Advanced Machining Center Setup & Development of CNC Post-Processors

Transcription

1 Advanced Machining Center Setup & Development of CNC Post-Processors Eduardo Luís Mateus Chocalheiro Mechanical Engineering Department, Instituto Superior Técnico, Technical University of Lisbon, Lisbon, Portugal Abstract Multi-Axis Machine Tools are increasingly used in the manufacture of parts, because they allow their execution with fewer fastenings and consequent gain in overall accuracy, as well as executing more complex geometries. Among Multi-Axis machines, those who have had greater application and evolution at industry level are the Numerical Controlled (NC) Milling Machines. However, the installation of these production units requires a great number of procedures at startup level, the preparation of clamping devices, tools and, especially, the Computer Aided Design and Machining softwares (CAD/CAM). This master thesis is developed in the field of installation of a 5-Axis Machining Center and its auxiliary systems for its normal functioning. This work was held at the worksohp of Instituto Superior Técnico of Lisbon and it was developed from the reception of the equipment, through network settings to the startup operation. It was also necessary to apply certain procedures for proper installation and functioning of the machine, installing CAD/CAM softwares and develop Post-Processors. This master thesis work made it possible to consolidate the installation procedures and startup of 5-Axis Machining Centres, as well as perform a comparative evaluation between some CAM softwares and the development of Post-Processors. Keywords: Machining; Multi-Axis; CAD/CAM; Post-Processor. 1. Introduction Thirty years ago, industrial companies wondered if it would be worthy to replace the conventional machines by CNC technology. Nowadays, the 5 Axis CNC s, are presented as valuable alternatives to the traditional 3 Axis. In any business, investing in the latest technologies has the repercussion being placed ahead of the rest of the market. In the other hand, companies that remain faithful to traditional machines ended up losing competitiveness in an increasingly global market. Keeping those things in mind, and because Instituto Superior Técnico is well known for being one of the major engineering universities, the DMG DMU 50 Eco was the chosen 5 Axis CNC Machining Center to integrate NOF, the renewed workshop center. It is renowned for the wide range of geometries that can perform. However, it demands a previous study by an engineer to withdraw as 1

2 much as possible from the machine. With the increase of machine s versatility, comes the increasing number of machining planes, which triggers the project s complexity. Thus, the use of Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM), or CAD/CAM software becomes essential. Those softwares allow designing new parts, creating the required numerical control program in a short time and making geometrical changes in real time. The CAD/CAM technology enables to save more time, resources and production costs with efficiency and accuracy. Once the design and machining project are done, it is required to upload a file with the machining paths to the machine s controller. The interface works as a translator between these two technologies is the so called Post-Processor. There are many different controllers trademarks and a vast number of CAD/CAM software s. Therefore, specific Post-Processors need to be developed for each controller-software set. Once the equipment have been chosen, it is necessary to select which tools are going to be use depending on the diversity of geometries required. The more diverse and versatile they are the more productive capacity they have. 2. Brief Review of 5 Axis Machining 5 Axis machining is, at this time, the cutting-edge technology that many companies have access to. Machines with this technology can not only position the tool but also in any rotated position of the 3D coordinate system. To understand the characteristics of a machine, it is required to understand the basic terminology of 5 Axis machining centers. In a common 3 Axis milling machine, the linear axis that define them are the X, Y and Z. In 5 Axis milling, there are 2 of 3 additional rotary Axis, which rotate around the three linear Axis: A rotate around X, B around Y and C around Z. Figure 1 3D Coordinate System Whenever is possible, a 3+2 milling type should be chosen, i.e. using both rotary Axis only for position the workpiece: it s easier to program, the cut is faster and there are fewer risks of unexpected collisions between parts. Moreover, it s the most suitable mode for roughing. Even though 5 Axis machining is impressive to watch, it is slower, harder to program and makes the machine less rigid. It s better to create a part with 5 sided machining instead of simultaneous 5 Axis. 2

3 2.1 Machining Centers Long before CNC machine tools were invented, metalworking was handmade on conventional milling machines. The operator had total control over the operations and machines were purely actuated by mechanical mechanisms. The first NC (Numerical Control) machines appeared late in 1940 s. They were huge, heavy and extremely difficult to operate. The instructions were given by punched paper cards containing a sequence of G-Code to position tools. Nevertheless, NC machines were adequate for mass production and, for this reason, throughout 1950 s they have become the industry standard. But with the development of computers as we know, they started to upload the G-Code into machines and the concept of Computer Controlled Machine (CNC) emerged in In 1972, another revolution emerged with the development of first CAD/CAM software. Finally, in 1989 came out to the market the first 3D software and CNC machines have become the industry standard. Even if today many CNC Machinig Centers had the possibility to directly program in the controller the tool paths required for a certain job, CAD/CAM softwares would still be the most used tool to create new workpieces. However, each CAD/CAM-controller set requires a unique Post- Processor to translate those two different technologies. In the past years, Post-Processors have evolved in order to include a huge amount of G-Code list and tailored functions to optimize the machining process, making it the most efficient and fast as possible. 3. DMG DMU 50 Eco Setup-ups Any mechanical equipment must be select according to the workshop needs. On a CNC Machining Center, some characteristics such as feed rate, spindle speed, the dimensions of the table or the capacity of the tool magazine must be taken into account. Figure 1 present CNC Machinig Center available in NOF Figure 2 CNC Machining Center DMG 50 Eco: 1) Spindle; 2) Controller; 3) Tool magazine; 4) Chip deposit; 5) Machine s floor; 6) Power distribution; 7) Rotating table 3

