The reasons are few, but simple and compelling. First, if a thorogh design development has been completed using Revit, very few design changes should be required during the course of construction. Second and probably most compelling is the staff during construction phase services. In many design firms a different - most often, more senior - team will handle a project in construction. These team members may not be familiar with Revit at all.
Aspects of Revit's DWG Export
Revit has the ability to export DWG, DGN, DXF and some other formats with a decent level of consistency; however, there are a few aspects to note before embarking on converting an entire project.
- Revit does not export DWGs in STB plot style format. If you require this format, be prepared to use some scripts or custom programming to assign your standard named plot styles.
- Revit assigns lineweight and linetype overrides at the entity level. Although there are other options to assign overrides to new layers or exclude overrides completely, they are not very useful. If you don't use overrides, you lose all lineweights and linetypes anyway and using new layers for overrides only complicates a clean conversion by creating layers such as A-WALL-1, A-WALL-2, A-WALL-3, etc. If your standard use of CAD dictates using a plot style table for defining plotted lineweights, again be prepared to do some work.
- Using the "Shared" option for coordinates on exported files has no effect on plans when you're exporting sheets. Data in model space of exported sheets is positioned only in relation to the paper space viewports.
- Lines within blocks are not on layer 0; they are on the layer of the category or sub-category of the entire component. This is particularly troublesome if families are made with confusing categorization. For example, an elevator door was made as a Specialty Equipment family, but with a sub-category called "Doors". When exported, the lines within the block were on A-DOOR, but the block itself was inserted on layer A-EQPM.
- Text and Dimensions all export into the "Standard" style. There is no setting in Revit to modify this.
- Hatches for columns are exported onto the same layer as the column.
Pre-Conversion Tips for Revit
If you happen to be working on a Revit project and know (or even think) you will be exporting to CAD format, be aware of these potential pitfalls.
- Don't use solid white Fill Regions to mask model elements in any views. The latest versions of Revit have the new Masking Region tool which will behave as expected (like a wipeout) when exported.
- Don't coverup unwanted text or dimensions with opaque background text. AutoCAD does not have an analogous object, so you end up with two, overlapping text elements in the exported CAD file.
- Use appropriate sub-categories for custom families. If you have many custom families all under Generic Models, they will most likely be exported onto one layer.
- Try not to 'fudge' a design by drawing it with Model Lines or Detail Lines. These will export as line layers, not as building layers such as walls, doors or windows.
- Because unreferenced view tags cannot be hidden or removed during the exporting process, make it a habit to hide all temporary sections, callouts or elevations during your project. Hopefully, Autodesk will soon develop a way to mitigate this problem - perhaps by allowing visibility (on/off) access to the various view types within a project.
- Protect your export layers template file. Every time you use it to export CAD formats from Revit, it is appended with any additional data which may be in the Revit file at the time - including all linked and imported CAD files. This is an additive process ONLY. The export template will not 'clean' itself when it is needed for the next project. I usually create a clean master template then ZIP the file so no one can accidentally use it in a live Revit project.