INSPECCION

PRUEBA DE FUGA

INSPECCION POR LIQUIDOS PENETRANTES

INSPECCION POR PARTICULAS MAGNETICAS

INSPECCION POR CORRIENTES EDDY

INSPECCION POR ULTRASONIDO

INSPECCION POR EMISIÓN ACUSTICA

INSPECCION POR RADiOGRAFIA

INSPECCION POR RADIOGRAFIA POR NEUTRONES

INSPECCtON POR TERMOGRAFIA

lnspection Equipment and Techniques

Visual Inspection .............................................................. 3 Laser Inspection .............................................................. 12 Coordinate Measuring .Machines ................................................ 18 Machine Vision and Robotic lnspection Systems ................................. 29

Visual lnspection

VISUAL INSPECTION is a nondestruc­tive testing technique that provides a means of detecting and examining a variety of sur­face flaws, such as corrosion. contamination, surface finish. and surface discontinuities on joints (for example, welds, seals, solder con­nections, and adhesive bondsl. Visual inspec­tion is also the most widely used method for detecting and examining surface cracks, which are particularly important because of their relationship to structural failure mecha­nisms. Even when other nondestructive tech­niques are used to detect surface cracks, visual inspection often provides a useful sup­plement. For example, when the eddy current examination of process tubing is performed, visual inspection is often performed to verify and more closely examine the surface distur­bance.

Given the wide variety of surface ílaws that may be detectable by visual examina­tion. the use of visual inspection may en­compass different techniques, depending on

e product and the type of surface flaw 0eing monitored. This article focuses on sorne equipment used to aid the process of visual inspection. The techniques and appli­cability of visual inspection for sorne prod­ucts are considered in the Selected Refer­ences in this article and in the Section "Nondestructive Inspection of Specific Products" in this Volume.

The methods of visual inspection in vol ve a wide variety of equipment, ranging from ex­amination with the naked eye to the use of interference microscopes for measuring the depth of scratches in the finish of finely pol­ished or lapped surfaces. Sorne of the equip­ment used to aid visual inspection includes:

• Flexible or rigid borescopes for illuminat­ing and observing interna!. closed or oth­erwise inaccessible areas

• Image sensors for remote sensing or for the development of permanent visual rec­ords in the form of photographs, video­tapes, or computer-enhanced images

• Magnifying systems for evaluating surtace finish. surface shapes (profile and contour gaging). and surface microstructures

• Oye and fluorescent penetrants and mag­netic particles for enhancing the observa­tion of surface cracks (and sometimes near-surface conditions in the case of .nagnetic particle inspection)

This article will review the use of the equip­ment listed above in visual inspection. ex-

Borescopes cept for dye penetrants and magnetic parti­cles, which are discussed in the articles ''Liquid Penetrant Inspection" and "Mag­netic Particle Inspection," respectively, in this Volume.

A borescope (Fig. l) is a long, tubular optical device that illuminates and allows the inspection of surfaces inside narrow

Eyepiece lens Relay iens

~: l!fi é (a)

Eyepiece lens

Focus1ng ring Control handles

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(b)

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(e)

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lnterchangeaole tms

L1t;;rit gu1ae extt

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1 1

To :ght source

Fig. 1 Three typical designs of borescopes. (a) A rigid borescope with a lampo! the distal end. (b) A flexible fiberscope with a light source. (e) A rigíd borescope with a light guide bundle in the shaft

4 / lnspedion Equipment and Techniques

55°~ r:r:::n -y-c=r 8ottom1ng

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f • 2 Typicat·directions and field af view with rigid 1 9 • borescopes

tubes or difficult-to-reach chambers. The tube, which can be rigid or flexible with a wide variety of lengths and diameters, pro­vídes the necessary optical connection be­tween the viewing end and an objective lens at the distan t. or distal. tip of the borescope. This optical connection can be achieved in one of three different ways:

• By using a rigid tube with a series of relay len ses

e By using a tube (normally flexible but also rigid) with a bundle of optical fibers

