NX Best Practices

Fixing the Load Options

When trying to access the shared folder and files on the STARLab shared Drive, you need to create a file to specify where that folder is on your computer.

• Make sure NX is not running on your computer

• On your computer go to "C:\Program Files\Siemens\NX1872\UGII"

• Create a text file,

• Rename the file "load_options.def",

• Using a text editor (notepad, wordpad, textpad, Atom.io), open "load_options.def"

• Add the following lines to it

• In the first line change what is inside the quotations to the path on your computer for the shared drive. For example, on my computer the path is "\\\\ME-TSERVE\\STARLab_Shared\\Shared CAD Folder Trial\\..."

  • Note that every backslash needs to be doubled for a path to be recognized

  • Also after the last pair of slashes three periods are added. This "..." tells the load options to search the entire directory contained within "CAD Files" when searching for a file

• Save and close "load_options.def"

• Open NX

• Check out a license

• Open a file from the shared drive that you pointed NX to above

• If all CAD files open, then no other steps need to be taken and this change will be permanent on your computer

General Modeling Practices

• Make sure Google-Sync is done loading before opening. If the drive is not finished syncing, you can end up creating multiple copies of files. Files with (1) at the end are created when there are conflicted copies.

• Make sure Google-Sync is done saving/uploading before closing or locking your machine.

• Close NX when not in use. We have a limited number of licenses so please remember to close NX when you're not actively using it as that will block someone else from working on the files.

• Don't use "recently opened" files. If a file has been "up-reved" since the last time you opened NX you can end up working on an old version of the file which will result in someone having to redo some amount of modelling.

• Avoid using "Concentric mates" whenever possible. The NX feature locks 6 DOF when a concentric mate only actually locks 2. This will make it much more difficult to find incorrectly mated parts and can even hide missing parts

• Name features, sketches and critical mates!!! When done creating a part and it is ready to be used in a production assembly, review all features in your feature tree and name them appropriately. The shortcut to rename a feature is to click on it once and press "F2" (same shortcut for editing a cell in excel, google sheets, and renaming a file/folder on windows machines). As a part evolves and becomes more complex having appropriately named features and sketches will allow someone to search and navigate a part made by someone else. "Extrude 18" doesn't give any information to the next user, but "Holes to connect camera lens" can be quite useful. As feature trees expand beyond 10-15 entities modifying and updating the parts becomes quite difficult if there are no names. Critical mates are those for moving parts that may need to be changed for different configurations

• Dimension features in parts as they will be measured during manufacturing. This may require going back and re-dimensioning features and sketches after you've made the part. The reason for doing this is that the complexity of the dimensions, the dimension stacking and the reference features should be representative of the manufactured part.

• When possible, make sketches internal to their feature. If a sketch can be made internal to the feature, right click on the feature and there will be an option "Make sketch internal". Doing this will reduce the size of the feature tree and make it easier to navigate.

Useful NX Shortcuts

• To switch to the Drafting environment "Ctrl+Shift+D"

• To switch to the Modelling environment press "Ctrl+M"

• To show/hide the GCS press W

• To show/hide different things press "Ctrl + W", note this doesn't let you hide the GCS

Importing Step Models

When downloading parts from McMaster Carr only use the STEP file format. All others will not import properly into NX...because NX.

When importing a step model TO NX, first drag the STEP file into somewhere in the Sandbox folder

Next in NX click Open File and navigate to the STEP file and select it.

Depending on the file being imported either 1 or 1 million NX prt files will be created. To make sure that these files will not interfere with other parts the naming needs to be solved.... morgan needs to figure something out that won't break things. If this becomes a problem, yell at morgan, he needs to solve this still

Useful NX Hidden Features

These features are built into NX, but are hidden by default and you will need to search for them to find and use them

• In the Drafting Environment to use the hole callout feature search for "Feature parameters"

• In Modelling Environment to get a mass estimate of the assembly you are working on search for "Measure Body"

  • To use this feature for an assembly, you will need to switch to wireframe mode and show all hidden edges

  • Then select the entire assembly by dragging a box around the entire assembly to select all parts

  • This method is faster but will be retired

• In the Modelling environment to get a mass estimate of the assembly select the Analysis tab along the top ribbon

  • Select the Weight Management

  • Check the "Use Spreadsheet" option

  • THEN click work part

  • THEN click OK..

