9th American Waterjet Conference
August 23-26, 1997: Dearborn, Michigan

Computer Aided Manufacturing for 3D Abrasive Water Jet Machining

A. Henning
Fraunhofer Society
Institute. for Produktion Engineering and Automation
Nobelstr. 12, 70569 Stuttgart, Germany
Tel.: +49 (0) 711 - 970 1775
Fax: +49 (0) 711 - 970 1004
e-mail: henning@ipa.fhg.de

ABSTRACT

This paper presents a systematic approach to computer aided manufacturing for two and three dimensional abrasive water jet applications. On the turning point to using the abrasive water jet in industrial production lines e.g. in automobile or aerospace industry as well as in machine shops for high quality and precision machining further demands have to be satisfied. With the described system for water jet cutting the actual shape of the tool can be modeled and so the cutting edge contour can be simulated. Using the results of the simulation both linear and angular tool path correction can be realized and an optimized cutting path is generated. Also machining strategies to meet demands for high contour accuracy, high cutting speed and high surface quality are integrated. With computer aided manufacturing production cost can be decreased by reducing labor and machine resources spend for preparation, the machining process can be optimized, and industrial standards for accuracy, reliability and efficiency will be met. Organized and Sponsored by the Waterjet Technology Association.

1. INTRODUCTION

Abrasive water jets have become a most recent tool in mechanical machining. With its great advantages of geometrical and material flexibility and its ability of cutting hard-to-machine materials the technology is more and more spreading among firms serving as job-shops many different fields of industry for two-dimensional cutting. Here the operators obtain a high level of operator's expertise and qualification. For two-dimensional machining tasks operators do all from programming and selection of the optimal cutting parameters to controlling the machining process with little external support (Henning, 1996)
In three-dimensional machining this cannot be done manually. Here computer support is needed. Other than in two-dimensional machining the operator can not generate the tool path using drawing programs. With three-dimensional machining generation of the tool path and selection of the process parameters is much more complex. The cutting conditions in three dimensional machining vary over the entire cutting path with changing cutting angles and work piece geometry. Even when only cutting sheet metal with chamfers the actual cutting depth continuously varies with the angle of attack. Therefore process parameters must continuously be adapted to the actual cutting conditions which cannot be done manually.
For three as well as for two dimensional cutting computer support is necessary. Several applications for computers support in water jet cutting. With computer based process control systems the reliability and the roughness of the process can be improved (Kischkat, 1996). With such also labor cost can be reduced when controlling many machines by one operator. For three shift machining this may be essential. Another field for computer support is the generation of the CAD model. Here approaches to complete process cycles from scanning to machining can be found (Kille, 1995). The third major support for water jet machining is the use of data bases or expert systems to archive and cumulate knowledge and experience to ease the selection of the parameters (Singh, 1995).
The subject that is discussed in this paper deals with the computerized generation of the optimal cutting path with suitable machining strategies. Conventional machining strategies always meant to manually iterate experiments until the demanded accuracy was achieved. Today's approaches using numerical modeling and simulation of the process not only save plenty of machining time but also can optimize the process towards various criteria. With such a tool geometrical accuracy of the cutting edge can be improved and therefore new approaches to precision machining can be introduced.

2. COMPUTER AIDED MANUFACTURING (CAM)

In the last ten years, demands on production processes have severely changed. With the fast growing of global markets, product development delays and life cycles of the products have been reduced. This comes along with the development and using of computers in product development as well as in the production process itself. Besides computer aided design (CAD) that is used for creating computerized models of the work piece, more and more companies employ computer aided manufacturing (CAM) to support the actual production process (Schwarz, 1994)

2.1 What is CAM

There are many approaches to CAM available on the market. Depending on the expected use, they can cover a large variety of functions, from scanning of hardcopy plots or prototypes of the product (e.g. seals) and contour analysis over tool path generation with tool correction up to off-line programming. Other functions such as process simulation, logistic functions, nesting algorithms, and resource planning modules can also be integrated (Kille, 1995). There is a large number of possible elements that can be integrated in such a CAM system. The most important function, though, is the generation of the tool path. In the past this has been a very time wasting and costly task for the operator. With the use of CAM systems the programming of the NC-code can be done off-line. This saves resources of labor and machining. Also, with simulation tools the accuracy of machining outcome can be improved.

