Howto create M:N-relations in drupal (without CCK)

Hi,

I develop a drupal-modul (6.x) Where I need a M:N relation between two node-types.
My module is about fever-curves, presented as table and charts. I havend found anything like that in the repository...

My relations:
fever_type (a drupal-node)
fever_element (a internal relation under "content-type"-level)
fever_timestamp (a drupal-node)
fever_value (a internal relation under "content-type"-level)

1 fever_type has many fever_element
1 fever_type has many fever_timestamp
1 fever_timestamp has many fever_value
1 fever_type has many fever_value

fever_type <--- fever_timestamp
    ^                 ^
    |                 |
fever_element <--- fever_value

How can I create those relations? The node_example.module ( http://api.drupal.org/api/file/developer/examples/node_example.module/6 )
just creates a very simple entity ...
Which module can I take as a template / for hints ?

thnx

Magnus