Wood

that serves the metal [of a utensil]

is tamei

and metal

that serves the wood [of a utensil]

is tahor.

How?

A gate-key (some say: lock) of wood

and its pins are of metal (the pins are the parts that open the lock, and the wood is the shaft they are fastened to; according to those that translate this as a lock, the pins are the interior parts that actually lock it shut),

even one,

is tamei;

a gate-key of metal

and its pins are of wood

is tahor.

A ring of metal

and its signet is of almog (a type of wood)

is tamei,

a ring of almog

and its signet is of metal

is tahor.

The tooth of a plate (a small piece of metal that projected from a thin sheet of hammered metal, and which was used as a key),

and of a gate-key,

and of a standard key

is tamei on its own (even if separated from the plate, a tooth, as opposed to a pin, did not need a shaft to function).

פרק יג משנה ז

The Ashkelonian grapnels (these were each a long wooden pole affixed to the center of metal hoop, with several hooks dangling from the hoop; some say they were used to retrieve items that had fallen into a well, others that they were used to hang flasks of water to cool)

that broke

and their hook remains (attached to each broken-off piece)

these are tamei (as the hooks can still be used).

The pitchfork,

and the winnowing fork (which had more tines),

and the rake (which had even more tines),

and similarly a head (i.e. hair) comb,

that had one of their (wooden) tines removed

and made of (i.e. replaced with) metal,

these are tamei;

and on all of these

Rabbi Yehoshua said

the Sages have created a new novelty,

and I do not have with what to answer it.  (He held it to be a tremendous novelty to say such by the case of the pieces of the broken grapnel, where they could not be used for their original purpose; in the case of the single tine, he found it to be very novel to say that this was enough to make it tamei.)