Machine Bridge / Gantry

In a Coordinate Measuring Machine (CMM), the Machine Bridge or Gantry is the critical structural component that forms the moving axis system. It typically consists of a rigid, often box-shaped, beam that spans the measurement volume. This bridge moves along one axis (commonly the X-axis) on precision rails or air bearings, while carrying the carriage (Y-axis) and Z-axis ram that holds the probing system. Its primary function is to provide a stable, vibration-dampened platform that ensures the probe can be positioned with extreme accuracy and repeatability anywhere within the machine's working envelope, directly impacting the CMM's overall measurement precision and volumetric accuracy.

The bridge is driven by a servo motor or linear motor coupled with a precision ball screw or linear drive system. It moves along hardened and ground guideways or on a cushion of air (air bearings). Position feedback is provided by a linear scale (e.g., glass or reflective) mounted along its travel path. The rigidity and low thermal expansion of the bridge structure minimize deflection and distortion during acceleration, deceleration, and probing, ensuring that the geometric relationship between the probe and the measured part remains constant.