2.2 Different Approaches to CAM

There are already many professional CAM solutions for all kinds of applications on the market. They can be divided into different categories due to the supplier on the market (Warnecke, 1989):

- machine orientated
- application orientated
- process orientated

Machine orientated CAM solutions are mostly sold by the machine suppliers to work with their own machines with their specific controls and specific NC-codes. This machine specific CAM is in many cases only designed to serve this specific brand-name. Adapting it to machines of other brand-names is very difficult or even impossible (fonshoff and Kader, 1993). Application orientated CAM solutions are designed to serve special jobs. They consist of modules that support the user with his special demands. Since they are designed for this application they can cover the whole process from design to manufacturing. For rapid prototyping e.g. all modules from scanning to programming and control of the machining process are available (Rauh, 1994).
Most CAM systems on the market consist of a basic program to supply the global functions as e.g. graphics or import and export devices and process specific modules. Often also associated with a CAD system, the program provides all functions the operator or programmer needs. Such solutions are also capable to serve as an integrated system for all tooling methods (milling, turning, cutting etc.) for a whole machine shop. The usability of the program here depends on the programming effectiveness and on the quality of the single process modules. CAM systems are often specialized on one special tooling methods as. Here, the program offers a large variety of items that can be used for this special technology. Although the programs offer modules for other tooling methods as well, they are often only added by using the functionality of the original modules. Therefore it is very important for the user to analyze which method he uses most when choosing a CAM system.
For tooling methods as water jet machining, that are not as widely spread as milling or turning, CAM solutions are mostly derived from conventional solution of other methods as e.g. EDM wire cutting. Machining with EDM is similar to water jet cutting by means of creating and controlling the cutting path. Yet there are major differences in geometric aspects and the influence and interaction of process parameters.

3. DEVELOPMENT OF A CAM SYSTEM FOR WATER JET MACHINING

3.1 Deficiencies of Conventional CAM Solutions for Water Jet Cutting

Conventional CAM solutions for cutting with the water jet are commonly derived from other machining methods like EDM, laser or even milling. These machining and cutting methods use very similar machine controls, so that programs can easily be converted. In many cases it might be good enough only to create the bare NC-code for the cutting task from a CAD-model. For high quality cutting more and specific functionality of the system is needed to compensate the effects of the jet lag or the taper of the kerf: This is not provided by conventional CAM systems, since those effects are unknown to the tooling method they were originally designed for. Working with milling tools e.g. will always lead to the same material removal. The radius and the shape of the tool are usually constant (see figure la). Changes of the shape due to alternating the velocity of the tool as we find it with water jet cutting, do not occur. Tool correction other than a linear offset is not necessary for milling technology. With water jet cutting, the tools taper cannot be described like a milling tool with parallel edges. Depending on the speed and other parameters, changes on the shape do occur. In figure 1 b, a schematic shape of the water jet beam being described as the 'tool' is shown. We do not only find non parallel edges as described by Guo (1994) but also an increasing displacement of the tool's center with increasing cutting depth.
With conventional 2D-CAM solutions, this changing jet tool shape has mostly been ignored. They assumed parallel edges and so the operator had to find the combination of parameters to fulfill these requirements. Doing so, the operator looses the possibility of optimizing the process regarding other important aspects as e.g. cutting speed or contour accuracy.

3.2 Approach to CAM for Water Jet Cutting

A specific CAM solution for water jet cutting has to cover a large variety of functions. It also has to take into account the real shape of the water jet tool as well as the critical points as e.g. corners and curves. The following specific functions have to be employed:

- Modeling of the tools taper
- Machining strategies
- Tool correction (linear, angular) with optimization
- Simulation of the machining process

According to usability studies, the CAM system has to meet industry standards. Therefore we have integrated users interface, where the operator or the programmer can e.g. select certain machining strategies that he considers the best solution. The program here serves as an advisory tool that proposes a choice from analyzing the situation. The decision though must be made by the operator from his expertise. Depending on the level of the operators experience and reliability of the system, these decisions can also be automated.
For the modeling the shape of the tool depending on the parameters the operator can feed the system with new data when using new materials or parameter combinations. For this, a user interface was created where the operator enters data that he can measure directly like the top and bottom diameter of a circular or square cut.

3.3 Modeling of the jet tool shape

The most important module of the CAM system is the modeling of the tool's shape. It is used to simulate as well as to calculate amount and direction of the tool correction. In literature, many models for the cutting kerf can be found (Hashish, Kim and Zeng, Blickwedel, etc.). Most of them model the maximum cutting depth from the given parameters. The model that is used here, models the shape of the tool from the process parameters. As shown in figure 2a, a geometric model of the tool water jet is created. It consists of two functions that describe the diameter and displacement of the tool against the moving direction (figure 2b). In figure 2c the side view of the tool gives an idea of the jet lag that occurs under these working conditions. The front view (figure 2d) shows the increase of radius with the cutting depth.
This model is based on empirical experience and functional dependencies of the parameters (e.g. Guo, 1994). From these data, significant codes are derived to set up a geometrical model that can be effectively used for tool correction and simulation. Since this is a semi-empirical model, the operator can easily enter new parameters for his working conditions (material, hydraulic parameters etc.).

