Simulating Drilling Processes with DEFORM-3D

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1 Simulating Drilling Processes with DEFORM-3D Due to the number of revolutions of a drill necessary to establish characteristic behavior, drilling simulations in DEFORM are time consuming. Therefore, every effort will be made to optimize problem size. Considerations include keeping the workpiece as small as possible while capturing geometry (both in diameter and thickness), using the largest element which can adequately capture chip geometry, and possibly pre-shaping the workpiece to eliminate the necessity to simulate the transient point penetration before the drill reaches full depth. We will describe the use of the open preprocessor for defining this problem. If the user understands this process, use of the template should not pose a problem. This tutorial will use the example of a 6mm two flute twist drill running at 400RPM with a 0.15mm/rev feed. Creating a New Problem Create a new problem from the main DEFORM window. Assign a name. The preprocessor will open with the given problem name. Many of the icons which will be referenced throughout this tutorial are identified in the screen capture below. Icons in DEFORM use Tip Boxes with the icon name. If the mouse is held over the icon for a couple seconds, the icon name will be shown. These icon names are used throughout this tutorial. Simulation Controls Interobject Relationship s Object Positioning Object Tree Add Object

2 Setting Simulation Controls Go to the Simulation Controls window. Turn on Heat Transfer mode. Change units to SI Change the simulation title to Drilling Simulation. Click OK to exit. Defining the workpiece geometry The workpiece geometry can be imported from an.stl file. If the workpice shape is a simple cylinder or box, it can be created using the geometric primitives in DEFORM. (This feature is available in DEFORM-3D version 5.1 and later. Contact your distributor or DEFORM support if you do not have this version). For now, we will work with a simple, solid workpiece. The workpiece will be round, with a diameter roughly 20% larger than the drill. The thickness should be large enough that the full tip taper can be engaged in the workpiece. For this tutorial the drill diameter is 6mm. The tip taper is 1.5mm. We will create a workpiece 7mm in diameter, and 1.7 mm thick. Object 1 should exist by default, and should be named Workpiece. If there is not already a workpiece defined, click the Add Object button to add one. On the General window in DEFORM, be sure the object name is Workpiece, and the Object type is Plastic. Go to the Geometry tab, and select Geo Primitive. Select Cylinder, Enter a radius of 3.5 and a height of 1.7, and click Create. Close the Geometry Primitive window. Defining the drill geometry Complex geometries such as a drill must be imported from a CAD system. The most common format is an.stl file. (IDEAS and PATRAN mesh neutral files (.unv and.pda) can also be used if the software is available) The.stl file should be a single, closed, watertight surface. Multiple surfaces which are not stitched will likely cause problems with mesh generation. Many CAM packages do not produce watertight surfaces. Contact your distributor or DEFORM support if you have questions about.stl file generation.

3 It is best to position the center of the drill tip at the X,Y,Z origin in your CAD system. Click the Add Object icon at the bottom of the object tree window. Change the object name to Drill. Be sure the object type is Rigid. Go to the Geometry window. Click the Import Geometry button, and select Drill.stl. Check the geometry. There should be One surface No free edges No invalid edges No invalid orientations An excessive number of surface polygons will cause increased computation and preprocessing times due to increased searching and sorting requirements. Typically 1000 to 20,000 surface facets should be adequate to describe a geometry, depending on complexity. The number of facets can generally be controlled in the.stl export utility in the CAD system. Use the Save icon in the upper left corner of the user interface to save the current information in the preprocessor. Generating a mesh on the workpiece In the course of a metal cutting simulation, DEFORM will regenerate a mesh dozens or hundreds of times as the material and mesh become distorted. The new meshes are generated based on user defined parameters to keep fine elements where they are needed for resolution, and place coarse elements in other areas. We define a maximum and minimum element size, and criteria for refining the mesh. For drilling, the minimum element size should be roughly ½ of the feed per cutting edge. (so ¼ the feed/revolution on a 2 flute drill, 1/6 the feed/rev on a 3 flute drill, etc). We will define an initial mesh using windows to specify localized mesh refinement areas. Then we will define the remeshing parameters that will be used for automatic mesh regeneration while the simulation is running. Initial Mesh Parameters: Be sure the Workpiece is selected in the object tree. Go to the Mesh screen and select the Detailed Settings tab. On the General tab, change Type to Absolute. The minimum element size is determined by the feed. For a two flute drill with 0.15mm feed/revolution, the feed per cutting edge is 0.075mm. To get two elements in the chip thickness, use a minimum element size of 0.04mm ( rounded up).

