A stereolithography apparatus comprises a build platform (8) connected to a movable frame (11) via a ball joint (12). The ball joint (12) permits a limited range of movement of the build platform in all directions. A locking mechanism (13) is configured to lock the ball joint (12) stationary for calibration of the orientation angle of the build platform (8) in relation to the exposure arrangement (6), so that the lower surface (9) of the build platform is parallel with the second upper surface (7) of the exposure arrangement (6). The ball joint (12) comprises a ball member (14) attached to the build platform (8) and a socket member (16; 16-1, 16-2) connected to the frame (11). A vertical guide (18) is arranged in the frame (11). The socket member (16) is arranged movable vertically inside and along the guide (18) to permit a limited vertical movement of the socket member (16) in relation to the frame (11). The locking mechanism (13) is configured to lock also the socket member (16; 16-1, 16-2) stationary for calibration of the vertical position of the build platform to determine a zero level of the build platform (8) in relation to the second upper surface (7) of the exposure arrangement (6).
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