Flow drilling screws are used to connect different materials of small thickness. High axial forces in combination with a high rotational speed create frictional heat, which heats up the metal sheet so that it softens.
The screw tip sinks into the material and forms a funnel with an internal thread, projecting beyond the bottom surface of the metal sheet. Thus an accurate metric thread is formed which ensures a tight screw connection.
This method offers all advantages of a metric connection and is mainly used in car body manufacturing.
Graph of flow drilling process
The rotational speed has a significant effect on the quality of the fastening process in all stages of the operating process: At the locating stage, the slow rotation of the bit facilitates the smooth mating with the screw slot. Flow drilling requires a high rotational speed in order to ensure a rapid heating up of the material. The rotational speed defines the process cycle time for forming the thread and screwing in the screw. A low rotational speed improves the accuracy of the screw tightening torque.
During the flow drilling process, the metal sheets are pressed together to avoid the formation of gaps. The hold-down force prevents gap formation between the metal sheets that are to be connected. A gap may arise through high forces upon the screw or through material throw-up during the flow drilling process. The hold-down force can be set variably in order to avoid damage of the work piece.