Aug. 11, 2025
Mechanical Parts
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, the most common being mechanical and laser sensors, though optical and white light sensors do exist. Depending on the machine, the probe position may be manually controlled by an operator, or it may be computer controlled. CMMs 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.
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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, with typical accuracy in the order of microns. When the probe contacts (or otherwise detects) a particular location on the object, the machine samples the axis position sensors, thus measuring the location of one point on the object's surface, as well as the 3-dimensional vector of the measurement taken. This process is repeated as necessary, moving the probe each time, to produce a "point cloud" which describes the surface areas of interest. The points can be measured either manually by an operator, automatically via Direct Computer Control (DCC), or automatically using scripted programs; thus, an automated CMM is a specialized form of industrial robot.
A common use of CMMs is in manufacturing and assembly processes to test a part or assembly against the design intent. The measured points can be used to verify the distance between features. They can also be used to construct geometric features such as cylinders and planes for GD&T so that aspects like roundness, flatness, and perpendicularity can be assessed.
Coordinate-measuring machines include three main components:
These machines are available as stationary or portable.
The accuracy of coordinate measurement machines is typically given as an uncertainty factor as a function over distance. For a CMM using a touch probe, this relates to the repeatability of the probe and the accuracy of the linear scales. Typical probe repeatability can result in measurements within one micron or 0. inch (half a ten thousandth) over the entire measurement volume. For 3, 3+2, and 5 axis machines, probes are routinely calibrated using traceable standards and the machine movement is verified using gauges to ensure accuracy.
The first CMM was developed by the Ferranti Company of Scotland in the s[1] as the result of a direct need to measure precision components in their military products, although this machine only had 2 axes. The first 3-axis models began appearing in the s (made by DEA of Italy and LK of the UK), and computer control debuted in the early s, but the first working CMM was developed and put on sale by Browne & Sharpe in Melbourne, England. Leitz Germany subsequently produced a fixed machine structure with moving table.[citation needed]
In modern machines, the gantry-type superstructure has two legs and is often called a bridge. This moves freely along the granite table with one leg (often referred to as the inside leg) following a guide rail attached to one side of the granite table. The opposite leg (often outside leg) simply rests on the granite table following the vertical surface contour. Air bearings are the chosen method for ensuring friction-free travel. In these, compressed air is forced through a series of very small holes in a flat bearing surface to provide a smooth-but-controlled air cushion on which the CMM can move in a nearly frictionless manner which can be compensated for through software. The movement of the bridge or gantry along the granite table forms one axis of the XY plane. The bridge of the gantry contains a carriage which traverses between the inside and outside legs and forms the other horizontal axis. The third axis of movement (Z axis) is provided by the addition of a vertical quill or spindle which moves up and down through the center of the carriage. The touch probe forms the sensing device on the end of the quill. The movement of the X, Y, and Z axes fully describes the measuring envelope. Optional rotary tables can be used to enhance the approachability of the measuring probe to complicated workpieces. The rotary table as a fourth drive axis does not enhance the measuring dimensions, which remain 3D, but it does provide a degree of flexibility. Some touch probes are themselves powered rotary devices with the probe tip able to swivel vertically through more than 180° and through a full 360° rotation.
CMMs are now also available in a variety of other forms. These include CMM arms that use angular measurements taken at the joints of the arm to calculate the position of the stylus tip, and can be outfitted with probes for laser scanning and optical imaging. Such arm CMMs are often used where their portability is an advantage over traditional fixed-bed CMMs: by storing measured locations, programming software also allows moving the measuring arm itself, and its measurement volume, around the part to be measured during a measurement routine. Because CMM arms imitate the flexibility of a human arm, they are also often able to reach the insides of complex parts that could not be probed using a standard three axis machine.
