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The third component is supported at one end only, cantilever fashion, and moves horizontally x-direction relative to the machine base.. The third component is supported on two legs that

Fixed bridge

L-shaped bridge

In addition, the two types of machine can be characterized to some degree by the levels of accuracy they each achieve (although there is a considerable degree of overlap based on the design of an individual machine), by the size of part they can handle, and by application The prospective buyer/user of a CMM cannot make an intelligent choice of the type of machine that will best meet his needs, let alone the specific make and model of CMM, until he thoroughly evaluates and plans both the specific intended application of the CMM and the overall manufacturing and quality context in which it will operate Table 1 provides a general comparison of CMM types, applications, and levels of measurement accuracy a CMM user can expect

Table 1 Typical CMM specifications

Minimum measurement Application CMM type Bearing type

mm in

Laboratory quality (a)

Laboratory grade Vertical, moving bridge Air bearings <0.003 <0.0001

Clean room Vertical, moving bridge Air bearings <0.003 <0.0001

Production machines

Open shop Vertical, moving bridge Air bearings <0.013-0.025 <0.0005-0.001

Sheet metal Horizontal (with fixed x-y axis) Recirculating bearing packs <0.025-0.050 <0.001-0.002

Clean room and shop Vertical, moving bridge; all horizontal Air/roller bearings <0.050 <0.002

(a) These CMMs have specific environments into which they must be installed to maintain their rated

accuracies

Experience clearly shows that the user who simply buys a CMM and places it in use without considering the impact of the unit on the manufacturing process will almost invariably fail On the other hand, the user who determines the results desired from the CMM, the resources required to achieve those results, and the effect of the CMM on the overall manufacturing process will likely realize the maximum benefits a CMM can provide

Evaluation and planning are further discussed in the section "CMM Implementation" in this article

Vertical CMMs

Vertical CMMs, which have the probe or sensor mounted on the vertical z-axis, have the potential to be the most accurate

type Vertical CMMs in general can be more massive and can be built with fewer moving parts than their horizontal counterparts They are therefore more rigid and more stable Their limitation, however, is the size of part they can conveniently handle, because the part to be measured must fit under the structural member from which the probe descends

In an attempt to overcome this limitation, various designs have been produced As a result, within the overall category of vertical CMM, there are designs utilizing moving or fixed bridges, cantilevers, gantries, and so on Each design approach

is of course a compromise, because as the size of the work envelope of the machine and the travel distance along the axes increase, so also do the problems of maintaining rigidity and accuracy A gantry design has proved to be an effective solution to the problem of increasing the capacity of a CMM while maintaining a high level of accuracy A cantilever design, on the other hand, presents some inherent problems associated with isolating the CMM from floor vibrations and maintaining high precision due to the overhanging unsupported arm

Cantilever-type CMMs employ three movable components moving along mutually perpendicular guideways The

probe is attached to the first component, which moves vertically (z-direction) relative to the second The second component moves horizontally (y-direction) relative to the third The third component is supported at one end only, cantilever fashion, and moves horizontally (x-direction) relative to the machine base The workpiece is supported on the

worktable A typical machine of this configuration is shown in Fig 2(a) A modification of the fixed-table cantilever configuration is the moving-table cantilever CMM shown in Fig 2(b)

Fig 2 Motion of components in cantilever-type CMMs (a) Fixed table (b) Moving table

The cantilever design provides openness and accessibility from three sides, making it popular for small, manual CMMs

Its cantilevered y-axis places a size limitation on this configuration Because of the small y-z assembly, this configuration

is lightweight and provides fast measuring speeds in direct computer control (DCC) applications

Bridge-type CMMs employ three movable components moving along mutually perpendicular guideways The probe is

attached to the first component, which moves vertically (z-direction) relative to the second The second component moves horizontally (y-direction) relative to the third The third component is supported on two legs that reach down to opposite sides of the machine base, and it moves horizontally (x-direction) relative to the base The workpiece is supported on the

base

90% of all CMM sales

Fig 3 Motion of components in bridge-type CMMs (a) Moving bridge (b) Fixed bridge (c) L-shaped bridge

