Architips From Cheap Tricks, Apr 95 Issue Published By: Shu Associates, Inc. 120 Trenton St., Melrose, MA 02176-3714 (617)662-0020 (FAX Same) eshu@world.std.com

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This column is a primer on using DataCAD’s DXF read\write capabilities. While DXF has some limitations, it is a very useful tool that people use on a daily basis. With the help of this primer and some planning and preparation, you can work with virtually any CAD program available.

DXF stands for AutoCAD Data Transfer Format. It is owned by Autodesk Inc., as such it is a proprietary standard. Because of AutoCAD’s popularity it has also become a de facto industry standard as well. Keep in mind that DXF is an AutoCAD drawing format.

Successful, accurate, and hassle-free DXF’ing is a result of understanding the capabilities and limitations of the DXF protocol and taking into account those limitations when creating your work.

We are the developer and publisher of the DataCAD macro DXF-PREP (Cheap Tricks Ware B26), which is designed to prepare your DataCAD drawing for DXF translation. A star (*) at the end of an item below indicates that DXF-PREP will correct that situation.

The limitations of the DXF protocol can be broken up into two areas. 1) A limitation in the DXF standard itself (e.g. the DXF protocol does not support attribute data) and 2) non-equivalencies between the different CAD software (e.g. DataCAD’s associative hatch has no equivalent in AutoCAD R12.)

The following are limitations of the DXF protocol that affect both the source and target CAD software: a) Direct compatibility of four fonts: SIMPLEX, COMPLEX, ROMANS, and ROMANC *; b) Direct transfer of four line types, solid, dashed, dotted, and dot-dash; c)the DXF protocol will only support 3-D entities (polygons) with a maximum of 4 vertices and no thickness (3-D only.)

Going from DataCAD to AutoCAD the following limitations exist:

• a) layer names must be all CAPITALS and contain only DOS characters;*
• b) DataCAD’s Associative hatch does not transfer;
• c) Single point polylines will cause an abort* (problem usually caused by a user error);
• d) DataCAD’s DXF process (5.x & above) will tessellate (bust up) the higher order 3-D entities (i.e. slabs) into individual polygon surfaces (3 or 4 vertices). The coincident edges of the coplanar polygons will be invisible to give the appearance of it being one entity (3-D only);
• e) DataCAD’s 2-1/2 D entities should be squashed to 2-D only*;
• f) Nested symbols will not transfer*;
• g) Symbols must have unique names*; and
• h) unused symbol definitions should be purged.

Going from AutoCAD to DataCAD, if the AutoCAD drawing uses offset polylines to draw walls (or any other parallel lines), these polylines should be exploded to regular lines, prior to creating the DXF file.

We recommend the following procedure to set up for DXF’ing:

In DataCAD’s CONFIG program select one of the DXF compatible fonts as your default character set. Now create a clean default drawing that you will use only for DXF.

For each drawing you wish to export via DXF:

• a) make sure the limitations of DXF protocol have been observed, are accounted for, or have been corrected (DXF-PREP is great for this.) Go to Directory\SymFiles\PurgeSym to purge all unneeded symbol definitions: Go to the LYRUTIL Macro and save out the drawing;
• b) Open up a new drawing using the DXF default file you created previously. Go to LYRUTIL and load in the drawing you saved out previously. Now go to File I/O, then DXF\WriteDXF to create the DXF file.

For each drawing you wish to import via DXF: start with your clean default drawing as noted above and go to File I/O\DXF\ReadDXF and choose the DXF file to import. If you don’t know the base units the drawing was created in, you should check the actual size of line of known dimension. If it is the wrong size you can either use Enlarge to adjust the drawing by a factor or you can reset the DXF import Scale and go through the ReadDXF process again.

Here are some additional DXF Quick Tips from Mary Kadlubowski:

• 1. When you DXF from DataCAD to AutoCAD, be aware that the layer order is alphabetized.
• 2. You can fool AutoCAD by renaming similar fonts to AutoCAD names.
• 3. Renaming linetypes with AutoCAD names helps the transfer, and spacing can be adjusted in DataCAD by enlarging the whole drawing with x=1, y=1, z=1, line factor = 1024.
• 4. If you DXF 2 layers with the same name, AutoCAD creates one layer with both sets of entities.
• 5. Plan for a "0" layer as AutoCAD always creates one as the first layer anyway.
• 6. AutoCAD polylines DXF’d to DataCAD can be exploded into 2D via the Polyline macro.