rotary kiln maintenance
The continuity of operation of a lime sludge kiln requires strict maintenance control. The rotary kiln is among the largest type of moving machines made and is subjected to extreme temperatures, power failures, atmospheric conditions, varying loads, and other operating conditions which affect its wear and alignment. It should be erected under the supervision of an experienced erecting engineer.
Even though great care has been taken in the design and the construction of the concrete piers, in some cases settling or tipping of the foundation can occur, throwing the carrying mechanisms, bases, and rollers out of alignment. If this condition is not corrected, it will lead to continual trouble with the kiln shell and the riding rings and rollers. It is advisable when making the original installation of the kiln to establish bench marks away from the kiln foundation. The kiln alignment should then be checked from the bench marks within six months after initial installation, and annually thereafter.
The thrust rollers are designed to carry the full downhill thrust of the kiln, with the riding ring and roller faces lined up for full bearing across the width. Carrying rollers should be set parallel to the axis of the kiln or cut slightly to avoid excessive downhill thrust, which might be detrimental to the thrust roller.
In aligning the kiln by adjusting or cutting the rollers, it is necessary to cut all rollers equally rather than taking all of the cut on one set of rollers. If only one set of rollers is cut to move the kiln in a given direction, and another set of rollers cut to move the kiln in the opposite direction, such action, if continued, would cause various sets of mechanism rollers to work against each other. If this action is carried to extremes, misalignment could occur between any one set of rollers and the corresponding riding ring, resulting in unequal and aggravated wear between the riding ring and roller surfaces. Operators should be discouraged from adjusting only the most easily accessible rollers, which would be on the discharge end mechanisms. A record of all roller adjustments should be kept as an aid to maintaining proper alignment.
When a kiln with a hot charge is stopped for any reason, such as a power failure, it is imperative to keep the kiln rotating with the auxiliary drive. Failure to do so may result in a warped or distorted kiln shell. It is difficult to return a warped shell to its original condition, and operating a kiln with a bowed or warped condition will place an excessive load on various mechanism piers. This is particularly troublesome in multiple support kilns.
Sometimes a warped kiln can be returned to somewhat its original alignment by carefully re-heating the kiln on the side opposite from the warp to draw it back in line. Even at best, though, constant attention must then be given to the carrying mechanisms to provide an alignment which will not cause additional damage or excessive wear.
Sometimes the only way in which to correct a warped or bowed condition or misaligned shell is to cut out a portion of the shell, realign the ridng rings and carrying rollers, and weld the shell section back in. This might result in a slightly disjointed shell, but the items of major importance, namely the riding rings and rollers, are then realigned.
Some kilns are installed without auxiliary drives. This is false economy, since the small additional cost of the auxiliary drive in the initial installation provides good insurance against much more serious difficulties.
The main gear, usually a spur gear, is made in halves with full machined teeth to permit reversing of the gear to obtain a double life. This gear is bolted to a gear flange which is welded to the kiln shell. The driving pinion is mounted on a jackshaft which is coupled to the low speed shaft of an enclosed gear reducer. The gear reducer and jackshaft assembly is fixed to the foundation on the same slope as the kiln, and is provided with adjusting bolts and lugs on the base plate to provide for alignment of the drive. The driving motor is usually connected to the high speed shaft of the gear reducer through a multiple V-belt drive. The motor is also mounted on the same slope as the kiln. Ball bearing motors should always be used, since oil will run out of the bearings on a sleeve bearing motor.
The main gear and pinion should be maintained in proper mesh. Improper meshing of the teeth results in a jerky or vibrating motion of the kiln. Too small a clearance will cause bottoming of the main gear on the base of the pinion teeth. Proper adjustment of the carrying rollers to compensate for the wear on the tires and rollers should prevent this condition.
If a minimum adverse clearance is allowed to continue with a resulting scoring of gear teeth and peening of pinion teeth, it will be necessary to reverse the gear and pinion before such action is normally necessary and then reset the drive accordingly.
Large, slow moving equipment such as rotary kilns will have a low natural frequency of vibration which in some cases could coincide with a kiln speed. Were this to occur, there would be a pronounced vibration of the kiln on the supporting rollers, and knocking and pounding in the main gear and the gear reducer. If such a condition were allowed to continue, the foundations and the kiln could be severely damaged.
The design of the kiln installation insures a natural frequency of vibration well out of the range of recommended operating speeds. Consequently, the kiln speed should never be changed without first investigating the effect which the increased speed might have on the vibrational characteristics of the kiln.
Confirm the original centerline of carrying mechanism bases. To do this, establish an offset centerline, preferably with piano wire, along the side of the kiln from the first to the last support where visibility is unobstructed, as shown in Figure 1. This offset centerline should be equidistant from the centerline marks on the carrying mechanism bases at the two extreme supports of the kiln. By tramming from this offset centerline, determine if the centerline marks on the carrying mechanism bases are all in line.
