These two get mixed up constantly. The difference comes down to one thing: whether it drills its own hole.
Self-tapping screws
Cut or form threads as they go in. They need a pilot hole first — you're doing the drilling separately. Good for thicker materials (over 3mm) and when you need precise thread control. Two steps.
Thread-cutting type
Sharp edges shave material as they go. Better for harder metals. You can take them apart and reassemble without damaging the thread.
Thread-forming type
Deform the material around the hole instead of cutting. Creates a tighter fit but needs more torque to drive.
Self-drilling screws
Have a built-in drill bit tip. They drill, thread, and fasten in one motion. No pilot hole needed. Fastest option for sheet metal. One step.
When to use which
If you're working with sheet metal and want speed, self-drilling wins. If you need to fasten something thick — 4mm steel or concrete — self-tapping with a pilot hole is the way to go.
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