mech world: waterjet machining-background

Waterjets (or abrasivejets) are fast, flexible, reasonably precise, and in the last few years have become friendly and easy to use. They use the technology of high pressure water being squirted through a small hole to concentrate an extreme amount of energy in a small area to cut stuff.

"A machine shop without a waterjet, is like a carpenter without a hammer - Sure the carpenter can use the back of his crow bar to hammer in nails, but there is a better way..."
You have already heard the terms "Waterjet" and "Abrasive jet". It is important to understand that Abrasive jets are not the same thing as water jets, although they are nearly the same. Water Jet technology has been around since the early 1970s or so, and abrasive jets extended the concept about 10 years later by adding abrasive to the mix.

Both technologies use the principle of pressurizing water to extremely high pressures, and allowing the water to escape through a very small opening (typically called the "orifice" or "jewel"). The restriction of the tiny orifice creates high pressure and a high velocity beam, much like putting your finger over the end of a garden hose.
Water jets use the beam of water exiting the orifice (or jewel) to cut soft stuff like diapers, candy bars, and thin soft wood, but are not effective for cutting harder materials.

The inlet water is typically pressurized between 20,000 and 60,000 Pounds Per Square Inch (PSI). (Or 1300 - 6200 "bar" if you prefer metric). This is forced through a tiny hole in the jewel, which is typically 0.007" to 0.020" in diameter (0.18 - 0.4mm) This creates a very high velocity beam of water!

Abrasive jets use that same beam of water to accelerate abrasive particles to speeds fast enough to cut through much harder materials:

(top): A diagram of an abrasive jet. Notice that it is just like a water jet with more stuff underneath the jewel. The high velocity water exiting the jewel creates a vacuum which pulls abrasive from the abrasive line, which then mixes with the water in the mixing tube to form a high velocity beam of abrasives.

(bottom): An actual photograph of the same nozzle, with the guard removed, cutting out some parts.

People often incorrectly use the word "waterjet" when they really mean "abrasivejet". Also, people sometimes say "abrasivejet", "abrasive waterjet", or "AWJ", which mean the same thing. Don't worry. If you accidentally call an "abrasivejet" a "waterjet". Nobody will laugh at you, as it is fairly common to do so. Likewise, their are multiple spellings for the terms "water-jet", "waterjet", "water jet", etc. Any of these variations is ok to use, though perhaps "waterjet" and "abrasivejet" are the most common.