• By using a tube (normally flexible) with wiring that carries the image signa! from a charge-coupled device (CCD) imagíng sensor at the distal tip

These three basic tube designs can have either fixed or adjustable focusing of the objective lens at the distal tip. The distal tip also has prisms and mirrors that define the direction and field of view (see Fig. 2). These views vary according to the type and application of borescope. The design of illumination system also varies with the type of borescope. Generally, a fiber optic light guide and a lamp producing white light is used in the illumination system. although ultraviolet light can be used to inspect sur­faces treated wíth liquid fluorescent pene­trants. Light-emitting diodes at the distal tip are sometimes used for illumination in vid­eoscopes with working lengths greater than 15 m (50 ft).

Rigid Borescopes Rigid borescopes are generally limited to

applications with a straight-line path be­tween the observer and the area to be observed. The sizes range in lengths from 0.15 to 30 m (0.5 to 100 ft) and in diameters from 0.9 to 70 mm i0.035 to 2.75 in.). Magnification is usually 3 to 4 x, but powers up to 50x are available. The illumination svstem is either an incandescent lamp locat­ed at the distal end (Fig. la) ora light guide bundle made from optical fibers \Fig. lcJ that .::onduct light from an externa! source.

f ig. 3 Typical chomberscope. Courtesy oí Lenox lnstrument Campony

(a) (b)

Fi • 4 T.wo views. down a combustor c~n with the. distal tip in the sorne position .. A fiberscope with smoller 9 ct1ometer f1bers ond 40% more f1bers in the rmoge bundle prov1des better resolution (o¡ thon o f1berscope

with lorger fibers (b). Courtesy of Olympus Corporotion

The choice of viewing heads for rigid borescopes (Fig. 2) varies according to the application. as described in the section "Se­lection" in this article. Rigid borescopes generally ha ve a 55º field of view. a!though the fields of view can range from l O to 90º. Typically. the distal tips are not inter­changeable. but sorne mode!s (such as the extendable borescopes) may have inter­changeable viewing heads.

Sorne rigid borescopes have orbital sean (Fig. le). which invo!ves the rotatíon of the optical shaft for scanning purposes. De­pending on the borescope model, the amount of rotation can vary from 120 to 370º. Sorne rigid borescopes also have mov­able prisms at the tip for scanning.

Rigid borescopes are available in a vari­ety of models having significant variations in the design of the shaft. the distal tip, and the illumination system. Sorne of these de­sign variations are described be!ow.

Basic Design. The rigid borescope typica!­ly has a series of achromatic relay lenses in

the optical tube. These lenses preserve the resolution of the image as it traveis from tht objective lens to the eyepiece. The tube di ameter of these borescopes ranges from .+ tL 70 mm (0.16 to 2.75 in.). The illuminatior system can be either a distal lamp or a iigh· guide bundle, and the various features ma\ include orbital sean. various viewing heads and adjustable focusing of the objective lens

Miniborescopes. instead of the conven tional relay tenses. miniborescopes have , single image-relaying rod or quartz fiber ir the optical tube. The lengths of minibore scopes are 110 and 170 mm (·U and 6.7 in.1 and the diameters range from 0.9 to 2.7 mrr (0.035 to 0.105 in.). High magnification (ur to 30x) can be reached at minima! foca lengths, and an adjustable focus is not re quired. because the scope has :m intinit depth of field. The larger sizes háve for ward, si de view. and forward-obliqu. views. The 0.9 mm (0.035 in.l diam size ha only a forward view. Miniborescopes hav an integral light guide bundle.

Visual lnspection / 5

,:"ii':

(a) (b)

f ig. 5 Videoscope imoges (a) ínside engine guide vones (b) of on engine fuel nozzle. Courtesy of Welch Allyn, lnc.

Hybrid borescopes utilize rod lenses combined with convex lenses to relay the image. The rod lenses have fewer glass-air boundaries; this reduces scattering and al­lows for a more compact optical guide. Consequently, a larger light guide bundle can be employed with an increase in illumi­nation and an image with a higher degree of contras t.