The moving-bridge design overcomes the size limitations inherent in the cantilever design by providing a second leg,

which allows for an extended y-axis The second leg does, however, reduce access to the unit The limitations of this

configuration usually occur because of walking problems associated with retaining drive through just one leg Higher speeds, achieved by increasing dynamic forces and reducing machine setting time, accentuate the problem Vertical moving-bridge CMMs can be controlled manually (Fig 4) and with DCC hardware (Fig 5)

Fig 4 Vertical moving-bridge CMM inspecting a case-hardened cast steel automotive crank-shaft using a

touch-trigger probe This manually operated unit has a granite table

Fig 5 Vertical moving-bridge CMM, which incorporates a granite base, inspecting a cast aluminum alloy

four-wheel-drive transaxle gear case The DCC motor drive that controls all of the axial movements is equipped with

a remote joystick controller

structure that can achieve fast x-z moves The moving table in larger machines can become massive, with decreased

throughput capability The influence of part weight on accuracy becomes a consideration for large parts

of these bridges is fixed at each end to the machine base The other bridge, which is an inverted L-shape, moves

horizontally (x-direction) on guideways in the fixed bridge and machine base

two-axis saddle that allows movement in the horizontal (x-y) directions (Fig 6) High accuracy can be achieved with this

design As in the fixed-bridge configuration, however, part mass and table considerations can restrict measuring volume and speed

Fig 6 Schematic of column CMM illustrating movement of probe, column, and table components

Column CMMs are often referred to as universal measuring machines rather than CMMs by manufacturers Column units are considered gage-room instruments rather than production-floor machines

Gantry CMMs employ three movable components moving along mutually perpendicular guideways (Fig 7) The probe

is attached to the probe quill, which moves vertically (z-direction) relative to a crossbeam The probe quill is mounted in a carriage that moves horizontally (y-direction) along the crossbeam The crossbeam is supported and moves in the

x-direction along two elevated rails, which are supported by columns attached to the floor

Fig 7 Schematic of gantry CMM illustrating movement of probe, crossbeam, and elevated rails

The gantry design has relatively restricted part access unless utilized in very large machines The machine is physically large with respect to the size of the part Large axis travels can be obtained, and heavy parts are not a problem, because the weight of the part can be decoupled from the measurement system by proper design of the machine base (foundation

in a large machine) This is not as practical in smaller CMMs, and this configuration is most popular for large machines

The gantry configuration was initially introduced in the early 1960s to inspect large parts, such as airplane fuselages, automobile bodies, ship propellers, and diesel engine blocks The open design permits the operator to remain close to the part being inspected while minimizing the inertia of the moving machine parts and maintaining structural stiffness

Horizontal CMMs

Horizontal CMMs, which have the probe mounted on the horizontal y-axis, are generally used in applications in which

large parts must be measured for example, automobile bodies or airplane wings Horizontal CMMs require no bridge over the part because the part is approached from the side Therefore, there is substantially less restriction on the sizes of the parts that can be measured

Most horizontal CMMs, however, do not measure to state-of-the-art levels of accuracy, because of the high cost of achieving such accuracy in machines capable of handling large parts In general, it is more cost effective to accept accuracy in the 0.050 mm (0.002 in.) range when gaining the part-handling capabilities of large horizontal machines and

to use vertical designs when finer measurement is demanded

In any CMM design, fewer moving parts and joints will result in higher potential levels of accuracy This principle has been applied to a class of horizontal CMM called process control robots In these units, fixed members that move together provide the flexibility and capacity of a horizontal CMM, along with the higher accuracy of a vertical design The horizontal direction of attack makes these CMMs the logical design for production applications in which horizontal machine tools are used

In choosing a CMM, the buyer must take into consideration not only the degree of accuracy required but also the location

of the unit and the measurements to be taken In general, when dealing with smaller parts where measurements of very high accuracy are required, the potential user is likely to to best served by a machine located in a clean room environment When on-line process control is desired, the appropriate shop-hardened CMM that can be located in the shop itself is the best solution