4 The size ratio sets the size of the largest elements, in areas where no refinement is required. For metal cutting simulations, a very aggressive size ratio of 10 is used. Larger size ratios may lead to substantial increases in the time required for mesh generation while the simulation is running. Enter a size ratio of 10. Go to the Weighting Factors tab. Slide the Mesh Windows bar to 1, and all other weighting bars to 0. Go to the Mesh Window tab. Click the + button to add a window to the list. Now click on the point at the center of the workpiece as the center point of the window. Figure 1: Mesh window. Arrow indicates resizing handles. Using the handles, resize the window so it covers only the center top of the workpiece, as shown in Figure 1. Click Surface Mesh to generate the mesh on the surface of the workpiece. Rotate the workpiece to check the mesh definition. The mesh should be fine on the top, and coarse

5 on the bottom. Once you are satisfied with the mesh definition, click Solid Mesh to complete mesh generation. Click the Save icon to save the data. Runtime Mesh Parameters: After the initial mesh is generated, we will no longer use the mesh window, so delete it (using the - button). Go to Weighting Factors, and change mesh window weighting to 0. Set Strain weight to about 0.65, and Strain Rate weight to about These values will be used for mesh refinement while the simulation is running. Generating a mesh on the tool The tool mesh is not as critical as the workpiece mesh. For a rigid object, the.stl geometry is always maintained for deformation contact calculations. The mesh is only used for temperature calculations. We will define a 20,000 element mesh, weighted towards the cutting tip. Select the drill from the object tree. Go to the mesh screen, and set the number of elements to 20,000. Go to the weighting factors screen, and set the mesh window weighting to 1, and all others to 0. Go to the mesh windows tab, and define a mesh window that covers the tip of the drill. Assign a Size Ratio Relative to Elem Outside Window of 0.3. Generate a surface mesh, then a solid mesh. Save the data. Assigning Material Go to the material screen. Select the workpiece from the object tree. Select AISI-1045(Machining) from the Steel folder. Click the Assign Material button. The steel name should now appear next to the workpiece in the object tree. Select the drill from the object tree. Select Carbide (15%) from the material list under Die Materials. Click the Assign Material button. The carbide name should appear with the drill in the object tree.

6 Assign movement controls Go to the movement screen. We need to assign translational and rotational movement to the drill. Select the drill from the object tree. In the open preprocessor, rotational speed must be defined in radians/sec, and translational speed must be defined in mm/sec (or in/sec for English problems). For this simulation, rotational speed is 400 RPM. Converting this to radians/second. 400 RPM * 2 * / 60 = 41rad /sec. On the Rotation1 tab, assign a constant angular velocity of 41 Rad/Sec. Define the center of rotation as 0,0,0, and the axis as Z. Now go to the Speed/Force tab. For this simulation, the feed is 0.15mm/rev. At 400 RPM this is 0.15*400 = 60mm/min, or 1mm/sec. Assign Simulation Controls Select the workpiece from the object tree. Select the Boundary Conditions screen. Velocity We will fix the velocity of all nodes on the side of the workpiece, and we will assign heat exchange boundary conditions to all surfaces. On the BC Type tree, select Velocity. Set the direction to X, and click on the side of the workpiece. Click the Add Boundary Conditions (+) icon. X,Fixed should appear in the boundary condition tree. Set the direction to Y, click on the side of the workpiece, and click Add Boundary Conditions. Repeat for Z. When you are completed, you should have the lines X,Fixed; Y,Fixed; and Z,Fixed below the Velocity entry in the boundary condition tree.

