Self-Drilling Screws

Point too short. The drill point must break through before the thread engages. If the thread reaches un-drilled material first, the screw spins in place, heats up and burns the point. This is the classic self-drilling failure.

The stack is thicker than the drill capacity, and the gap counts. ASTM C1513 defines drill capacity as the total thickness of material the screw is designed to drill including any gap between the layers. Two 2 mm sheets held 3 mm apart are a 7 mm drilling problem, not a 4 mm one — which is the commonest way a rating gets exceeded without the metal thickness ever exceeding it. Drill-point numbers are not a shared scale either: one commercial table puts #3 at roughly 2.79–5.59 mm in mild steel while one major maker's own No. 3 is about 2.1–6 mm. Quote the product's rated capacity, not the point number — grip range and drill capacity are different figures.

Where the rating comes from, so you know how far it travels. The drilling test is not in the product standard — ISO 15480 refers it out to ISO 10666, which sets mild-steel test plates, axial force 150–350 N, 1,800–2,500 rpm for smaller sizes and 1,000–1,800 rpm for ST5.5–ST6.3, and a maximum drilling time from 3 to 13 seconds by size. So a rating means "passed on the specified plate at the specified thrust and speed" — it is not a promise about your material, your gap or your tool, and it is the honest reason two compliant suppliers publish different capacities.

Speed too high. The point cuts. Excessive speed anneals it, and once the hardness is gone it will not drill.

“Stainless” self-drillers. Standard stainless will not drill steel. What is sold as stainless is usually hardened SUS410 or a bi-metal part — stainless body with a carbon steel point. Corrosion resistance and price differ substantially; ask which one you are buying.