Hybrid borescopes ha ve lengths up to 990 mm (39 in.), with diameters ranging from 5.5 to 12 mm (0.216 to 0.47 in.). Ali hybrid borescopes have adjustable focusing of the objective lens and a 370º rotation for orbital sean. The various viewing directions are forward. side, retrospective, and forward­oblique.

Extendable borescopes allow the user to construct a longer borescopic tube by join­ing extension tubes. Extendable borescopes are available with either a fiber-optic light guide or an incandescent lamp at the distal end. Extendable borescopes with an inte­gral lamp have a maximum length of about 30 m ( 100 ft). Scopes with a light guide bundle have a shorter maximum length (about 8 m, or 26 ft), but do allow smaller tube diameters (as srnall as 8 mm. or 0.3 in.). lnterchangeable viewing heads are also available. Extendable borescopes do not have adjustable focusing of the objective lens.

Rigid chamberscopes allow more rapid inspection of larger chambers. Chamber­scopes (Fig. 3) have variable magnification (zoom). a lamp at the distal tip, and a scanning mirror that a!lows the user to observe in dífferent directions. The higher

illumination and greater magnification of chamberscopes allow the inspection of sur­faces as muchas 910 mm (36 in.) away frorn the distal tip of the scope.

Mirror sheaths can convert a direct-view­ing borescope into a side-viewing scope. A mirror sheath is designed to fit over the tip of the scope and thus reílect an image frorn the side of the scope. However, not ali applications are suitable for this device. :\ side. forward-oblique. or retrospective viewíng head provides better resolutíon and a higher degree of image contrast. A rnirror sheath also produces an inverse image and may produce unwanted reílections from the shaft.

Scanning. In addition to the orbital sean feature described earlier. sorne rigid bore­scopes have the ability to sean longitudinally along the axis of the sha.ft. A movable prisrn with a control at the handle accomplishes this scanning. Typically, the prism can shift the direction of view through an are of 120º.

Flexible Borescopes Flexible borescopes are used primarily in

applications that do not have a straight passageway to the point of observation. The two types of flexible borescopes are flexible fiberscopes and videoscopes with a CCD image sensor at the distal tip.

Flexible Fiberscopes. A typical fiber­scope (Fig. lb) consísts of a light guide bundle, an image guide bundle, an objective leos. interchangeable viewing heads, and re mote controls for articulation of the distal tip. Fiberscopes are :wailable in diameters from 1 A to ! 3 mm (0.055 to 0.5 in.) and in

lengths up to 12 m (40 ft). Special quartz fiberscopes are available in lengths up to 90 rn (300 ft).

The fibers used in the light guide bundk are generally 30 µm (0.001 in.) in diameter. The second optical bundle. called the image guide. is used to carry the image formed by the objective lens back to the eyepiece. The fibers in the irnage guide must be precisely aligned so that they are in an identícal relative position to each other at their ter­minations for proper irnage resolution.

The diameter of the fibers in the image guide is another factor in obtaining good image resolution. With smaller diameter fi­bers, a brighter image with better resoiution can be obtained by packing more fibers in the image guide. With higher resolution. it is then possible to use an objective lens with a wider field of view and also to magnify the image at the eyepiece. This allows better viewing of objects at the periphery of the image (Fig . .+). Image guide fibers range from 6.5 to 17 µm (255 to 670 µin.).

The interchangeable distal tips provide various directions and fields of view on a single fiberscope. However. because the tip can be articulated for scanning purposes. distal tips with either a forward or side viewing directíon are usually sufficient. Fields of view are typically .+O to 60º. al­though they can range from !O to 120º. M.ost fiberscopes provide adjustable focusing of the objective lens.