If the intermediate piers are not in line, determine whether they or the end piers have settled before proceeding with the alignment work. Some changes may be required in order to bring either the bases or piers into line, depending on whether or not the settling has reached its final stages. If no further settling is anticipated, or in cases where no settling has taken place, the offset centerline should be permanently located by setting lead or brass markers into the piers or floor. The true centerline should be clearly marked on each carrying mechanism base by tramming from the offset centerline.
Check the setting of the kiln shell in relation to the true centerline of the bases. This can be done by stringing a cord with a plumb bob attached to each end over the top of the shell as near the riding rings as possible. In cases where there are irregularities in the shell, the cord should be strung over the wearing faces of the riding rings. This cord must be long enough to permit the plumb bobs to hang free beneath the kiln, as shown in Figure 1. The midpoint of the distance between these plumb bobs is the center of the kiln shell at that position. Mark this point on the bases and proceed with the same operation at the next support. Rotate the shell 90 degrees and repeat the markings at each support. The mean of the four marks at each quarter point through a complete revolution will then indicate the path from support to support of the true centerline of the kiln shell.
If condition (2) applies, the carrying rollers must be adjusted so that the true centerline of the kiln shell is made to coincide with the true centerline of the bases. The carrying rollers must be kept parallel to the centerline of the kiln shell and carrying mechanism bases, as shown in Figure 1. In making these adjustments care must be taken to maintain the proper clearance at the feed end and discharge end air seals and at the main drive gear and pinion.
This check should be made when the kiln is shut down and at a time when the shell is not distorted by either radiant heat from adjacent kilns or the sun. Tape the circumference of each riding ring so the distance from the outside diameter of the riding ring to the center of the kiln can be determined for each ring. Set up a transit or level on top of the kiln over the feed end riding ring. Adjust transit so the line of sight is parallel to centerline of the kiln at the feed end and discharge end riding rings. Check the distance from the riding ring to line of sight for each intermediate ring, repeating this check at quarter points around the circumference. Knowing the radius of each riding ring, the average misalignment at each mechanism can now be determined. Correct this misalignment by making the necessary adjustments to the carrying rollers. The slope of the kiln can be checked with the transit at this time.
With the kiln now in correct vertical alignment, a simple gauge can be constructed to check vertical alignment at each mechanism without having to shut down the kiln and go through the elaborate measurements outlined above. This consists of a gauge pin or tram just long enough to reach from the mark of the true centerline on the carrying mechanism frame to kiln shell. A pin should be made for each mechanism frame. Gauging the distance between the frames and the shell will indicate the extent of wear on the rollers and riding rings. The carrying rollers can then be moved in to return the kiln to its original elevation.
Since the shell may not be perfectly round at the planes where the measurements are taken, reference points should be established on the shell so that the vertical distances between the mechanism bases and the shell will always be gauged at the same points on the circumference of the shell. Reference points can be made by welding four -in. nuts to the shell in each plane where measurements are to be taken. These nuts should be spaced 90 degrees apart. A bolt can then be turned into each nut to a point where the head of the bolt will just touch the gauge pin. The bolt is then welded to the nut. Four of these bolts are used at each mechanism to provide an average reading.
The floating type riding rings should not wobble as the kiln rotates, since it is impossible to obtain full contact between the carrying rollers and riding ring under such conditions. Figure 2 shows a method for determining the amount of runout in a ring.
Two pointers are constructed of angle iron and placed as shown. By using two pointers the effect of any kiln float is eliminated. These pointers should be mounted on the kiln pier away from the carrying mechanism and should extend to the centerline of the kiln. The edge of each pointer should be approximately one inch from the machined outer edge of the riding ring.
Measurements are taken from reference marks on the pointers to the machined sides of the riding ring. A set of readings taken at 16 equally spaced points around the circumference of the ring will indicate the location and magnitude of maximum runout. If the runout at any location exceeds it must be corrected by relocating the retaining bands and riding rings.
When rollers are set parallel to the centerline of the kiln, the roller shafts should bear against the downhill bearing caps. This can be checked by tapping the bearing caps with a hammer. The loaded caps will emit a solid sound.
Check each downhill bearing cap to make certain there is no excessive downhill thrust on any cap. This is done by cutting the roller to just relieve the pressure of the roller shaft against the cap. Note how much the adjusting screw was turned. Then return the roller to a setting which will just produce a light roller shaft force against the downhill bearing cap. After all the rollers are adjusted the kiln will bear against the lower thrust roller. Care must be taken to avoid excessive thrust roller loading.
Sometimes, when starting up a new kiln with the carrying rollers set parallel to the axis of the kiln, the carrying rollers and riding rings will not make 100% contact throughout the complete revolution of the kiln. In such cases it is better to let the rolling surfaces wear in to obtain full contact rather than to adjust the rollers to obtain full contact immediately.
