Overview of How Waterjet Cutting Works
Meet Father Waterjet.
Dr. Norman Franz is considered the father of the waterjet. He was the first to study and the use ultrahigh-pressure water as a cutting tool. Ultrahigh-Pressure 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, he dropped heavy weights onto columns of water, forcing that water through a tiny orifice succeeding in delivering short bursts of very high pressures that cut through wood and other materials.
His later 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. Waterjet technology provides a range of capabilities and advantages that prove very effective in the battle for cost reduction and improved productivity.
Waterjet technology is now recognized as the most versatile and fastest growing process in the industry. It compliments other technologies such as milling, laser, EDM, plasma and routers. Waterjet cutting technology uses no noxious gases or liquids, nor do they 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 proven that it can do things that other technologies are unable to. Waterjet technology is used in cutting the finest, details in stone, glass and metals; to high speed hole drilling of titanium; to cutting of food, to the killing of pathogens in beverages and dips.
How High Pressure water is Created
The basic technology for creating high pressure water is complex. Water flows from a pump, through pipes and out a cutting head. To generate and control water at pressures of 60,000 psi requires high technology. At these pressures a slight leak can cause permanent erosion damage to components if not properly designed.
Essentially, there are two types of waterjets:
- Pure waterjet
- Abrasive waterjet.
Machines are designed to employ either waterjet only, abrasive waterjet only, or a combination of both.
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.
Direct Drive Pump
The direct drive pump operates in the same manner as a low-pressure “pressure washer” that is typically used to pressure wash a house or deck prior to repainting. It is a triplex pump that gets the movement of the three plungers directly from the electric motor. Direct drive pumps deliver a maximum continuous operating pressure 10 to 25% lower than intensifier pumps units (20k to 50k for direct drive, 40k to 60k for intensifiers).
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.
Why is there so much interest in waterjet cutting?
Partial list of the benefits to using a waterjet:
- Cheaper than other processes.
- Cuts virtually any material:
- pre hardened steel
- mild steel
- exotics like Titanium, Inconel, Hastalloy
- gummy 304 stainless
- (most steels cut at the same speed, whether hardened, or not.)
- Copper, Brass, Aluminum: They are a cinch!
- brittle materials like glass, ceramic, quartz, stone.
- laminates
- flammable materials
- Cut thin stuff, or thick material
- Makes all sorts of shapes with only one tool.
- Cuts wide range of thickness’ to reasonable tolerance up to 2” (50mm) thick
- Up to 5” (127mm) or thicker where tolerance not important, or in soft materials.
- No Heat Generated / No heat affected zones - this is cold cutting!
- No mechanical stresses
- Cuts virtually any shape:
- Fast Setup:
- Only one tool to qualify / No tool changes required
- Fast turn around on the machine. Make a part, then 2 minutes be making a completely different part from a completely different material.
- Leaves a satin smooth finish, thus reducing secondary operations
- Clean cutting process without gasses or oils
- Makes its own start holes
- Your "scrap" metal is easier to recycle or re-use (no oily chips!)
- Very easy to learn.
- Trade off tolerance vs speed from feature to feature on your part.
- Can easily switch between high production, and single piece production, on the same machine, with no extra effort.
- Are very safe.
- Draw the part / cut the part. That easy!
- No "scaley" edges, which makes it easier to make a high quality weld
- Machine composite materials, or materials where dissimilar materials are glued together
- Machine stacks of thin parts all at once.
When comparing with Lasers:
- Abrasive waterjets can machine many materials that lasers cannot. (Reflective materials in particular, such as Aluminum and Copper.
- Uniformity of material is not very important to an Abrasivejet.
- Abrasive jets do not heat your part. Thus there is no thermal distortion or hardening of the material.
- Precision abrasive jet machines can obtain about the same or higher tolerances than lasers (especially as thickness increases).
- Your capital equipment costs for water jet are generally much lower than that for a laser. For the price of a laser, you can purchase several abrasive jet-machining centers.
- Abrasive jets can machine thicker materials. How thick you can cut is a function of how long you are willing to wait. 2" (50mm) steel and 3" (76mm) aluminum is quite common. I heard of people doing up to 10" (250mm) steel, and 24" (600mm) thick glass with high horsepower systems. Once you get over 2" (50mm) thick it is very difficult to get precision, however. Lasers seem to have a maximum of 0.5" (12mm) - 0.75" (19mm).
- Abrasive jets are safer. No burnt fingers, no noxious fumes, and no fires. (You still have to keep those fingers out of the beam.)
- Abrasive jets are more environmentally friendly.
- Maintenance on the abrasive jet nozzle is simpler than that of a laser, though probably just as frequent.
- Abrasive jets are capable of similar tolerances on thin parts, and better on parts thicker than .5"
- Abrasive jets do not loose much "focus" when cutting over uneven surfaces
- While lasers are often faster on thinner materials...
- ...it may be cheaper and faster to simply buy two or three abrasive jet machining centers to do the same work
- ...you can stack materials, so you are cutting multiple thin parts simultaneously.
- ...you can run additional cutting heads in parallel on a single machine
- Modern Abrasive jets are typically much easier to operate and maintain than lasers, which means that every employee in your shop can be quickly trained to run one!
When comparing with EDM:
- Abrasive jets are much faster than EDM.
- Abrasive Jets machine a wider variety of materials (virtually any material).
- Uniformity of material is not very important to an Abrasivejet.
- Abrasive jets make their own pierce holes.
- Abrasive jets do not heat the surface of what they machine.
- Abrasive jets are capable of ignoring material aberrations that would cause wire EDM to lose flushing.
- Abrasive Jet machining is useful for creating start holes for wire insertion later on. (a mill could do the job, but only after spotting the hole, changing tools to drill a pilot, then changing tools again to drill out the hole).
- New technology allows Abrasive jets to obtain tolerances of up to +/-.003" (0.075mm) or better (I have personally done some +/-.001" (0.025mm) work, but that's the exception, not the norm, and only on certain shapes and materials.)
- No heat affected Zone with Abrasive jets.
- Abrasive jets require less setup.
- Make bigger parts.
- Many EDM shops are also buying waterjets. Waterjets can be considered to be like super-fast EDM machines with less precision. This means that many parts of the same category that an EDM would do can be done faster and cheaper on an abrasivejet, if the tolerances are not extreme.
When comparing with Plasma / Fine Plasma:
- Provides a nicer edge finish
- Don’t heat the part
- Cuts virtually any material
- More precise
- Plasma is typically faster
When comparing with Flame cutting:
- Provides a much nicer edge finish
- Won’t heat the part
- Cuts virtually any material
- More precise
- Flame cutting is typically faster
When comparing with milling:
- Setup and fixturing typically involves placing the material on the table with a waterjet
- Cleanup is much faster with a waterjet
- Programming is easier and faster
- Machine virtually any material, including:
- brittle materials
- pre hardened materials
- otherwise difficult materials such as Titanium, Hastalloy, Inconel, SS 304, hardened tool steel....
- Waterjets are often used for complimenting or replacing milling operations.