Videoscopes with CCD probes involv, the electronic transmission of color or black and white images to a video monitor. The distal end of electronic videoscopes con-

6 / lnspection Equipment and Techniques

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f ig 6 Typical resolution of CCD videoscopes with 0

• 90° field al view (a), 60° field of view (b), 30° field of view (e). Source: Welch Allyn, lnc.

tains a CCD chip, which consists of thou­sands of light-sensitive elements arrayed in a pattern of rows and columns. The objec­tive lens focuses the image of an object on the surface of the CCD chip, where the light is converted to electrons that are stored in each picture element, or pixel, of the CCD device. The image of the object is thus stored in the form of electrons on the CCD device. At this point. a voltage proportional to the number of electrons at each pixel is determined electronicalty for each pixel

(a)

(b)

f ig. 7 lmage ~roma videoscope (a) anda fiberscope (b). In sorne fiberscope images, voids between individue glass f1bers can create a haneycomb pattern that adds graininess to the image. Courtesy of Welc

Allyn, lnc.

F • 8 View through a measuring fiberscope with 19 • reticles for 20° and 40° field-of-view lenses.

Courtesy of Olympus Corporation

site. This voltage is then amplified, filtered, and sent to the input of a video monitor.

Videoscopes with CCD probes produce images (Fig. 5) with spatial resolutions of the order of those described in Fig. 6. Like rigid borescopes and flexible fiberscopes, the resolution of videoscopes depends on the object-to-lens distance and the fields of view, because these two factors affect the amount of magnification (see the section "Magnification and Field of View" in this article). Gene rally, videoscopes produce higher resolution than fiberscopes, although fiberscopes with smaller diameter fibers (Fig. 4a¡ may be competitive with the reso­lution of videoscopes.

Another advantage of videoscopes is their longer working length. With a given amount of illumination at the distal tip, videoscopes can return an image over a greater length than fiberscopes. Other fea­tures of videoscopes include:

Visual lnspection í 1

(a) (b)

(e) (d)

f ¡ g 9 T urbine flaws seen through a flexible fiberscope. (a) Crack near a fuel burner nozzle. (b) Crack in an outer • combustion liner. (e) Combustion chamber and high pressure nozzle guide vanes. (d} Campressor damage

showing blade deformation. Courtesy of Olympus Corporation

• The display can help reduce eye fatigue (but does not allow the capability of di­rect viewing through an eyepiece)

• There is no honeycomb pattem or irreg­ular picture distortion as with sorne fiber­scopes (Fig. 7)

f ig. 1 O ln-service defects as seen through a borescope designed for automotive servicing. (a) Carbon on valves. (b) Broken transmission gear tooth. (e) Differential gear wear. Caurtesy of Lenax lnstrument Company

8 i lnspection Equipment and Techniques

F • 11 Operator viewing a weld 21 m (70 ft) 19. inside piping with a videoscope. Courtesy

oí Olympus Corporation

e The electronic form of the image signa! allows digital image enhancement and the potential for integration with automatic inspection systems

• The display allows the generation of ret­icles on the viewing screen for point­to-point measurements

Apedal Features Measuring borescopes and fiberscopes

contain a movable cursor that ailows mea­>Urements during viewing 1 Fig. 8). When the object under measurement is in focus. lhe movable cursor provides a reference for Jimensional measurements in the optical plane of the object. This capability eiimi­nates the need to know the object-to-lens distance when determining magnifícation factors.

Working channels are used in bore­scopes and fiberscopes to pass working devices to the distal tip. Working channels are presently used to pass measuring instru­ments. retrieval devices. and hooks for aid­ing the insertion of thin. flexible fiber­scopes. Working channels are used in flexible fiberscopes with diameters as small as 2.7 mm (0.106 in.). Working channels are also under consideration for the application and removal of dye penetrants and for the passage of wires and sensors in eddy cur­rent measurernents.

Selection Flexible and rigid borescopes are avail­

able in a wide variety of standard and customized designs. and severa! factors can íntluence the selection of a :>cope for a

·ricular application. These factors inciude ising. iilurnination. magnitication. work-

1ng length. direction of view. and envíron­menr.