Horizontal-Arm CMMs Several different types of horizontal-arm CMMs are available As with all CMMs, the

horizontal-arm configuration employs three movable components moving along mutually perpendicular guideways

Horizontal-arm CMMs are used to inspect the dimensional and geometric accuracy of a broad spectrum of machined or fabricated workpieces Utilizing an electronic probe, these machines check parts in a mode similar to the way they are machined on horizontal machine tools They are especially suited for measuring large gearcases and engine blocks, for which high-precision bore alignment and geometry measurements are required Four-axis capability can be obtained by incorporating a rotary table

Horizontal arms for large machines have a lower profile than vertical arms For some applications, horizontal access is desirable For others, it is restrictive and a rotary table is usually required, thus increasing the cost

(Fig 8a) The ram is encased in a carriage that moves in a vertical (z) direction and is supported on a column that moves horizontally (x-direction) relative to the base

Fig 8 Schematic illustrating three types of horizontal-arm CMMs (a) Moving ram (b) Moving table (c) Fixed

table Length of table/base is usually two to three times the width Units with bases capable of accommodating two to three interstate buses situated end-to-end have been built for major automotive manufacturers

Moving-Table Type. In this configuration, the probe is attached to the horizontal arm, which is permanently attached at

one end only to a carriage that moves in a vertical (z) direction (Fig 8b) on the column The arm support and table move horizontally (x- and y-directions) relative to the machine base The moving-table horizontal-arm CMM unit shown in Fig

9 is even more versatile because of the introduction of a rotary moving table

Fig 9 Horizontal-arm moving-table CMM equipped for operation in hostile environments is shown inspecting a

cast iron automotive engine block The unit incorporates a rotary table, a motorized programmable probe, and

an environmentally protected automatic probe changer With its process control robot, this CMM is designed for easy integration and installation in manufacturing cells

style at the arm support and moves in a vertical (z) direction (Fig 8c) The arm support moves horizontally (x- and

y-directions) relative to the machine base Parts to be inspected are mounted on the machine base The versatility of the fixed-table horizontal-arm design is readily illustrated by its ability to inspect a relatively plane surface such as a door (Fig 10) as well as an array of cylindrical surfaces encountered in a valve assembly (Fig 11)

Fig 10 Horizontal-arm fixed-table CMM using a metal scribing tool to inspect a steel sheet metal automotive

door taken from the production line This manually operated unit has a cast iron table

Fig 11 Horizontal-arm fixed-table CMM using an edge-finder probe to inspect a precision cast aluminum alloy

valve body This manually operated unit incorporates a cast iron table

Coordinate Measuring Machines

David H Genest, Brown & Sharpe Manufacturing Company

Key CMM Components

As stated earlier, it is virtually impossible for the CMM buyer to select the appropriate unit without carefully evaluating which functions are needed, where the unit will operate, under what conditions, who will operate it, and so on Every CMM offers trade-offs and its own cost-to-benefits ratio The CMM that provides state-of-the-art measurement, for example, may not be the best choice if the user cannot provide the correct operating environment The best CMM is the one that matches the application in terms of design, capacity, accuracy, protection, and so on Once these factors have been considered, the CMM buyer should assess the characteristics and capabilities of the following major components of the CMM

Base/Bed Construction The first major component to examine is the overall construction of the machine The CMM

must be rigidly built to minimize errors introduced by unintended movement between the machine members and in their movement along the axes of travel Some designs (such as the cantilever) require a more massive structure to provide the same degree of rigidity than do others (such as the gantry type) that have more inherent rigidness If the CMM is to operate in the shop, the user should select a design having the same toughness and shielding as a machine tool If the CMM will be used in a laboratory, a less protected model can be selected

Traditionally, all measuring machines had cast iron bases or beds, but today granite is also frequently used because of its mass and thermal stability In the past, it was essential that the bed of the measuring machine be perfectly flat, but with the advent of computer-based error compensation, this characteristic has become far less vital

