外文 Part program automatic check for three axis CNC machines.pdf

Part program automatic check for three axis CNC machines Roberto Licaria,*, Ernesto Lo Valvob, Mario Piacentinia aUniversita di Palermo, Palazzo Steri Piazza Marina, 61-90133 Palermo, Italy bUniversita di Catania, Catania, Italy Abstract The simulation and verifi cation of NC codes for CNC machining is a very important task. The aim of this work is to limit the number of cutting tests needed to verify the right writing of the part program for a CN milling machine in the intent of saving time, human resources and money. This is obtained through the Boolean operation among solids, in AutoCAD environment, of the volume covered by the tool during the operations ruled by the part program. # 2001 Published by Elsevier Science B.V. Keywords: CNC machines; AutoCAD; Part program 1. Introduction We have recently attendedtothe irreversibledevelopment of computers, that now are cheaper, more friendly and, consequently, more diffused in modern industries. Compu- ters have been used in the industry sector for several years in different stages: in the design stage using CAD systems; in the process planning stage using CAPP systems; in the production stage using CAM systems. Since afewyearsago,thesethreestages were isolated one by another and each stage should have answered specifi c problems and questions. Sometimes it happened that the production stage imposed some essential conditions to the other stages (as an example, when the designer establishes the tolerance for the piece; or when it is necessary to make some change to the piece during the production stage since there is an impossible or diffi cult machine production), but the three stages were strictly separated. Moreover, it was thought that it was impossible for different programs, written by different programmers with different logics, to speak to each other. Recently, different programs tried to communicate in order to solve some problems, but this is very diffi cult to be achieved. There exist a number of programs which are able to perform that way, but they are not universal programs: they are very specialized programs which can be used only in specifi c fi elds using powerful computers. As a matter of fact,itisnecessarytodevelopauniversalsoftwareeasytobe used by a simple, common and very cheap PC. Numerical Control machines are very commonly used for their ability to help industries to achieve an increase in productivity and in quality at the lowest costs. As a matter of fact, NC machines are faster and more precise than tradi- tional ones and they work very accurate surfaces, but are more expensive and it is more diffi cult to use them than the traditional ones. Moreover, it is necessary to compile a specifi c program to be read by the machine control unit in order to obtain the data needed to exactly move the tool. This program (called part program) is written using a particular programming language that can be read by every NC machine (machines have to be similar: turning machines, end milling machines, etc.). The fi rst problem we meet using NC machines is that when the programmer makes a mistake in writing the part program, the piece will not be realized the way we want, but it will have a different shape or different features. But it could be more dangerous (and also expensive) if the pro- grammingmistakegivesthetoolamotioncommandthatcan generate a collision between the tool and the fi xed parts of the machine, because of the speed of the NC machine tool is higher than that of the traditional machine tool. We have other problems using NC machines, for example how to choose the right depth or feed rate or how to choose the shape of the workpiece in order to minimize the material waste. As a rule, in order to solve these problems some cutting tests are realized, but they are very expensive to be implemented since they are a waste of human resources, time, materials and money. Moreover, not all the problems are very easy to be solved byimplementingonetestonly,sothatthetesthastobemade again over and over. It should be really useful to make virtual cutting tests using computers instead of NC machines and as much useful should be the possibility to Journal of Materials Processing Technology 109 (2001) 290293 *Corresponding author. 0924-0136/01/$ see front matter # 2001 Published by Elsevier Science B.V. PII: S0924-0136(00)00812-8 display the space regions crossed by the machine tool during the processing work. The diffusion of electronic realistic representation sys- temsofmechanicalpiecessuggestsustousetheminorderto realize a virtual simulation of the cutting tests for the three axis end milling machine 15. Our task was to create a software which can directly read and interpret the part program and display it using the AutoCAD solid modeler. Our software makes it possible to compare the tested piece on the screen either with the project piece or with the workpiece, and it shows the tool path, so that dangerous collisions can be monitored. 2. The cutting process The cutting process is the result of an interference between the tool and the workpiece, and it can be simulated by a number of Boolean operations between primitives. The tool, an end milling tool, can be represented by a revolution AutoCAD solid. The tools swept volume can be represented by surfaces, while edges and vertexes of this volume are created by the tool motion. Every primitive creates its own swept volume, depending on the motion direction. For example, a cylindric tool can move following a line which can be parallel or orthogonal to the tool axis. In the fi rst case,the swept volume is a higher cylinder, in the second case the swept volume is a combination of a box and two half cylinders. A cutting process on a circular line can be represented by the motion of a closed polyline (the tools cross-section) around a revolution axis (Fig. 1). These solids can be subtracted from the solid representing the workpiece, in order to simulate the end milling cutting process. 