Critical assembly operations on die-cast metals or aluminium using tri-lobe screws?

The Smart Thread feature offered by electrical solutions is the key to solving them

Assembling different materials by screwing may seem a simple operation, but it is not. Especially when the processes involve die-cast metals or aluminium with tri-lobe screws. However, assembly operations may also be critical when using self-threading, self-tapping or self-drilling screws on other types of materials, or where there is a risk of stripping the screws, because the tightening torque is lower than the thread cutting torque, compromising the mechanical tightness of the components.

To circumvent these conditions – which, remember, can affect component tightening quality and safety, and therefore the quality of the finished product – the ideal situation is to have a final tightening torque that is less than or equal to the initial threading/tapping torque, and only computer-controlled electric screwdrivers can achieve this kind of tightening. Let’s see how.

The parameters in play in such critical assembly operations are speed, angle and torque. In fact, high speeds and high torques are required in the initial thread forming step. The speed is then reduced when the screw is about to come to the end of its travel and the screwdriver automatically changes the torque to tighten the components to the desired torque without stripping the thread.

This system therefore avoids the need for post-process checks and prevents components that are not tightened properly from continuing through the production process, with the risk of letting them reach the end where they would be permanently rejected, wasting the accumulated manufacturing costs.

New generation electric screwdrivers are combined with power supply and control units that can parametrise the speed, angle, initial torque, and final torque to adapt them to different assembly requirements. Electric tightening technologies are now very advanced and there are many types of electric screwdriver available with different accuracies and programming levels, as well as a very fair quality/price ratio compared to the past.

We remind you that the factors for determining the right tightening torque are screw type, material types, joint geometry and the many friction coefficients that significantly affect the percentage of useful torque transmitted to the screw. Therefore, bear in mind that the DIN screw strength classes given in the reference tables are indicative and correspond to approximately 70–90% of the yield point torque.

An important regulatory reference for screw tightening torques is EN 898-1 (“Mechanical properties of fasteners made of steel - Part 1: Bolts, screws and studs with specified property classes - Coarse thread and fine pitch thread”), which defines the minimum breaking and testing loads for coarse and fine pitch metric screws.

For more details on these topics, Fiam publishes the “The industrial tightening process” manual.

If you have a critical assembly operation that you are unable to solve properly, our Technical Assistance Team is here to help you.