The typical causes of failure with self-drilling screws are explained below:
- Thickness to be drilled:
Before starting a fixing, you must choose a screw with the right diameter for the thickness you want to drill. As a general rule, make sure that the length of the screw bit is greater than the total thickness to be drilled, including the thickness of both sheets and any intermediate chamber (air, insulation, etc.) that there may be between them.
To do this, look up the range of thicknesses recommended for each diameter and type of screw:
To fix sheets that are very thick (6 mm ÷ 12 mm), we can use beam screws (Index reference AUTO) because having an extra-long drill bit guarantees that the screw will completely drill through the total thickness of the sheets before the screw thread starts threading.
In contrast, for sheets that are very thin, we can use screws with a reduced drill bit tip (Index reference ARE), which will make an initial stamp in the sheet where the screw thread will be housed, preventing it from going too far.
- Material to be drilled:
Carbon steel self-drilling screws are hardened, so they can drill through materials such as steel, brass and aluminium.
However, stainless steel self-drilling screws can´t be hardened with heat treatment, so it´s recommended to use them exclusively for materials with less resistance, such as aluminium.
If you need to use stainless self-drilling screws on carbon steel, bimetal screws should be used, which are made with an A2 stainless steel body + a hardened carbon steel drill bit tip, welded together.
- Installation torque:
When installing a self-drilling screw, we must be aware of the installation torque. Excessive torque can cause it to break when the head comes into contact with the sheet. That´s why it´s a good idea to use electric drills with torque control.
- Drilling speed:
It´s also important to observe a suitable turning speed while drilling. A turning speed that´s too fast can cause excessive heating in the screw bit, causing it to become blunt and making installation more difficult.
In the following table, you can find the recommended drilling speed for each screw depending on its diameter and the material to be drilled.
- Fixing elastic materials:
The use of self-drilling screws is not recommended when the materials to be fixed are elastic, for example, when there are intermediate pieces made of rubber or soft plastic. An excessive installation torque may exert a high compression force on these elastic materials which may in turn exert an excessive tensile force on the screw in the medium term, causing it to break.
Ultima revisione: FAQ26 rev0
