the present invention provides for a drop - in seat unit as well as a method of manufacturing the drop - in seat unit that does not require the worker to affix one end of the spring to a frame rail and stretch the spring from one of the frame rails to the other frame rail . the ends of the spring are affixed to the frame rails while the springs are in an un - stretched state . once the plurality of springs is attached to the two frame rails , the rails are mechanically pulled away from each other , thus created a larger open area between the frame rails and thus extending the springs . once the rails are properly positioned in this extended state , support members are positioned between the two frame rails and one end of each support member is affixed to each frame rail , traversing the space or gap between the two frame rails . the support members fix the distance between the two frame rails . once the springs are attached to the two frame rails and the support members are affixed to the two frame rails , the completed drop - in seat unit or seat box assembly is unclamped and released from the manufacturing apparatus and is ready for shipment or use in further construction of the sofa or chair .