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Part program automatic check for three axis CNC machines Roberto Licaria,*, Ernesto Lo Valvob, Mario PiacentiniaaUniversita di Palermo, Palazzo Steri Piazza Marina, 61-90133 Palermo, ItalybUniversita di Catania, Catania, ItalyAbstractThe simulation and verification of NC codes for CNC machining is a very important task. The aim of this work is to limit the number ofcutting tests needed to verify the right writing of the part program for a CN milling machine in the intent of saving time, human resourcesand money. This is obtained through the Boolean operation among solids, in AutoCAD environment, of the volume covered by the toolduring the operations ruled by the part program. # 2001 Published by Elsevier Science B.V.Keywords: CNC machines; AutoCAD; Part program1. IntroductionWe have recently attendedtothe irreversibledevelopmentof 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 indifferent stages: in the design stage using CAD systems;in the process planning stage using CAPP systems; in theproduction stage using CAM systems.Since afewyearsago,thesethreestages were isolated oneby another and each stage should have answered specificproblems and questions. Sometimes it happened that theproduction stage imposed some essential conditions to theother stages (as an example, when the designer establishesthe tolerance for the piece; or when it is necessary to makesome change to the piece during the production stage sincethere is an impossible or difficult machine production), butthe three stages were strictly separated.Moreover, it was thought that it was impossible fordifferent programs, written by different programmers withdifferent logics, to speak to each other. Recently,different programs tried to communicate in order to solvesome problems, but this is very difficult to be achieved.There exist a number of programs which are able toperform that way, but they are not universal programs:they are very specialized programs which can be used onlyin specific fields using powerful computers. As a matter offact,itisnecessarytodevelopauniversalsoftwareeasytobeused by a simple, common and very cheap PC.Numerical Control machines are very commonly used fortheir ability to help industries to achieve an increase inproductivity and in quality at the lowest costs. As a matter offact, NC machines are faster and more precise than tradi-tional ones and they work very accurate surfaces, but aremore expensive and it is more difficult to use them than thetraditional ones.Moreover, it is necessary to compile a specific program tobe read by the machine control unit in order to obtain thedata needed to exactly move the tool. This program (calledpart program) is written using a particular programminglanguage that can be read by every NC machine (machineshave to be similar: turning machines, end milling machines,etc.).The first problem we meet using NC machines is thatwhen the programmer makes a mistake in writing the partprogram, the piece will not be realized the way we want, butit will have a different shape or different features. But itcould be more dangerous (and also expensive) if the pro-grammingmistakegivesthetoolamotioncommandthatcangenerate a collision between the tool and the fixed parts ofthe machine, because of the speed of the NC machine tool ishigher than that of the traditional machine tool.We have other problems using NC machines, for examplehow to choose the right depth or feed rate or how to choosethe shape of the workpiece in order to minimize the materialwaste. As a rule, in order to solve these problems somecutting tests are realized, but they are very expensive to beimplemented since they are a waste of human resources,time, materials and money.Moreover, not all the problems are very easy to be solvedbyimplementingonetestonly,sothatthetesthastobemadeagain over and over. It should be really useful to makevirtual cutting tests using computers instead of NCmachines and as much useful should be the possibility toJournal 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)00812-8display the space regions crossed by the machine tool duringthe processing work.The diffusion of electronic realistic representation sys-temsofmechanicalpiecessuggestsustousetheminordertorealize a virtual simulation of the cutting tests for the threeaxis end milling machine 15.Our task was to create a software which can directly readand interpret the part program and display it using theAutoCAD solid modeler. Our software makes it possibleto compare the tested piece on the screen either with theproject piece or with the workpiece, and it shows the toolpath, so that dangerous collisions can be monitored.2. The cutting processThe cutting process is the result of an interferencebetween the tool and the workpiece, and it can be simulatedby a number of Boolean operations between primitives.The tool, an end milling tool, can be represented by arevolution AutoCAD solid. The tools swept volume canbe represented by surfaces, while edges and vertexes of thisvolume are created by the tool motion. Every primitivecreates its own swept volume, depending on the motiondirection. For example, a cylindric tool can move followinga line which can be parallel or orthogonal to the tool axis. Inthe first case,the swept volume is a higher cylinder, in thesecond case the swept volume is a combination of a boxand two half cylinders.A cutting process on a circular line can be represented bythe motion of a closed polyline (the tools cross-section)around a revolution axis (Fig. 1). These solids can besubtracted from the solid representing the workpiece, inorder to simulate the end milling cutting process.3. AutoCADAutodesks AutoCAD was the most popular and verypowerful CAD software for PC since it was introduced in1982. It has always been providing AutoLISP and ADSprogramming interfaces in order to develop customizedapplications. ADS is more efficient and easier to be usedthan LISP and it has been offered as an alternative interfacesince version R11. ADS uses ANSI-C as the programminglanguage since it has been the most widely accepted lan-guage for the development of miscellaneous applications. Itcan 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 ithas switched to ACIS standard since version R13. With thenewly released R14, we decided to upgrade our softwarebecause of the ACISs faster computing efficiency and moreprecise description of solids. Moreover, it reduces the size ofthe drawing files. As a