urdf_to_collada disrupts the robot model

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Hello all

I am interesting in creating a single Collada model out of my robot URDF. I probably doing some thing wrong her a simple example :
my rob.URDF file is :

    <?xml version="1.0"?>
<robot name="my_rob">
  <link name="base_cylinder">
    <visual>
      <geometry>
        <cylinder length="1" radius="0.1"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 .8 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0" />
      <geometry>
         <cylinder length="1" radius="0.1"/>
      </geometry>
    </collision> 
  </link>

  <link name="head_box">
    <visual>
      <geometry>
        <box size="0.4 0.4 0.4"/>
      </geometry>
      <origin xyz="0 0 0" rpy="0 0 0" />
      <material name="white">
        <color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0" />
      <geometry>
        <box size="0.3 0.3 0.3"/>
      </geometry>
    </collision>
  </link>

  <joint name="my_box_joint" type="fixed">
    <parent link="base_cylinder"/>
    <child link="head_box"/>
    <origin xyz="0 0 0.7"/>
  </joint>

</robot>

in the RVIZ my model seems OK :

but after i execute :

rosrun collada_urdf urdf_to_collada rob.URDF rob.dae

my rob.dae file looks like this in MeshLab :

What am i doing wrong ?