The interfacing software named PSG Slice has been developed using the. computer programming language C. Since, the software has a mouse driven

Transcription

1 CHAPTER 6 DEVELOPMENT OF SLICING MODULE FOR RAPID PROTOTYPING MACHINE 6.1 INTRODUCTION The interfacing software named PSG Slice has been developed using the computer programming language C. Since, the software has a mouse driven graphical user interface, it requires mouse driver and graphics driver programs suitable for the system hardware. The software works using C language and CAD package. For modelling and STL file creation Pro/ENGINEER (or any other solid modelling software, which is having SLA interface) is used [41], CAD package helps us to model even very complicated shapes. This software provides the facility to rotate, zoom and shade the model. C programming language is used for the source code development because of its powerful programming capacity and portability [42, 43]. Using the above-mentioned features of CAD package and C, the software has been customized to generate NC program. Customizing involves writing programs in turbo C and creating special screen menus. The software is completely user interactive and the user need not have any previous knowledge in C Programming [44], Inside this package main screen, one has to select the different modules from the screen menu. The user is guided by a series of prompts and messages supported by menu options. The component is modelled using CAD package. The slicing module can be used for slicing the model into different slices and SLA output is taken for each slice. 6.1

2 After getting STL file, NC program can be obtained by going to the main screen and choosing the appropriate menu option at any stage. The tool path simulation facility is a very powerful tool to check the feasibility of the prototyping. It gives warning to the user regarding laser path interfacing. Demo program helps the user in understanding the principle of rapid prototyping. The data transfer of the NC codes from the computer to the RP machine is made through RS-232/C serial port. 6.2 WORKING PRINCIPLE OF PSG SLICE PACKAGE The main steps in Rapid Prototyping are as follows: The component is modelled using CAD package. A STL file is generated. For taking STL output, CAD package interface option is been used. The model is cut into different slices and SLA output for each slice is taken. Fig. 6.1 shows different stages in RP software. 6.2

3 Fig. 6.1 Stages in RP Software 6.3 SOFTWARE DEVELOPMENT PROCESS The flowchart in Fig.6.2 indicates the different steps involved in the development of the software. 6.3

4 Fig. 6.2 Software development phases 6.4

5 6.3.1 Requirement phase In this phase, current developments in the field of work and the actual requirements of the software are studied. In PSG Slice, the orientation selection is a user defined one and will generate a path in different types of hatch patterns. Quickslice used in commercial RP machines also will give the possible six orientations, but not the optimum one. Current developments in layer manufacturing are, towards the automated selection of optimum build direction and optimum path, which will minimize the overall cost. By considering build time, support structure requirement and accuracy, the optimum orientation is selected. Build time affects the overall cost in building the component. To minimise the cost, the component should be manufactured as quickly as possible. So, there is a need to develop software for the selection of optimum build direction and to generate the path in such a way that the overall cost involved in the building of the component is minimized Specification phase The functional and performance requirements of the software are finalized and the Software Requirement Specification (SRS) document is prepared. The basic issues a SRS must address are > Functionality > Performance > Design constraints > External interfaces 6.5

6 Slice process Slice is the SL program used to define the geometric pattern which the laser, scans to solidify the prototype. The STL file triangle information is processed into layer vector data. The slice program for Stereolithography has evolved to the point that only a few characteristics need to be fine-tuned [45]. This is true whether a very quick concept model or a functioning prototype is required [46], However, flexibility has been intentionally retained in the software to allow optimization of the process [47], Scale Scale is simply the size ratio between the part to be built and the CAD model size. Using a scale of 1 will provide a rapid prototype of full size without reduction in size. Using a scale of 0.5 will build the part to half size. When making a RP&M pattern for any moulding process, it is possible to accommodate the shrink factor for the final part. Some SLA software is capable of scaling a file in all three axes separately [48, 49, 50], This is very helpful when either a CAD-produced support or a third party support-generating software is available. It is then possible to use a cube STL file and generate a box support of any x, y and z ratio [51, 52], Layer thickness The slice program converts three-dimensional STL files into two-dimensional cross sections. These cross section can be sliced from the CAD s x, y, or z-axis. The slice axis, by definition, is then perpendicular to the planes created by this cross section. When a layer thickness is assigned during the slice process, these 6.6