In the early days of coordinate measurement, mechanical probes were fitted into a special holder on the end of the quill. A very common probe was made by soldering a hard ball to the end of a shaft. This was ideal for measuring a whole range of flat-face, cylindrical, or spherical surfaces. Other probes were ground to specific shapes, for example a quadrant, to enable measurement of special features. These probes were physically held against the workpiece with the position in space being read from a 3-axis digital readout (DRO) or, in more advanced systems, being logged into a computer by means of a footswitch or similar device. Measurements taken by this contact method were often unreliable as machines were moved by hand and each machine operator applied different amounts of pressure on the probe or adopted differing techniques for the measurement.[citation needed]
A further development was the addition of motors for driving each axis. Operators no longer had to physically touch the machine but could drive each axis using a handbox with joysticks in much the same way as with modern remote controlled cars. Measurement accuracy and precision improved dramatically with the invention of the electronic touch trigger probe. The pioneer of this new probe device was David McMurtry who subsequently formed what is now Renishaw plc.[2] Although still a contact device, the probe had a spring-loaded steel ball (later ruby ball) stylus. As the probe touched the surface of the component, the stylus deflected and simultaneously sent the X,Y,Z coordinate information to the computer. Measurement errors caused by individual operators became fewer, and the stage was set for the introduction of CNC operations and the coming of age of CMMs.
Optical probes are lens-and-CCD systems, which are moved like the mechanical ones, and are aimed at the point of interest, instead of touching the material. The captured image of the surface will be enclosed in the borders of a measuring window, until the residue is adequate to contrast between black and white zones. The dividing curve can be calculated to a point, which is the wanted measuring point in space. The horizontal information on the CCD is 2D (XY) and the vertical position is the position of the complete probing system on the stand Z-drive (or other device component).
There are newer models that have probes that drag along the surface of the part while 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,[3] laser line scanning,[4] and white light scanning,[5] 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.
Probing systems for microscale metrology applications are another emerging area.[6][7] There are several commercially available coordinate measuring machines that have a microprobe integrated into the system, several specialty systems at government laboratories, and any number of university-built metrology platforms for microscale metrology. Although these machines are good and in many cases excellent metrology platforms with nanometric scales, their primary limitation is a reliable, robust, capable micro/nano probe.[citation needed] Challenges for microscale probing technologies include the need for a high-aspect-ratio probe giving the ability to access deep, narrow features with low contact forces so as to not damage the surface and high precision (nanometer level).[citation needed] Additionally, microscale probes are susceptible to environmental conditions such as humidity and surface interactions such as stiction (caused by adhesion, meniscus, and/or Van der Waals forces among others).[citation needed]
Technologies to achieve microscale probing include scaled-down version of classical CMM probes, optical probes, and a standing wave probe,[8] among others. However, current optical technologies cannot be scaled small enough to measure deep, narrow features, and optical resolution is limited by the wavelength of light. X-ray imaging provides a picture of the feature but no traceable metrology information.
Optical probes and laser probes can be used (if possible in combination), which change CMMs to measuring microscopes or multi-sensor measuring machines. Fringe projection systems, theodolite triangulation systems, and laser distance and triangulation systems are not called measuring machines, but the measuring result is the same: a space point. Laser probes are used to detect the distance between the surface and the reference point on the end of the kinematic chain (that is, the end of the Z-drive component). This can use an interferometrical function, focus variation, light deflection, or a beam-shadowing principle.
Whereas traditional CMMs use a probe that moves on three Cartesian axes to measure an object's physical characteristics, portable CMMs use either articulated arms or, in the case of optical CMMs, arm-free scanning systems that use optical triangulation methods and enable total freedom of movement around the object.
Portable CMMs with articulated arms have six or seven axes that are equipped with rotary encoders, instead of linear axes. Portable arms are lightweight (typically less than 20 pounds) and can be carried and used nearly anywhere. However, optical CMMs are increasingly being used in the industry. Designed with compact linear or matrix array cameras (like the Microsoft Kinect), optical CMMs are smaller than portable CMMs with arms, feature no wires, and enable users to easily take 3D measurements of all types of objects located almost anywhere.