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Fig. 1 :2 Schematic of line projection method for monitoring the surface roughness on fast-moving caoles

Fig. 13 Setup used in the in-plan! trials of the line proiection method far monitoring the surfoce rougnness oí cables. Courtesy of P. Cielo, National Research Council of Canada

Focusing and Resoiution. If portions of long objects are at different planes. the scope must ha ve sufficient focus adjustment to achieve an adequate depth of tield. If the scope has a fixed focal length. the object will be in focus only at a specific lens­to-object distance.

To allow the observation of surface detail at a desired size. the optical system of a borescope must also provide adequate res­olution and ímage contrast. !f resolution is adequate but contrast is lacking, detaii can­not be observed.

[n general. the optical quaiity of a rigid borescope improves as the size of the lens increases: consequently. a borescope with the largest possible diameter should be used. For fiberscopes. the resolution is de­pendent on the accuracv of alignment and . -the diameter of the fibers in the ímage

bundle. Smaller-diameter tibers provide more resolution and edge contrast (fig . .+), when combined with good geometrical alignment of the fibers. Typical resolutions of videoscopes are given in Fig. ó.

lllumination. The required intensny of the light source is determined by the retlec­tivity of the surface, the area of surface to be illuminated. and the transmission !osses over the tength of the scooe. At working lengths greater than 6 m (2Ó ftL rigid bore­scopes with a lamp at the distal end provide the greatest amount of illumination o ver the widest area. However. the heat generated by the light source may deform rubber or plastic materials. Fiber-optic illumination in scopes with working lengths less than 6 m CO ftl is always brighter and is suitabte for heat-sensitive applications because fiiters can remove ínfrared frequenc1es. Because

µm

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100

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f ig. 14 Examples af signals obtained with the apparatus shawn in Fig. 13. (a) Acceptable surface raughness. (b) Unacceptable surface roughness

the amount of illumination depends on the diameter of the light guide bundle, it is desirable to use the largest diameter possi­ble.

Magnification and field of view are interrelated: as magnification is increased. the field of view is reduced. The precise relation­ship between magnification and field of view is specified by the manufacturer.

The degree of magnification in a particular application is deterrnined by the field of view and the distance from the objective lens to the object. Specifically. the magnification in­creases when either the field of view or the lens-to-object distance decreases.

Working length. In addition to the obvi­ous need for a scope of sufficient length. the working length can sometimes dictate the use of a particular type of scope. For exam­ple, a rigid borescope with a long working length may be limíted by the need for addi­tional supports. In general. videoscopes al­low a longer working length than fiber­scopes.

Direction of View. The selection of a viewing direction is influenced by the loca­tion of the access port in relation to the object to be observed. The following sec­tions describe sorne críteria for choosing the direction of view shown in Fig. 2. Flexible

Visual lnspection I 9

fiberscopes or videoscopes. because of their articulating tip. are often adequate wíth either a side or forward viewing tip.

Circumferential or panoramic heads are designed for the inspection of tubing or other cylindrical structures. A centrally lo­cated mírror permits right-angle viewing of an areajust scanned by the panoramic view.

The forward viewing head permits the inspection of the area directly ahead of the viewing head. lt is commonly used when examining facing walls or the bottoms of blind holes and cavities.

Forward-oblique heads bend the viewing direction at an angle to the borescope axis. permitting the inspection of comers at the end ofa bored hole. The retrospective view­ing head bends the cone of view at a retro­spective angle to the borescope axis. pro­viding a view of the area just passed by the advancing borescope. It is especially suited to inspecting the inside neck of cylinders and bottles.

Environment. Flexible and rigíd bore· scopes can be manufactured to withstand a variety of environments. Although most scopes can operate at temperatures from - 34 to 66 ºC (-30 to 150 ºFl. specially desígned scopes can be used at tempera­tures to 1925 ºC (3500 ºF). Scopes can also be manufactured for use in liquid media.