7 Figure 2: Boundary condition selection on the side of the workpiece. Heat Exchange with the Environment Select Heat Exchange with the Environment under the boundary conditions tree. Click All on the Pick Nodes window in the bottom left corner of the user interface window. Click the Add Boundary Conditions icon to assign heat exchange boundary conditions over the full workpiece. Select the drill in the object tree. Select All surfaces, and Add Boundary Conditions. Save the data. Simulation Controls Go to the Simulation Controls icon along the top of the user interface. We need to define the numerical parameters for running the simulation. Step controls In DEFORM, the deformation is subdivided into hundreds or thousands of incremental time steps. The user defined time step gives the simulation a starting point for calculations. If it is too large, the simulation module will automatically reduce it to a more suitable value.

8 For drilling, we would like to have about 1 degree of rotation per time step. We can round this off to about 300 steps per revolution. The drill makes 400 rev/min, which means 1 revolution takes about 1/6 of a second. (1/6) / 300 gives a time step of seconds per step. Go to Steps on the simulation controls screen. Set the Solution Steps Definition to With Constant Time Increment, and assign a value of sec. It is about 3.3mm from the bottom of the workpiece to the top of the drill tip. We can assume 3.5 mm of penetration is necessary for the drill to go completely through the workpiece. At 1 mm/sec, this means that 3.5 seconds, or 7000 steps will be required to completely drill through this sample. Enter 7000 as the number of simulation steps. This is an estimate of the number of steps that will be calculated. Due to remeshing and automatic time step control, the actual number may be more. DEFORM will adjust for this automatically. However, a secondary stopping control can be defined. Stopping controls There are a large number of stopping controls which can be set in DEFORM. A simulation will run until it reaches one of the predefined stopping controls, or until it is stopped by the user. We will stop when the primary tool travel (the drill) reaches 3.5 mm in the direction of travel. So set Primary Die Displacement to [0,0,3.5]. Displacement is only suitable for linear motion. If only rotational motion is defined, the stopping control must be time based. Processing conditions The heat transfer convection coefficient is defined under Process Conditions. The default value is appropriate for still air (dry cutting). If coolant is used, appropriate values can be entered in this field. Oil based coolant - 7E-4 btu/in^2.sec.f (English units) or 2. ( SI units) Water coolant - 3.5E-3 btu/in^2.sec.f (English units) or 10. (in SI units) Click OK to get out of the simulation controls window, then save the data. Object Positioning Click the Object Positioning icon on the top of the user interface, near the Simulation Controls icon.

9 The drill should be pre-positioned in the CAD system so it is on the Z axis (x=y=0) in the CAD system. If this is not possible, place a pointed cone on the center back of the drill. The tip of this point can be used as a reference point in positioning. For this tutorial, the drill is already positioned along the correct axis. We will position the drill so it is touching the workpiece. Select Interference positioning. Make the Drill the positioning object, and the Workpiece the reference object. Make the approach direction Z. Click Apply. The drill should be moved so the tip is just touching the workpiece. Click OK to accept this positioning and exit object positioning. Save the data. Inter-Object Data The Inter-Object setting allows the user to define relationships between objects, including friction, heat transfer between objects, etc. Click the Inter-Object icon, which is right next to the Positioning icon. The system will prompt the user to define default relationships. Click Yes. The drill is automatically defined as the master object, and the workpiece as the slave. In DEFORM simulation, the object causing deformation will always be the master, and the object being deformed will be the slave. Click Edit to define friction and heat transfer values. Friction modeling is still a matter of some discussion amongst researchers. We have found that, in the absence of better information, values in the range of 0.5 to 0.6 give reasonable results. Enter a value of 0.6 for constant friction. Go to the Thermal tab and enter a heat transfer coefficient of 40. The Friction Window tab allows localized friction values to be defined. We will not use this feature. Click Close to exit the editing screen. The values you entered should now appear in the table. Self contact We need to add one more contact relationship. Since the chip is likely to touch the workpiece, we need to instruct the system to search for this possible contact mode.