3.4 Jet Tool Path Correction

As mentioned before tool path correction is one of the central elements of CAM systems. In two dimensional e.g. sheet metal cutting with two or three axis water jet cutting machines tool path correction can be achieved by simple linear offset. This can be done manually by the operator generating the NC code himself or with the support of the machine control. The most important part is here to find the right combination of cutting parameters. For three dimensional machining tool path correction is much more complex. Additional to the linear offset also angular corrections are possible. In all cases of 3D-machining though the correction is to be determined in vectorial manner with perpendicular displacement to the contour surface. Three different methods of tool path correction are implemented in the CAM system:

- linear offset to compensate the top tool width
- angular correction to compensate the variance of tool width
- angular correction to compensate the effects of the jet lag
- adaptation of the feed rate

All different types of corrections can be implemented individually according to the needs of the actual machining job. Since all angular corrections do result in a slight increase of cutting depth little reduction of the feed rate might be necessary to obtain the required surface quality. The basic tool correction that can be done in two dimensional machining as well is the linear offset. Here the cutting path keeps the distance of the tool top radius from the programmed contour (figure 3b). With this linear offset the contour accuracy can be improved very well compared to cutting on the very contour (figure 3a). In two dimensional cutting this kind of cutting path correction can be done by creation an equidistant to the contour. In three dimensional cutting the direction of the offset is normal to the contour and parallel to the work piece surface. With this method alone a moderate contour accuracy can be achieved when selecting the process parameters for parallel cutting edges.
In addition to linear correction of the tool path with angular correction the cutting process can be optimized in means of accuracy and cutting speed. For this, though, five-axis cutting machines are necessary. There are two angles that can be changed for compensating the shape of the tool. The first angle compensates deviations of the tool width in cutting direction (figure 3c). Using this kind of correction parallel cutting edges are not necessary, and the feed rate can be increased.
The second angle points in cutting direction. With this the effects of the jet lag can be minimized (figure 3e). This is very important at critical points such as e.g. corners and at critical contour elements as small radius curves. Here typical geometrical errors as e.g. overshooting can be avoided with the second angular correction.

3.5 Machining Strategies

The definition of machining strategies is another vital module of a CAM system. Here the machining process is optimized according to the given demands. Typical optimizing criteria are the following:

- high cutting speed (little machining time)
- high contour surface quality
- high contour accuracy (size, shape)

The cutting parameters and the cutting path have to be adjusted according to the optimizing criteria or weighted combination chosen. Strategies for compensation of geometrical errors do include more than only changing the given tool path and adding some angles to it. For best results also the cutting speed has to be adapted to the actual cutting depth and for the actual cutting situation. So the feed rate ought to be decreased before the process comes to a critical point. Thus adding a speed profile to the cutting path, it has to be split into smaller segments according to the actual needs.
As shown in figure 4 there are major differences between the top contour (figure 4b) and the shape of the bottom contour (figure 4c). Especially corners and small radius curves deviations from the expected contour occur. These critical points have to be treated separately at all machining strategies.

3.5.1 Corner

Corners are a very sensitive subject with water jet cutting. Due to the jet lag a deviation from the expected and programmed contour occurs. The deviation increases with the cutting depth and has the shape of an overshot (see figure 4b). This cannot be allowed for industrial cutting with exactly defined contours. Therefore the cutting path has to be changed in order to compensate this effect. As shown in figure S, three different strategies are implemented into the CAM system. The easiest compensation strategy is to stop at the corner, waiting for the jet lag to catch up to the tool position (figure 5a). When starting off for cutting the next edge, damage on the outside edge will occur, though. Another strategy is to fillet the tool path at the corner with the radius of the tool and decelerating at the same time (figure 5c). With this strategy, the corner is most likely to be filleted as well due to the very low feed rate in the center point of the circle. At concave corners the same strategies are used (figure 5d). Here the minimum contour radius that can be achieved is the radius of the tool. Recent developments have shown that a radius of 0.2 mm and below can be realized. The sharpest corner and the best results can be achieved by doing the loop (figure 5b). The cutting contour for both edges is created by straight cutting of the edges before and after the corner. Therefore maximum contour accuracy at the corner can be expected by following this strategy. But this also means an extra need of space and machining time. With three dimensional tool path correction the geometric accuracy at the corner can be improved at minimum waste of machining time by compensating the effect of the jet lag through angular correction.