Certain nonrepetitive applications such as reverse engineering, rapid prototyping, and large-scale inspection of parts of all sizes are ideally suited for portable CMMs. The benefits of portable CMMs are multifold. Users have the flexibility in taking 3D measurements of all types of parts and in the most remote and difficult locations. They are easy to use and do not require a controlled environment to take accurate measurements. Moreover, portable CMMs tend to cost less than traditional CMMs.
The inherent trade-offs of portable CMMs are manual operation (they always require a human to use them). In addition, their overall accuracy can be somewhat less accurate than that of a bridge-type CMM and is less suitable for some applications.
Traditional CMM technology using touch probes is often combined with other measurement technologies. This includes laser, video, or white light sensors to provide what is known as multisensor measurement.[9]
Further reading:ISTE Automation Product Page
The safety of coordinate measurement machines is covered in the technical rule VDMA .[10]
To verify the performance of a coordinate measurement machine, the ISO series is available. This series of standards defines the characteristics of the probing system and the length measurement error:
The ISO series consists of the following parts:
The inspection article is fixed firmly to the table so that it does not move or deflect while being probed. It is held by clamp-type fixtures or other means. The touch-trigger or other probe type is suspended from the head with adapters and a shaft threaded to a ball known as the “stylus.” The probe assembly has an integrated sensor that communicates the stylus touches and the position of the tip. This data is streamed back to the probe and machine controllers. The whole system talks to the software incorporated into the computer workstation. The machine is programmed by the CMM software, and acquired data is mathematically fit, organized, compared, and reported by the software as well.
Detailed Breakdown of Major Components of a CMM
There are varied approaches towards achieving the measurement goals to be realized with a CMM. A lot depends on the use-case, accepted technical association’s rules and standards, accuracy, and repeatability needs. Three-dimensional contoured shapes cannot be verified by hand tools such as micrometers, calipers. and gage pins. Not even height gages or other 2-dimensional measurement tools will do the job. Many relationship measurements such as GD&T callouts such as parallelism, concentricity, etc., require considerable time to set up and measure with dial indicators or “old school” techniques. Calipers and micrometers touch the feature being measured in only two points of contact. Measurements vary due to the inspector’s grip on the gage and workpiece, with readings varying from one inspector to the next.
Measurements must be written down and therefore slow the process and are subject to transposition errors. So, in many cases, CMMs are the answer. When evaluating which type of CMM design and features are best for your needs, you should always test some of your parts to your specifications on the various types of CMMs being considered, and weigh the pros and cons.
The coordinate measuring machine (CMM) has been around for decades, and it has come a long way in its evolution. For these measurement systems, competition amongst the OEMs is part of what has taken the technology to new heights. Also, third-party software developers, service providers, dealers, customers, and competing technologies drive improvements in CMM capabilities and efficiencies as well. The main purpose of the CMM is precise dimensional shape and size measurement, which includes the position and orientations of features on the part and the relationships between features.
CMMs come in all sizes to suit the size of parts and accuracy needed for them. Also, budget and other practicalities are considerations. There are a small number of machine types that have evolved and are prevalent in the market. This article intends to present an overall look at the basic characteristics of their structural layouts. This might be helpful towards understanding which style suits your needs best when looking for a system.
This is the most common style. Its key design feature from which it gets its name is an upright “carriage” structure consisting of two vertical beams that support a horizontal (bridge) beam connecting the two vertical elements. The bridge moves back and forth transiting the table, moved by an electric servo motor, and its position is tracked by precision measuring scales. The scales consist of a fixed, coded tape that is interpreted by reader heads that are affixed to the moving structural element. Older systems communicated via an analog system that read position via an electronic signal, interpreted by a decoder, followed by transmission of the position signal to the machine controller. Analog systems have been supplanted by digital scales that communicate via hexadecimal data stream communication (although there are still analog systems in use). Digital scales are now the standard.
Suspended from the bridge beam is a vertical moving beam (Z-axis) that is also motorized, and position read by a scale of its own. It has a moving main structural element that carries the probe head and probe. The bridge design is optimized for accuracy, at a slight loss of access to the measurement table (meaning diminished access to the inspection article). Due to its accuracy and prolific acceptance and distribution, bridge CMMs come in many sizes. They are offered in sizes from smallish, maxing out at microwave oven-sized parts, to largeish, maxing out at the size of a Smart Car.
