Arm assembly and voice coil motor

A voice coil motor comprising:

• a rotatable boss member having upper and lower surfaces and first and second sides, with pins projecting from the upper and lower surfaces;
• upper, lower and middle arm plates each formed from a thin, non-magnetic metal sheet, each having a free end and each extending from the first side of said boss member, the upper and lower arm plates having through-holes extending therethrough such that the pins, which project from the upper and lower surfaces of the boss member, respectively fit in the through-holes of the upper and lower arm plates to fix the upper and lower plates to the boss member, the middle arm plate being positioned between and parallel to the upper and lower arm plates and being fixed to said boss member with a thermoplastic resin;
• a coil member fixed to the second side of said boss member;
• a plurality of suspension members, each having a front end equipped with a recording/reproducing head and a rear end fixed to the free end of one of the upper, lower and middle arm plates;
• and
• a magnetic circuit having
• an upper yoke having a lower surface;
• a lower yoke having an upper surface such that the upper surface of the lower yoke opposes the lower surface of the upper yoke;
• at least one lower permanent magnet having upper and lower surfaces, the lower surface of the at least one lower permanent magnet being attached to the upper surface of the lower yoke;
• and
• supports fixed to the upper and lower yokes for separating the upper and lower yokes to define a magnetic gap between the upper surface of the at least one lower permanent magnet and the lower surface of the upper yoke, the coil member being swingable in the magnetic gap.

An arm assembly for a magnetic disk drive, comprising:

• a rotatable boss member having an upper, lower and side surfaces with pins extending from the upper and lower surfaces;
• upper, lower and middle arm plates, each arm plate having a recording/reproducing head attached thereto, the upper and lower arm plates having holes therein, the pins, which extend from the upper and lower surfaces of the rotatable boss member, being fitted respectively in the holes in the upper and lower arm plates;
• and
• a resin attaching the middle arm plate to the side surface of the rotatable boss member.