The thrust mechanisms on modern kilns are designed to carry the full thrust of the kiln. On many older kilns, however, it is necessary to carry much of the thrust by adjusting the carrying rollers, since the thrust mechanisms were not designed to take the full thrust load. To float the shell of such a kiln, thus reducing or even eliminatingcompletely the thrust on the downhill thrust roller, the carrying rollers are set at an angle as shown in Figure 3. The illustrations exaggerate the amount of adjustment to show the principle involved more clearly. Any such adjustments must be performed carefully with each roller to avoid excessive pressure with resultant wear from developing on any one roller.
The main gear is equipped with adjusting screws to facilitate centering of the gear on the shell.
This gear must run true. Several points around the circumference of the gear should be checked with a feeler gauge for uniform contact across the full face of the teeth. Flange bolts should be inspected periodically to be sure they are tight at all times. A tight fit can be achieved by pulling each bolt up tightly, then heating the bolt to about 350 F and advancing the nut an additional 20 degrees.
The pinion and gear must mesh properly, as shown in Figure 5. Pitch lines are scribed on both sides of the gear and pinion at the factory. The pinion should be set so the pitch lines on the gear and pinion are 1/16 in. apart when the shell is cold. When the shell is hot the pitch lines should be from 0 to 1/16 in. apart. In no case should the pitch lines overlap, since this would cause excessive wear and overloading of the pinionshaft and bearings.
The mesh of the gear and pinion should be checked at regular intervals. Any wear or adjustment on the carrying rollers will change the mesh. If the pinion is meshing too deeply, the kiln should be raised to its original position by moving the carrying rollers in toward the centerline of the kiln. It should not be necessary to back the drive out to obtain the correct mesh. While the drive is equipped with adjusting screws, these are mainly for use in the initial alignment of the drive. Sometimes it becomes necessary to back the drive away from the kiln to relieve a serious condition which, if permitted to continue, would result in damage to the gear and pinion. This must be considered only a temporary expedient, and the drive should be returned to its correct location immediately upon returning the shell to its true centerline in the recommended manner.
Sometimes it becomes necessary to reface riding rings or rollers. A grinding rig, shown in Figure 6, can be constructed on the job and used to reface the riding rings while the kiln is in operation. The coil springs and adjusting bolts serve as a stop so that the high spots on the ring will be ground off first. In operating this refacing tool, a reference mark should be established on the side of the riding ring being ground, and the adjusting screw on the tool turned one revolution per revolution of the kiln to assure even grinding across the face of the ring. Any circumferential ridges on the ring must be ground off first.
If it is not feasible to operate the tool throughout a full 24-hour period, it is a simple matter to lower the grinding table on the adjusting bolts, thereby removing the carborundum blocks from contact with the riding ring. This tool can also be adapted for refacing the carrying roller by tipping the rig up on its side and setting it against the face of the roller. This can be more easily accomplished on the outer side of the carrying mechanism due to space limitations, especially at the thrust mechanism. These rollers can also be refaced by using a lathe and cutting tool arrangement, or it may be more expedient and economical to put the kiln on cribbing and reface the rollers in the machine shop. In either case, whether the ring and roller are to wear in over a period of time or whether they are to be refaced, it is essential to watch the following:
Use high grade lubricant with specifications shown in Figure 7. After a new kiln has been in service for one month, drain oil and clean reservoir. Refill with new oil. Thereafter change oil every six months.
Use high grade lubricant with specifications shown in Figure 8. The oil level should be checked at frequent intervals and maintained at the proper level. When starting a new kiln the level should be checked daily. The oil should be changed after the first month of operation and at six-month intervals thereafter.
Floating type riding rings should be lubricated between riding rings and filler bars with a graphited grease. Initially, apply lubricant to the inner surface of the riding ring at all spaces between filler bars. Subsequent applications of lubricant need be made only at four evenly spaced points around the circumference of the ring. The lubricant can be most easily applied with a hand gun with extended nozzle.
tire and trunnion grinding: critical rotary drum maintenance
A rotary drum in any industrial process setting, whether it is serving as a dryer, cooler, kiln, or agglomeration drum, is a critical component in the process loop; in most cases, if the rotary drum is offline, the remainder of the process must go down as well, resulting in downtime and lost production. In order to minimize downtime and circumvent potential issues, operators must regularly inspect the drum and carefully follow preventative maintenance procedures.
One of the easiest and most effective preventative maintenance procedures is tire and trunnion grinding the practice of resurfacing the tire (riding ring) and trunnion wheel components to like-new condition. Since damage to these components has a compounding effect on the rest of the drum, keeping tires and trunnion wheels in good condition is the starting point for avoiding more severe issues.
As the drums foundational support, problems that start in the tires and trunnion wheels work their way to other parts of the drum if not promptly addressed. When left untreated, the resulting stress on other components continues to build, exacerbating existing issues and promoting more severe damage.
