Best Practice to work with CAD files in Revit
Although BIM is getting popular these days or become mandatory for a few sectors, many are still relying on AutoCAD or other CAD programmes. For instance, the survey is normally provided in DWG format and many manufacturers' details are still available in CAD only. Therefore, it would be good to know how to work with DWGs and understand the associated rules to prevent any related causes.
Especially, in Revit, working with DWG is not desirable as it will impact some critical settings such as line styles. Moreover, the linking or importing decisions are varied by the users even though importing CAD is not recommended.
The most critical rule that you must remember. DO NOT directly IMPORT CAD files into the project. Always link them so that they can be managed easily and creating an in-place model and store in it would be a better choice.
Why people keep telling me that DWG can't be imported?
Every BIM manager in this world will tell you not to import CAD files, but they don't really tell you why. The main reason is that when you import CAD files, it will also bring all the settings from CAD, meaning that line styles, line weight, etc will come with the file.
Moreover, it can be easily abandoned in the project as people forget to delete them once it is no longer needed, which will increase the model size significantly if you leave them in the project. These days, there are a number of plug-ins/add-ins that will allow you to select abandoned CAD files, so you can find suitable software. Otherwise, use Dynamo.
Clean-up CAD prior to Linking into Revit
Before bringing it to the project, you need to do some pre-works.
❶ Duplicate the file and keep the original.
❷ If there are XREFs, detach them if not required. Otherwise, bind them to the drawing.
❸ SETBYLAYER, which will allow the removal of colours, line types, or line weight overrides from objects.
❹ EXPLODE to break all the block references (Revit doesn’t like blocks so make sure to explode all).
❺ QSELECT to isolate the lines.
❻ Delete all the layers and lines that you don't need and all layouts. If you want to have better control, you can customise the layers or add layers as required.
❼ LAYDEL if you want to delete all the lines and blocks in the layer.
❽ OVERKILL to clean up duplicated/overlapping lines.
❾ PURGE to delete unused layers, lines and blocks.
❿ Audit and type 'Y' to finish.
How to link the CAD files?
❶ Create a view to place the dwg file
❷ Set the Workset if it is a work-sharing model.
❸ Go to the Insert Tap and select Link CAD.
❹ Browse the location of the file.
❺ Check Current View only. Uncheck if it should be shown in all views.
❻ Leave the colours, layers/levels, and import units.
❼ Uncheck the Correct lines that are slightly off-axis and orient to views to bring the original location.
❽ Positioning - Centre to Centre and manually re-position the file as CAD files and Revit don't share the same coordinate system. Once it is positioned, you do not need to do it again if you reload the file.
❾ Select Place at - Associated Level if Current view only is not checked.
❿ Click OK
Visibility Control of CAD files
There are two options to change the lines and colours of the linked CAD, which are Object Style and Visibility/Graphics Override. Object style will change them globally, meaning that it will be applied to all the views in the project, whereas the latter will be only view-specific.
Manage the CAD files.
❶ Go to Manage tap and select Manage Links
❷ Select CAD Formats and it will show all the CAD files linked in the project.
❸ Now, it is possible to Reload From, Reload, Unload, Import, and Remove the CAD files.
Reload From - If the file location is changed, use this option to re-link the CAD file.
Reload - Once you made the change in the same file, simply select reload to update the CAD file.
Unload - Temporarily remove the CAD file in the project.
Import - The CAD file will be bound to the project.
Remove - Permanently remove the CAD file in the project.
There are some cases in which you need to 'import' CAD files for details or object reference. in that case, there is a workaround to avoid any issues.
Insert the CAD file in In-Place Model.
Insert the CAD file in Family (don't matter the category of the family).
To Sum Up
It is quite common practice to work with CAD files as many firms are still using CAD software. It is important to understand and set the workflow to coordinate with them and maintain the good quality and health of your models. However, combining CAD and BIM will benefit the performance due to the limitation and constraints in Revit. Make sure to AUDIT and LINK the CAD file.