  • THEN wait for upto five minutes depending on the size of the model, for an excel sheet to be opened and populated. Be warned that NX, Excel and probably your entire computer will be unusable while this is populated.

Space Systems Thermal Best Practices

• When modelling MLI, use a 2D Null Shell. The null shell models the MLI as mass-less, which reduces the chance of convergence issues and eliminates the need to apply a material to the MLI mesh. As MLI has a very low mass, it has a very low thermal time constant and therefore typically needs very short time steps to converge. In-plane conductivity for MLI is negligible, and radiation is the dominant heat transfer method. See the image below for an example of an appropriate mesh collector for MLI. Radiation properties can be assigned to the top and bottom of the mesh, and the top and bottom are created as two layer shells with an effective emissivity assigned between these layers.

• Always use 2D meshes wherever possible to minimize the number of elements. 2D meshes can be used for parts that are a constant thickness in a single plane, and where the temperature gradient through the thickness of the part is not needed.

• Never (or almost never) keep fasteners in a thermal model. It will drastically increase the number of elements in the mesh, increasing the computational time, without giving valuable information (do you really need to know the temperature profile through each fastener?). Instead, replace the thermal path through each fastener with a thermal coupling between the two parts that are joined by the fastener. When the fastener is deleted you can also delete the hole that the fastener went through and replace it with a face with a larger radius where the heat flows through (effective contact area for the joint). This will also reduce the number of small features in the model, reducing elements.

• The magnitude of the thermal coupling created by a fastener can be calculated in a number of ways:

  • Referencing published test data.

  • Creating a breakout model of the joint.

  • Applying the conduction equation to the fastener.

• When replacing a bolted joint with a thermal coupling, first remove the fastener from the model and create the faces on either side of the joint where the coupling will be applied by creating an "Offset Curve" of the hole where the fastener was. The offset curve should have a radius that is 1.5 times the diameter of the original fastener hole (ref Spacecraft Thermal Control Handbook by Gilmore). Use the Divide Face process described in point 2 of the General NX/Modelling Best Practices section below to turn the curve into a separate face, and then use the Delete Face feature and select the inside face of the hole to remove the hole from the model.

• The TMG thermal solver will sometimes create additional elements for solution purposes (for example for radiation and MLI). If an element number that shows up in the MSGF (run log) file cannot be found by searching the elements in your model, search for the element number in the REPF file and you should find a statement "TMG element 237768 is XXX" , that tells you the associated element.

• Tips to try if a model is not converging:

  • Reducing the very high couplings and increasing the very low couplings - sometimes having a combination of very high and low conductances will cause numerical problems.

  • Make very low mass meshes null shells. Sometimes carrying these high value into the matrices might cause numerical problems

  • Remove temperature-dependent values.

  • Check if thermostats are the issue by replacing them with a constant heat load. If a thermostat is causing troubles, using a proportional controller may help.

  • Decrease relaxation factor used by solver (under "Edit Solver Parameters").

  • Decrease time step size.

General NX/Modelling Best Practices

• A session of NX should be restarted every couple days (don't leave the same session running for days at a time and just open different files, fully shut down the application and reopen it ). Leaving a session running for an extended period increases the likelihood that a temp file (.tmp) can get deleted by an automated program on your computer and corrupt the file. Once the file is corrupted you will not be able to save, and when closed the file will (best case) revert to the previous version you had saved successfully.

• When modelling features on the surface of a part (like keep out zones) where only an outline is needed, create a sketch of the outline you want and then use the pre/post application feature "Divide face". You select the sketch (or any other curves you want, such as an offset curve) as the tool option, and the face the sketch is on as the faces to divide (as shown below). The projection direction should be normal to face.

• If you are getting an unknown error or having difficulties saving, you can try a part cleanup (follow the steps below):

  • File -->Utilities -->Part Cleanup.

  • Toggle on all options except the Serious Delete Actions

  • Then hit Apply or Ok