Autocad 2000i Save problem

I construct 3D solids by adding/subtracting 3d boxes. Prior to saving I can "modify/solids editing" without any problems. When I later retrieve this file and attempt to "modify/solids editing" it says "No solids detected." I've saved under different file formats(DXF2000,R12..,R14..) with the same result. I'm new to this and assume I'm missing something obvious. Any suggestions?

Maybe the boxes are comprised of surfaces.

Could you explain?

Do a "list" on the boxes to check if they are 3D Solids.

If I create a box by draw/solids/box and then do a "list" it says the box is a 3d Solid. If I then save this box as a DXF file and then retrieve it, the "list" command says it's a Block Reference? Is there anyway to get it back to a 3D Solid so I can continue editing?

If I then save this box as a DXF file and then retrieve it, the "list" command says it's a Block Reference?

How are you retrieving it?

What happens if you Explode the Block Reference?

Also, why are you saving as DXF if you want to continue working on it?

I use file/open to retrieve, and if I do an "explode" to this file it converts it into lines.

I'm saving as a DXF because I'm using it in a program called MESHCAM which generates toolpaths for use on another program called Mach3 which controls my NC Router. If I'm unable to edit my DXF files, I have to start from scratch everytime I have a change in a project. I'm a little overwhelmed right now because I'm new to both 3D modeling and CNC. I've had success making simple drawing/shapes and cutting them out. It wasn't until I had an actual part whose drawing I couldn't finish in one sitting did I discover this problem.

In any event I appreciate the help.

Why don't you just save your model as dwg and then save out a dxf for your CAM program. Then, if you need to back and make changes, just open your dwg file, make the changes and then save out a new dxf file. There should be no reason to start over every time.