Abrasive jet and Water Jet technologies have been around for years. Waterjet cutting has been a specialty technology used in a wide variety of industries since about 1970. Around 1993, big advances in the technology were introduced that have caused this technology to become very popular for machine shops. There are now a lot of companies making a lot of money by replacing and complementing conventional machining with water jet cutting methods.
Over the last 10 years, abrasivejet machining has taken off like wildfire. Thousands of job-shops have sprung up around the world.
Above: Pictures of some "typical" abrasivejet made parts. (Photos courtesy of OMAX Corporation )
We hope you find this information useful. We have tried to be as accurate as possible. Feel free to send us an email if you find any errors or omissions, etc.They are quick to program (make money on short runs.) They are quick to set up, and offer quick turn-around on the machine. They complement existing tools, used for either primary or secondary operations. They make parts quickly out of virtually any material. They do not heat your material. All sorts of intricate shapes are easy to make. They are money making machines. (On the next page, you will find a much longer list of advantages )
Most of the information contained here specifically applies to 2D machining, but is general enough to be useful for those researching abrasive and water jets in general.
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.
(Above: Pure-water "Waterjet". Graphic waterjet nozzle courtesy of
OMAX Corporation )
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:
_
(Abrasivejet graphic courtesy of
OMAX Corporation )
(Left): 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.
(Right): An actual photograph of the same nozzle, with the guard removed, cutting out some parts.
_
Above: On the left is a typical
waterjet nozzle. On the right is an abrasivejet nozzle. On
the far right is a picture of an abrasive nozzle installed on a
machine. The white tube protruding from the side of the abrasive
nozzle is where the abrasive comes in from, and is a dead give-away
that it is an abrasivejet you are looking at.
| Water Jet Nozzle | AbrasiveJet Nozzle | ||
| Soft rubber | Hardened tool steel | Plastic | |
| Foam | Titanium | Nylon | |
| Extremely thin stuff like Foil | Aluminum | Graphite | |
| Carpet | Hard Rubber | Many ceramics | |
| Paper and cardboard | Stone | Carbon Fiber | |
| Soft Gasket material | Inconel® | Composites | |
| Candy bars | Hastalloy | mild steel | |
| Diapers | Copper | Stainless Steel | |
| Soft, or thin wood | Exotic materials | Kevlar |
|
| ...All sorts of other soft stuff | Hard, or thick Wood | Granite | |
| Glass (even bullet proof!) | Mixed materials |
||
| Marble | Brass | ||
| In Fact, there are very few materials that abrasivejets can't cut! | |||
Note: Many machines let you swap nozzles in a matter of minutes. Alternately, you can simply turn off the abrasive, and get a somewhat inefficient water jet from your abrasive jet nozzle.
Limitations to water only nozzles:
Typically, the only problems that arise with a water only nozzle will be with the jewel (the orifice with the tiny hole that the water squirts through).
Jewels can crack,
plug, or form deposits on them. Cracking and plugging happens as a
result of dirty inlet water, and is typically avoided with proper
filtration. Deposits accumulate gradually as a result of minerals in
the water. Depending on your water supply, slightly fancier filtering
may be necessary. Jewels are easily replaced in about 2 - 10 min., and
are typically cheap ($5-$50). There are
also diamond orifices for sale for $200.00 and up, which can last
longer in many applications. Which is better, will depend on your
exact needs.
This means that (for some machines) you can make good money off single part and low volume production!
![[picture dragon artwork machined with an abrasive waterjet]](dragon.jpg)
Pictured here is a dragon machined from 1" thick bullet proof glass, and inlay of marble and granite. Notice the fine detail possible. (picture courtesy of OMAX corp.)
While most money will probably be made in thickness' under 1" (25mm) for steel, It is common to also machine up to 4" (100mm). How thick it is possible to cut is dictated by the time it takes. Cutting speed is a function of thickness, and a part twice as thick will take more than twice as long. People make low tolerance parts and roughing out metal up to 5-10" thick (125mm-250mm), but those people are very patient, and probably have no other way to do it. Typically, most money is made on parts 2" (50mm) thick or thinner.
![[picture of 2in ss304 machined with an abrasive waterjet]](part2.jpg)
Pictured here is a 2" (50mm) thick piece of 304 stainless steel. In 1993 when this part was first cut, It took just under 3 hours with a very small 10 horsepower pump and old control software to machine this to a tolerance of +/-.005" (0.125mm). Today, using a 40 HP direct drive pump, and modern control software, this could be machined to the same tolerance in under an hour (including programming, setup, etc.) . (picture courtesy of OMAX corp.)
