A pneumatic grinding machine includes a fixed upper casing, a lower casing, an annular wall, an upper sealing member, a lower sealing member, a rotor, multiple vanes, a central shaft assembly and a grinding tray assembly. An inlet port of the annular wall is disposed above an outlet port and the inlet port and the outlet port are isolated from each other by a middle stopper ring. The upper sealing member has an upper stopper ring and an upper cover body. The lower sealing member has a lower stopper ring and a lower cover body. A dustproof sheath is disposed around the second bearing for preventing the splashing dusts from contacting with the second bearing.

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