3.5.2 Curved Contour

Machining curves with the abrasive water jet is also a sensitive matter. Especially when machining curves with a small radius the cutting contour shows significant geometric deficiencies. Depending of the diameter of the cutting curve the cutting contour varies at the same cutting conditions (figure 6). The effect is bigger for smaller diameters. This can be explained by the constant jet lag that always points against the actual feeding direction. With small diameters at the same jet lag the radial deviation is larger than with bigger diameters (figure 7). Because of the non linear jet lag tool the contour at curved cuts also shows a curved shape. (figure 6). Conventional approaches to achieve high accuracy at curved cuts have mostly reduced the feed rate in order to abate the effect of the jet lag to a minimum. With the lower speed the shape of the tool will change again to a wider cutting shape at the bottom. To achieve high contour accuracy at an economical cutting speed three dimensional tool path correction both angular and linear is needed.

3.5.3 Straight contour

At straight contours conventional 2D cutting systems can achieve a high degree of contour accuracy as well. For this thorough selection and close control of the cutting parameters is necessary, though. With a certain combination of cutting parameters that depend on material properties, cutting depth, and other machining conditions parallel cutting edges can be realized. But this also means that there is only one feed rate that can be used for machining (Guo, 1994). Therefore optimization is only possible in one criteria. The operator has to consider which optimization criteria he will follow: contour accuracy or cutting speed. With 3D-angular tool path correction the maximum cutting speed can be used without losing contour accuracy. The only limit is set by the minimal allowed surface quality of the cutting edge.

3.6 Cutting Edge Simulation

The major problem with water jet machining is to determine the cutting edge 1 contour before carrying out the actual job. Thus high precision machining has always been dependent on the expertise of the operator. and a matter of iterating approaches to meeting the demanded accuracy. With the previously described modeling of the jet tool shape (see chapter 3.3) not only correction of the cutting path is possible but also simulation of the actual cutting. edge contour. With this the tool path can be optimized without wasting machine resources. The outcome can be simulated that in most cases tolerances are kept from the. first piece without iterating experiments.
In figure 8 a sample of the simulation is shown. The work piece was to be cut with maximum possible speed. With angular correction of the tool path it was possible to., obtain both demands for high accuracy and minimal cutting time. In figure 8b and 8c one part of the simulation is magnified. It is very good to observe in figure 8b that big deviations from the expected contour do occur. With angular correction in figure 8c the deviations are reduced to a minimum.

3.7 Data Import and Export

Import and export functions are vital for communication of the CAM with its environment. This being a standalone solution it is possible for all users to adapt easily with standard file interfaces. Geometric data can be imported in many different formats like e.g. CNC, DXF. These data are converted to the internal used Vector-Paint-Sets. After processing and simulation these can be exported to the machine control for machining. Most controls req4ire machine specific NC-data, though. Here converter for most controls are available or can be added rapidly.

4. CONCLUSION

Water Jet technology has become one of the most recent technologies for machining metal and non-metal materials. Because of its unique features and its high flexibility, its use is more and more spreading not only among job-shops but as well in industrial machining. Thus having reached a turning point from laboratory use where the operator can control and optimize the process with his personal expertise to using water jet technology as an efficient and economical tool for manufacturing different demands have to be met. Here computer support is needed to reduce preparation time and optimize and control the machining process.
With this computer aided manufacturing system for abrasive water jet cutting high precision machining with the abrasive water jet can be realized in both two and three dimensional applications. On the base of three dimensional geometrical jet shape modeling tool path correction both linear and angular can be carried out for maximum precision of the cutting edge. Additionally the use of machining simulation replaces laborious experiments for optimization of the machining outcome. With the application of specific machining strategies the outcome can be optimized regarding machining time, contour accuracy and surface quality.

5. REFERENCES

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Henning, A. , "Entwicklung eines Miniatur-Abrasiv-Schneidkopfes" (Development of an Abrasive Cutting Head for small Jet Diameters), Study Thesis, Universitat Hannover, Germany, 1992.

Kille, K. , "CAD/CAM Integration into .Production Line," Precise Processing by Advanced Water Jet Technology, Japanese-German Joint Seminar, Shimizu and Sendaj, Japan, 1995

Kischkat, R. , "Process Control for High Pressure Water Jet Cutting Systems," 13th International Conference on Jetting Technology," Cagliari, Italy, 1996.

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Schwarz, H: , "SimulationsgestUtzte CAD/CAM-Kopplung fUr die 3D-Laserbearbeitung mit integrierter Sensorik," Dissertation of Helmut Schwarz at Lehrstuhl filr Werkzeugmaschinen und Betriebswissenschaften der Technischen Universitiit Milnchen, Springer Verlag, Germany, 1994.

Singh, P., "Development of a Windows-based Expert System for Abrasive Waterjet Cutting," 8th American Water Jet Conference, pp. 717-726, Quantum Industries International, Inc., Bethlehem, PA 18018, 1995.

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