Tapping Screws: Which Type?
“Self-tapping” gets used for three very different screws. Grab the wrong one and you’ll strip it, crack the material, or watch it spin without ever biting. Here’s how to tell them apart.
All three make their own threads — that’s why they all get called “self-tapping.” The real questions are how they do it, and whether you drill a hole first.
Thread-forming (sheet-metal)
Pushes material aside to form threads instead of removing it — like the common Type A, AB, and B sheet-metal screws.
- Best in: thinner sheet metal, plastic, soft material — where there’s give
- Plus: strong hold, because no material is lost
- Watch: too thick or hard and it fights you or strips
Thread-cutting
A cutting edge or flute carves a thread, like a tiny tap — Type 23, 25, and F are common.
- Best in: thicker or harder metal and castings, where forming would build up stress and crack the part
- Plus: cutting relieves that stress
- Watch: the thread it leaves is a touch less strong than a formed one
Self-drilling (TEK)
A little drill point on the tip bores its own hole and threads in one shot — no pre-drilling.
- Best in: fastening steel and metal panels fast
- Watch: the point is rated for a limited steel thickness (a point number, roughly #1 to #5)
Why a TEK screw just spins
The most common mistake is a self-driller on steel too thick for its point. If the screw spins without pulling in, that’s usually it — step up to a longer drill point, or pre-drill and use a thread-cutting screw instead.
Quick way to choose
Thin sheet, plastic, soft metal
→ Thread-forming, pre-drilled
Thick or hard metal, or a casting you don’t want to crack
→ Thread-cutting, pre-drilled
Steel panel or framing — skip the drill
→ Self-drilling TEK — check its point can handle your thickness
About the pilot hole
For thread-forming and thread-cutting screws, the pilot hole size matters. Looking down the hole:
The right size depends on the screw and the material, so drill a test hole in scrap of the same stock before you commit — or tell us the screw and material and we’ll point you to the right drill.
Tell us the job, we’ll match the screw
Bring a sample or describe what you’re fastening into and how thick it is. We’ll sort the type, point, and pilot so the first one goes in clean.