Kelim, Chapter Eleven, Mishnah Six
A metal spindle-knob:
1) Rabbi Akiva says it is susceptible to impurity
a) But the sages say it is not susceptible.
b) If it was only plated [with metal] it is clean.
2) A spindle, a distaff, a rod, a double flute and a pipe are susceptible to impurity if they are of metal, but if they are only plated [with metal] they are clean.
3) If a double flute has a receptacle for the wings it is susceptible to impurity in either case.
Section one: A metal-spindle knob is placed on top of the spindle used in spinning wool. According to the sages, it does not have its own name and therefore it is clean. Rabbi Akiva holds that it does have its own name, so it is susceptible.
Rabbi Akiva agrees that if it is only covered with metal, it is considered a simple wooden vessel, and it is not susceptible.
Section two: All of these vessels have their own name and therefore they are all susceptible to impurity, if made of metal. However, if they are only covered with metal, but their base is wood they are not susceptible, because they do not have a receptacle.
Section three: If the double-flute has a receptacle in which to receive “wings” which somehow make other types of musical noises, then it is susceptible even if it is made of wood. Wooden vessels are not-susceptible only if they have no receptacle.