7 cross sections are derived at increments of that layer thickness [53], The stepping of the elevator in the same increment generates the actual part layer thickness [54], This converts the two-dimensional cross section into the three dimensional layer of the actual prototype. The thickness of each slice of the part affects the surface texture, the accuracy in the z-axis and the build speed [55]. Currently, these layers can be from 0.1 to mm thickness. Layer thickness is one of the most widely used variables in rapid prototyping. It is true that surface texture can usually be improved during postprocessing. However, when accuracy is a paramount in the slice axis, a layer thickness value of mm or less should be considered. Dimension in the slice axis is rounded off in increments of the layer thickness, not necessarily from the bottom of the part but from the origin. Contrary, to a popular misconception, thicker layer does not always result in shorter build time. Depending upon laser power, resin photosensitivity, and the area being scanned, the quickest layer to build is generally between and mm thick SRS structure The purpose of SRS document is to describe the functional and performance requirements of the software. It also describes the interfaces of the system. This document acts as a guide throughout the software development cycle. It will give the work to be done in each phase of the development process. It will be the base for validating the final delivered system. 6.7

8 a) General description SLA software should select the optimum build direction and it should have to generate the path at an optimum hatch angle. This software should provide the build time and the cost for each orientation of the component. The input to the software is *.STL file which is generated by anyone of the modelling software like Pro/E or IDEAS. b) Specific requirements Input and Output Input file to this SLA software is *.STL file format.the STL Format is a triangulation of the surface of the 3D model. The STL format has a list of facets with each triangle which is described by three unique vertices, and a normal pointing towards the exterior of the model. Expected outputs of SLA software are optimum orientation, manufacturing codes, build time and the overall cost. External interface requirements The CAD packages require a platform of Windows or any UNIX operating system. The CAD packages used for modeling and STL file generation require workstations or PCs. User Interfaces PSG SLA DISPLAY READ INPUT SLICE THICK SCALE SLICE ROTATE X HATCH SPACE ROTATE Y BUILD ROTATE Z BUILD TIME OPTIMUM COST INPUT AND MESSAGES Fig. 6.3 Screen Layout 6.8

9 User Interfaces include screen formats, menus and command structure. Fig. 6.3 shows the screen layout. User Commands READ INPUT SCALE SHOW MODEL ROTATE X, Y, Z OPTIMUM SLICE THICKNESS SLICE MODEL HATCH SPACING SCAN TYPE BUILD BUILD TIME COST Functional requirements SLA software must have the following capabilities, Read the STL file that is generated by 3-D modeling package. Orient the component in different orientations. Calculate the time required to build the part in those selected orientations. Compare and choose the optimum orientation in such a way that it would minimize the build time (minimize the part s height) Orient the part according to the user requirement. Optimize the path generation 6.9

10 Performance requirements > Minimum response time > Optimum processing time for each activity. Exception handling RP software should respond to the unexpected situations when some errors occur. RP software should give the optimum orientation. If any error occurs during reading of STL file or during the selection of orientation it should give the message in the message box Design phase The aim of design methodologies is not to reduce the process of design to a sequence of steps but to provide a guideline during the detailed design. Structured design methodology views every software system as having some input that is converted into desired output. The software is viewed as a transformation function that transforms the given inputs into the desired outputs, and the central problem of designing software system is considered to be properly designing the transformation function. The aim is to design the system so that programs implementing the design would have nice hierarchical structure, with functionally cohesive modules and few interconnections between modules as possible. The overall strategy is to identify the input and output streams and primary transformation that have to be performed to produce the output. High-level modules are then created to perform the major activities, which are later refined. The first step is to construct the Data Flow Diagram (DFD) for the problem. The details extracted from the problem are represented in the format of Data Flow 6.10

11 Diagram. The DFD during design represents how the data will flow in the system. Structuring a DFD for design activity, in which the designer visualizes the eventual system, its processes and data flows. DFD must show what transformations are needed in the software. Fig. 6.4 Data Flow Diagram for PSG Slice 6.11