consequence, ACIS is able to handlevery complicated models much better than AME can.4. The developed softwareOursoftwareiswrittenusingClanguagetobeexecutedinAutoCAD 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 beread and interpreted.The part program contains some instructions about thetool path (Gxx instructions), geometrical characteristics(like points coordinates or joint radius), technological char-acteristics (feed rate, spindle speed, etc.): the softwareinterprets the geometrical instructions only.The software runs inside AutoCAD and the operator candraw the workpiece or load it as an external file, choose thetool shape (there are four types of tool: cylindric, cylindricball-end, half sphere, sphere) and its dimensions: the soft-ware calculate and draw the cross-section of the tool that isan AutoCAD polyline (Fig. 2). Now the operator has to loadthe 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) 290293291The procedure, first of all, analyses the geometricalcharacteristics and organizes them in a chronological order(for example: the X-coordinate of the start point of a genericmotion is called oldX, the X-coordinate of the end point iscalled valX.After this first step, the software interprets the motioninstructions (G00, G01, G02 and G03) given by the partprogram and draws them through the AutoCAD commandsExtrude and Revolve. The Extrude AutoCAD com-mand can add the 3D to a 2D closed polyline, whereas theRevolve AutoCAD command realizes a revolution solidfrom a 2D closed polyline.TheG00instructionrepresentsthemotionsofthetoolwhenit does not touch theworkpiece: in our work it is representedby a prismatic AutoCAD solid. The cross-section of thissolid is the same as the tool and it is obtained by Extrudecommand. The simulation of this motionis useful in order toverify the possibility of a collision with fixturing.The G01 instruction represents the motions of the toolwhen it touches the workpiece: in our work it is representedby a prismatic AutoCAD solid. The cross-section of thissolid is still the same as the tool and it is obtained by theExtrude command. The Extrude AutoCAD commanduses a segment; its start point has oldX, oldY, oldZas coordinates and its end point has valX, valY, valZas coordinates.NoticethatinordertoexecutetheExtrudecommand,itisnecessary tohave the Z-axis aligned with that segmentandthe polyline lying on the XY plane. For this reason we havecreated the same instructions to change the AutoCADcoordinate system (UCS: user coordinate system).The G02 and the G03 instructions are represented byrevolution solids. These solids are created by the rotation ofthe cross-section of the tool around a revolution axis. Thisaxis starts from the center of the fillet and is perpendicular tothe XY plane.As the part program does not include the informationsneeded by AutoCAD in order to draw this solid, it wasnecessary for us to realize some calculation subroutines inorder to obtain the essential information from the partprogram data. Now the operator can start the simulation.Hechoosestheworkpiece(orhedrawsit)andhechoosestheshape and the dimension of the tool; the software automa-tically draws a polyline and puts it in the so-called restpoint, far away from the workpiece. The tool has aprogramming point: it is the point that follows the partprogram trajectory (Fig. 3).When our software processes a G00 or G01 instruction, ithas two options:1. The start point Z-coordinate is different from the endpoint Z-coordinate: we have a vertical motion and thesoftware draws a cylinder with the same radius of thetool and h ? ?Z2? Z1?.2. The start point Z-coordinate is the same as the end pointZ-coordinate: the tool moves on the XY plane and thesoftware makes a copy of the polyline cross-section ofthe tool and moves it towards the start point of themotion. The software changes the UCS (the Z-axis isaligned with the segment from the start point to the endpoint) and rotates the polyline since it has to beperpendicular to the Z-axis. Now the polyline can beextruded and the software draws a solid representing thetool motion (Fig. 4).When our software processes a G02 or a G03 instruction,the tool moves on the XY plane and the software makes acopy of the polyline cross-section of the tool and moves ittowards the end point (G02) or the start point (G03) of themotion. The result of this procedure is the revolution axis,the revolution of the polyline and the drawing of a solidrepresenting the tool motion (Fig. 5).At the end of the simulation, the operator can see on themonitor of his PC the complete tool path. But he has now aFig. 3. Tools programming point.Fig. 4. G01 command simulation.292R. Licari et al./Journal of Materials Processing Technology 109 (2001) 290293CAD file: this tool path is an AutoCAD solid, which can bemeasured, which perspective can be changed, which volumecan be calculated. He can also use another AutoCADcommand: the Subtract command by which he obtainsthe final shape of the workpiece and he can measure it, hecan change the viewpoint or obtain geometric informationon volume, center of gravity and so on.We have tested our software using some part program andthe results have been very flattering: it was very easy, fastand cheap to make these simulations. In Fig. 6 is reported asample image obtained with our software.Thesamplerepresentsaworkpieceof300mm?300mm ? 50mm mold-base steel with a central hole (dia-meter of 175 mm). The image shows the fixturing and themachinedpart witha ball-endtool (diameter of40 mm).Theapproximated simulation time for this piece is less than1 min using a Personal Computer equipped with a Pentium133 MHz processor and 32 MB memory RAM. So it ispossible to repeat the cutting simulation by changing thegeometrical parameters in a few minutes, in order to opti-mize the programming stage.5. ConclusionsThe part program check is very expensivein terms of timeand human resources if it is manually made. Thanks to theability to correct programming mistakes after the post-processing stage, many cutting tests can be cancelled, andthe machine functionally used. The developed software is avalid support to easily and quickly verify the part program.As a matter of fact, it can be integrated with a powerfulcommon 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 thecutting process. Moreover, since the rendering image ofsimulated results can closely represent the machined part,this software provides a better approach for NC simulationand verification on Personal Computer.The s