Coordinate measuring machines designed for use in the laboratory or clean room are usually built with granite beds Horizontal-arm CMMs and any CMM intended for use on the factory floor are still built with a cast iron base because of their larger size and the levels of accuracy required in these applications Steel is also far easier than granite to fabricate and use in integrated systems involving automated materials handling and similar functions

Key elements of the structure (such as bearing ways and scales) may be of stainless steel, granite, aluminum, or more exotic materials, such as ceramic One principal factor, however, is that all these critical components must be of the same material This reduces the effects of expansion and contraction due to temperature changes The buyer wants a machine that is thermally stable, and the manufacturer can achieve this stability in several ways A CMM whose key components are of ceramic will be thermally stable because ceramic reacts very slowly to temperature changes By the same token, a CMM with aluminum components will provide thermal stability because these components react very quickly to temperature changes In both cases, the user knows with a high degree of certainty the thermal condition of the CMM

Bearing Construction Bearings have a direct impact on the accuracy of all CMMs because of their effect on every

motion of the machine along its axes Bearing assemblies can be of the air or roller type or can consist of recirculating bearing packs

bearings are the best choice because they move without friction and are therefore the most accurate Air bearings are not suitable on CMMs located on the shop floor, because the bearings need constant cleaning and require costly air-activated vibration isolation systems, which slow the measuring speed and decrease the accuracy of the CMM The isolation system also increases the potential for failure because of its complexity and generally fragile design

choice Although they provide somewhat lower levels of potential accuracy than air bearings, roller bearings are sturdier and can function in an atmosphere containing some dirt and dust They are not suitable for CMMs in hostile environments

available from recirculating bearing packs such as those typically used on grinding machines Because these bearing packs are completely sealed, they permit operation in harsh factory environments, and they can handle the weight of larger, more rigid machines Their inherent stiffness provides volumetric accuracies and repeatabilities equal to those of air bearings at significantly higher accelerations, which shorten measurement cycle times

Scales and Encoders The scales of a CMM show where the probe is located on the x, y, and z axes within the work

envelope of the machine Because CMMs measure to such a high degree of resolution, the scales used are machine readable and cannot be read by the naked eye The CMM encoder reads the scales and inputs this measurement data into the computer for computation A machine readout amplifies the same measurement data so that they are accessible to the operator by the naked eye The buyer should make sure that the readout on any CMM is both visible and decipherable

There are many different kinds of scales for example, rotary, wire, magnetic, linear metal, and glass The most accurate scales are glass because this material permits finer etching of measurement lines than steel or other commonly used materials The resolution of the finest scales is to the submicron level The buyer should make sure that the scales of any machine intended for shop use are thoroughly sealed and protected from the outside environment

Sensors and Probes Sensors and probes are the devices through which CMMs collect their measurement input A

broad variety of probes are currently available, each with its own application, permitting users to obtain very accurate measurements of virtually any type of part feature, contour, surface, and so on

operator manually brings a hard probe into contact with the object being measured and reads (via the machine) the coordinates of the measurement from the machine scales Hard probes are available in a variety of configurations (ball, tapered plug, and edge) and continue to have broad application and utility, primarily on manual machines

Of the various types of hard probes, ball probes are used to measure the distance from one point to another Tapered-plug probes are used for such measurements as the distance between the centerline of two holes Edge probes are used to locate edge points

The shortcoming of all hard or fixed probes is that they depend on a subjective element in the performance of the machine operator Every operator has his own touch, a light or heavy hand in moving and bringing the probe in contact with the part Just as one operator will use more force than another to close a micrometer on a part and thus obtain a variation in measurement, the CMM operator can unknowingly influence measurements taken with a hard probe

Touch-trigger probes operate like an electronic switch that closes when the probe comes in contact with the workpiece measured The development of touch-trigger probes was a major break-through for CMM technology

Not only do touch-trigger probes remove a major source of operator error, they greatly increase the flexibility of measurements and facilitate direct computer-controlled CMMs Because of their considerable advantages, touch-trigger probes are replacing hard probes in many applications Touch-trigger probes are also available in many configurations and types, that is, with extensions, special mounts, multiaxis motorized heads, and so on