality of the cut versus speed. The cut will usually have striation (“rooster tail”) along the edge where the water/abrasive eroded away the material. Striation is a function of material, thickness, speed, and nozzle size. Slower cuts usually result in less striation. Faster cuts usually result in more striation.

Good Applications:

1. Soft materials like rubber, thin plastic, or leather
2. Hard materials like steel, tool steel, or stone
3. Exotic materials like ceramic, composites, and glass
4. Food products like pizza, cheese cake, and vegetables
5. Energetic materials like extruded propellant
6. Gummy materials like elastic and cookie dough
7. Multiple layers like stacks of sheet metal or fiberglass
8. Programs that have contoured corners like letters in signs
9. Multiple nozzles to improve production volume
10. Nested parts to reduce scrap waste

What makes a bad application for a Waterjet?

1. Depth of cut is not possible. The stream must pass all the way through the material. However, it is easy to cut bevels or contoured surfaces with multiple axis options.
2. Interrupted cuts like tubes and pipe. Materials that have a hard-soft-hard cross sections tend to blow out the bottom surface because the stream becomes unfocused in the soft regions.
3. Very large amounts of perforations like screen mesh. Very high numbers of cycle will wear out valve seals prematurely. Conventional punch presses are a better technology.
4. Low commodity products like scrap metal, scrap tires, and grass. Waterjets add value to your products but they add cost. A good waterjet application will re-coupe operating costs by 1) improving an existing process by making better cuts or increasing production volume 2) eliminating post-processing or 3) making applications possible that were not possible before.

Why use a Waterjet instead of saws, plasma, laser, EDM, or milling machines?

Every cutting technology has advantages and disadvantages. Waterjets are best known for cutting speed and versatility in cutting a wide variety of materials. They are easily adaptable to automated material handling. Like every cutting tool, however, there will be costs associated with tool wear plus the cost of consumable abrasive (if applicable).

• Saws vs. Waterjets
• Band saws and scroll saws are best for rough cutting thick sections and tubular sections
• Unlike waterjets, saws cannot cut intricate contoured shapes or very hard tool steels.
• Saws do not allow nested parts
• Saws cannot cut from the center of a plate.

• Plasma vs. Waterjets
• Plasma offers extremely fast cutting of steel & aluminum
• Plasma offers low cost & no consumable media
• Plasma leaves molten slag leaves very rough edge quality
• Plasma produces a heat affected zone (HAZ)
• In stainless steel, HAZ is a chromium depletion zone which will rust unless heat treated.

• Laser vs. Waterjets
• Lasers can cut faster and more acturately on to ½" thick or less steel
• Lasers provide a clean cutting process
• Lasers cannot cut 4 - 6 inch thick steel
• Lasers cannot cut shiny surfaces like aluminum because the molten slag acts like a mirror which burns optics
• Lasers cannot powder coat laser cut parts without chemical removal of organics
• Lasers cannot cut some combustibles like corrugate & foam (toxic isocyanate)
• Lasers introduce a heat affect zone and degrades material
• Lasers are expensive to support multiple nozzles
• Lasers costs start at $350,000 to $900,000. Waterjets cost $100,000 to $180,000.

• EDM vs. Waterjets
• EDM is more accurate than waterjets. Waterjet accuracy is typically 0.005" whereas EDM accuracy is 0.0001" accuracy
• Waterjets cut much faster.
• Waterjets cut a much larger variety of materials including plastic, stone, rubber and other non-conductive materials.

• Milling Machines vs. Waterjets
• Milling machines can cut a variety of materials with very close tolerance.
• Milling Machines allows accurate depth of cut (X,Y,Z)
• Milling Machines are less expensive to operate (No consumable media)
• Milling Machines are less expensive to buy than waterjets ($45,000 to $100,000)
• Milling Machines cannot nest parts as closely (typical cutter is 3/8" - 1" diameter)
• Must deal with chips
• Milling Machines require substantial clamping or the rotating tool will move the part
• Tool contacts the part, so may have tool breakage and wear
• Must be careful not to distort thin parts
• Milling Machines are less adaptable to automatic feeding (conveyors, indexers, robot arms, etc) because of need for clamping

How do I decide the best Waterjet vendor?