Rotary drum alignment is a primary contributor to overall drum performance and longevity. Much like tires and trunnion wheels, the alignment of a drum affects several components, and when allowed to run out of alignment, can result in a wide range of problems.
While there are many potential consequences of misalignment, one especially challenging result is the lack of control over axial thrust. The inability to properly manage axial thrust promotes excessive wear on one of the thrust rollers, which has the potential to cause thrust roller bearing failure. In turn, bearing failure can lead to the drum dismantling the thrust roller shaft. If dismantling of the thrust roller shaft occurs, the drum is able to roll off the trunnion rollers, causing catastrophic failure and severe damage to breechings, seals, trunnion base weldment, concrete piers, gears (or chain and sprocket), tires, and the drum shell.
Drum vibration amplitude should be kept at a minimum in order to keep the drum running as smoothly and efficiently as possible, minimizing stress on all components. Excessive vibrations not only cause undesirable chattering, but they also quickly propagate to the rest of the drum, causing:
Allowing a drum to run with an increased vibration amplitude will eventually lead to catastrophic failure. A vibrations study can be performed to measure the vibration amplitude (peaks) and determine how aggressive the vibrations are. Vibration amplitude assesses the amount of displacement occurring, measured in micrometers (m).
A rotary drum is supported at four points where the tires (riding rings) meet the trunnion wheels. These four points of contact bear the weight of the drum and must be carefully balanced to provide even weight distribution and avoid putting excess strain on any of the components.
Additionally, wear may also be indicated by a chattering or vibrational sound. Furthermore, wear on one component (the tire or trunnion) is typically mirrored on the corresponding component, but this does not hold true in all settings. Any sign of wear, whether on one or both surfaces, should be immediately addressed.
Sometimes called reconditioning, tire and trunnion wheel grinding is a relatively simple process in which trained technicians use a specialized grinding machine to remove any surface wear to reveal the undamaged surface underneath.
Depending on the severity of the wear and the diameter of the wear surface, this process may take anywhere from a few hours to a few days. In many cases, the drum can remain in operation during the grinding process, so production need not be affected.
If wear exceeds greater than 10% of the original diameter, FEECO recommends replacement of the component to avoid problems associated with an altered slope, which can have greater ramifications on the overall process.
If the drum were not aligned following grinding, the misalignment of the drum, however minuscule, would immediately begin causing wear to tires and trunnion wheels, bringing them back to their pre-grinding condition.
While manual methods are available, rotary drum alignment is typically carried out using a laser alignment system. This advanced system reduces potential for human error, improves accuracy, and speeds up the alignment process.
Tires and trunnion wheels serve as the support system for a rotary drum, and as such, have a compounding effect on the system if allowed to wear. Tire and trunnion wheel grinding is a critical preventive maintenance tool in prolonging rotary drum life and promoting efficiency.
FEECO is a leader in rotary drum maintenance for dryers, kilns, coolers, and agglomeration drums. Our experienced technicians can assess damage to load-bearing surfaces and resurface tires and trunnions quickly and accurately. Technicians can then realign the drum for optimum mechanical stability. FEECO can also provide replacement tires and trunnions in cases of severe wear. For more information, contact our Customer Service Team today!
11: materials and processes for cutting, grinding, finishing, and polishing | pocket dentistry
Glaze ceramicA specially formulated ceramic powder that, when mixed with a liquid, applied to a ceramic surface, and heated to an appropriate temperature for a sufficient time, forms a smooth glassy layer on a dental ceramic surface (see natural glaze).
Abrasive processes have been used since prehistoric times. Hunting and gathering instruments were shaped and sharpened by chipping and abrading one surface against another over 10,000 years ago to produce sharp edges on hard natural materials. Spear points, arrowheads, scraper tools, and hoes were made by chipping, grinding, and honing the surfaces and edges of relatively hard rocklike materials. Sandstone was used to produce smoother surfaces on the Egyptian pyramids.
Grinding wheels of a primitive type were created over 4000 years ago by taking a cylindrical stone with an abrasive surface and spinning it against metals and ceramics to adjust their shapes, reduce rough areas, and produce smoother surfaces. These processes were refined over subsequent millennia to produce, by the Middle Ages, metal daggers, swords, spears, and shields of relatively high quality. The Chinese introduced the first coated abrasives in the thirteenth century by embedding seashell fragments in natural gums that were spread on a parchment backing.
In the early 1900s, abrasive technology advanced further through the development and use of alumina grains, diamond particles, and silicon carbide grit. New products in the form of powders, slurries, particle-embedded discs and wheels, and burs of different types soon emerged for use in dentistry. The further refinement of dental handpieces, air or abrasive technology, and methods of bonding abrasives to various binders has led to major processing breakthroughs that have rapidly advanced the quality of treatment in the current era of restorative dentistry, particularly with adhesive and esthetic dentistry.