As long as you
are not machining a material that is hazardous, the spent abrasive and
waste material become suitable for land fill. The red color of garnet
abrasive also looks nice in your garden. If you are machining lots of
lead or other hazardous materials, you will still need to dispose of
your waste appropriately, and recycle your water. Keep in mind,
however, that very little metal is actually removed in the cutting
process. This keeps the environmental impact relatively low, even
if you do machine the occasional hazardous
material.
In most areas, excess water is simply drained
to the sewer. In some areas, some water treatment may be
necessary prior to draining to sewere. In a few areas, a "closed
loop" system that recycles the water may be required.
The pumps do use a considerable amount of
electricity, though, so there is some additional environmental (and
cost) impact due to this.
Nope! If you are seeing 6" (150mm) thick steel being cut on a "waterjet", what you are really looking at is an "abrasivejet". The water is accelerating the abrasive. The steel is being cut by the abrasive, not the water!
How long will a mixing tube last?
A "worn" mixing tube is like a worn tool bit: It is difficult to say when a mixing tube is fully worn, but as it wears, it becomes a less effective cutting tool. (although once it starts to go bad, the wear rate accelerates). For precision work, a new mixing tube performs better than a used one. How long a mixing tube will last depends on a number of factors, including the sales person that you talk to. Numbers from 20 to 80 hours are fairly typical, although it is possible that they may wear faster, or last longer, depending on circumstances.
So what's the real cost?
When looking at costs such as mixing tubes and jewels that are expensive wear parts, consider the "total cost of operation", and compare it with the productivity of the machine. When you make such a comparison you will quickly see that an abrasivejet will probably be the most profitable machine tool in your shop - by far. Consider that your operating cost of the machine will vary between $20 and $35 per hour, but for "typical" jobs you will earn between $60 and $150 per hour, with $120/hour being quite typical*.
* I have also
seen shops do special work at prices between $500 and $2000 per hour.
Although not the norm, I occasionally see shops that find some
niche market that cannot be done any other way, or where the alternate
methods are very expensive. These guys make loads of money, and
often are quite secretive about how they do it.
Price varies
considerably depending on regional factors such as competitors and
local markets. Reasearch this carefully when looking to purchase
a machine.
When pricing the work, it is often more
sensible to price based on a "per part" price, instead of "per
hour". Often profits can be maximized this way, and it is
possible to then realize the benifits of faster cutting machines,
and/or machines with multiple nozzles.
Obtainable tolerances vary greatly from manufacturer to manufacturer. Most of this variation comes from differences in controller technology, and some of the variation comes from machine construction. Recently, there have been significant advances in the control of the process allowing for higher tolerances. A machine from 1990 may be capable of tolerances of 0.060"-.010" (1.5mm-0.25mm) Today, some machines are capable of making some parts +/- 0.001" (0.025mm), or even better in special circumstances (though +/-0.002" is perhaps more realistic).
When purchasing a machine, be sure measure parts that come off the machine you are going to buy. Some manufactures stretch the truth a bit when quoting tolerances, or they quote the positioning accuracy of the mechanics of the machine, which does not necessarily translate into the cutting accuracy in the final parts. The reality of it is that Manufactures of abrasive jet equipment are in a tough spot when trying to advertise obtainable tolerances because of these and other factors:
Harder materials typically exhibit less taper, and taper is a big factor in determining what kind of tolerances you can hold. It is possible to compensate for taper by adjusting the cutting speed, and/or tilting the cutting head opposite of the taper direction.
As the material gets thicker, it becomes more difficult to control the behavior of the jet as it exits out the bottom. This will cause blow-out in the corners, and taper around curves. Materials thinner than 1/8" (3mm) tend to exhibit the most taper (which is perhaps the opposite of what you might expect.), and with thicker materials, the controller must be quite sophisticated in order to get decent cuts around complex geometry.
Obviously, the more precise you can position the jet, the more precise you can machine the part. Generally speaking, though, it is much easier to find precise tables, than it is to find machines that can make precise parts. (More on why this is in "control of the abrasivejet" below.)Stability of table
Vibrations between the motion system and the material, poor velocity control, and other sudden variances in conditions can cause blemishes in the part (often called "witness marks"), as shown in a severe case below:
The hardware that is out there varies greatly in stability and susceptibility to vibrations. If the cutting head vibrates relative to the part you are cutting, then your part can be ugly.
