Self-drilling screws are a commonly used fastener that quickly fixes two metal sheets together without a pre-drilled hole .
ISO 15480 defines the drilling range of sheet thickness. It says clearly that, for example, the sheet working with a ST4,2 self-drilling screw should range from 1.75mm to 3.0mm in thickness.
It makes sense to tell users the biggest value that a screw can work on so that users won't blame supplier supplier when a ST4,2 screw fails on a 4-mm-thick plate.
Why set a lower limit? If a ST4,2 screw can drill through 1.75-mm-thick steel plate, what is the problem of drilling through a 1.5-mm one?
A self-drilling screw not only drills but also fastens sheet metals together. The drilling point and thread size must be considered together to correctly "engineer" a screw.
Again, let us take a ST4,2 screw for example. According to ISO standards, the pitch of a ST4,2 self-tapping thread is 1.4mm.
To properly fasten metal sheets, the screw pitch must be smaller than the plate thickness. Below are two images comparing the differences between fastening a sheet with correct thickness and a sheet that is too thin. Correct fastening should have engagement with the threads, if the sheet is too thin, the contact with the screw wii be loose and too flimsy. By setting the lower limit of sheet thickness to 1.75mm, the screw will have at least 1 full thread engaging into steel. This is why ISO specifies a lower limit of plate thickness.
|A screw working with sheet metal of the correct thickness.
|A screw working with sheet metal of incorrect thickness (too-thin).
But things have exceptions. The above stiation only covers ISO standard self-drilling screws. If we still need to drill a sheet that is thinner than screw pitch, a reduced point screw can be considered. Based on the relationship between thread and drilling tip, self-drilling screws can be classified into two categories:
1. Standard Drilling Point (below image): Minor Diameter(Yellow) < Drilling Diameter(Red) < Major Diameter(Blue). When using this kind of screw, note that pitch must be smaller than plate thickness.
2. Reduced Drilling Point: Drilling Diameter < Minor Diameter < Major Diameter. In this case, pitch must be bigger than plate thickness to cause an effective joint.
You are welcome to consult Sun Through about how to decide screw dimensions to meet your specific applications or if you have any other questions about fasteners.