The intraoral surfaces of virtually every direct and indirect restoration must be contoured by grinding, finishing, and polishing procedures. The goal of these procedures is to produce the smoothest surface possible in a limited time. A single type of abrasive cannot be used effectively for all types of dental materials. Different abrasives are used for the three major classes of materials: ceramics, metals, and resin-based composites.
Why are abrasives different? The abrasive instruments used for metals must be able to remove metal particles quickly and efficiently without generating excessive heat or becoming clogged with debris. Although the flexible discs used for resin composites can be used for metals, they are incapable of removing large amounts of metal quickly. Instead, silicon carbide discs are required for cutting metal sections such as casting sprues, and bonded abrasive wheels or points are used for rapid adjustment of surface contours. Likewise, although diamond burs have been developed for grinding and finishing zirconia frameworks, specialized Zir-Cut (Axis/SybronEndo, Coppell, Texas) coarse blue wheels with embedded diamond particles may be more effective because they may not wear out as fast as diamond burs and they can remove relatively large amounts of zirconia from framework surfaces rather efficiently. Therefore, suitable finishing and polishing instruments should be utilized for the respective dental materials.
The form of polishing instruments also affects the rate of material removal and the surface finish. For example, regular and extra-thin Sof-Lex (3M ESPE St. Paul, MN) contouring and polishing discs, useful for finishing and polishing resin-based materials, are provided in the stiff and flexible disc forms to allow either light or heavy pressure to be applied, depending on the amount of composite that needs to be removed. The polymer backing allows the discs to be used either in the dry or wet condition. The regular Sof-Lex discs are available in four grades of abrasiveness, coarse (black), medium (dark blue), fine (medium blue), and superfine (light blue). The extra-thin discs and gapped strips are also provided in these four abrasive grades, but their colors are different. A word of caution is important here. The abrasive discs and strips of other manufacturers may not follow the same convention for Sof-Lex discs (i.e., darker colors corresponding to coarser abrasives and lighter colors to finer abrasives).
The Astropol finishing and polishing system (Ivoclar Vivadent, Amherst, NY) for composites and ceromers is also provided in different forms and levels of abrasiveness. Astropol is a comprehensive finishing and polishing set that consists of four differently shaped polishers in three grit sizes for interdental and occlusal applications, small flame, large flame, cup, and disc forms. The grit sizes are designated as (1) finish (Astropol F for the removal of excess material and prepolishing); (2) polish (Astropol P, for polishing of restorations: especially those made from microfilled composite materials); and (3) high-gloss polish (Astropol HP, that is best suited for hybrid composites). Similar types of abrasive tools are available from other manufacturers and suppliers of dental products.
In summary, dental abrasives are used for tooth cleaning (dental prophylaxis), occlusal adjustment of tooth enamel and restoration surfaces, contouring of material (acrylic, composite, metal, and ceramic) surfaces, finishing and debris removal (grinding and air-particle abrasion), and fine polishing to produce glossy surfaces. The abrasives can be provided in the form of powders, pastes, diamond burs, and abrasive stones, discs, wheels, points, and cups. The best choice for any dental application depends on the initial surface quality, material type, and specific purpose or need. The specific need can vary from cutting or rough grinding to final polishing to achieve a desired luster or gloss.
Finished and polished restorations provide four benefits of dental care: better gingival health, chewing efficiency, patient comfort, and esthetics. Patients can detect a surface roughness change of less than 1m (Jines et al., Research Brit Dent J, 196:4245, 2004) by tongue proprioception. Surface changes greater than 1m can also lead to increased bacterial adhesion as well as surface staining. A well-contoured and polished restoration promotes gingival health by resisting the accumulation of food debris and pathogenic bacteria. This is accomplished through a reduction in total surface area and reduced roughness of the restoration surface. Smoother surfaces have less retention areas and are easier to maintain in a hygienic state when preventive oral home care is practiced because dental floss and the toothbrush bristles can gain more complete access to all surfaces and marginal areas.
Tarnish and corrosion activity of some metallic materials can be significantly reduced if the entire metal restoration is highly polished. Oral function is enhanced with a well-polished restoration because food glides more freely over occlusal and embrasure surfaces during mastication. More importantly, smooth restoration surfaces minimize wear rates on opposing and adjacent teeth. This is particularly true for restorative materials such as ceramics, which contain phases that are harder than tooth enamel and dentin.
Rough material surfaces lead to the development of high, two-body contact stresses that can cause the loss of functional and stabilizing contacts between teeth or a reduction in the vertical dimension of occlusion. Rough surfaces on ceramics also act as stress concentration points. Finishing and polishing these surfaces can improve the strength of the restoration, especially in areas that are under tension, such as the perimeter of ceramic-ceramic crowns where unsupported areas of veneering ceramic are present. Finally, esthetic demands may require the dentist to finish and polish highly visible surfaces of restorations differently from those that are not accessible. Although a mirrorlike polish is preferred for previously mentioned reasons, this type of surface may not be esthetically compatible with adjacent teeth in highly visible areas, such as the facial surfaces of maxillary anterior teeth. Fortunately, these surfaces are not subject to high contact stresses and they are easily accessible for cleaning. Subtle anatomic features and textures may be added to these areas without affecting oral health or function.
