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 bemade to optimize problem size.

Considerations include keeping the workpiece as small as possible while capturinggeometry (both in diameter and thickness), using the largest element which canadequately capture chip geometry, and possibly pre-shaping the workpiece to eliminatethe 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 userunderstands 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 witha 0.15mm/rev feed.

Creating a New ProblemCreate a new problem from the main DEFORM window. Assign a name. Thepreprocessor will open with the given problem name. Many of the icons which will bereferenced 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 theicon for a couple seconds, the icon name will be shown. These icon names are usedthroughout this tutorial.

Add Object

SimulationControls

ObjectPositioning

InterobjectRelationships

Object Tree

Setting Simulation ControlsGo 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 geometryThe workpiece geometry can be imported from an .stl file. If the workpice shape is asimple 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 distributoror 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 enoughthat 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 aworkpiece 7mm in diameter, and 1.7 mm thick.

Object 1 should exist by default, and should be named “Workpiece.” If there is notalready 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 theObject type is “Plastic” .

Go to the Geometry tab, and select Geo Primitive. Select “Cylinder,” Enter a radius of3.5 and a height of 1.7, and click “Create.”

Close the Geometry Primitive window.

Defining the drill geometryComplex geometries such as a drill must be imported from a CAD system. The mostcommon 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 problemswith mesh generation. Many CAM packages do not produce watertight surfaces.Contact your distributor or DEFORM support if you have questions about .stl filegeneration.

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 objectname 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 andpreprocessing times due to increased searching and sorting requirements. Typically 1000to 20,000 surface facets should be adequate to describe a geometry, depending oncomplexity. The number of facets can generally be controlled in the .stl export utility inthe CAD system.

Use the Save icon in the upper left corner of the user interface to save the currentinformation in the preprocessor.

Generating a mesh on the workpieceIn the course of a metal cutting simulation, DEFORM will regenerate a mesh dozens orhundreds of times as the material and mesh become distorted. The new meshes aregenerated based on user defined parameters to keep fine elements where they are neededfor 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 meshregeneration while the simulation is running.

Initial Mesh Parameters: Be sure the Workpiece is selected in the object tree. Go tothe 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 0.15mmfeed/revolution, the feed per cutting edge is 0.075mm. To get two elements in the chipthickness, use a minimum element size of 0.04mm (0.0375 rounded up).

The size ratio sets the size of the largest elements, in areas where no refinement isrequired. For metal cutting simulations, a very aggressive size ratio of 10 is used. Largersize ratios may lead to substantial increases in the time required for mesh generationwhile 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 otherweighting 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 ofthe 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, asshown in Figure 1.

Click Surface Mesh to generate the mesh on the surface of the workpiece. Rotate theworkpiece to check the mesh definition. The mesh should be fine on the top, and coarse

on the bottom. Once you are satisfied with the mesh definition, click Solid Mesh tocomplete mesh generation.

Click the “Save” icon to save the data.

Runtime Mesh Parameters: After the initial mesh is generated, we will no longer usethe 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 0.35.

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 .stlgeometry is always maintained for deformation contact calculations. The mesh is onlyused 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 ofelements to 20,000.

Go to the weighting factors screen, and set the mesh window weighting to 1, and allothers 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 surfacemesh, then a solid mesh.

Save the data.

Assigning MaterialGo to the material screen.

Select the workpiece from the object tree. Select AISI-1045(Machining) from the Steelfolder. Click the “Assign Material” button. The steel name should now appear next tothe 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 appearwith the drill in the object tree.

Assign movement controlsGo 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 bedefined in radians/sec, and translational speed must be defined in mm/sec (or in/sec forEnglish problems).

For this simulation, rotational speed is 400 RPM. Converting this to radians/second.

400 RPM * 2 * 3.1415 / 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.

VelocityWe will fix the velocity of all nodes on the side of the workpiece, and we will assign heatexchange boundary conditions to all surfaces.

On the BC Type tree, select Velocity. Set the direction to X, and click on the side of theworkpiece. Click the “Add Boundary Conditions” (+) icon. “X,Fixed” should appear inthe boundary condition tree.

Set the direction to Y, click on the side of the workpiece, and click “Add BoundaryConditions.”

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.

Figure 2: Boundary condition selection on the side of the workpiece.

Heat Exchange with the EnvironmentSelect Heat Exchange with the Environment under the boundary conditions tree. ClickAll 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 conditionsover 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 controlsIn DEFORM, the deformation is subdivided into hundreds or thousands of incrementaltime steps. The user defined time step gives the simulation a starting point forcalculations. If it is too large, the simulation module will automatically reduce it to amore suitable value.

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 of0.0005 seconds per step.

Go to Steps on the simulation controls screen. Set the Solution Steps Definition toWith Constant Time Increment, and assign a value of 0.0005 sec.