Here are a few quiglines for choosing a Waterjet System vendor:

1. CNC Chassis Accuracy

There are many ways to produce high pressure water. But the key to accuracy is in the CNC chassis. You must ensure that a machine tool's accuracy is tightly controlled so that the overall desired shape will be dimensionally accurate. Otherwise, circles will be elliptical and squares will be trapezoidal to some degree. If the mahine tool is not "tight", then edges will not be straight and there will be poor cut quality. This is especially a problem with cantilever designs or machines driven from one side only. In general the finished part accuracy of most materials will be around 0.005". It is difficult to achieve better due to the nature of high pressure jet mechanics and inherent 21/2 degree draft angle. Here a few guidlines when selecting the chassis.

• Buy a mid-rail gantry - Romeo Engineering uses electronic gearing to drive a master motor and a slave motor for smooth accurate motion.
• Don't buy a cantilever chassis - Cantilever designs, high-rail gantries, and single-side drives are never accurate. Cantilever arms have notoriously poor "chatter" during rapid acceleration/deceleration. This causes excessive "rooster tail washout" especially in smaller circles and corners. Cantilevers are less expensive than gantries because the manufacturer saves costs of the additional motor, amplifier, rails, etc. But YOU pay the penalty of low accuracy.
• Don't get a chassis with rails bolted to the tank - Tanks swell up to 0.015" and put rails in a bind. All Romeo Engineering machines have the drive train detached from the tank to prevent distortion.
• Ensure the chasis is ballscrew driven - Romeo Engineering designs include ballscrews which have been certified to be matched sets. Rack & Pinion systems are only accurate to around 0.015 inches.
• Get closed loop servo motors only - Do not accept open loop stepper motors. Machines that use servos with so-called "follower rack & pinion" are even worse.
• Ensure that the machine is mechanically accurate first and foremost - Do not accept a machine that has been electronic compensated to meet the specification. Electronic compensation is a software trick used to map out position errors. The accuracy is short lived.
• Witness the accuracy for yourself and get a written report - For quality assurance, all Romeo Engineering machines are checked with a high precision laser interferometer capable of measuring sub-micron accuracy.
• Ensure the structure is made of steel - Romeo Engineering's waterjet chassis are built entirely of heavy structural steel and not aluminum extrusion.
• Ensure all guideways are precision machined flat and straight - Romeo Engineering uses the world's largest surface grinder. Our design dictates that the straightness and flatness are permanent features of the rigid structure. Do not accept machines with shims or spacers.

2. CNC Controller

• Ensure that the controller is RS-274 complaint - It is the United States standard for CNC programming architecture. Some controllers use proprietary code that is not supported by other formats.
• Ensure the controller is right for you - There are "traditional or conventional" type controllers and PC based controllers. Traditional controllers have fast dedicated purpose processors. However, most traditional controllers are adapted from milling machines and do not include special software features desired for waterjet cutting. The vast majority of controllers in the waterjet industry are now PC based because they tend to cost less and are much easier to upgrade as technology changes.
• If the controller is PC based, learn if the controller is a CPU driven or DSP driven - CPU driven controllers are not stable. The Romeo Engineering RE2000NT Waterjet Controller is DSP driven using special microchips designed exclusively for servo motion control. Romeo Engineering designed the RE2000NT Waterjet Controller from the ground up dedicated for waterjet cutting. It runs on Windows 2000 platform and includes all sorts of clever software functions exclusively for waterjet cutting.
• Know how large a part program the controller can handle - Romeo Engineering's RE2000NT allows unlimited part program size. Competitive controllers are sometimes limted to less than 5,000 lines.

3. Nozzle Design

• Get the specifications about life cutting speeds for abrasive nozzles - KMT Waterjet Systems focusing tubes use a special low binder formula of carbide for long life and wear resistence. KMT Waterjet Systems focusing tubes are also a full 3/8" diameter for durability. Some competitve brands have a small diameter with weak thinner walls, which cause the carbide to fracture easier.
• Get prices up front for technician service and spare parts - Spares include orifices, focusing tubes, valve packing, pump seals, and check valves. Be aware that the bd price may not reflect the long term cost of spare parts.

4. Abrasive Delivery System

• Do not accept vacuum orifice feeders - They tend to surge because light particles are drawn first, then heavier particles. The result is more striation and wear on the nozzles.
• Do not accept vibratory feeders - They depend on inconsistent in-coming source electricity voltage and frequency, so they cannot be calibrated accurately. Romeo Engineering uses a special positive displacement abrasive feeder design. It even has a display meter to illustrate the pounds per minute of abrasive delivery. Accurate abrasive delivery is very important because the consumable cost of abrasive is the highest percentage of operation cost. Hence, the more accurate abrasive delivery, the less waste and less cost.

