The invention relates to a sub-aperture polishing tool, comprising a support member including an attachment feature for attachment to a sub-aperture polishing machine. At an end of the support member is a polishing head comprising: a base structure attached to or integral with the support member, the base structure being arranged to provide a non-flat surface. The polishing head also comprises an outer shell, at least part of the outer surface of which defines the working surface of the polishing tool, the outer shell being affixed to the base structure so as to enclose a cavity between the outer shell and the non-flat surface. A viscoelastic material filling the cavity, located between the outer shell and the base structure.

A gear grinding unit, capable of grinding a tooth flank of a gear to have fine surface roughness without requiring any special shaping work such as dressing, includes a threaded grinding part which is rotatable around a rotation axis (S.sub.1) and engages with a tooth of a gear to be ground. Through a rotational motion of the threaded grinding part, a tooth flank of the gear is ground. The threaded grinding part includes a plurality of flexible abrasive sheets each including a virtual circle having a diameter that equals a root diameter (D.sub.A) of the threaded grinding part and a bulge part bulging radially outward from the virtual circle. With the bulge parts of the abrasive sheets mutually shifted around the rotation axis (S.sub.1), the abrasive sheets are overlapped along the rotation axis (S.sub.1) to form the threaded grinding part.

A gear grinding unit, capable of grinding a tooth flank of a gear to have fine surface roughness without requiring any special shaping work such as dressing, includes a threaded grinding part which is rotatable around a rotation axis (S.sub.1) and engages with a tooth of a gear to be ground. Through a rotational motion of the threaded grinding part, a tooth flank of the gear is ground. The threaded grinding part includes a plurality of flexible abrasive sheets each including a virtual circle having a diameter that equals a root diameter (D.sub.A) of the threaded grinding part and a bulge part bulging radially outward from the virtual circle. With the bulge parts of the abrasive sheets mutually shifted around the rotation axis (S.sub.1), the abrasive sheets are overlapped along the rotation axis (S.sub.1) to form the threaded grinding part.