Special scopes are requíred for use in pressures above ambient and in atmo­spheres exposed to radiation. Radiation can cause the multicomponent tenses and image bundles to turn brown. When a scope is used in atmospheres exposed to radiation. quartz fiberscopes are generally used. Scopes used in a gaseous environ­ment should be made explosionproof to minimize the potential of an accidental explosion.

Applications Rigid and flexible borescopes are avail­

able in different designs suitable for a vari­ety of applications. For example. when in­specting straight process piping for leaks. rigid borescopes with a 360º radial view are capable of examining inside diameters of 3 to 600 mm I0.118 to 24 in.). Scopes are also used by building inspectors and contractors to see inside walls, ducts. large tanks. or other dark areas.

The principal use of borescopes is in equipment maintenance programs. in which borescopes can reduce or eliminate the need for costly teardowns. Sorne types of equipment. such as turbines. have access ports that are specifically desjgned for bore­scopes. Borescopes provide' a means of checking in-service defects in a variety of equipment. such as turbines ( Fig. 9). auto­motive components ( Fig. 1 Ol. and process piping (Fig. 11 ).

Borescopes are also extensiveiy used in a variety of manufacturing industries to c:n-

1 O I lnspection Equipment and Techniques

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sou~ce ~I len?as 1 ,,-: ~ --ft- - -----1 WJ Ob1ect

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Fig. 1 5 Schematic of an optical comporotor

sure the product quality of difficult-to-reach components. Manufacturers of hydraulic cylinders, for example. use borescopes to examine the interiors of bores for pitting, scoring, and tool marks. Aircraft and aero­space manufacturers also use borescopes to verify the proper placement and fit of seals. bonds. gaskets, and subassemblies in diffi­cult-to-reach regions.

Optical Sensors* Visible light. which can be detected by

the human ;ye or with optical sensors, has sorne advantages over inspection methods based on nuclear, microwave. or ultrasound radiation. For example. one of the advan­tages of visible light is the capability of tightly focusing the probing beam on the inspected surface ( Ref 1 ). High spatial res­olution can result from this sharp focusing, which is useful in gaging and profiling ap­plications ( Ref 1 ).

Sorne different types of image sensors used in visual inspection include:

• Vidicon or plumbicon television tubes • Secondary electron-coupled ( SEC) vidi-

cons • Image orthicons and image isocons • Charge-coupled device sensors • Holographic plates ( see the article .. Op­

tical Holography" in this Volume)

Television cameras with vidicon tubes are useful at higher light levels (about 0.2 lm!m2 • or 10-:- ftc). while orthicons. isocons. and SEC vidicons are useful at lower light levels. The section "T elevision Cameras ..

"'Example 1 tn th1s section wo.i:-i adatited with f?erm1ssion from P. Cído. Optical Tt'chn1t1ta's for lndtotrta! lnspcc~ uon. A1.2aJcmrc Pn::s~. 19K8.

in the article "Radiographic Inspection" in this Volume describes these cameras in more detail.

Charge-coupled devices are suitable for many different information-processing ap­plications, including image sensing in tele­vision-camera technology. Charge-coupled devices offer a clear advantage over vacu­um-tube image sensors because of the relia­bility of their solid-state technology. their operation at low voltage and low power dissipation. extensive dynamic range, visi­ble and near-infrared response, and geomet­ric reproducibility of image location. Image enhancement (or visual feedback into robot­ic systems) typically involve the use of CCDs as the optical sensor or the use of television signals that are converted into digital form.

Optical sensors are a!so used in inspec­tion applications that do not involve imag­ing. The articles "Laser Inspection" and "Speckle Metrology·· in this Volume de­scribe the use of optical sensors when laser light is the probing too!. In sorne appiica­tions, however. incoherent light sources are very effective in non-imaging inspection ap­plications utilizing optical sensors.