5. Service and Training

• Determine if the vendor will be able to support you operation over an extended period
• Does the vendor have a history of rejected machines?
• Does the vendor have used machines available?
• Is emergency service available?

How important is water quality?

The life of waterjet orifices, seals, and components is directly proportional to the purity of water. The supply water to the intensifier must meet specifications established by KMT Waterjet Systems. A very important parameter is Total Dissolved Solids (TDS), which is an indication of the amount of minerals dissolved in the water. For waterjet cutting, it is desirable to have a TDS around 50 parts per million. Do not reduce TDS below 5 PPM or the water will become too aggressive. The TDS can be measured with a resistivity meter.

What are the water quality specifications?

At 60,000 psi, water is compressed approximately 14% by volume. Under this immense pressure, dissolved solids are forced out of the solution. Even though water appears "clean", it still contains dissolved minerals. Those minerals act as microscopic abrasive to erode high pressure orifices & seals or to build up grime that plugs up nozzles. This is especially true when using .005 inch or smaller diameter jewels. The following table shows the acceptable level of water quality. Some areas of the United States can use straight tap water. In other areas it may be necessary to condition the water using a simple water softener, reverse osmosis, or de-ionizer. (Note that if de-ionized water is in fact required, then we need know prior to manufacturing the pump because it may require different seals).

Water Quality Specifications:

To reduce maintenance and increase the life of high pressure orifices, seals, etc. the water supply should comply with the following water quality specifications.

* Do not reduce beyond this amount or the water will become too aggressive.

Do I need a water softener, de-ionizer, or reverse osmosis?

In many areas of North America, tap water straight from the municipal supply is fine to supply the intensifier pump. Most areas however do have relatively "hard water", which means it contains calcium and magnesium. A water softener is inexpensive and in general will increase the life and performance of the intensifier pump.

When the amount of TDS (total dissolved solids) is too high, then you have two options: De-ionization (DI) or Reverse Osmosis (RO). Do not use a water softener with DI or RO systems. We generally recommended the de-ionizer over reverse osmosis. The benefit of a DI filter is that for every gallon of water input, you get one gallon output. For example, a 25 horsepower intensifier pump requites 0.5 GPM of clean water. That is equivalent to 720 gallons per 24-hour day. A suitable size reverse osmosis unit will have about 35% receovery, so you would expect to loose 1,300 gallons per day down the drain.

The main problem with DI systems is that the resin beads will become depleted and must be regenerated. The regeneration process uses powerful chemicals that you do not want stored on site. Therefore, you typically use an outside company who can regenerate the resin for you. Culligan is a major brand who will come to your site to replace the tanks for you for a fee. Silex is another brand who will ship you the resin and you can install it yourself, thereby eliminating the cost of a service call. The resin itself is not expensive.

What does resistivity/conductivity mean?

The majority of impurities in potable water are in the form of dissassociated ionized mineral salts such as NaCl and CaCO3. Because such ions by definition carry an electrical charge, their presence is directly related to the ability of the water to conduct electricity, or resistivity. Hence, resistivity (and its reciprocal conductivity) are the best parameters by which to gauge the purity of water. Resistivity is a measure in ohm-cm and its reciprocal conductivity is measured in mhos-cm or siemens. It is important to realize that resistivity/conductivity is a measure of only ionic impurities and is not affected by particulates, bacteria, or other organic contamination. We usually use a 10k light to indicte when the filter needs to be regenerated.

What size nozzle do I need?

Proper nozzle size is a function of cutting speed, thickness of material, pump size and cost. In general, larger nozzles will cut faster and will cut thicker materials because there is more momentum.

NUMBER OF WATERJET NOZZLES SUPPORTED BY PUMP HORSEPOWER

What abrasives are available?

The most common abrasives are garnet and copper slag. Most abrasive is sold in 60 and 100 pounds sacks. A typical pallet weighs about 1,000 pounds. Garnet is a pinkish mineral that is mined all over the world. It is commonly used on sandpaper and it is available in 60 to 120 grit depending on the speed and finish desired. Garnet is more expensive, but generally gives the best cutting results. We recommend garnet for cutting glass. Copper slag is a by-product of copper smelting and is sold under the trade name Sharpjet. Like garnet, slag is available in 60 to 120 grit. Copper slag is about half the cost of garnet, but generally cuts a little slower and does not give as smooth of finish. Both are safe for sanitary landfill assuming that the material being cut meet all local and EPA regulations. Other types of abrasive include silica quartzite.

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