Soft cutting tool controller

At present, the global manufacturing industry is speeding up to the digital and intelligent era. Intelligent manufacturing has more and more influence on the competitiveness of the manufacturing industry. Intelligent manufacturing is essentially information-based manufacturing under the condition of ubiquitous perception for the whole life cycle of products. Intelligent manufacturing technology relies on modern 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, energy consumption, and operation costs, while improving production efficiency and product quality.

Waterjet cutting plays a crucial role in many manufacturing applications. For example, aircraft engines consist 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, an array of irregular holes must be machined. Because of the confined 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 consuming and costly. Waterjet cutting (Figure 1) completes rough machining 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 is significantly reduced. Furthermore, because of the cold processing 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 profile cutting. Contradictory to a rigid cutting tool like a milling cutter, the jet is bent backwards while it is cutting (Figure 2). 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 developed for rigid cutting tools, such as milling, turning, drilling, grinding machines, etc. Rigid cutting tools no doubt make up the mainstream of machining methods and the machining market. Developers of even the most sophisticated CNC controllers have paid little attention to soft 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 control method” a “smart waterjet”. Let us see how “soft cutting tool control method” is different from the traditional CNC control method.

The traditional CNC control method assumes that the machining profiles at the top and bottom of the workpiece are identical, and thus its control strategy is focused simply on 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.

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

Figure 2 Waterjet is bent backward while it is cutting a glass part

The path of motion often consists of straight lines and/or arc 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 put together into a software program, such as a G –code program. This program is then processed by the motion controller into motion commands for servo or step motors, which is called “interpolation”. This interpolation process basically breaks up the line segments into motor steps and is essentially a “geometrical interpolation”. Even though some speed-vs-material data are offered to the user in selection of cutting speed and acceleration/deceleration is applied at certain points, the change of speed along the cutting path and the rates 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 machine is moving. This sets limits to the interpolation calculation time 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 local cutting speed, the cutting models, and the path geometry, aiming to correct part taper and other geometrical inaccuracy 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 are used 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 better cut quality without sacrificing the cutting speed by using the cutting model to set the optimized length and speed of the corner crossing feature. Sometimes, it is even necessary to use an additional round of interpolation, “kerf width compensation”, to compensate for the kerf width variation as the cutting speed varies, which is essential when cutting a 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 changing smoothly, at the increment of a motor step, from slow to fast. The features of corner crossing are also visible at external corners.

Figure 3 A preview of a cutting program with the “soft cutting tool control method”

Some may wonder why traditional CNC controllers cannot be used to do the programming work in the “soft cutting tool control method”. Traditional CNC controllers are configured to do the interpolation in the controller firmware while the machine is moving. Because of the use 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 interpolation calculation fast enough to keep up with the pace of the machine 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. The main 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 cutting models can be more conveniently enhanced and upgraded. Because the infrastructure of the “soft cutting tool control method” is quite different from that of traditional CNC control method, its controller software and hardware is usually not available from the mainstream market of CNC motion control. Several waterjet companies have developed their own versions of the “soft cutting tool controller”.

An advantage of the “soft cutting tool control method” is illustrated in Figures 4 and5. A waterjet cut part out of 20mm thick aluminum with a traditional 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 100mm thick aluminum with a “soft cutting tool controller”. The difference between the top and bottom profiles of the part is much smaller than that in Figure 4, even though the thickness is much larger.

Figure 4 A waterjet cut part out of 20mm aluminum with a traditional CNC controller

Figure 5 A waterjet cut part out of 100mm aluminum with a “soft cutting tool controller”

The “soft cutting tool controller” offers benefits beyond higher cutting speed and quality. Because cutting models and 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 a new 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 digital and 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 technological breakthroughs in several articles. Even though the height and quality of these articles are unfortunately limited by the author’s knowledge and vision, the author hopes that they serve the purpose of opening the dialog and more people will share their insights, visions, and opinions. The author sincerely welcomes feedback, corrections, and discussions. Feedback can be received at my email address:  zengjiyue@lionstek.com.