The older RS274D Gerber spec does not include the aperture definitions in the actual data file. These apertures are defined separately and transmitted either in a separate file or on paper. If you are going to convert RS274D files you need to have the apertures on hand so that you can enter them into the dialog box.

Unfortunately there is no standard format for describing the apertures (that's why RS274X was invented!). However for several popular formats we have developed an aperture translator that automatically imports the aperture description. See APTGEN which ships with GBF2DXF.

The Gerber language basically has two types of commands: Draws, which drag a round or square aperture from point to point; Flashes, which image a special shape at one location. Depending on what you need to do with the DXF file, GBR2DXF can create different entities in DXF.

Draws The program can translate each draw into a polyline whose width matches the draw diameter. For CAD systems that do not support a line width properly the program can output the draw using zero width polyline to outline the area covered by the circuit trace.

Flashes flashes are translated as blocks. The GBR2DXF program defines a block for each aperture definition. It then inserts each block at the same coordinates where the corresponding flash was placed. An option is available that flattens these blocks.

A screen shot of a Gerber test file, testpat.gbr, that will be translated into AutoCAD DXF. We ran the GBR2DXF translator with three different setting combinations as shown below.

Download testpat.gbr [RS274X Gerber File 8KB].

You can see that the polyline endpoints are not compensated to take into account the finite draw aperture width. Also note how AutoCAD does not miter polyline bends. This option produces the smallest DXF output file.