[Gmsh] Conformal mesh on two parts

[Krzysztof Bzowski]

Hi,
I have two parts, which need to be meshed. One part is located inside 
another. To simplify the problem, imagine two cubes: one inside another.

http://i.stack.imgur.com/h87gD.png

Geometries are stored in STL files. From CAD point of view, one file 
contains an internal cube, another contains cube with a hole for an 
internal cube. All dimensions fits (there are no intersections). My task 
is to create a conformal tetrahedral mesh suitable for mechanical FEM 
analysis - without duplicated nodes on shared faces. Each element should 
contain information to which part it belongs. File format does not 
matter but Abaqus is preferred.

I decided to use gmsh, but I cannot mesh both parts together.
I tried the following commands:

  1. File -> Open Internal cube
  2. Add volume to internal cube
  3. File -> Merge External cube
  4. Add volume to external cube

This produces mesh but elements in internal cube are duplicated. It 
looks like gmsh does not treat hole (in external cube) as a void. I also 
tried to use Coherence in gmsh but it failed with error:

 > Error   : Self intersecting surface mesh, computing intersections 
(this could take a while)

 > Error   : Surface mesh is wrong, cannot do the 3D mesh

I assume, that my geometries are fine:

  - Internal cube: http://pastie.org/10673562
  - External cube: http://pastie.org/10673564

How to create such a mesh which satisfy my requirements?

Best regards,
Krzysztof