10 Click the + button, and a None None relationship will appear in the table. At the bottom of the window, Change Master to Workpiece, and Slave to Workpiece. Click on the Drill-Workpiece pair in the table, and click Apply to Other Relations. This will copy friction and contact thermal data to the Workpiece-Workpiece pair. Contact Generating initial contact conditions can identify potential geometry problems, and improve the initial calculations. After a simulation is running, the program updates contact conditions automatically. The Contact BCC function finds any nodes on the slave object that are within the tolerance distance of the master, and assigns a contact condition to them. Click the hammer icon to set the tolerance, then click Generate All to generate the contact. If you rotate the workpiece, you will see contact nodes between the drill and the workpiece. Click OK to exit the Inter-Object menu. Save the Data. Generating the Database Click the Database Generation icon, next to the Inter-Object icon. The database name will be the same as the one you specified when you opened the problem. If you want to create variations on the problem, you can enter a different name. Click the Check to run the automatic data checking. DEFORM will mark errors with red circles. This indicates a situation which will not allow the situation to run. The user must return to the preprocessor and correct the situation before continuing. Some conditions will be marked with yellow. These indicate potential problems, which will not necessarily cause a simulation to stop, but may lead to incorrect results. The user should identify the source of any of these marks before continuing. TRGVOL (Volume Compensation). DEFORM has a volume control feature for use with forging simulations where a few percent change in volume can significantly influence a simulation result. This feature will never be used for a machining simulation, so this warning can always be ignored. If there are no other errors or warnings, the database can be generated.

11 After database generation is completed, click Close to exit Database Generation. Then click Exit to exit the preprocessor. Running the Simulation Special Control File For some functions, DEFORM uses control files in the working directory. From Windows, open My Computer, and change to the problem directory (generally c:\deform3d\problem\drilldemo. Select File->New->Text File. When the file is created, rename it to STRAIN_DST.DAT. Windows will warn you about changing extensions. Click OK to continue. When DEFORM runs, it will check for the presence of this file, and use a different mesh generation scheme which maintains better resolution in the chip. Starting the Simulation On the main menu, the simulation can be started using the Run button. Click this now to start the simulation. Run (option) contains additional options. If a multiple processor license is available, the multiple processor settings can be defined here. DEFORM also includes a feature to send to one or more addresses when a simulation stops. The function can be set up under the Options->Environment tab on the top line menu. Scroll right to the tab. The server name is your outgoing mail server (generally mail.yourdomain.com or smtp.yourdomain.com or a similar address). Simulation Graphics The simulation function can be used to monitor the current status of the simulation. While in simulation graphics mode, several functions are available under a right-mouse pop-up menu. The following zoom and pan functions can be used: CTRL- LeftMouse dynamic rotation CTRL-Right Mouse zoom window Shift-LeftMouse pan Shift-RightMouse- dynamic zoom.

12 Post-Processing while Running While a simulation is running, DEFORM renames the database file to FOR003. This file can be opened in the postprocessor like any other database file. However, while the simulation is running, the last step may change. If the user tries to view the last step after it has changed, the post-processor will crash. It will not cause data loss. Occasionally it may stop the simulation. If the simulation stops, click the Continue button to resume running. To post-process a simulation that is running, open the post-processor. If the problem does not load automatically, type FOR003 in the file open window. If the post-processor malfunctions, close it and re-open it. A note about file sizes. Windows and other 32Bit operating systems have a 2GB file size limit. Drilling simulations will almost certainly exceed this limit. DEFORM handles this by creating a new database when the file size approaches 2GB. The existing file is renamed, with the step number appended to the problem ID.

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