It is about 3.3mm from the bottom of the workpiece to the top of the drill tip. We canassume 3.5 mm of penetration is necessary for the drill to go completely through theworkpiece. At 1 mm/sec, this means that 3.5 seconds, or 7000 steps will be required tocompletely 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 andautomatic time step control, the actual number may be more. DEFORM will adjust forthis automatically. However, a secondary stopping control can be defined.

Stopping controlsThere are a large number of stopping controls which can be set in DEFORM. Asimulation will run until it reaches one of the predefined stopping controls, or until it isstopped by the user.

We will stop when the primary tool travel (the drill) reaches 3.5 mm in the direction oftravel. So set Primary Die Displacement to [0,0,3.5].

Displacement is only suitable for linear motion. If only rotational motion is defined, thestopping control must be time based.

Processing conditionsThe heat transfer convection coefficient is defined under Process Conditions. Thedefault value is appropriate for still air (dry cutting). If coolant is used, appropriatevalues 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 PositioningClick the Object Positioning icon on the top of the user interface, near the SimulationControls icon.

The drill should be pre-positioned in the CAD system so it is on the Z axis (x=y=0) in theCAD 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 positionthe drill so it is touching the workpiece.

Select Interference positioning. Make the Drill the positioning object, and theWorkpiece the reference object. Make the approach direction –Z. Click “Apply” . Thedrill should be moved so the tip is just touching the workpiece. Click OK to accept thispositioning and exit object positioning.

Save the data.

Inter-Object DataThe 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 isautomatically defined as the master object, and the workpiece as the slave. In DEFORMsimulation, the object causing deformation will always be the master, and the objectbeing 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 havefound that, in the absence of better information, values in the range of 0.5 to 0.6 givereasonable 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 usethis feature.

Click Close to exit the editing screen. The values you entered should now appear in thetable.

Self contactWe need to add one more contact relationship. Since the chip is likely to touch theworkpiece, we need to instruct the system to search for this possible contact mode.

Click the ‘+’ button, and a “None – None” relationship will appear in the table. At thebottom 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.

ContactGenerating initial contact conditions can identify potential geometry problems, andimprove the initial calculations. After a simulation is running, the program updatescontact conditions automatically.

The Contact BCC function finds any nodes on the slave object that are within thetolerance 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 thecontact. If you rotate the workpiece, you will see contact nodes between the drill and theworkpiece.

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 namewill be the same as the one you specified when you opened the problem. If you want tocreate variations on the problem, you can enter a different name.

Click the Check to run the automatic data checking. DEFORM will mark errors with redcircles. This indicates a situation which will not allow the situation to run. The usermust return to the preprocessor and correct the situation before continuing.

Some conditions will be marked with yellow. These indicate potential problems, whichwill not necessarily cause a simulation to stop, but may lead to incorrect results. The usershould identify the source of any of these marks before continuing.

TRGVOL (Volume Compensation). DEFORM has a volume control feature for usewith forging simulations where a few percent change in volume can significantlyinfluence 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.

After database generation is completed, click Close to exit Database Generation. Thenclick Exit to exit the preprocessor.

Running the Simulation

Special Control FileFor some functions, DEFORM uses control files in the working directory. FromWindows, open My Computer, and change to the problem directory (generallyc:\deform3d\problem\DrillDemo. Select File->New->Text File. When the file iscreated, rename it to STRAIN_DST.DAT. Windows will warn you about changingextensions. Click OK to continue.

When DEFORM runs, it will check for the presence of this file, and use a different meshgeneration 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 tostart 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 e-mail to one or more addresses when asimulation stops. The e-mail function can be set up under the Options->Environmenttab on the top line menu. Scroll right to the E-mail tab. The server name is youroutgoing mail server (generally mail.yourdomain.com or smtp.yourdomain.com or asimilar address).

Simulation GraphicsThe 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-mousepop-up menu.

The following zoom and pan functions can be used:CTRL- LeftMouse – dynamic rotationCTRL-Right Mouse – zoom windowShift-LeftMouse – panShift-RightMouse- dynamic zoom.

Post-Processing while RunningWhile a simulation is running, DEFORM renames the database file to FOR003. This filecan be opened in the postprocessor like any other database file. However, while thesimulation is running, the last step may change. If the user tries to view the last step afterit has changed, the post-processor will crash. It will not cause data loss. Occasionally itmay stop the simulation. If the simulation stops, click the Continue button to resumerunning.

To post-process a simulation that is running, open the post-processor. If the problemdoes 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. Drillingsimulations will almost certainly exceed this limit. DEFORM handles this by creating anew database when the file size approaches 2GB. The existing file is renamed, with thestep number appended to the problem ID.