4 Rough Milling Generic Operations Beginning in the transportation, throughout technical trainning, network settings and tools selection, all stages are important, and a right choice of the required equipment to achieve the order specifications is a crucial issue. Selecting the right operating conditions is very important in order to fulfill the desired quality requirements at the lowest price. So as to make the machine more versatile, and execute the biggest number of geometries, a tool set was chosen as wide and varied as possible to the machine s tool magazine. Position Tool Name Cut 1 Center drill bits CDB_D2 2 Spher Mill SM_D2 2 3 Drill D_D3 3,5 4 End Mill EM_D4 4 5 Tap M4 T_M4 4 6 Spher Mill SM_D6 6 7 Drill D_D7 7 8 End Mill EM_D8 8 9 Tap M8 T_M8 8 Empty Inserts tool TIT_D Inserts tool RIT_D Inserts tool FIT_D Inserts tool RIT_D Inserts tool FIT_D16 16 Empty Table 1 DMG 50 Eco Set of Tools 4. Posts-Processors Development Firstly it is required to explore the Sinumerik 810D functioning, the controller s machine, in order to understand what pre-cycles it has at disposal. These cycles reduce significantly the number of lines required for the same job just to create a good and efficient Post-Processor that explores all the machine s potential. Besides ShopMill, the integrate CAM solution in the machine s controller, three different CAD/CAM softwares were chosen in order to know which would be more appropriate to machine s controller: MasterCam, SolidCam and NXCam. Because all three softwares are very different from each other, the time spent exploring them was the same. However, the time spent developing new Post-Processors wasn t. MasterCam is one of the leading CAD/CAM software in the industry, because it is reliable and very thorough. It was the software tested in the first place, but a dedicated Post-Processor was not developed, because it already existed one in the school. To implement in the machine, few changes had to be applied in the Post-Processor code. The second tested software was SolidCam, which is an add-on to the existing SolidWorks CAD software. Even being recent in the market, it presents lots of machining operations like 4

5 MasterCam, However, this software works based in solid geometries. Just like MasterCam, the software comes with some generic Post-Processors, for educational purpose. One of them was specific for Sinumerik 810D, but only with 3 Axis. Thus the challenge was to adapt this Post-Processor to the NOF Machining Center. After several weeks, this solution was putted apart, once the controller does not accept the required rotating Axis. As described, this machine only rotates B and C Axis. Assuming we want to rotate A Axis, the Post-Processor needs to do a coodinates' change, in order to transform this command into a relative move between B and C axis. Since this problem couldn t be solved, the solution was directed to another CAD/CAM software. Finally, the last tested software was the NXCam. It is very similar to SolidCam in terms of interface and performs as well as the others. Like the others softwares, NXCam comes with a range of non-industrial Post Processors' libraries. However, the biggest advantage of it is to have dedicated software to help developing Post-Processors, called Post Builder. As can be seen in Figure 2, the command list of this Post-Processor is organized almost like a flow chart, making the creation of it much easier. Figure 3 Post Builder In every single job, it is required for the operator to set a reference point in the workpiece relative to machine s home position. However, when the table rotates, this relative point is lost. So as to solve this problem, it is necessary to declare a new cycle (blue box in Figure 3). This so called Cycle800 is used to rotate any type of surface so that it can be machined. By calling the appropriate NC functions, the cycle converts the active workpiece zero and tool offsets to refer on the inclined surface, taking account of the kinematic chain on the machine. 5. Experimental Work To validate the developed Post-Processors, some experimental work was done. Firstly, it was manufactured an Aluminum dummy table. It is a very handy device when it is necessary to machine a workpice until the very near its base. 5

6 Secondly, a case study was projected to test all CAD/CAM softwares, as well as to compare the way they are machined. This workpiece was fixed to the Aluminum dummy table using cyanoacrylate. a) b) Figure 4 a) Aluminium Dummy Table; b) Study Case Geometry For being a common material in many engineering applications, very versatile and easy to machining, the Aluminium was the chosen material to realize this case study. As such, it was taken into account the mechanical properties of it in the definition of the machining parameters, wich are presented in the table below. Operation Tool Spindle Speed (rpm) Feed rate (mm/min) Stepdown (mm) Facing End Mill D Contour End Mill D Box End Mill D Drill BH_D Thread drilling M_M Tilt Facing End Mill D (SolidCam = 0.2) Drill 90 o BH_D Table 2 Case study machining speeds 6. Results and Discussion As all softwares are different, the comparison was based on the programing time of a same geometry. The Table 3 contain the operation times. As show in Table 3, the programing times differed only by a few minutes in the first three cases. When it needs to be done the same geometry using ShopMill, the time spent is three times more. If there is the need to rotate the machine table, the programing time of that operation increases an order of magnitude. When the geometry entails the rotation of any angle, the user is forced to 6

