A spinstand having a vacuum chuck clamping mechanism for securing magnetic discs includes a cap (9) and a base (10). A disk-to-be-clamped is held between the cap (9) and the base (10). Vacuum, applied through a port (4) in a mounting screw (12) of the base, provides the clamping force. The vacuum is held constant using a circumferential seal (27) on a piston (2) of the cap, which extends into a cylindrical countering bushing (3) extending from the base. In one form, the cap (9) is centered about a spin axis (SA) extending through the base (10), using a hardened pin (1) extending from the cap and a locating bushing (19) affixed to the base. When the cap (9) is inserted into the base (10), the pin (1) prevents a piston (2) in the cap from contacting the inside walls/sealing surface (11) of the centering bushing (3) of the base. To remove the cap, positive air pressure is applied to the air passage (4), collapsing the seal (27), and forcing the cap (9) out of the base (10) without causing wear on the seal (27) or the sealing surface (11) of the centering bushing (3).