Waterjet Cutting in the Era of Intelligent Manufacturing

At present, the global manufacturingindustry is speeding up to the digital and intelligent era. Intelligent manufacturing has more and moreinfluence on the competitiveness of the manufacturing industry. Intelligent manufacturing is essentially information based manufacturing under the condition of ubiquitous perception for the wholelife cycle of products. Intelligentmanufacturing technology relies onmodern sensor technology, network technology, automation technology,anthropomorphic intelligent technology,and other advanced technologies. It is the deep integration of information technology, intelligent technology and equipment manufacturing technology.The realization of intelligent manufacturing can shorten the product development cycle, reduce resources, energyconsumption, and operation costs, while improving production efficiency and product quality.

                                                                                       Figure 1. Waterjet cutting of a titanium alloy part for aircraft engine.

Waterjet cutting plays a crucial role in manymanufacturing applications. For example, aircraft enginesconsist of titanium alloy parts that are difficult to machine,therefore waterjet cutting is the machining process of choice. 

For the engine rectifier shown in Figure 1, anarray of irregular holes must be machined. Because of theconfined space of the hole, the traditional milling method requires the use of a delicate milling cutter to remove material bit by bit. The milling cutter is worn out quickly.The whole machining process is very time consumingand costly. Waterjet cutting (Figure 1) completes roughmachining of each hole within 25 minutes, followed by a finishing process of milling or electrochemical machining.Compared to over 150 minutes of machining time using only the milling method, the total machining time issignificantly reduced. Furthermore, because of the coldprocessing nature of waterjet cutting, the waterjet-machined part is free of thermal distortion and heat affected zones.

However, waterjet cutting cannot replace the milling process entirely because waterjets, as soft cutting tools,come with drawbacks. The waterjet can only do profilecutting. Contradictory to a rigid cutting tool like a millingcutter, the jet is bent backwards while it is cutting (Figure2).The curvature of the jet during cutting depends on the workpiece parameters (such as material and thickness),the cutting speed, and the jet parameters (such as pressure,nozzle diameter, abrasive flow rate, etc.). The kerf width of a cut also varies along the thickness direction; this is also known as “taper” error. It is very difficult to use such a soft cutting tool to machine a part precisely.The traditional CNC control method was developedfor rigid cutting tools, such as milling, turning, drilling, grinding machines, etc. Rigid cutting tools no doubt make up the mainstream of machining methods andthe machining market. Developers of even the most sophisticated CNC controllers have paid little attention tosoft cutting tools, such as waterjet cutting machines, and have never offered any solution to the problems caused by the deformation of the soft cutting tool. Therefore,the waterjet industry is forced to come up with its own solution. A new control method has thus been developed. We may call it a “soft cutting tool control method.” We also call a waterjet machine with the “soft cutting tool controlmethod” a “smart waterjet.” Let us see how the “softcutting tool control method” is different from the traditional CNC control method.

The traditional CNC control method assumes that themachining profiles at the top and bottom of the workpiece are identical, and thus its control strategy is focused simplyon the top of the workpiece. In the “soft cutting tool control method,” the focus of control strategy is on the jet exit point at the bottom of the workpiece.

The path of motion often consists of straight lines and orarc segments. In the traditional CNC control method, each of these segments is assigned a certain speed of motion.These segments’ geometrical and speed data are puttogether into a software program, such as a G–codeprogram.

This program is then processed by the motion controller into motion commands for servo or step motors, which iscalled “interpolation.”This interpolation process basically breaks up theline segments into motorsteps and is essentially a“geometrical interpolation.”Even though some speed vs-material data are offered to the user in selectionof cutting speed and acceleration/deceleration is applied at certain points,the change of speed alongthe cutting path and therates of acceleration/deceleration do not reflect the difference in workpiece and jet parameters, not to mention speed optimization.The interpolation is often implemented while the machineis moving. This sets limits to the interpolation calculationtime and the ability of looking forward and backward in the machining program, which is necessary for optimization. In the “soft cutting tool control method,” “geometrical interpolation” is only the first round of the interpolation process.

