Base member, spindle motor, and hard disk drive device

A base member of a spindle motor, comprising:

• an upper surface;
• and
• a lower surface, wherein
• the base member is a casting, and the upper surface and the lower surface are casting surfaces of the base member,
• the upper surface includes a protrusion that is integrally cast with the base member and extending upward,
• the lower surface includes a first lower casting surface and a second lower casting surface positioned higher than the first lower casting surface,
• the first lower casting surface and the second lower casting surface are adjacent to each other with a stepped portion interposed between the first lower casting surface and the second lower casting surface, the stepped portion increasing in height from the first lower casting surface to the second lower casting surface,
• the stepped portion has a uniform height difference, and
• when a tangential line is drawn to the stepped portion in a cross section passing through a central axis of the protrusion and extending perpendicular to the stepped portion in plan view, the tangential line extends through between a lower edge point of an outer periphery surface of the protrusion on the second lower casting surface side and a lower edge point of the outer periphery surface of the protrusion on the first lower casting surface side.