12 Fig. 6.4 shows the data flow diagram for PSG Slice. The *.STL file output from the modelling software is given as input to the SLA software. SLA software will read this file and convert this file to another format, which consists of each triangle s x, y & z co-ordinates and the maximum height of the model in each axis. If the maximum size of the model exceeds the platform size then by using SCALE option it must be scaled down. Next, this updated data file is sent to AUTO ORIENT. In this module, the orientation which will minimize the build time is chosen. Rotate command is optional. If the user is not satisfied with the current orientation then the user can use this command to re-orient the model. After that the model is sliced to the user-specified thickness. This slice data file is sent to the NC code module where the slice data is extracted to generate NO code for manufacturing. From the codes, the overall build time required is calculated and using the cost of machine, operator, workstation and material, the cost involved in building the part is calculated Detailed design, implementation and testing procedure In this phase, different algorithms are tried out for the most efficient one. After finalizing the algorithm, coding is done using a programming language that is more suitable to the application. After the implementation, the software is tested with reference to the Software Requirement Specification (SRS) document. Each module of the software is tested separately, and then they are integrated to give the final output. These three phases of software development process are explained in the following sections. Each activity of the project is allocated with approximate time. A time chart showing the time break up for each activity is prepared. 6.12

13 Development of slicing algoiithm for rapid prototyping machine This chart is used to track the time duration of each activity while processing. It is prepared by identifying the work elements from the Software Requirement Specification document. Deficiencies in the existing software are listed out and the new functional requirements are also identified, and SRS is prepared to indicate the performance, functional requirements, desired input / output and the software/ hardware requirements. Data flow diagram is also prepared, which will give an idea about how the data flows between input and output. Detailed design and coding are the two activities, which will consume more time. After finishing all designs, the software has to be tested with reference to SRS. The final step is to prepare the report indicating the final results and the scope for further improvement Slicing module In the present work, the software is developed for slicing CAD models. The software takes STL files representing CAD models as its input. It slices the input STL model and generates NC codes for creating prototype layer by layer in RP machine developed. The details about the slice format and slicing procedure are discussed here. Working principles of different modules of the software are explained. Then, the outline of the methodology adopted for developing the package is discussed. Development of major modules of the package like Graphical User Interface, STL file reading module, re-orientation module, slicing module, NC code generation module and build time estimation module are explained in detail [56, 57, 58], 6.13

14 Tessellation (STL Format) Fig. 6.5 STL representation Some interface format is required for integrating the RP system with CAD system. A method of mapping a triangular mesh over a surface or solid model is devised by the manufacturers of rapid prototyping systems, which offers the possibility of an easy universal input from CAD to RP [59, 60], Almost all RP manufacturers accepted this approach called Tessellation. The STL format has a list of facets with each triangle described by three unique vertices and a normal direction pointing towards the exterior of the model [61], The STL file format includes both an ASCII and a binary version. While creating the STL file for the CAD model, the chord height deviation between the actual model surface and the triangular facets can be controlled [62], The accuracy in this approach depends upon the acceptable chordal error between the plane of the triangle and the surface it is approximating. This value has no relevance for polygonal surfaces and the concept of tessellating such an object is quite efficient. 6.14

15 However, this may not be very efficient for a curved surface where a high accuracy requirement will result in a very large data file. In reality, some compromise is made or the model is split in order to: > keep file sizes down > vary chordal accuracy > overcome tessellation problems Splitting a CAD model into parts allows the designer to keep the file size down to a manageable number of triangles by ensuring that only those features, which are important, are fine faceted [63], The transfer of 3D model to STL may be done in three different ways. The 3D model may be converted into: (a) A general neutral exchange format like IGES and then to STL. (b) An intermediate format like DXF before being converted to STL. (c) STL directly. Any 3D model to be fabricated in RP has to be converted into a triangulated model, using surface triangulation techniques. Post-Processing operations such as orientation determination, support creation and slicing are done on the STL model [64], The STL is used as: (i) (ii) Neutral exchange format for transferring a 3D model to RP. A secondary representation of a 3D model on which all post processing operation is performed. (iii) An internal format for storing the secondary representation. 6.15