The “behaviors” of the soft cutting tool are described with mathematical cutting models. In these mathematical models, cut surface roughness, kerf width variation along the depth direction, jet lag distance (the distance of jet exit point at the bottom lagging behind the entry point at the top of workpiece ) are related to workpiece and jet parameters as well as machining path geometry. Following the “geometrical interpolation,” a second round of interpolation, “speed interpolation,” assigns an optimized speed to each motor step based on the cutting model and the path geometry. This is usually done by looking through the program codes forward and backward the entire machining path for the purpose of optimization.

A third round of interpolation, “jet angle interpolation,”adds compensation tilting motion steps based on the localcutting speed, the cutting models, and the path geometry,aiming to correct part taper and other geometrical inaccuracies caused by the soft cutting tool. In addition to these three basic rounds of interpolation, special treatments are also given for specific scenarios. For example, at the lead-in/lead-out point, speed control and tilting motion areused to ensure that the jet penetrates the entire thickness of the workpiece prior to entering the part profile and to have a clean cut without residual “bridging” and sags or crests,typical lead-in/lead-out defects. At external corners, a corner crossing technique can be applied to achieve a bette rcut quality without sacrificing the cutting speed by using the cutting model to set the optimized length and speed ofthe corner crossing feature. Sometimes, it is even necessaryto use an additional round of interpolation, “kerf width compensation,” to compensate for the kerf width variationas the cutting speed varies, which is essential when cuttinga thick part with high precision.

Figure 3 is a preview of a cutting program generated with the “soft cutting tool control method.” At the lead-in/lead-out and at corners, color changes gradually from blue to red, representing optimized cutting speed changingsmoothly, at the increment of a motor step, from slow tofast. The features of corner crossing are also visible atexternal corners.

Some may wonder why traditional CNC controllers can not be used to do the programming work in the “softcutting tool control method.” Traditional CNC controllers are configured to do the interpolation in the controller firm ware while the machine is moving. Because of theuse of multiple rounds of interpolation and a number of mathematical cutting models in the “soft cutting tool control method,” traditional CNC controllers often do not have the adequate “power” to process the interpolationcal culation fast enough to keep up with the pace of themachine motion. They also lack the software architecture to process the cutting program forward and backward multiple times. In the “soft cutting tool control method,”interpolation is often implemented in a PC and then the processed motion commands are downloaded to the controller before starting the machine motion. Themain functions of the motion controller hardware are reduced to data buffering and data dispatching, without further interpolation.

Another benefit of doing interpolation in the PC is that interpolation algorithm and cuttingmodels can be more conveniently enhanced andupgraded. Because the infrastructure of the “soft cutting tool control method” is quite differentfrom that of a traditional CNC control method, itscontroller software and hardware is usually not available from the mainstream market of CNCmotion control. Several waterjet companies have developed their own versions of the “soft cuttingtool controller.”

An advantage of the “soft cutting tool control method” is illustrated in Figures 4 and 5. A waterjetcut part out of 20 mm thick aluminum with atraditional CNC controller is shown in Figure 4.The part looks perfect at the top surface. Even though the program has been manually modified to slow down the cutting speed at corners, defects are obvious when looking at the bottom of the part.Figure 5 shows a waterjet cut part out of 100 mm thick aluminum with a “soft cutting tool controller.” The difference between the top and bottom profiles of the partis much smaller than that in Figure 4, even though thethickness is much greater.

The “soft cutting tool controller” offers benefits beyond higher cutting speed and quality. Because cutting modelsand advanced algorithms are used, programming in the “soft cutting tool control method” becomes very easy. Users only need to provide an e-drawing of the part, select a material from a list, and enter the material thickness. The smart software will take care of the rest of the programming. The costly trials-and-errors for anew cutting job can be skipped. A new user can learn the software and start cutting parts on his first day of work. Job data of the cutting time, the amount of abrasive usage,the total length of cutting path, material size, and all other process parameters are readily available for job quotation,production planning, and job report.

To embrace the challenge of the rapidly evolving digitaland intelligent era, waterjet cutting technology needs many more and faster technological developments, above and beyond the “soft cutting tool control method.” The author intends to introduce and discuss other new technologicalbreakthroughs in several articles. Even though the height and quality of these articles are unfortunately limited by theauthor’s knowledge and vision, the author hopes that they entry training, permits, and other costly preventive measures that would otherwise come into play. This leads to shortened project timeframes, faster returns to production, and a lower total bill for the client.