Example 1: Monitoring Surface Rough­ness on a Fast-Moving Cable. A shadow projection configuration that can be used at hígh extrusion speeds is shown in Fig. 12. A linear-filament lamp is imaged by two spherical lenses of focal length f 1 on a large-area single detector. Two cylindrical lenses are used to project and recollimate a laminar light beam of uniform intensity. nearly 0.5 mm (0.02 in.) wide across the wire situated near their common focal plane. The portion ot the light beam that is not intercepted by lhe wire is collected on

the detector, which has an altemating cur­rent output that corresponds to the defect­related wire diameter fluctuations. The wire speed is limited only by the detector re­sponse time. With a moderare detector bandwidth of 100 kHz, wire extrusion speeds up to 50 mJs (160 ft/s) can be accept­ed. Moreover, the uniformity of the nearly coilimated projected beam obtained with such a configuration makes the detected signal relatively independent of the random wire excursions in the plane of Fig. 12. It should be mentioned that the adoption of either a single He-Ne laser or an array of fiber-pigtailed diode lasers proved to be inadequate in this case because of speckle noise, high-frequency laser amplitude or mode-to-mode intert'erence fluctuations. and line nonuniformity.

An industrial prototype of such a sensor was tested on the production line at extrud­ing speeds reaching 30 mis ( 100 ft/s). Figure 13 shows the location of the sensor just after the extruder die. Random noise introduced by vapor turbulence could be almost com­pletely suppressed by high-pass filtering. Figure 14 shows two exampies of signais obtained with a wire of acceptable and unacceptable surface quaiity. As shown. a roughness amplitude resolution of a few micrometers can be obtained with such a device. Subcritical surface roughness levels can thus be monitored for real time control of the extrusion process.

Magnifying Systems In addition to the use of microscopes in

the metallographic examination of micro­structures ( see the article .. Reolication .\1i­croscopy Techniques for ~DE.-. in this Vol­ume), magnifying systems are also used in visual reference gaging. When tolerances are too tight to judge by eye :ilone. optical comparators or tooimakers · microscopes are used to achieve magnifications ranging from 5 to 500x.

A toolmakers' microscope consists ot a microscope mounted on a base that carnes an adjustable stage, a stage transport mech­anism. and supplementary lighting. Mi­crometer barreis are often incorporated in to the stage transport mechanism to permit precisely controiied movements, and digital readouts of stage positioning are becoming increasingly available. Various objective tenses provide magnifications rangmg from 10 to 2oox.

Optical comparators (Fig. 15) are mag­nifying devices that project the silhouette of small parts onto a large projection screen. The magnified silhouette is then compared against an optical comparator chart. which is a magnified outline drawing of the work· piece being gaged. Opticai comparators are available with magnifications ranging from 5 to 500X.

Parts with recessed contours can also be successfu!ly gaged on optical comparators. This is done with the use of a pamograph. One arrn of the pantograph is a stylus that traces the recessed contour of the part, and the other arm canies a follower that is visible in the light path. As the stylus moves. the fo!lower projects a con tour on the screen.

ACKNOWLEDGMENT

ASM INTERNATIONAL would like to

thank Oliver Darling and Morley Melden of Spectrum Marketing, Inc., for their assis­tance in preparing the section on bore­scopes. They provided a draft of a textbook being developed for Olympus Corporation. Thanks are also extended to Virginia Torrey ofWelch Allyn, Inc., for the information on videoscopes and to Peter Sigmund of Lind­hult and Iones, Inc., for the information on instruments from Lenox, Inc.

REFERENCE

l. P. Cielo. Optical Techniques for Indus­trial Inspection, Academic Press. l 988. p 243

SHECTED REFERENCES

• Robert C. Anderson. Inspection of .'vier­ais: Visual Examinarían, Vol l. :\.meri­can Society for Metals, 1983

• Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, ASTM A 262. Annual Book of A.Sn-t Standards. American Society for Testing and Materials

• Detecting Susceptibility to Intergranular Attack in Fenitic Stainless Steeis. ASTYi A 763, Annual Book ofASTJf Standards. American Society for Testing and Yiate­rials

• Detecting Susceptibility to Intergranular Corrosion in Severely Sensitized Auste-

Visual lnspection Í l 1

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