16 6.4 SOFTWARE DESIGN AND STRUCTURE Structure of the software The software PSG Slice is mouse driven and requires no previous knowledge in C language. The Graphical User Interface (GUI) of PSG Slice has twenty-two button controls, a view window, an input and message window, and a status bar to make the user interaction easy. The software when initialised prompts a screen with twenty-two button controls and the user is free to click any button and execute the function. The status bar of PSG Slice displays the default values of all the process parameters considered in the software and updates the display when the user changes them. The user is guided by a series of prompts and messages when he/she goes wrong. The view window displays the input model when the input file is successfully read. It is also used to display the slicing process, build process and build time calculation. PSG Slice accepts three dimensional design files from all standard software packages that can produce files in STL format [65]. It is important for STL files to be relatively free from defects before reading them into PSG Slice. The user is free to set the major process parameters like slice thickness, hatch spacing, and line width compensation and hatch pattern. Another important process parameter is that the laser power is not controllable by the NC codes fed into the control system of RP machine developed, and it is the operator s job to set an optimum laser power according to the travel speed of laser delivery system. The build space of RP machine developed is 500 x500 x 500 mm, so the input models larger than this size are automatically scaled down by the software. 6.16

17 The user is allowed to re-scale the input models, but the maximum dimensions of the model are not allowed to exceed the build space of the machine. The 3D display of input model and its dimensions help the user to understand its orientation. Since the orientation of the model plays an important role in minimising build time and improving surface finish, PSG Slice provides options for the user to re-orient the model by rotating X, Y and Z axes separately. An input model could be sliced number of times with different values of sice thickness, but the software keeps only the data about the latest sliced model in memory for generating NC codes. If a new model is read or slice thickness value is changed, the slice data will be deleted. PSG Slice allows the user to specify line width compensation, which is necessary for boundaries to be cured accurately. The default value for line width compensation is half of the hatch spacing and it could be changed to a user-desired value to minimise the heataffected zones in nearby areas of the model. Hatch spacing and hatch pattern are important process parameters from prototype s integrity point of view. The software is capable of generating NC codes to make prototypes with different simple hatch patterns including X hatch, Y hatch and alternative X and Y hatch. In all types of hatch patterns, the user can vary hatch spacing and have a boundary trace. The boundary trace improves surface finish of the prototype by clear edge definitions. The NC code generation module generates NC codes to make slices with the process parameters set by the user. The user is also allowed to vary with the datum point and output file name. Varying datum point helps the user to build models in a selected area of the platform [66]. PSG Slice provides two options for the user to generate NC codes for a sliced model - fast build and slow build. 6.17

18 As the name indicates, slow build is purposefully made slow and it provides a means of verification by displaying the generated NC codes, beside the 2D display of laser path. Where as fast build generates NC codes with a maximum speed at which the computer can do, and displays the laser path in 3D view. The build time estimation tool finds an estimation of time required for building a model in RP machine developed. It is possible for the user to optimise the model orientation by choosing one, which gives the least time estimation. PSG Slice does not create support structures automatically; the user has to include necessary supports in the model itself according to its orientation. The supports provided in the model can be removed after post curing Working principle of the software The model data file refers to the file containing model data after scaling and reorientation of data extracted from the input STL file [67], Usually STL files are created with default co-ordinate system, so it may fall anywhere on the model and some of the co-ordinate values may be negative. The negative values of model co-ordinates in STL file may cause problems while re-orienting and slicing the model. In order to avoid such problems, PSG Slice makes all co-ordinate values of the input model positive and stores them in the model data file. Since the software does not use the details about normal vectors of the facets representing the model, the software does not store them in the model data file. The model data file is overwritten each time when the model is re-scaled or reoriented. PSG Slice makes use of a temporary file while rotating the input model. The format of temporary file is same like the model data file, but it may have negative co-ordinate values since the model is rotated about its centre point. 6.18

