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CMM (Coordinate measuring machine)

CMM (Coordinate measuring machine)

A coordinate measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, including mechanical, optical, laser, and white light. Depending on the machine, the probe position may be manually controlled by an operator or it may be computer controlled. CMM typically specify a probe's position in terms of its displacement from a reference position in a three-dimensional Cartesian coordinate system (i.e., with XYZ axes). In addition to moving the probe along the X, Y, and Z axes, many machines also allow the probe angle to be controlled to allow measurement of surfaces that would otherwise be unreachable.


Description:

A common use of CMM is in manufacturing and assembly processes to test a part or assembly against the design intent. In such applications, point clouds are generated which are analysed via regression algorithms for the construction of features. These points are collected by using a probe that is positioned manually by an operator or automatically via Direct Computer Control (DCC). DCC CMMs can be programmed to repeatedly measure identical parts; thus an automated CMM is a specialized form of industrial robot.


The typical 3D "bridge" CMM allows probe movement along three axes, X, Y and Z, which are orthogonal to each other in a three-dimensional Cartesian coordinate system. Each axis has a sensor that monitors the position of the probe on that axis, typically with micrometer precision. 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. This process is repeated as necessary, moving the probe each time, to produce a "point cloud" which describes the surface areas of interest.


Coordinate-measuring machines include three main components:

1. The main structure which includes three axes of motion. The material used to construct the moving frame has varied over the years. Granite and steel were used in the early CMM's. Today all the major CMM manufacturers build frames from aluminium alloy or some derivative and also use ceramic to increase the stiffness of the Z axis for scanning applications. Few CMM builders today still manufacture granite frame CMM due to market requirement for improved metrology dynamics and increasing trend to install CMM outside of the quality lab. Typically only low volume CMM builders and domestic manufacturers in China and India are still manufacturing granite CMM due to low technology approach and easy entry to become a CMM frame builder. The increasing trend towards scanning also requires the CMM Z axis to be stiffer and new materials have been introduced such as ceramic and silicon carbide.


2. Probing system: 

New probing systems:

There are newer models that have probes that drag along the surface of the part taking points at specified intervals, known as scanning probes. This method of CMM inspection is often more accurate than the conventional touch-probe method and most times faster as well.


The next generation of scanning, known as noncontact scanning, which includes high speed laser single point triangulation, laser line scanning, and white light scanning, is advancing very quickly. This method uses either laser beams or white light that are projected against the surface of the part. Many thousands of points can then be taken and used not only to check size and position, but to create a 3D image of the part as well. This "point-cloud data" can then be transferred to CAD software to create a working 3D model of the part. These optical scanners are often used on soft or delicate parts or to facilitate reverse engineering.


3. Data collection system: typically includes a machine controller, desktop computer and application software.


Availability:

These machines can be free-standing, handheld and portable.


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