3. AutoCAD Autodesks AutoCAD was the most popular and very powerful CAD software for PC since it was introduced in 1982. It has always been providing AutoLISP and ADS programming interfaces in order to develop customized applications. ADS is more effi cient and easier to be used than LISP and it has been offered as an alternative interface since version R11. ADS uses ANSI-C as the programming language since it has been the most widely accepted lan- guage for the development of miscellaneous applications. It can also use all portable ANSI-C libraries. We developed our software in 1996 using AutoCAD R12. At that time AutoCAD used AME for solid modeling, but it has switched to ACIS standard since version R13. With the newly released R14, we decided to upgrade our software because of the ACISs faster computing effi ciency and more precise description of solids. Moreover, it reduces the size of the drawing fi les. As a consequence, ACIS is able to handle very complicated models much better than AME can. 4. The developed software OursoftwareiswrittenusingClanguagetobeexecutedin AutoCAD ADS environment with some instructions pecu- liar to AutoCAD commands execution 6,7. The software is divided into two fundamental parts: ? The first part creates an interface between the part pro- gram and the AutoCAD ambient. ? The second part makes the part program data ready to be read and interpreted. The part program contains some instructions about the tool path (Gxx instructions), geometrical characteristics (like points coordinates or joint radius), technological char- acteristics (feed rate, spindle speed, etc.): the software interprets the geometrical instructions only. The software runs inside AutoCAD and the operator can draw the workpiece or load it as an external fi le, choose the tool shape (there are four types of tool: cylindric, cylindric ball-end, half sphere, sphere) and its dimensions: the soft- ware calculate and draw the cross-section of the tool that is an AutoCAD polyline (Fig. 2). Now the operator has to load the part program and the simulation can start. Fig. 1. Tools swept volume.Fig. 2. Tool options. R. Licari et al./Journal of Materials Processing Technology 109 (2001) 290293291 The procedure, fi rst of all, analyses the geometrical characteristics and organizes them in a chronological order (for example: the X-coordinate of the start point of a generic motion is called oldX, the X-coordinate of the end point is called valX. After this fi rst step, the software interprets the motion instructions (G00, G01, G02 and G03) given by the part program and draws them through the AutoCAD commands Extrude and Revolve. The Extrude AutoCAD com- mand can add the 3D to a 2D closed polyline, whereas the Revolve AutoCAD command realizes a revolution solid from a 2D closed polyline. TheG00instructionrepresentsthemotionsofthetoolwhen it does not touch theworkpiece: in our work it is represented by a prismatic AutoCAD solid. The cross-section of this solid is the same as the tool and it is obtained by Extrude command. The simulation of this motionis useful in order to verify the possibility of a collision with fi xturing. The G01 instruction represents the motions of the tool when it touches the workpiece: in our work it is represented by a prismatic AutoCAD solid. The cross-section of this solid is still the same as the tool and it is obtained by the Extrude command. The Extrude AutoCAD command uses a segment; its start point has oldX, oldY, oldZ as coordinates and its end point has valX, valY, valZ as coordinates. NoticethatinordertoexecutetheExtrudecommand,it isnecessary tohave the Z-axis aligned with that segmentand the polyline lying on the XY plane. For this reason we have created the same instructions to change the AutoCAD coordinate system (UCS: user coordinate system). The G02 and the G03 instructions are represented by revolution solids. These solids are created by the rotation of the cross-section of the tool around a revolution axis. This axis starts from the center of the fi llet and is perpendicular to the XY plane. As the part program does not include the informations needed by AutoCAD in order to draw this solid, it was necessary for us to realize some calculation subroutines in order to obtain the essential information from the part program data. Now the operator can start the simulation. Hechoosestheworkpiece(orhedrawsit)andhechoosesthe shape and the dimension of the tool; the software automa- tically draws a polyline and puts it in the so-called rest point, far away from the workpiece. The tool has a programming point: it is the point that follows the part program trajectory (Fig. 3). When our software processes a G00 or G01 instruction, it has two options: 1. The start point Z-coordinate is different from the end point Z-coordinate: we have a vertical motion and the software draws a cylinder with the same radius of the tool and h ? ?Z2? Z1?. 2. The start point Z-coordinate is the same as the end point Z-coordinate: the tool moves on the XY plane and the software makes a copy of the polyline cross-section of the tool and moves it towards the start point of the motion. The software changes the UCS (the Z-axis is aligned with the segment from the start point to the end point) and rotates the polyline since it has to be perpendicular to the Z-axis. Now the polyline can be extruded and the software draws a solid representing the tool motion (Fig. 4). When our software processes a G02 or a G03 instruction, the tool moves on the XY plane and the software makes a copy of the polyline cross-section of the tool and moves it towards the end point (G02) or the start point (G03) of the motion. The result of this procedure is the revolution axis, the revolution of the polyline and the drawing of a solid representing the tool motion (Fig. 5). At the end of the simulation, the operator can see on the monitor of his PC the complete tool path. But he has now a Fig. 3. Tools programming point. Fig. 4. G01 command simulation. 292R. Licari et al./Journal of Materials Processing Technology 109 (2001) 290293 CAD fi le: this tool path is an AutoCAD solid, which can be measured, which perspective can be changed, which volume can be calculated. He can also use another AutoCAD command: the Subtract command by which he obtains the fi nal shape of the workpiece and he can measure it, he can change the viewpoint or obtain geometric information on volume, center of gravity and so on. We have tested our software using some part program and the results have been very fl attering: it was very easy, fast and cheap to make these simulations. In Fig. 6 is reported a sample image obtained with our software. Thesamplerepresentsaworkpieceof300mm? 300mm ? 50mm mold-base steel with a central hole (dia- meter of 175 mm). The image shows the fi xturing and the machinedpart witha ball-endtool (diameter of40 mm).The approximated simulation time for this piece is less than 1 min using a Personal Computer equipped with a Pentium 133 MHz processor and 32 MB memory RAM. So it is possible to repeat the cutting simulation by changing the geometrical parameters in a few minutes, in order to opti- mize the programming stage. 5. Conclusions The part program check is very expensivein terms of time and human resources if it is manually made. Thanks to the ability to correct programming mistakes after the post- processing stage, many cutting tests can be cancelled, and the machine functionally used. The developed software is a valid support to easily and quickly verify the part program. As a matter of fact, it can be integrated with a powerful common CAD software that can manage a solid modeler. Thanks to it, the designerprogrammer is able to immedi- ately see the piece and the mistakes of the shape or of the cutting process. Moreover, since the rendering image of simulated results can closely represent the machined part, this software provides a better approach for NC simulation and verifi cation on Personal Computer. The si