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How to inspect your flow drill fastening joints in five steps

Having a high-quality flow drill fastening process is crucial for every car manufacturer. However, inspecting flow drill fastening joints in a production line can be challenging. Learn from our experts how to run a quality check of your joints in five easy steps.

Fixação por perfuratriz de fluxo Soluções de junta Indústrias Artigos Tópicos Soluções de junta Produtos Tipo de conteúdo

Step 1: Coating of the fastener

Before we even start to evaluate the completed joint, the first step should always be the overall visual inspection of the fastener. To be protected from corrosion, the fasteners are evenly coated in zinc-nickel or zinc flake. When a large surface of the coating is damaged, a high risk of corrosion is possible.

Our experts recommend:

If you want to protect your parts from unwanted corrosion, always make sure to check your fasteners for uniform coating before using, and ensure the fasteners match the part description on the packaging.

Step 2: Position of the fastener drive

Checking the final position of the fastener head is very important. Incorrect installation will impact the strength and corrosion resistance of the joint. The parts being assembled can shift during the joining process if not properly secured which can weaken the integrity of the installation. It is important to differentiate between process spots and structural spots because they do have different requirements.

2.1 Process spot

Process spots don’t have structural-relevant properties and are only used for stabilizing the joint location. They often appear in combination with structural adhesives to ensure the joint does not move until the adhesive is cured completely.

K-Flow Process Spot requirements
The joint quality of process spots are OK when:
• The fastener drives completely to depth which rigidly supports the material stack from moving.
• There is a clamp load generated by the joint using a pre-hole in the top sheet.

The quality is NOK when:
The fastener was installed at an angle leaving a slight gap on the underside of the head of the fastener to the surface.

When NOK, our experts recommend checking…

… the rotation speed in the thread forming process
If the rotation speed is for example too low, the desired thread rolling torque gets exceeded and the process controller fails the install. This results in an unseated and incomplete NOK joint.

… the torque in the final tightening
If the final torque is too low, this can result in an unsecured joint with little or no clamp load. In this case, adjust the torque parameters in the last process step and ensure the tool tightens the fastener to the correct specification.

2.2 Strength spot

A structural spot has high strength properties, especially when used with structural adhesive which is a hybrid joint. Numerous variants of flow drill fastening structural spots can be found in the body shop, specifically in the underbody, front & crash sections, and on the framing line.

K-Flow strength spot
A strength spot is only OK when:
• The fastener head is seated flush to the top surface all around the fastener head to the material surface (no gaps).

The quality is NOK when:
• The fastener is stripped which is tested using a torque wrench on the fastener head.
• When there are any gaps under the head of the fastener.
• Only one side of the fastener drive supports the material surface.

When NOK our experts recommend checking…

… the positioning of the robot
If the angle position of the robot is not absolutely perpendicular (or Normal) to the surfaces being joined, the fastener can enter the assembly incorrectly and cause a failed joint. This results in a fastener install that is weaker and open a corrosion pathway on that joint. Adjust the robot approach angle to top surface.

… the clamping of the parts
When the car body parts are insufficiently clamped or supported from movement during the flow drill fastening process due to the high thrust loads being applied, it can result in one-sided support.

… the fit of the components to each other
Parts being assembled need to fit together well with no gaps between them. They system will pre-load the parts to hold them in place during fastening, but clamps and support is required to stop them moving during the process.

… the rotation speed in the thread forming process
When the incorrect speed (RPM) is used on the spindle, the joint stack can result in gaps under the fastener head . This is due to the system controls experiencing higher threading torque during this stage than is expected and stop the process to protect from further damage. This can be caused by installation RPM being too low for the materials being joined (steel slower, aluminum alloy is faster).

… the torque in the final tightening
Another issue that results in a NOK joint is failing to achieve the final seating torque. If the resultant torque is too high, the fastener cannot be tightened enough in the final process step and the fastener stops midway.

Step 3: Pre-hole coverage

Fastener fully covering the pre-hole

Fastener not covering the pre-hole

Flow drill fastening is often used because the process can be performed from 'one side' of the assembly and does not generally require a pre-hole in the top surface material. Exceptions are very thick stacks, castings, or complex mixed -material combinations, or special joint requirements such as thermal conductivity. In these cases, the industry uses materials with a pre-hole and the fastener drive fulfills one more function: pre-hole protection.

Joints not using a pre-hole are much less prone to the possible effects of corrosion. When a pre-hole is used and any under-head gaps are present, there is the possibility of corrosion due to moisture entering under the fastener head and into the layers of the joint. This is where structural adhesives and sealants in hybrid joints play an important role.

If the pre-hole is not fully covered our experts recommend checking…

… the correct diameter of pre-hole is present in the top sheet, then that the robot programming is placing the fastener in the center of the pre-hole (typically 7mm diameter pre-hole for an M5 fastener).

Step 4: Gap between materials

A perfect joint is more than just a flush fastener drive or the coverage of a pre-hole. The quality of a joint also depends on the fit of the material sheets to each other. Gaps within the layers can cause corrosion or air bubbles, rupture the adhesive and influence the strength properties negatively.

K-Flow joint with gap between layers

Gap between material layers

Our experts recommend checking…

… the material thickness
If the material thickness exceeds tolerance, it takes more friction to heat the material and the fastener can not push through the material as easily to provide the required joint strength and tightness.

… the torque in the final process step
Sometimes it’s just a small adjustment that can help to improve the joint quality. Especially in multi-material design, the individual requirements of the single material layers have to be considered when programming the parameters which is part of the initial joint development.

… the rotation speed when piercing the material
When the rotation speed is too high in the first process step, the material gets too hot. This causes issues when the system pushes the material through to the back and form a boss for the next phase, equally too little rotational speed (RPM) can take longer cycle times and not create enough heat to flow the material layers correctly.

Step 5: Movement of the fastener

When the process passes all the previous steps, it is very unlikely that it will fail, but there is one last step to check before approving the joint. It is the manual measurement of the tightness of the fastener - or at least the checking of "breakaway" torque test done manually. If the fastener can be rotated in a counterclockwise  direction, then something went wrong in the joining process and the joint is not validated and should be deemed defective.

K-Flow movement fastener

Fastener movement often results in loose sheets or parts (when adhesive is not used)

Our experts recommend checking…

… the torque in the final process step
A possible error is that the torque was set too high for the final tightening. The thread strips and damages the boss. This can also occur when adhesive gets into the joint inadvertently and lubricates the joining process which can cause torque overshoot.

… if there is adhesive under the fastener drive
When adhesive is placed directly under the fastener location, the friction gets reduced and the fastener is often over-tightened. This results in a loose fastener. It is much less common on joints with no pre-hole in the top material.

… the material thickness and heat-treat
When production materials are used in the assembly process, the thickness and material hardness (tensile strength) can sometimes vary, and the set torque can suddenly be too low.


These five tips are useful to look for improvements that will produce reliable joints, but inspections are only part of this process and joint development should be undertaken on all structural joints. We always recommend an engineering based process development in both the design phase and before the start of production. In our 16 innovation centers worldwide, we make sure that your joining process is viable, reliable, and provides you with the data to support your manufacturing goals. Check our Innovation Center page and get in contact with us if you want to know more.

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