Because your cutting tool is basically a beam of water, it acts like a "floppy tool". The jet lags between where it first enters your material and where it exits.
Above: Bottom of jet lags behind cutting head. The controller needs to be aware of this behavior, and compensate for it, in order to get high tolerances.
This can be a source of error in the following places:
As the jet makes its way around a radius, the jet lag causes a tapering effect. Therefore it is necessary to slow the jet down, and let the tail catch up with the head. (And / or tilt the cutting head to compensate)
As the jet enters the corner, the traverse speed must slow down to allow the jets tail to catch up. Otherwise the tail lag will cause the corner to "blow out" a little.
As the jet exits the corner, the feed rate must not be increased too quickly, otherwise the jet will kick back and damage the part.
When the jet slows down, its kerf width grows slightly.Acceleration / Jerk:
Any sudden movement (like a change in feed rate) will cause a slight blemish as well. Thus for highest precision it is necessary to control the acceleration as well as feed rate, and even Jerk ("Jerk" is a change in acceleration.).
Some nozzles produce more taper than others. Longer nozzles usually produce less taper. Smaller diameter nozzles also produce less taper. Holding the nozzle close to the work piece produces less taper as well. (And, of course, it is possible to tilt the cutting head to elliminate the taper in most cases.)Speed of cutting:
Kerf width, which is the width of the cutting beam, determines how sharp of an inside corner you can make. About the smallest practical abrasivejet nozzle will give you a kerf width of .015" (0.38mm) in diameter. Higher horsepower machines require larger nozzles, due to the amount of water and abrasive that they flow through.
Some waterjet (water only) nozzles have very fine kerf widths (like .003" / 0.076mm). Likewise, it is possible to make ultra-small abrasivejet nozzles, but they can be problematic.
Kerf width is typically compensated for by the controller by specifying a "tool offset", where the jet is moved 1/2 of its diameter away from the edge of the part when it cuts.
Variations in waterjet pump pressure can cause marks on the final part. It is important that the pump pressure vary as little as possible while machining is in progress to prevent these. (This becomes an issue only when looking for better than +-.005" (0.125mm) tolerances, however). Typically it is older Intensifier type pumps that exhibit this problem. Some newer intensifiers, and as far as I know all crankshaft driven pumps have smoother pressure delivery, and this is usually not an issue.
Abrasive jets are capable of anywhere from +-0.02" to +-0.001" (0.5mm - 0.025mm) depending on the above factors. What distinguishes one machine from another is how easy those tolerances are obtained. If you had a nozzle attached to any X, Y table capable of positioning to +-.001" (0.025mm), then, in theory, in 0.5" (13mm) thick steel, you could perhaps machine +-0.002" (0.05mm) or so. This is given either software to compensate for jet behavior, and/or an experienced operator tweaking the machine through trial and error. I have personally been able to produce parts in the slightly better than +-0.001" (0.025mm) range on an OMAX 2652, which as far as I know is the most precise machine on the market (other than an OMAX 2626xp), but that usually requires cutting the part once, measuring the error, then cutting it again, and is only possible on certain materials and geometries.Buying a machine? Look at, and measure parts that come off the machine. Measure the first part, then cut the same part at different locations on the table to get an idea of repeatability. Ideally, have the seller do so while you watch, to prevent cheating. (One way to cheat is to slow the cutting way down, and another is to simply use a different machine - It happens.) Also, don't forget to check out the buyers guide which you can link to from the home page of this web site, or the waterjet equipment manufacturers listing page..
| Picture |
Description |
Approximate Cutting time |
 |
2.5" x 2.5" Box cut from
0.5"
thick mild steel (63 x 63mm from 12mm steel) |
5 minutes |
|
the same part as above,
only in 3" (76 mm) mild steel |
2.25 hours |
 |
8" wide Electrical Panel
cut
from
0.06" mild steel (200 mm from 1.5mm steel) |
1-3 minutes |
 |
3" wide gear cut from
0.25" thick nylon (75mm from 6mm nylon) |
1.25 minutes |
 |
10" wide thingy cut from
1"
thick titanium (254 mm wide from 25 mm thick titanium) |
22 minutes |
 |
7" tall horse cut from
0.25"
thick aluminum (178 mm cut from 6 mm thick aluminum) |
4.8 minutes |
