Manufacturing method of a sheet glass material for magnetic disk and manufacturing method of a glass substrate for magnetic disk

A manufacturing method of a glass substrate for a magnetic disk, the method including:

• a dropping process for dropping a lump of molten glass;
• a pressing process for sandwiching at substantially the same time the lump from both sides of a dropping path of the lump using a pair of die surfaces that face each other, and performing press forming to the lump to obtain a sheet glass material;
• and
• a machining process for machining the obtained sheet glass material,
• at least one of the pair of dies having a concave shape with respect to the dropping path of the lump,
• the die having the concave shape with respect to the dropping path of the lump being formed by bonding a plurality of plates or thin films including materials each having a different coefficient of thermal expansion, the coefficient of thermal expansion of the material constituting the plurality of plates or thin films in contact with the lump is higher than the materials constituting the plurality of plates of thin films not in contact with the lump, and the die having the concave shape being flattened by pressing the lump with the pair of dies, and
• the machining process including a grinding process for grinding the principal surfaces using fixed abrasive grains, and a polishing process for polishing the principal surfaces using loose abrasive grains after the grinding process.