In summary, the goals of finishing and polishing procedures are to obtain the desired anatomy, proper occlusion, and reduction of roughness and the depth of gouges and scratches produced by the contouring and finishing instruments. The instruments available for finishing and polishing restorations include fluted carbide burs, diamond burs, stones, coated abrasive discs and strips, polishing pastes, and soft and hard polymeric cups, points, and wheels impregnated with specific types and sizes of abrasive particles. The polished surface should be smooth enough to be well tolerated by oral soft tissues and to resist bacterial adhesion and excessive plaque accumulation. When plaque deposits exist on restorative material surfaces, they should be easily removable by brushing and flossing.
Particles of a substrate material (workpiece) are removed by the action of a harder material that makes frictional contact with the substrate. This contact must generate sufficient tensile and shear stresses to break atomic bonds and release particles from the substrate. With rotary instrumentation, the blades of a carbide bur or the tips of abrasive particles transfer the force to the substrate. These tensile and shear stresses are induced in both the substrate and the rotary instrument. The instrument will fail to cut, grind, or polish if the stress that develops in any part of the cutting or grinding surface exceeds the strength of the instrument blade edges or particle bond strength to the binder compared with the strength of the substrate (workpiece). As a result, blade edges will become dull, and abrasive particles will fracture or tear away from their binder. Such degradation of finishing instruments is discussed in more detail in a later section.
Subtle differences distinguish the cutting, grinding, and polishing processes. A cutting operation usually involves the use of a bladed instrument or any other instrument in a bladelike fashion. Substrates may be divided into large separate segments, or they may sustain deep notches and grooves by the cutting operation. High-speed tungsten carbide burs have numerous regularly arranged blades that remove small pieces or segments of the substrate as the bur rotates. As shown in Figure 11-1, A, the unidirectional cutting pattern reflects the action of the regularly arranged blades on a tungsten carbide bur. The pattern produced by a diamond bur is shown in Figure 11-1, B. The surface of a coarse diamond bur is shown in Figure 11-1, C. When 30-fluted finishing burs have been used on a surface, the regular pattern of the cutting blades is discernible only if the surface is magnified for inspection. On the other hand, a separating wheel is an example of an instrument that can be used in a bladelike fashion. A separating wheel does not contain individual blades, but its thin blade design allows it to be used in a rotating fashion to grind through cast metal sprues and die stone materials.
A grinding operation removes small particles of a substrate through the action of bonded or coated abrasive instruments. Grinding instruments contain many randomly arranged abrasive particles. Each particle may contain several sharp points that run along the substrate surface and remove particles of material. For example, a diamond-coated rotary instrument may contain many sharp diamond particles (see Figure 11-1, C) that pass over a tooth during each revolution of the instrument. Because these particles are randomly arranged, many unidirectional scratches are produced within the material surface, as illustrated in Figure 11-1, B, which shows a tooth surface ground by a diamond bur. Cutting and grinding are both considered predominantly unidirectional in their action. This means that a cut or ground surface exhibits cuts and scratches oriented in one predominant direction.
Different types of burs have unique effects on surfaces. In general, a carbide bur with more blades will produce a smoother surface than a carbide bur containing fewer blades. For example, a 16-fluted carbide bur produces a smoother finish than an 8-fluted carbide bur, but the latter removes material more rapidly. Similarly, the coarsest diamond bur removes material more quickly but leaves a rougher surface. (See Figure 11-2 for scanning electron microscopy [SEM] images of carbide and diamond burs.)
Bulk reduction can be achieved through the use of instruments such as diamond burs, tungsten carbide burs, steel burs, abrasive wheels, and separating discs. Whereas diamond burs and abrasive wheels provide this action by grinding, steel and carbide burs remove materials through a cutting action of the hard blades. Abrasive-coated discs are popular instruments for bulk reduction of resin-based composite restorations. For bulk reduction of composites, the clinician should choose 8- to 12-fluted carbide burs or abrasives with a particle size of 100m or larger and sufficient hardness (9 to 10 Mohs hardness). SEM images of the surface finishes produced on a resin-based composite by a coarse diamond, a 12-fluted carbide bur, a 16-fluted carbide bur, and two types of finishing or polishing systems are shown in Figure 11-3. For bulk reduction of ceramics and metals, the user should follow the manufacturers instructions to minimize the time required. In some cases, instruments used in the dental lab may be different from those used chairside, so the abrasiveness of an unknown instrument should be tested on a scrap piece of the material that will be used for a specific task.
