When it comes to Engineering-To-Order (ETO) products, neither the exact number nor the form of the components in them can be predefined. Thus, existing assembly models and generative design techniques are not adequate to support development of design automation tools for ETO products. ETO companies usually use custom libraries with past case designs that are adjusted to a customer’s requirements. This method is not cost effective and it is prone to human errors. In this work, we present the Automatic Assembly Synthesis Model (AASM), connecting a Knowledge Based Engineering (KBE) system and a CAD system to automate routine design tasks for ETO mechanical products.