Typical Applications
Best applications
Applications that may not be ideal, but might still make sense
Applications that are generally poor
What are abrasivejets good at? What are the "best" applications for them? When are they not as good? This document looks at what abrasivejets do well, and what they don't do well, to help you decide if they're appropriate for your application.
Best applications
Materials and thickness
To date, the most profitable precision applications involve metals such as aluminum, tool steel, stainless steel, mild steel and titanium in thicknesses up to about 1" (2.5 cm). These materials are widely used and found in many engineering applications. They are also very straightforward to machine on a PC-controlled precision abrasivejet system, and part tolerances of ±0.005" (?0.1 mm) with good surface finishes are readily attained.
In thicknesses of 3/16" (4.7 mm) to 1" (25 mm), machining on an OMAX is typically faster than other machine tools; for thicknesses over 1" (25 mm), the abrasivejet process begins to slow, and the surface finish and part accuracy along the lower edge of the part begin to degrade. Square and sharp inside corners in particular begin to become a problem, because they are slow and because the accuracy at the lower part of the cut begins to degrade.
Above 2" (50 mm) in thickness, the process slows and accuracy to ?0.005" (?0.1 mm) or better is more difficult to obtain. Running more horsepower to the nozzle (through higher pressure or water flow rates) can speed up the process in thicker materials, but not necessarily improve the accuracy.
Shapes
An abrasivejet can make almost any two-dimensional shape imaginable—quickly and accurately—in material less than 1" (25 mm) thick. The only limitation comes from the fact that the minimum inside radius in a corner is equal to ½ the diameter of the jet, or about 0.015" (0.4 mm) or 0.010" (0.25 mm) with the OMAX Mini-Jet nozzle.
Intricate parts with lots of sharp inside corners that can be made quickly and accurately in 1" (12 mm) thick material become slow and inaccurate in 3" (75 mm) material. Thus, the simpler the shape and the more it involves rounded corners and long straight runs, the thicker it can be made yet remain cost-effective.
Production Quantities
One of the real strengths of the OMAX PC-based programming system is the very short time required to program and set up the machine to make a part. A part that takes a full day for an experienced programmer and machinist to program and set up on a vertical machining center may only take a few minutes to do on an OMAX.
For this reason, the OMAX is ideal for short-run parts or parts made in batches of one to several hundred. This feature also benefits job shops that may have to stop a project in mid-stream in order to perform a rush job for a special customer. It's also ideal for prototype shop or repair shops that make parts as needed or one at-a-time.
The OMAX quickly accommodates changing from one part or material to another. The OMAX can also be used to make large production runs, but in these cases the ease of set-up and programming are less important features.
Piercing
One of the key features of abrasivejets is their ability to pierce materials without requiring a mechanically drilled starter or pilot hole. There are, however, some limitations.
The OMAX offers the options of Stationary piercing, Dynamic piercing, or Wiggle Piercing. Stationary piercing creates a round pierce hole approximately 0.035" (0.9 mm) in diameter in most materials up to about 1" (6 mm) thick. In harder materials over 1" (6 mm) thick, the jet begins to reflect back on itself while piercing. This can cause a pierce hole to lose roundness and straightness. In addition, it greatly slows the piercing process.
A piece of tool steel over 2" (50 mm) thick may take considerable time to pierce directly as the jet stalls out against its own reflection. "Wiggle" pierce eliminates the risk of a stalled jet by automatically moving the nozzle back and forth over a line 0.090" (2.2 mm) long. This greatly speeds the piercing process and results in a pierce line approximately 0.090" (2.2 mm) long and 0.040" (1 mm) wide. In order to use the "wiggle" pierce feature, the initial lead-in line where the pierce takes place must be at least 0.100" (2.5 mm) long.
Dynamic piercing is used as an alternative to wiggle piercing in areas where there is enough room to pierce as the nozzle is moving. This method of piercing can be faster than either stationary piercing or wiggle piercing, if enough room is available to use it.
Piercing brittle, laminated or composite material can be a challenge. When the jet initially strikes the material, the impact can cause brittle material such as glass or stone to crack; it can cause laminated or composite material to delaminate or break apart. A low-pressure pierce using minimum water pressure (sometimes as low as 10,000 psi (69,000 kPa) can eliminate this problem in most materials. OMAX Machines come equipped with low pressure piercing capability, as well as a special brittle mode of operation that ramps the pressure up slowly to help avoid cracking and delamination.
However, test pierces in composite and laminated material should be made to determine if low-pressure piercing will actually work in any given application. Some composites are very non-uniform; for them, there is no guarantee piercing will consistently work. In such cases as these, the use of a mechanical drilling system to create a small pierce hole is a reasonable alternative.
You can also use the optional Drill Head attachment to pre-drill holes automatically. This can be useful where materials do not pierce as desired with the abrasivejet.
Issues regarding part accuracy
Even the most precise X-Y table will not guarantee accurate parts from an abrasivejet system. The problem generally has to do with the flexible nature of the cutting jet itself and that fact that it does not have a sharply defined cutting edge. The following issues are of greatest concern:
Jet Drag
As a jet moves through the material, the lower section of the jet lags behind the upper section. In a straight cut, this does not present a problem. However, when it comes time to make a corner or bend, the jet must be slowed to control the amount of lag; otherwise, the part will be undercut or overcut.
Jet drag is exactly the kind of phenomenon that the OMAX software (based on a computer algorithm that models the cutting action of the jet) is designed to overcome, and it usually works quite well. However, for material over 2" (5 cm) thick, it becomes very difficult to compensate for all the jet behaviors, making it impossible to obtain the same tolerances possible in thinner material.