19 Data from the temporary file are brought back to the model data file after making the co-ordinate values positive, PSG Slice uses a slice format similar to CLI format, in which the layers are represented by their thickness and the outline [68], The outline of a layer is a set of contours represented by polylines. Since PSG Slice slices models from bottom to top, slice data are stored in the ascending order of slice height. The slice data file generated by PSC Slice is same like one generated by other commercial software, which use CLI slice format and it is independent of the RP technology used for model building. The NC code generation module takes the slice data file as input and generates NC codes to create physical models in RP machine. The user specified parameters, hatch spacing and hatch pattern are considered in this module. The NC code file generated by PSG Slice mainly consists of G codes for moving the laser beam and the elevator table, and M codes for controlling the operations of the shutter and the re-coating mechanism. The build time estimation module gives an estimation of time for building a model in RP machine. It scans the NC code file and calculates the total time elapsed in moving the laser delivery system, and operating the shutter and the re-coating mechanism. It considers scanning speed, time delay in shutter operations, distance moved by the laser delivery system, number of shutter operations and re-coatings for the calculation. The time estimation is displayed only on the screen, not stored in hard disk. PSG Slice allows the user to edit the generated NC codes and send the NC code file to the machine through RS 232-communication port. 6.19

20 6.5 SOFTWARE METHODOLOGY The PSG Slice program consists of about 250 user defined functions arranged in 9 header files according to their usage. The main function of the program displays the GUI screen and continuously senses the mouse position with the help of software interrupts. When the user clicks a button, it calls the corresponding function. The software can be divided into nine modules and the methodology of developing each module is explained below Graphical user interface (GUI) The Graphical User Interface shown in Fig. 6.6 helps the user to interact with the software easily. The title bar displays the software name PSG Slice with a dotted background. The buttons are displayed with names indicating their function and the mouse pointer is restricted to move only inside the button window. The GUI module of the software leaves the view window, input / message window blank, and are handled by model display module and user input module respectively. The status bar at the bottom of the screen displays the current values of process parameters set in the software. The background for title bar, view window, button window and input / message window are made by drawing a bar with proper fill styles set by set fill style function. 6.20

21 Fig. 6.6 Graphical User Interface User inputs PSG Slice takes many types of inputs from the user such as characters, strings, integers and real numbers. The user defined functions developed for validating integers and real numbers read the inputs as strings and convert them to appropriate type after type checking. The function for validating integer type inputs gives error, if it finds any alphabet or special character in the input string. The function for validating real numbers does so in all the above cases, but the only exception is the decimal point Reading STL file The function used for reading STL files creates model data file with necessary manipulations on the data extracted from STL file. It checks for the availability of the user specified STL file in the current working directory, before opening the 6.21

22 file. The process of reading STL file is stopped, if any problem is found in the file format. The function scans through the STL file and finds the maximum and minimum co-ordinate values of the model in each axis. The new co-ordinate values are multiplied with a reduced scale factor value, if the model size is larger than the build space of RP machine. The model smaller than the build space of RP machine is not automatically scaled down Model display The 3D display of the input model is necessary for the user to get an idea of its shape, size and orientation. A user-defined function has been developed to display input models by reading the model data file. This function is called from the main function, when the model is re-scaled or re-oriented. Show model button in the GUI also allows the user to view the input model at any time. The model display function uses a size-fitting algorithm to display input models of any size within the view window. It calculates the magnification factor from model dimensions and multiplies it with facet co-ordinates. The formula for calculating the magnification factor of a model is given below. If (Max X >= Max Y) and (Max X >= Max Z) M_factor =160/ Max X Else if (Max Y >= Max X) and (Max Y >= Max Z) M-factor = 160 / Max Y Else M_factor =160/ Max Z Fig. 6.7 shows the model display in PSG Slice 6.22

23 Fig. 6.7 Model Display Re-orientation In most of the cases, the use of default co-ordinate system ends up with an unwanted orientation of the STL model. PSG slice has been developed with provisions for re-orienting the model represented by the input STL file. The orientation of a model is a very important factor in avoiding support structure requirements, minimising build time, improving surface finish and minimising the size of NC code file. The user can achieve any orientation by rotating the axes one at a time. Since the re-orientation module uses the model data file as its source, the re-orientations and re-scaling done previously are available to it. While rotating one particular axis, the vertex co-ordinates corresponding to that axis are not changed, and the vertex co-ordinates corresponding to the remaining two axes are rotated more like a 2D point. The re-orientation module rotates the STL model about a line parallel to the axis of rotation and passing through the centre point of the model. An imaginary line joining the vertex point and centre point of the model is formed for each vertex point and is rotated 6.23

