Waterjet bevel cutting made easy

By Dr. John H. Olsen

March 1, 2010

Improvements in waterjet technology have made bevel cutting with these machines more suitable for a greater number of fabricating shops by simplifying programming and operation and reducing the need for trial-and-error setup.

Figure 1
Complex Beveled Part

The abrasive waterjet (AWJ) industry continues to add features and capabilities that make abrasive waterjet cutting a more useful technology for fabricating shops. Among the latest enhancements is an easy-to-program system and tilting head for cutting beveled edges that is well-suited for making complex shapes and weld-ready parts (see Figure 1).

Using AWJ to make an angled cut is not really new. Five-axis systems that can do this have been around for years. However, programming these systems traditionally has been difficult and time-consuming, requiring extensive programmer expertise and much trial-and-error testing. Now that has changed.

A Bit of History

Some discussion about the basics of AWJ cutting and its history is in order. The challenge in making accurate parts with an abrasive waterjet has always been that the abrasive waterjet itself is not a rigid, fixed-dimension cutting tool. As the cutting nozzle moves, the jet is deformed, and the lower part of the jet lags behind the upper part, as shown in Figure 2).

The amount of lag is a complex function that involves the nature of the material being cut (mechanical properties and thickness); the characteristics of the cutting jet (size of orifice and mixing tube, water pressure, abrasive flow rate, type and size of abrasive); and the motion of the nozzle (velocity of cut--both speed and direction--acceleration, and even the acceleration change rate).

In AWJ cutting's early days, the substantial operator experience and trial-and-error programming required to accommodate this jet lag made it difficult to produce even the simplest shapes accurately. This difficulty severely limited the adoption of the technology as a viable fabricating and machining method. However, in the early 1990s, two innovations combined to make AWJ easier to use with greater accuracy.

1. Mathematical modeling of AWJ stream behavior became sophisticated enough to permit accurate, computer-based prediction of jet lag.
   
   
2. Personal computers became powerful enough and cheap enough to allow such models to be incorporated into a cost-effective control system that eliminated the need for trial-and-error testing and programmer expertise.

These advancements meant that accurate and repeatable 2-D parts could be produced in relatively thick material by a relatively inexperienced operator directly from a 2-D CAD file. Suddenly AWJ systems became viable tools for general machine and fabricating shops. Since that time mathematical models of AWJ behavior have continued to improve along with the computing capability and speed of PCs.

Models were expanded to predict not only jet lag, but also the natural taper occurring in the AWJ cut (see Figure 3). Hardware and computer-controlled systems then were developed to provide precise tilt of the nozzle assembly to correct for this taper. As a result, highly accurate 2-D parts could be made with straight perpendicular sides—automatically, with no intervention or hand programming by the operator—making AWJ technology more useful to even more machine and fabricating shops.

Now things have progressed to the next step: using AWJ systems to produce parts with intentional bevel, such as those shown in Figure 1, Figure 4, and Figure 5, without the need for an expert programmer or trial-and-error testing.

The Challenge

Developing a system to cut beveled part features has several challenges:

1. The geometry of the upper surface of the part and the lower surface of the part are no longer the same, as they are with 2-D parts. For a curved bevel, such as that shown in Figure 3, the distance the cutting jet must travel is substantially greater on the lower surface than on the upper surface. This means that the controller must automatically adjust cutting speed and tilt in the direction of travel to produce the desired part geometry and maintain the desired cut quality and surface finish
   
   

   This is an even greater challenge in transition areas where, for example, a vertical surface intersects or transitions into a beveled surface, or the desired degree of bevel changes from one value to another.
   
   

2. The thickness of the cut changes based on the amount of bevel, and the controller must automatically adjust cutting speed to maintain the desired surface finish. The cut thickness also affects the AWJ stream jet lag, which requires correction. During transitions from a bevel cut to a vertical cut, or to another degree of bevel, the thickness of the cut actually changes as the cut progresses, which makes the process even more complex.
   
   
3. The nozzle-tilting mechanism (see Figure 6 for an example) must be designed specifically for AWJ conditions. This means it must be able to function smoothly and reliably in a wet abrasive-laden environment.
   

   Figure 6
   Bevel-cutting Head

   In addition it ideally is designed to have a virtual point of rotation about the point where the cutting jet intersects the upper surface of the material being cut. Any variation from this means that changes in bevel angle or orientation will require automatic changes in X, Y, and Z locations, further complicating the entire machine.
   
   

4. The operator interface must be as simple as possible to permit a relatively inexperienced operator to make a part accurately. Experienced programming experts of traditional five-axis AWJ's just aren't readily available in the labor market.

From the Operator/Programmer's Point of View

Systems now available for making beveled parts meet all of these challenges. From the operator/programmer's point of view, the process is almost identical to that for making a traditional 2-D part. In fact, if the whole part is beveled, as in Figure 5, you simply specify the angle in the machine controller, as in Figure 9.

An alternate method is to specify the angle of each entity for which an angle is desired. You can specify a different angle or none at all for each entity of the path. The process for this second method is shown in Figure 7 and Figure 8 and is as follows:

1. The programmer draws or imports a 2-D CAD drawing that provides the geometry of the part's upper surface and specifies the desired surface quality for all of the part's elements. The CAD system then automatically creates necessary pierce points, lead-ins and lead-outs, and traverses to create a 2-D tool path (see Figure 6).
   
   
2. The programmer then identifies those elements of the part where a bevel is specified and enters the desired angle of bevel from vertical. In areas of transition from one bevel angle to another, or from a vertical section to or from a beveled section, the operator simply enters the desired angle of bevel at each end of the transition area (see Figure 7). He need not worry about the transitions between angle surfaces at the corners; this is handled automatically by the software within the machine controller.
   
   
3. The operator then loads the path into the controller (Figure 9).
   
   
4. All that's left is to secure the selected material onto the cutting table and position the nozzle at the desired starting part. The AWJ system now is ready to make the part. A preview is available (Figure 10).

Ongoing R&D

Ongoing R&D efforts in the AWJ industry have now made it possible to produce accurate beveled parts almost as easily as 2-D parts, with only one additional programming step. This capability should greatly increase the applicability of modern abrasive waterjet cutting systems to machine shops and fabricating shops with a need to produce weld-ready parts and complicated shapes with beveled edges.