Even though contouring can be achieved during the bulk-reduction process, in some cases, it requires finer cutting instruments or abrasives to provide better control of contouring and surface details. At the end of this process, the desired anatomy and margins should be established. The smoothness of the surface at this stage depends on the instrument used and may require extra steps to establish a smoother surface. Usually 12- to 16-fluted carbide burs or abrasives ranging in size from 30 to 100m provide a fine contouring action.
In general, finishing and polishing processes require a stepwise approach, introducing finer scratches to the surface of the substrate to methodically remove deeper scratches. This process may require several steps to reach the desired surface smoothness. Surface imperfections can be an integral part of the internal structure, or they can be created by the instruments that are used for gross removal because of the size of the abrasives or the flute geometry. Finishing provides a relatively blemish-free smooth surface. The finishing action is usually accomplished using 18- to 30-fluted carbide burs, fine and superfine diamond burs, or abrasives that are between 8 and 20m in size.
The purpose of polishing is to provide an enamel-like luster to the restoration. Smaller particles provide smoother and shinier surfaces. The speed of achieving a luster, however, depends on the hardness and size of the abrasive particles and the method of abrasion (e.g., two-body abrasion or three-body abrasion). Ideally, abrasive particles in the range of particle sizes up to 20m provide luster at a low magnification. At the end of this process, there should be no visible scratches. However, there will always be scratches that are detectable at higher magnification. The surface must be cleaned between steps because abrasive particles left on the surface from the previous step can cause deep scratches.
The quality of the surface finish and polish can be characterized by the measurement of the surface roughness using a profilometer, an optical microscope, or an SEM. In clinical practice, the surface luster is usually judged without magnification. Most of the time, surface smoothness is correlated with the luster, as in cases such as resin-based composite restorations. However, the smoothest surface does not necessarily provide the most lustrous surface. For industrial applications, reflectometers are used to measure the luster. However, it is difficult to use them successfully for dental applications because of the irregular contour and small size of dental restorations.
Polishing procedures, the most refined of the finishing processes, remove the finest surface particles. Each type of polishing abrasive acts on an extremely thin region of the substrate surface. Polishing progresses from the finest abrasive that can remove scratches from the previous grinding procedure and is completed when the desired level of surface smoothness is achieved. Each step is followed by the use of progressively finer polishing media until no further improvement in surface finish is observed. The final stage produces scratches so fine that they are not visible unless greatly magnified. Polishing should be terminated when no further change in surface luster or glossiness occurs during the application of the finest abrasive that is used for that application. Further attempts to improve the surface appearance may actually degrade the surface by generating heat and by smearing dislodged material across the surface.
Examples of polishing instruments are rubber abrasive points, fine-particle discs and strips, and fine-particle polishing pastes. Polishing pastes are applied with soft felt points, muslin (woven cotton fabric) wheels, prophylaxis rubber cups, or buffing wheels. A nonabrasive material should be used as an applicator while using polishing pastes. Felt, leather, rubber, and synthetic foam are popular applicator materials for buffing. A common feature of some of these materials is their porous texture, which allows fine abrasive particles to be retained during the buffing procedure.
Polishing is considered multidirectional, which means that the final surface scratches are oriented in many directions. Some examples of ground and polished surfaces are shown in Figure 11-3. Note that the differences in surface appearance are subtle because of the transitional nature of the grinding and polishing processes. If there were larger differences in the size of particles removed, the surface change would be more easily detected.
Heat generation during the cutting, contouring, finishing, and polishing processes of direct restorations is a major concern. To avoid adverse effects to the pulp, the clinician must cool the surface with a lubricant, such as an airwater spray, and avoid continuous contact of high-speed rotary instruments with the substrate. Intermittent contact during operation is necessary not only to cool the surface but also to remove debris formed between the substrate and the instrument. The effectiveness and the speed of the contouring, finishing, and polishing procedures will be greatly improved by the removal of debris.
Dispersions of solid particles are generated and released into the breathing space of laboratories and dental clinics whenever finishing operations are performed. These airborne particles may contain tooth structure, dental materials, and microorganisms. Such aerosols have been identified as potential sources of infectious and chronic diseases of the eyes and lungs and present a hazard to dental personnel andtheir patients. Silicosis, also called grinders disease, is a major illness caused by inhalation of aerosol particles released from any of a number of silica-based materials that are used in the processing and finishing of dental restorations. Silicosis is a fibrotic pulmonary disease that severely debilitates the lungs and doubles the risk for lung cancer. The risk for silicosis is substantial, because 95% of generated aerosol particles are smaller than 5m in diameter and can readily reach the pulmonary alveoli during normal respiration. Additionally, 75% of airborne particles are potentially contaminated with infectious microorganisms. Furthermore, aerosols can remain airborne for more than 24 hours before settling and are, therefore, capable of cross-contaminating other areas of the treatment facility. Aerosol sources, in both the dental operatory and laboratory environments, must be controlled whenever finishing procedures are performed. A concise and informative source of information on aerosol hazards has been written by Cooley (see Selected Reading).
