By: Richard Ward
(Modern Application News, December 1998)
Customized waterjet cutting software is now able to handle the varying cutting speeds required when dealing with complex part geometries, nesting, and tighter tolerance requirements.
Making quality, high-tolerance waterjet-cut parts used to be limited to those companies with experienced operators on the floor, and the CAD (computer-aided design) experts to support them. The problem has been that the stream exit can lag up to 2" behind the entrance at the top; and this causes havoc when approaching a corner or a sharp radius.
A perfectly vertical stream is only achieved at very slow speeds in corners and tight radii. If the entire part was cut at the "optimum corner or tight-radii" speed, the cutting time would be excessive and the cost of waterjet cutting would be unacceptable. The object is to cut as fast as possible all the time, and still achieve the required cut quality and tolerances. While maintaining consistent quality, straight lines can be cut several times faster than small circles. Optimizing cutting parameters can reduce the time by half, and produce better parts than if the cutting had been at all one speed.
Waterjet cutting is best described as a controlled erosion process; which means that the kerf of the cut begins to grow as soon as the waterjet stream slows down. This growth limits the ultimate tolerances achievable with a waterjet. The look-ahead feature of customized waterjet software enables these systems to reduce the problems associated with a sharp change in direction. Usually, it is possible to eliminate the reverse kickback of the stream, producing clean 90° inside and outside corners by selecting different options in the CAM (computer-assisted manufacturing) process.
A multiple head waterjet cutting nested parts.
By analyzing the part’s geometry, the software can check all the parameters, like line length and angle of intersection between the present and the next line or arc. If the change in geometry exceeds any limitations specified in the CAM process, calculations by the software vary the speed, acceleration, deceleration, and offset of the kerf value, automatically. The CAM software then modifies the code posted to the controller to simulate the necessary changes in speed, enabling the tail of the waterjet stream to "catch up."
If there is no change in direction, but rather a straight line that extends into a tight radius, it is necessary to slow down on the radius if cut quality is maintained. The software is designed to evaluate every arc independently an allocate a different speed to each, depending on its radius and length. To ensure smooth transitions between the arcs, the software will generate code that decelerates into tighter arcs (smaller radii), enabling the waterjet stream to be at the correct speed before entering the arc. Acceleration of the stream would also apply when the situation is reversed.
Nesting and Material Utilization Are Keys to Cutting Costs
Although ideally suited to small runs and custom work, adding multiple heads and CAD/CAM software customized for multiple head use transforms the waterjet into a high-production tool. Although there is an increase in consumable cost, the overhead cost remain constant, enabling two, three, or four parts to be cut in the same time frame required to cut one part. With a multiple-head waterjet, it is possible to compete with single-head lasers in thinner steels under certain conditions.
To set up the tool path for multiple head nesting, it should only be necessary to specify the number of waterjet cutting heads, the size of the sheet being used, and let the software do the rest. The program should calculate the optimum spacing of the heads to maximize material use and switch various heads on and off, either automatically or by informing the operator to do it manually, as required.
CAD/CAM software is now available that automatically tracks each sheet of material being cut, placing the material remaining in a remnant library. When a future part requires the same material, the library can be searched and the part nested in the appropriate remnant. It is also possible to create a sheet of any particular shape with parts nested into it, enabling scrap materials to be used economically. Nesting software also is capable of nesting parts within parts, ensuring that all available material is used.
There are occasions when straight lines are cut as common lines between parts. The lines need not be the same length, and the parts need not be the same shape. Good CAM software is able to nest and arrange the parts so that all lines that can be cut as common, will be. It also is possible to specify the lead-ins for each cut to start in the previous kerf already cut, removing the need for piercing the material each time another part is cut. Time and cost savings of about 45% are achievable with common line cutting. A system with multiple cutting heads and a good CAD/CAM package can produce six times more work than a competitor without such a setup.
Addressing all the features specifically available for waterjet cutting as well as all other 2-D cutting such as laser, plasma, and oxy-fuel, would require a live demonstration. The software is becoming so powerful that in several instances all that is needed is a controller that can read and act on the code. Generating good code and maximum material utilization are the job of the "brains" behind any successful 2-D cutting operation. Good CAD/CAM software is now available that can make up for other, more human limitations.
