Perpendicular to the cut direction at the moment of cutting — but for a hot-wire that's automatic. The wire is straight, it spans the two towers, and the foam is between them. Just make sure the wire's mounting tension keeps it taut + parallel to the X-Y plane (assuming standard hobbycutter orientation).
Angled wire setups exist in industrial machines but they need a 5+-axis kinematic model; not what this tool does.
Angled wire setups exist in industrial machines but they need a 5+-axis kinematic model; not what this tool does.