how to open and edit a dxf files without using autocad

With a little time invested you could easily write your own class, to create ARCs, Cirles, Lines, etc which could make up your DXF file. You can even short cut this task if you like because all that is required for the drawing profile is just the Entities section for the DXF file.

for example if you were to copy and paste the following text into you your favotie text editor and save it as Filename.DXF you could open it with AutoCAD and you will see a circle.

0
SECTION
  2
ENTITIES
  0
CIRCLE
  8
0
 10
2.0
 20
2.5
 30
0.0
 40
12.0
  0
ENDSEC
  0
EOF

Here is a link to help you with the DXF code and formating you can download some files from here.

http://www.wotsit.org/list.asp?fc=19