7 indicate which is the new reference point before the rotation, which value (in degrees) of tilt and another reference point after. This requires either the technical drawing to have more information than usual or commits the operator to manually calculates the new reference points. NXCam SolidCam MasterCam ShopMill Preparing 00:55 00:35 01:35 01:05 Facing 01:30 00:55 01:05 00:35 Contour 02:00 01:05 00:55 01:10 Box 01:40 01:10 01:10 01:25 Drill 02:05 00:40 00:40 01:10 Thread Drilling 01:10 00:45 00:45 00:30 Tilt Facing 01:45 01:10 02:30 20:35 Drill 90 o 01:45 00:55 02:35 02:05 Total (mm:ss) 12:50 07:15 11:15 28:35 Table 3 CAM Programing time of the case study After generating, the NC files were uploaded to machine s controller in order to execute the same geometry with 3 different softwares. The MasterCam file was not performed because it s Post- Processor, as said, were not developed to DMG 50 Eco. The case study running times are showed in Table 4. NXCam SolidCam ShopMill Facing 04:50 04:40 05:05 Contour 03:00 03:05 03:10 Box 02:45 02:35 04:30 Drill 00:20 00:20 00:35 Thread Drilling 00:25 00:20 00:55 Tilt Facing 02:15 06:45 03:05 Drill 90 o 00: :05 Total (mm:ss) 14:05 17:45 18:25 Table 4 Case study running times Once the technological data for each machining operation is the same, it would be expected very close running times. However, some values diverge 30%. These differences are due to the path and speed at which the tool approaches the workpiece (rapid moves in G00). Both NXCam and SolidCam allow users to refine these strategies, while in ShopMill you can only set the cutting moves. For this reason, NXCam is up to four minutes faster than ShopMill when performing the same operations. As mentioned above, SolidCam Post-Processor s is only working with three linear Axis. Therefore, it wasn t possible to run the 90 degrees hole and the rotated facing had to be done with a different technique, using a contour around the slope with a small stepdown. These facts resulted in longer operational times. 7

8 But more relevant than time spent in the machining process, is to assess whether final work is according to project and design requirements. Figures 4, 5 and 6 illustrate the final result in NXCam, SolidCam and ShopMill, respectively. a) b) Figure 5 NXCam Geometry: a) Overall Vision; b) Detail a) b) Figure 6 SolidCam Geometry: a) Overall Vision; b) Detail a) b) Figure 7 SolidCam Geometry: a) Overall Vision; b) Detail As figure 4 to 6 illustrate, the surface finish of all the pieces is quite similar because technological parameters where the same, regarding on some aspects. The only part with the same geometry and dimensional accuracy of the case study is the one done by NXCam. Despite having 8

9 good surface finish, the piece executed by SolidCam does not fulfill the project requirements, because it was done by a 3 Axis Post-Processor. Therefore, the 90 degrees drill couldn t be done and the tilted facing has a ladder format. Finally, the workpiece performed in ShopMill: apart from the tilted milling, all operations have the dimensions and geometry required by project. As mentioned, it s extremely difficult and time consuming to compute new reference points and rotation angles whenever there is the need to rotate machine s table and satisfy geometrical needs. In this case study calculation of new rotation angles resulted as expected. However, the geometry didn t, with visible defects showed in Figure 6 b). This was due of a deviated calculation of the reference points, both before and after rotation. 7. Conclusions Every step in the process of implementing a CNC Machining Center on a workshop is crucial. But the challenging part is the development of a Post-Processor. It forces you to deeply know the CAD/CAM software and its programing routines, as well as the machine. The easiest Post-Processor to develop, and the one which takes advantage of machine s potential, was the NXCam s. Having its own software to develop new Post-Processors, with an objectoriented programing style, makes it simple to create new ones. And because NXCam belongs to the same company of the machine s controller (Siemens), this software already has been prepared for similar programing routines. From the three tested CAD/CAM softwares, the choosing criteria lies not in the required time for programing the machining tool paths. Especially in a workshop like NOF, where programing and execution times are not as important as the final quality of produced products. Regarding the geometrical accuracy of the parts done in the machining center, it turns out that for simple geometries, requiring only three Axis, NXCam, SolidCam and ShopMill are much alike. However, when more complicated projects needs to be done, results are different. For more complex geometries, ShopMill won t perform as well as the others, since the programing difficulty increases with the increasing number of Axis required to perform the operation. As expected, for more complex geometries, requiring the rotating Axis, the recourse to CAD/CAM software is essential. Not only allows more flexibility in the machining tool paths programing, but also has a secondary function such as simulation of the different stages of manufacture and collisions verification between the tools and the workpiece. To NOF, the ideal choice is NXCam, which was the one that exhibited the best response to the referred needs. 9

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