Because they act at a larger radius, and resistance to rotation is force times radius.

Each tooth applies roughly similar force regardless of where it sits; what changes is the leverage.

Teeth at the outer rim produce a substantially larger resisting moment than the same teeth close to the bore.

External washers also usually carry more teeth, since there is more circumference to put them on.

The price is that the teeth mark the surface outside the fastener head, in full view.

Where appearance matters, internal teeth are chosen and the reduced grip accepted.

The teeth cut through paint, anodising and oxide to make direct metal-to-metal contact.

A bolted joint between painted or anodised parts is an electrical insulator, which matters where the connection is an earth or a bond.

External teeth score through the coating as the joint is tightened, producing multiple clean contact points around a wide circle.

That gives a lower-resistance and more reliable path than relying on the bolt shank.

Material and finish are chosen for conductivity and for galvanic compatibility with the parts, since the joint is now a metal contact in service.

Where bonding is a safety function, the connection is verified by measurement rather than assumed.

Anything finished or visible, anything hardened, and anything soft enough for the teeth to sink into.

On a painted or plated surface the teeth break the protective coating and leave exposed substrate at the joint - which then corrodes.

On a visible surface the ring of marks is permanent and obvious.

Against hardened steel the teeth cannot penetrate, so the washer flattens and does little.

In aluminium, plastics and composites the teeth embed too far, the joint settles and preload is lost.

Medium-hardness steels are where they work best, which is much of general fabrication and machinery.

No - adding one under the teeth removes the bite on that side and defeats the washer.

The teeth must reach both surfaces: the fastener face on one side and the joint face on the other.

A hardened flat washer beneath gives the teeth a smooth hard surface they cannot penetrate.

If the joint face must be protected, the correct answer is a different locking method rather than a stacked washer.

Where a large bearing area is needed as well as locking, a serrated flange nut provides both in one part.

The washer goes directly against the surfaces, under the part that turns.

In practice no - the teeth take a permanent set and the joint face is already marked.

Tightening twists each tooth flat, which is plastic deformation rather than elastic spring.

A used washer shows flattened teeth and polished contact points.

Refitting it into the existing marks gives very little additional grip.

They are inexpensive enough that replacement at each disassembly is normal practice.

In bonding applications reuse is particularly poor practice, since the electrical contact depends on fresh cutting.

Wedge washers are far more effective against transverse vibration, and they do not rely on biting the joint face.

A wedge pair works by cam faces steeper than the thread pitch, so any rotation backwards lifts the fastener and raises tension.

That mechanism does not depend on friction, spring force or surface penetration, which is why it performs so much better in vibration tests.

Wedge washers still have serrations against the outer faces and so still mark the surfaces, though for a different purpose.

They cost considerably more and must be used as matched pairs, correctly oriented.

External tooth washers remain the cheaper choice for light general work and the only one of the two used for electrical bonding.

Mechanically similar but softer, so the teeth blunt sooner and bite less into harder joint faces.

Stainless toothed washers are used where corrosion resistance is needed and the locking duty is modest.

Galling between stainless components is a real risk, and the washer's teeth against a stainless nut face are a good place for it to start.

In bonding applications stainless has higher electrical resistance than plated steel, which matters for that duty.

The joint face damage still occurs and still breaks any passive layer, so the corrosion argument is not entirely won by the material choice.

For exposed vibration-critical joints, a stainless wedge washer pair is the more capable option.