Aerosols produced during finishing procedures may be controlled in three ways: First, they may be controlled at the source through the use of adequate infection control procedures, water spray, and high-volume suction. Second, personal protective equipment (PPE) such as safety glasses and disposable face masks can protect the eyes and respiratory tract from aerosols. Masks should be chosen to provide the best filtration along with ease of breathing for the wearer. Third, the entire facility should have an adequate ventilation system that efficiently removes any residual particulates from the air. Many systems are also capable of controlling chemical contaminants such as mercury vapor from amalgam scrap and monomer vapor from acrylic resin.
Wear is a material removal process that can occur whenever surfaces slide against each other. The process of finishing a restoration involves abrasive wear through the use of hard particles. In dentistry, the outermost particles or surface material of an abrading instrument is referred to as the abrasive. The material being finished is called the substrate. In the case of a diamond bur abrading a tooth surface, as illustrated in Figure 11-4, the diamond particles bonded to the bur represent the abrasive, and the tooth is the substrate. Also, note that the bur in the high-speed handpiece rotates in a clockwise direction as observed from the head of the handpiece.
The rotational direction of a rotary abrasive instrument is an important factor in controlling the instruments action on the substrates surface. When a handpiece and bur are translated in a direction opposite to the rotational direction of the bur at the surface being abraded, a smoother grinding action is achieved. However, when the handpiece and bur are translated in the same direction as the rotational direction of the bur at the surface, the bur tends to run away from the substrate, thereby producing a more uncontrolled grinding action and a rougher surface.
Abrasion is further divided into the processes of two-body and three-body wear. Two-body abrasion occurs when abrasive particles are firmly bonded to the surface of the abrasive instrument and no other abrasive particles are used. A diamond bur abrading a tooth represents an example of two-body wear. Three-body abrasion occurs when abrasive particles are free to translate and rotate between two surfaces. An example of three-body abrasion involves the use of nonbonded abrasives such as those in dental prophylaxis pastes. These nonbonded abrasives are placed in a rubber cup, which is rotated against a tooth or material surface. These two processes are not mutually exclusive. Diamond particles may debond from a diamond bur and cause three-body wear. Likewise, some abrasive particles in the abrasive paste can become trapped in the surface of a rubber cup and cause two-body wear. Lubricants are often used to minimize the risk of these unintentional shifts from two-body to three-body wear, and vice versa. Thus, the efficiency of cutting and grinding will be improved with the use of lubricants such as water, glycerin, or silicone. Intraorally, a water-soluble lubricant is preferred. Excessive amounts of lubricant may reduce the cutting efficiency by reducing the contact between the substrate and the abrasive. Too little lubricant results in increased heat generation and reduced cutting efficiency.
Erosive wear is caused by hard particles impacting a substrate surface, carried by either a stream of liquid or a stream of air, as occurs in sandblasting a surface. Figure 11-5 illustrates schematically the processes of two-body abrasion, three-body abrasion, and hard-particle erosion. Most dental laboratories have air-driven grit-blasting units that employ hard-particle erosion to remove surface material. A distinction must be made between this type of erosion and chemical erosion, which involves chemicals such as acids and alkalis instead of hard particles to remove substrate material. Chemical erosion, more commonly called acid etching in dentistry, is not used as a method of finishing dental materials. It is used primarily to prepare tooth surfaces to enhance bonding or coating.
As stated previously, the inherent strength of cutting blades or abrasive particles on a dental instrument must be great enough to remove particles of substrate material without becoming dull or fracturing too rapidly. The durability of an abrasive is related to the hardness of its particles or surface material. Hardness is a surface measurement of the resistance of one material to be plastically deformed by indenting or scratching another material. The first ranking of hardness was published in 1820 by Friedrich Mohs, a German mineralogist. He ranked 10 minerals by their relative scratch resistance in relation to one another. The least scratch-resistant mineral, talc, received a score of 1 and the most scratch-resistant mineral, diamond, received a score of 10.
Knoop and Vickers hardness tests are based on indentation methods that quantify the hardness of materials. The tip of a Knoop diamond indenter has an elongated pyramidal shape, whereas the Vickers diamond indenter has an equilateral pyramidal design. Both tests involve the application of the indenter to a test surface under a known load (usually 100 Newtons, or 100 N). The depth of surface penetration is reported as hardness in units of force per unit area. Although a number of other factors affect a materials abrasiveness, the farther apart a substrate and an abrasive are in hardness, the more efficient is the abrasion process. On the basis of a comparison of hardness values for several dental materials listed in Table 11-1, it is expected that silicon carbide and diamond abrasives will abrade dental porcelain more readily compared with garnet, even though the abrasive particles for all three materials have very sharp edge characteristics.