Surface plates for smaller components are easily inserted into the gantry, making measurement simpler and less tedious for the operator.Programming a massive part is also much easier, as access is unrestricted through 360°, enabling smaller features that are not easily visible to be carefully measured and programmed.Heavy parts and large components can be loaded directly on the floor (in the measuring volume) this is a tremendous safety feature, and enables the programmer to easily walk up to the part during programming of certain features.Gantry coordinate measuring machines provide significant advantages over conventional bridges: The measuring range of gantry CMMs can vary from 1 x 2 x 1m XYZ to 4 x 10 x 3m XYZ, and even larger, specially built units can be purchased. The Z axis, located on the X-axis carriage, can be as long as 4 meters, but normally they are between 1.2 to 2.0 meters in depth. Larger gantry machines have six or eight columns, or more, depending on the length of the Y axis. The X-axis carriage runs along the two supported beams of the Y axis. Smaller gantry machines have four upright columns supporting large Y-axis beams, usually 1.5 to 2 meters in height. This requirement is specified by the manufacturer and should not be ignored. Most gantry machines are mounted directly to the floor and therefore must have a substantial foundation. Gantry CMM machines are used predominantly for very large or heavy parts that require the high precision of a bridge machine. The accuracy of most bridge systems is usually better than other types of coordinate measuring machines, and certainly, when considering a system for machined parts with higher tolerances, a bridge is hard to beat. Heavy components have to be lifted onto the plate, necessitating a crane or a lift truck and potentially causing a collision with the machine. For instance, accessibility to the part being measured is sometimes quite difficult because of the uprights holding the X-axis beam. However, there are both pros and cons associated with their function. Bridge machines are the workhorses of CMMs. Machines are built in sizes ranging from 300×300×300 mm XYZ to 2000 mm x 5000 mm x1500mm, with some exceptions. 95% of all bridge machines run on air bearings to allow friction-free movement and minimize mechanical interaction. Generally, only one side of the bridge is driven the other side is the slave side and is allowed to float freely. Most bridge machines are based on a precision granite plate with two legs supporting the X-axis carriage. Bridge machines have a low cost to build and the ability to maintain accuracy and repeatability over the long term. This process is repeated as necessary, moving the probe each time, to produce a "point cloud" which describes the surface areas of interest.ĭue to their basic structure and simplicity of build, bridge machines have become extremely popular. When the probe contacts (or otherwise detects a particular location) on the object, the machine samples the three position sensors, thus measuring the location of one point on the object's surface. Each axis has a sensor that monitors the position of the probe on that axis, typically with micrometer precision. The typical 3D "bridge" CMM machine allows probe movement along three axes, X, Y and Z, which are orthogonal to each other in a three-dimensional Cartesian coordinate system. The description found on Wikipedia is as follows:
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