Jet kerf vs. feed rate
In general, the slower a jet nozzle moves across the material being cut, the wider the cut it makes. This effect is negligible in harder, thinner material (for example ½" (12 mm) stainless steel), but it can be up to 0.005" (0.1 mm) or more in thick material or soft material.
The "offset" setting in the OMAX software is intended to correct for the cutting width of the jet. But because it is set by the operator and is based on the average width of the jet, it does not fully account for any slight variations in the cutting width as the jet slows for corners or speeds up for straight cuts.
Taper
Taper is worse on very thin materials, then it decreases as you approach 1" (1.2 cm) thick materials, then it starts to get worse again as the thickness increases. In very thick materials the taper becomes barrel shaped, where the top and bottom of the part may measure out accurately, but the middle of the part may be significantly off. The OMAX Quality of Minimum Taper helps adjust for this to some degree.
In addition, the hardness of the material affects the taper, with software materials exhibiting larger taper.
Standoff
A major cause of excessive taper is the standoff of the nozzle (that is, the distance between the nozzle tip and the material being cut). Generally, the greater the standoff, the greater the taper in the cut.
On the other hand, if the standoff is too small, the nozzle will tend to plug when it is first activated, (the initial jet of water does not have enough room to exit the mixing tube and is, instead, forced up the garnet feed tube). The standoff is typically about 0.040" (1 mm) . If the upper surface of the material being cut is irregular or curved, then the standoff will change as the nozzle moves across the surface. This will also cause the taper to change and will have some effect on the accuracy of the part.
All OMAX machines are designed so that the work surface and the nozzle are highly parallel, allowing for extremely low stand-off distances. For the highest possible accuracy, you can prepierce all the holes in the part at a stand-off distance that is good for piercing, and then cut the parts out at a very low stand off that is good for precision. For most parts, though, simply setting the stand off to 0.040" (1 mm) is fine.
Interrupted cuts
If the jet passes through the material being cut, then through a much softer material, water, or air, then again through material intended for parts, the lower cut will typically be less accurate and rougher than the upper cut.
For example, think of cutting rectangular cross-section tubing. When essentially flat material is stacked up and cut, any gaps between the material layers tend to be quite small (on the order of a few thousandths of an inch) and the cut quality is fairly uniform. But as the gaps get larger because of uneven material surfaces or intentional surface contours (such as corrugations), the cut quality can degrade substantially from the upper layers to the lower layers.
Surface quality
If the material being cut has a rough or patterned surface, the accuracy of the cut may be irregular, particularly when it comes to thicker material. This is because the effective nozzle standoff changes as the nozzle moves along the irregular surface.
Applications that may not be ideal, but might still make sense
It is critical to remember that the individual machine shop is the ultimate arbiter of which abrasivejet applications make sense. Oftentimes, abrasivejet cutting specialists tend to focus on production time and part accuracy as the critical elements in determining whether or not an application is appropriate. Yet these are not always reliable decision-making criteria. Keep an open mind in looking at how the following criteria may transform the efficacy of a potential precision abrasivejet application:
- Rapid set-up and programming for short run parts
The flexibility of the OMAX system in making short-run and prototype parts may far outweigh the fact that the actual manufacturing cycle time is longer for the OMAX than for another process. The machine shop should be aware of the potential premium value for quick-turnaround parts.
- Lack of a heat-affected zone when using a precision abrasivejet
Conventional processes (particularly laser or plasma) will often cause a heat-affected zone that must be removed by other means. The abrasivejet can eliminate this intermediate process, thereby saving money and time.
- Need for secondary machining
Sometimes a part just needs to be roughed out prior to extensive conventional machining. A plasma or laser may be faster and cheaper for roughing the part, but the resulting melted edge may be difficult and expensive to machine conventionally.
- Elimination of heat distortion with an abrasivejet
Some complex parts are prone to warpage due to heating of the material during machining. With an abrasivejet, there is no heat.
- Low capital cost of an abrasivejet compared to a laser
This is particularly true if the shop owner is currently farming out work to a laser shop because he can not afford his own laser or does not have enough work to justify it. An OMAX system is typically less than one-third of the price of even a low-power laser.
- The ability to closely nest parts
This may be very important for a customer who works in expensive material, such as titanium.
Applications that are generally poor
Despite compensating factors, some applications just aren't a good match for abrasivejet machining:
- Low-cost applications where accuracy really has no value
Applications that involve cutting non-engineered components such as thick slabs of insulating material. It's unusual for a machine shop to purchase a precision machine tool to slice up insulation
- Using a precision abrasivejet as a cross-cut saw
Except for special exceptions, it makes more sense just to buy a saw (exceptions may include applications that call for sawing hardened tool steel or exotic materials such as Inconel®). Note that if you choose to occassionally use an abrasivejet as a saw, the OMAX Make software includes a cut-off saw feature.
- Applications in heavy steel plate that are currently being performed satisfactorily with a torch or plasma
If accuracy and heat affect are not issues, torches and plasma are cheap, fast and flexible alternatives.
- Applications involving cutting round tubing
Standoff distance will change as you traverse the tube surface, resulting in a rough cut.
- Applications involving interrupted cuts
Unless the lower cut does not need to be as accurate as the upper cut or the customer can afford to sacrifice a piece of material to fill the gap.
- Applications involving wood
It's hard to beat a simple jigsaw, but some precision abrasivejet systems are being used to cut wood, typically for special intricate shapes.
- Parts that truly require a 5-axis machine
This is a much more specialized market.