Pros – Accuracy. Availability. Moderate cost. Widespread use and therefore access to service, expertise/consulting, and parts.Cons – More obstruction of access to inspection article envelope than cantilever or horizontal arm types.
This design has a structure that unlike the bridge-type CMM with two moving, vertical beams supporting the crossbeam, is supported by a guideway only on one side. It is therefore cantilevered out from that side as the only supporting leg. The side support has a guideway at the top along which the crossbeam moves (in the X direction, for example). The vertical probe support beam transits the crossbeam (Y direction) and has the mechanism that moves the up/down Z strut that carries the probe head. This design gives better access to the inspection table but suffers some loss of accuracy. Since the droop effect would multiply as it extends further from the Z beam, the cantilever design is better for smaller parts and when access takes precedence over accuracy.
Pros – Easy access through 3 out of the 4 sides to the inspection table. Better for automation and part change-out. Less expensive. Suited to smaller parts.Cons – A degree of reduced accuracy. Less common these days so support, parts, repair may be issues.
This layout was more popular in the past but is still marketed for specific needs today. It is a lower-cost solution, but allows open access to the table, albeit a little less so than the cantilever design. This machine has been very popular for automobile design and shape development, especially for clay modeling and sculpting. It has been prolific for whole-car body panel measurement where the attach points and relationship between panels are of concern. On a side note, non-contact scanners have all but taken over the job of body panel and stamped part contour analysis. Scanning has become the relied upon technology for compliance with Class-A, customer-facing smoothness and continuity. This is also true for reverse engineering, as-built, and tool development of stampings or non-metallic “body-in-white” for automobiles.
A common variation of the horizontal arm setup is pairing two towers with probing heads, like two CMMs in one. In this configuration, both sides of a car can be reached easily in one setup. Versions of horizontal arm CMM were a low cost, sufficiently accurate, granite table-based design. The auto body type versions are usually mounted to the floor instead of on a granite table, giving them a large measurement range.
Pros – Inexpensive and open access. Historically chosen for auto body digitizing and inspection.Cons – Getting harder to find parts for older table-based versions. Less accurate. Non-contact laser or white light scanning technologies have supplanted some of this machine design’s main market.
This is often thought of as a CMM for larger parts. But there are smaller versions as well. For the big ones, there are two large steel fabrications erected as the machine’s side structures. The crossbeam runs on top-mounted guideways down the length of those structures. With the Z-axis beam suspended from the cross member, it can transit the crossbeam while moving up and down to carry out the Z movement. Big gantries are mounted to the floor typically, and smaller ones have the usual granite surface plate.
Pros – Appropriate and accurate for large parts as the machine has lots of daylight between the probe and floor and other structures. Stable, rigid design for larger formats. This design has often been chosen as preferred for the shop environment.Cons – The least access to the table and inspection area where the inspection article is clamped. The large gantries are cost-prohibitive unless needed for tight tolerance, large parts. Laser trackers have become a mainstay for portable CMM inspection of large parts and tooling. Therefore, they have supplanted large gantry machines in many cases.
The most common CMM installation requires a climate controlled CMM room or metrology lab that houses one or more CMMs isolated from the shop environment. This controlled setup reduces inconsistencies and unknowns of an unstable environment. Temperature variations and thermal expansion are big concerns in metrology. However, there are several benefits of having the CMM close to the work such as in a work cell. One option might be to incorporate the CMM with automation. If a robot or other system is added, a lights-out operation is possible.
Not necessarily new but becoming more common are shop-floor CMMs. A shop floor CMM might even be operated by the same person who made the part. These machines eliminate commonly used air bearings, environmentally controlled rooms, and high voltage requirements of typical CMMs. They bring the machine to the work. They accommodate temperature variations in the shop via designed-in compensation hardware and software features.
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