24 through the user specified angle by keeping the centre point of the model as fixed end. The resulting values of the vertex co-ordinates are stored in a temporary file. The following Fig. 6.8 shows the imaginary lines used for rotating a rectangle through 30 in clockwise direction. The rectangle after rotation is shown with thick lines. Fig. 6.8 Use of imaginary lines for model rotation The vertex co-ordinates stored in the temporary file may have some negative values, so they are converted to appropriate positive values with the help of formulae used in STL file reading function. The model data file is overwritten with the facets representing the model after rotation, and the model display function is called to display the rotated model on the screen. The Fig. 6.9 shows the display of a STL model with its default orientation, and after rotating the X-axis through 45. Mode1 : ASK3 Mode1 size :- X axis : 25.00mm V axis : 25.00mm Z axis : 25.00mm Total facets : 128 z Model : ASK3 Model size - X axis : 25.00mm Y axis : 3S.3Gmm Z axis : 35.36mm Total facets : 128 K Fig. 6.9 Display of STL model before and after rotation 6.24

25 6.5.6 Slicing The slicing module of the software cuts the STL model with a horizontal cutting plane to make slices of uniform thickness and stores the details about them in a slice data file using CLI format. At first, the software checks for the availability of model data file in which the facets representing the model are stored. After making sure that the model data file is available in the current working directory, the software calculates the number of slices of the model for the slice thickness set by the user. The thickness of the last slice may be less than the one specified by the user, if the model height is not exactly divisible by the slice thickness. The slicing algorithm checks each triangle stored in the model data file, whether the cutting plane cuts it or not. There are four possible cases for a triangle not to be cut by the cutting plane as shown in Fig and Fig and the slicing algorithm simply leaves such triangles. A case of one vertex above the cutting plane and remaining two on the cutting plane is not included in this, and is considered as triangle just above the cutting plane. 6.25

26 Case 1: One vertex below the Case 2: One vertex above cutting plane and remaining the cutting plane and two above the cutting plane remaining two below the cutting plane Case 3 : One vertex above the cutting plane, one below the cutting plane and remaining one on the cutting plane Case 4 : One vertex below the cutting plane and remaining two on the cutting plane Fig Possibilities for a triangle to be cut by the cutting plane Case 1: Triangle completely above the cutting nlane Case 2: Triangle completely below the cutting nlane Case 3: Triangle just above the cutting nlane Case 4: Triangle just below the cutting nlane Fig Possibilities for a triangle not to be cut by the cutting plane 6.26

27 The co-ordinate details of the triangles, which are cut by the cutting plane are used to calculate the co-ordinates of line of intersection. The Z co-ordinates of line of intersection are equal to the height of the slice. Case 1 x2 y2 z2 NX, = X3 + (X, - X3) / (Z3 -Z,) x (Z3 - h) NY, = Y3 - (Y3 - Y,) / (Z3 -Z,) x (Z3 - h) NX2 = X2 - (X2 - X,) / (Z2 -Z,) X (Z2 - h) NY2 = Y2 - (Y2 - Y,) / (Z2 -Z,) X (Z2 - h) Case 2 NX, = X3 + (X2 - X3) / (Z2 -Z3) x (h - Z3) NY, = Y3 - (Y2 - Y3) / (Z2 -Z3) x (h - Z3) NX2 = X, + (X2 - X,) / (Z2 -Z,) x (h - Z,) NY2 = Y, + (Y2 - Y,) / (Z2 -Z,) x (h - Z,) Case 3 X2 y2 z2 NX, = X3 NY, = Y3 NX2 = X, + (X2 - X,) / (Z2 -Z,) x (h - Z,) NY2 = Y, + (Y2 - Y,) / (Z2 -Z,) x (h - Z,) Fig Slice data extraction 6.27

