#4 | 2020
SYN] Magazin
15/21

Reliable flow drill joint inspection

Flow drill fastening can be used for multi-material joints even if the joint is only accessible from one side. The quality of the joint can easily be inspected externally based on five parameters.


1. Fastener coating

Fasteners have a uniform coating of zinc-nickel or zinc flakes for corrosion protection. Coating damage leads to a higher corrosion risk. Prior to assembly, you should therefore always inspect the fasteners to ensure that there is a homogeneous coating to protect your components against corrosion. 
 


2. Flush fit of the fastener drive 

Check whether the fastener head is flush with the material surface. If not, the joint may be weak. In such cases, a distinction is made between strength spots and process spots.

Process spots are joints without structurally relevant properties that are only used for stabilizing the joining location. At these points, flow drill fastening is often used in combination with structural adhesives. The component is fixed in place by the flow drill fastener until the adhesive has fully hardened. In the case of process spots, heads with one-side contact may be tolerated. If the head has not made contact at all, the wrong speed may have been used or the torque may have been too low. If the speed is too low, the torque required for thread cutting will be exceeded. On the other hand, if the torque during final tightening is too low, the clamping force may be too low, or no clamping force may be created. 





Strength spots are joints with structurally relevant properties. In body-in-white production, there are a large number of strength spots especially on the floor pan, the front end, the impact zone and the body frame. In such cases, the fastener head must always be flush with the material surface. If the fastener only makes contact on one side or is there is no contact between the head and the material, there may be several causes:

  • Incorrect robot positioning 
  • Inadequate clamping of components  
  • Poor fit between components  
  • Incorrect tightening parameters during thread cutting




3. Pre-hole coverage 

Joints without pre-drilled holes are more effectively protected against corrosion than joints with such holes. Moisture can easily penetrate under the head of the fastener through the pre-drilled holes. If the hole is not fully covered by the fastener, you should check the programming of your robot and the diameter of the pre-drilled hole. 



4. Material gap  

Visible gaps between layers may cause corrosion or air bubbles, break the adhesive bond and impair the strength of the joint. In this case, check the material thickness and the torque and speed used for drilling through the material. If the material thickness is outside the specified tolerances, the fastener will not penetrate so easily, and the joint will not be sufficiently strong or tight.


 

5. Fastener movement  

Before approving a joint, the firm seating of the fastener should be checked by hand. If the screw can be turned anticlockwise, the joint must be classed as not-OK. There may be several reasons for a loose fastener, including excessive torque or adhesive under the fastener. The thickness and hardness of castings may also vary. As a result, the torque set may be too low and the joint may be faulty.