Tuesday, 22 June 2010

Linking CAD Files into Revit Projects: Key Considerations

I could go on for hours and hours talking about this subject as there are so many different scenarios which lead to different best practice methods for either linking or importing CAD Data. Instead, this post discusses the key considerations when linking CAD data into a Project.

Linked File Storage
  • Take a copy of your Original CAD Data and store it in a separate location (suggest a folder such as ...\BIM\Linked Data\). It’s important to do this as you will often need to clean up a sub-consultants drawing and remove unwanted links, blocks etc (and you don’t want to destroy the original file they sent you!). Its also good to be able to see and manage all linked information from one folder location.

Linked File Preparation
  • Clean up the copy to make sure you are only linking in ‘the information you need’ i.e. If you are using an OS Plan to trace the outline of a house, don’t load in half of London! Similarly, if you only need the building outline, dont load in all the spot levels, hatching etc which will really slow down the performance of your model.
  • Use the OVERKILL command in AutoCAD to remove duplicate lines (thanks for this tip J)
  • Purge, Delete, Bind any Xref information in the CAD file and Audit it.
  • Finally you need to consider the coordinates of the CAD Data. If you are importing Site Plan information, this is often located a large distance from the 0,0 origin. Revit used to have a limit of 2 miles. Anything over this limit would cause errors in graphical display. In 2011 this has been increased to 20 miles. You can read more about this here on the Autodesk Website 

So if your CAD data is a large distance from 0,0 you may have to move the geometry in AutoCAD nearer to 0,0 (by whatever method you decide – too many options to post here). This isn’t an issue once inside Revit as the coordinate system is set retrospectively inside the model (I will post a separate discussion about site coordinates, True North/Project North etc...)

Revit Model Preparation

  • Create a dedicated workset in Revit for the linked CAD file making sure you uncheck ‘visible by default in all views’. (if your not using central files ignore this part).

  • Make this it the ‘Active’ workset.
  • Decide which Level, Elevation, View, Reference Plane you would like to link the file into/onto and make it current (in my example this will be Level 0).
  • Under visibility graphics, make sure the link is visible in the required view (remember this is turned off in all views by default as noted above). Note the interface has changed slightly in2011 as shown below (now a drop down instead of a check box).

Link Procedure
  • From the Insert Tab choose Link CAD (Avoid using the Import CAD option)
  • Browse to locate your CAD file
  • Make sure ‘Current View Only’ is Unchecked (the workset takes care of this issue). Note: If your not using worksets you may want to consider using this to restrict the CAD data to one view only.
  • Colours & Layers are user specific but make sure you set the units to match the drawing (don’t rely on the ‘Auto’ option).
  • ‘Positioning’, ‘Place at’ and ‘Orient to View’ are user specific.
  • You can now link your CAD file into your project.
Remember, if you want to see your CAD file in other views, you will need to open the view and set the visibility graphics to show the CAD workset created earlier.


J said...

You may also want to add to the "CAD Prep list" the command OVERKILL as well. lines on top of lines can be problematic and create unnecessary CAD bloat...

Chris Senior said...

Thats an excellent command, thanks J. I never knew that one existed in AutoCAD. Great Tip!

Anonymous said...

how do you attach a cad detail to a revit section?

Chris Senior said...

You link a CAD detail into a Revit section using the Link Procedure noted above. Be sure to check the 'Current View' only button when linking.

Anonymous said...

if i link a cad detail rather than import, will the cad file be included when i upload the mep file to an architect?

Chris Senior said...

The CAD file (even though it is linked) is retained in the Revit model in its last preserved state when you send the model externally. It will pop up a notification that the CAD link is missing when the recipient opens the model but it should still display it correctly.

To be safe you can change your Manage Links settings to ‘Relative Path’. This will mean that if you send the recipient all your CAD drawings along with your revit model (and he/she downloads them into one single folder), when the Revit file is opened it will look for the CAD files in the immediate folder and reference them in.

Anonymous said...

I'm trying to link in a CAD file into an existing Revit model to show some details in a drawing. Only problem is that my ducts block some things in the CAD file from view. Is there a way to change the draworder such that the CAD file is placed in the foreground of the view, overlayed on top of the ducts? Thanks.

Chris Senior said...

Select the CAD file and from the options bar, change the dropdown from background to foreground. This only works with linked files, not imports. This is different to the draw order tools on the ribbon.

Anonymous said...

What about when i get error messages, for instance "error 1", revit can´t link cad format, my cad file is purged, audited, etc...

rgesner said...

Two notes:
1. This workset procedure applies only to model views and is irrelevant to links into drafting views. In drafting views, links are assigned to the view name workset, e.g. "View "Drafting View: Door Sill"
2. Be careful copying DWG links from other Revit projects. If Revit can't find the DWG, it converts the copied link into an import.

Chris Senior said...

Sorry annonymous, if you are getting error messages like that it could be a number of things to do with the cad file. Is it a true Autodesk dwg or has it been generated by a third party app? There could be elements which are rogue within the file, it could be simply that you need to copy and paste the geometry into a fresh AutoCAD file. It could be some 3D elements causing problems... the list goes on. Not one I could solve for you here! Best thing would be to extract half of the cad model and paste it into a new dwg file. If this still fails when linking in, you know the problem still exists in this part of the cad file. Chop it down again so you only bring in say a quarter and see if this works. If it does, then you know the rogue element/geometry is somewhere in that part of the model which is causing the problem. Keep going until you isolate what is causing the issue. Just a suggestion...

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