28 The formulae used for calculating X and Y co-ordinates of the line of intersection in each case are shown in Fig The order and relative positions of vertices of all triangles need not be same; hence the slicing algorithm checks the order and relative positions of vertices before applying the formulae. It applies the above formulae with slight changes; if the order and relative positions of the vertices are not same like the one shown in Fig The intersection lines resulting in cutting the triangles come across the cutting plane represents the outline of the cross section. These lines are displayed on the screen with a magnification factor used in model display function. Fig shows the display of slices created for a cooling fan model. Fig Display of slices Re-scaling The model represented by input STL file is re-scaled by reading the file again with a user specified scale factor and overwriting the already available model 6.28

29 data file. As explained earlier, the model co-ordinates are multiplied with the scale factor value before writing them in model data file. The re-scaling function calls the STL file reading function after deleting the model data file and slice data file. Deleting slice data file avoids the confusion of having two different data in model data file and slice data file. When the user clicks the scale button before reading a STL file, an error message stating the unavailability of input file name is displayed NC code generation NC code generation module hatches the cross sections obtained in slicing module and uses the hatch data for generating NC codes. It uses lines parallel to X-axis for X hatch and lines parallel to Y-axis for Y hatch. It uses an array of structures for storing the slice details in the RAM. Each element in the array stores a line representing the outline of the slice. The hatch line generation algorithm cuts the slice outline with lines parallel to X-axis or Y-axis and extracts the details about the points of intersection for each cut. It cuts the out line from one end to another end with a user-specified spacing. The NC code generation module uses G01 code to achieve linear movements of the laser beam. It uses M codes for operating the shutter and the re-coating mechanism. The M codes for closing and opening the shutter are included in the NC code file for leaving the areas of internal features uncured. The NC code generation module also takes care of jumps in hatch lines due to isolated areas coming in a cross section. Fig shows a circular cross section having a hole inside, hatched with lines parallel to X-axis and lines parallel to Y-axis. The adjacent hatch lines are joined 6.29

30 by using a line in between them so that they can be scanned in a raster fashion without any break. The thin lines shown in the Fig indicate the laser beam movements by keeping the shutter in closed condition. Fig Hatching in circular section Fig NC code generation with 3D display of laser path The NC code generation module displays the laser path in 3D view or 2D view. 3D display of laser path is achieved with the conversion formulae used in model display function. NC code generation with 2D display of laser path is made slow by giving a delay of 20 microseconds for each movement. 6.30

31 The slower process and the display of co-ordinate values supplied to NC code file, helps the user to check the correctness of generated NC code. Fig shows the process of NC code generation with laser path displayed in 3D view. With the use of software interrupt which checks the status, the user is allowed to stop the execution of NC code generation function Build time estimation The build time estimation module reads the NC code file and calculates the total distance moved by the laser delivery system, number of shutter operations and number of re-coatings. It also extracts the scanning velocity and delay in shutter operations from the NC code file. The following formula is used to calculate the time required for building the model by processing the NC code. Time (in seconds) = Distance moved / feed rate + No. of Shutter operations x Time delay + No. of re-coatings x Re-coating time. 6.6 SOFTWARE TESTING AND VALIDATION PROCEDURE The software is mainly tested for its re-orientation capability, slicing capability and NC code generation capability. The software successfully reads the model represented by lakhs of triangles. The software is able to rotate the test models about all axes with angles ranging from 0 to 360. The models are also successfully sliced with various slice thickness values. The software generates NC codes for all the models with various orientations. The display of laser path is closely followed and no deviation is found for any 6.31

32 value of hatch spacing. The software is able to generate NC codes with three different fill patterns. Fig shows the sample model of cooling fan fabricated in the RP machine developed. The line width compensation and the datum point specified by the user are successfully used in the NC code generation. Fig Sample Prototype from RP Machine developed The software is able to handle thin sections and isolated areas without any jumps. The build time estimation given by the software depends on the number of slices and hatch spacing used for the model, and matches with time taken by the machine. Thus, the working of RP machine developed is verified with PSG Slice. 6.7 SUMMARY The interfacing software named PSG Slice has been developed using the computer programming language C. The different modules used in the software and the methodology of developing each module are explained in this chapter. The software is tested for different models. The NC codes generated by the software are transferred to the RP machine developed for the required model and verified. 6.32