INTRODUCTION: The term “rotary” applied to the tooth cutting instruments describes a group of instruments that turn on an axis to perform work. Applied to dental procedures, the character of work performed is primarily cutting, abrading, burnising, finishing or polishing tooth tissues or various restorative materials. Many procedures in operative dentistry substantially involve rotary instrumentation. The bulk of tooth tissue removal is now accomplished using rotary instruments. HISTORY: In 1868 when Dr. Jonathan Taft wrote his text book of Operative Dentistry, cutting procedures on tooth enamel and dentin were carried out using thick bulk chisel and excavator which Dr. Taft’s own description were of good steel, well wrought and thoroughly tempered. Every step in their manufacture should be perfectly executed so that edge not only cut dentin but also enamel which is the hardest animal tissue. These instruments were heavy handled and as wide as 1/4 th inch at one cutting edge. Taft suggested that a heavy instrument with the sharp point and a lateral curve is often efficient in opening up the cavities and cutting down strong projections of enamel.
It is assumed that carious lesions treated by these types of instruments were extensive enough to give the access to these type of bulky instruments with a gross carious lesion, the chisel was used to gain entrance to the carious dentin and makes it removal by the hand excavators. Access to interproximal lesion was gained by wedging. The first rotary instruments used for cutting tooth tissues were actually drill or bur heads that could twisted in the fingers for a crude cutting a abrading action. Taft described then as ‘bur drills’ which can be made from the best steel, forged close to their proper size and be finally shaped on a lathe. The bulb is then cut into basic shapes by hand with sharp edged file. These simple rotary instruments twisted with the fingers were capable of a very limited lateral and end cutting action. The early bur drills ranged in diameter from 1/3 inch to about 1/5 inch and used for opening of the cavities. These bur chills were more regular and precise orientation and were particularly adapted to small and medium sized cavities. In addition these bur drills were used for making retaining point for fillings. One of the refinements of these bur drills was ‘Scranton’ drill a cross section such that it can be rotated in either direction to achieve its cutting action. In 1846 the finger ring was introduced with a drill socket attached for adapting a series of long handled burs or drills of assorted shapes. This ‘drill ring’ which was adapted to the middle or index finger with a socket that fitted against the palm, providing a seat for the blunt end of the bur drill. The first drill with flexible cable drive and first angle hand piece were invented by Charles Merry between 1858 and 1862. 2
In 1871, techniques were improved significantly when Morison modified and adapted the dental foot engine from singer sewing machine. This importance of this addition was the fact that for the first time, cutting procedures were carried out with the power source other than operatorâ€™s own handle. In 1883 the elective dental engine linked to the hand piece by a flexible cable arm was introduced. For the first time cutting was made possible from a power source other than human hand or feet. It is interesting to know that Dr. G.V. Black had no dental engine when he began produce in 1864. In 1910 belt driven hand piece on a jointed engine arm became available. This unit, utilizing the elective motor as its power source, was used only with minor changes until the 1950s, when the air turbine hand pieces was introduced to the profession. The changing character of rotary instruments from the crude hand twisted bur drills to electrically powered hand piece with high cutting efficiency brought a change in role of hand cutting instruments. The most important function of rotary instruments in operative dentistry is the action of cutting and abrading. Prior to the 1947 dental rotary instruments were made up of carbon steel. Industrially these burs were six bladed bin fabricated from blank by a special cutter and were called as small milling cutter. The rotational speed ranged up to 6500. In 1947 the tungsten carbide burs was introduced to the dental profession. This carbide bur was characterized by the hardening, being more than twice that of steel bur. In design and cutting potential as well as efficiency and life expectancy. 3
In 1945 Dr. R.B Black published a report in the non-mechanical preparation of cavities and introduced air abrasive technique on dental profession. In 1949 Walsh and Symmons published their initial findings relating to the removal of tooth tissue with diamond points at rotational speech up to 70,000 rpm. This report indicated the use of tighter forces and resulting increased cutting efficiency of these higher speed. In early 1950 the ball bearing hand piece was introduced, followed closely by the ball bearing contrangle. In 1953, following the work of Nelson the first fluid turbine type hand piece was introduced to the profession. This instrument was capable of rotational speeds of approximately 50,000 rpm with moderate torque. In 1954 air driven hand pieces were developed. A continuous belt driven hand piece was also introduced, making possible cutting speeds up to 150,000 rpm. By 1957 many dentists were using rotational speeds up to 3 lacks rpm. At that time all but one of the air turbine hand piece used a 1/16 inch shank friction grip type diamond point or carbide bur. The introduction of air bearing hand piece in early 1960s made possible an even greater rotational speed of approximately 500,000 rpm. Superimposed on this rapidly developing era of rotary instrumentation was the unique ultrasonic method of the tooth tissue removal. The unit introduced in 1953 was designed so that suitably shaped tips vibrating at frequencies ranging from 15,000 to 30,000 cycles per sec were used to remove tissue.
CLASSIFICATION: Classification of rotating instruments:a) Dental burs: The dental burs have series of cutting blades. Designs vary from that of a twist drill to a multibladed fissured routes. These instruments rotate in a specific direction (counter clock wise) to coincide with the way the blades are formed. b) Abrading tools:- These are bonded to its surface or impregnated within it are bits of hard substances (e.g. diamond, garnet or sand). The hard filler particles vary in size according to use. Whether it be for reducing hard enamel or for polishing a soft plastic can be rotated in either of the directions. c) Polishing agent:- Non-bonded abrasives or polishing agent. In the form of slurry such as pumice or polishing agent these are carried to working area with polishing brush, impregnated cloth wheel. Classifications according to speed: According to charbenaue Conventiional or low speed – Below 10,000 rpm Increased or high speed – 10,000 to 150,000 rpm (max range of belt driven equipment) Ultraspeed
- Above 150,000 rpm
According to Marzouk:a) Ultra low speed – (300 – 3000 rpm) b) Low speed (3000 – 6000 rpm) c) Medium high speed (20,000 – 45,000 rpm)
d) High speed (45,000 â€“ 100,000 rpm) e) Ultra high speed (100, 000 rpm and more) DENTAL BURS: The term bur is applied to all rotary cutting instruments that have bladed cutting heads. These includes instruments intended for such purposes as cavity preparation, finishing of metal restorations and surgical removal of bone. Manufacturing and materials for dental burs: The earliest burs, were handmade. These hand made burs were expensive and variable in dimensions and performance. The dimension, nomenclature and shapes of modern burs are directly to related to those of first machine made burs introduced in 1891. Earlier burs made up of steel. Steel burs performed well, cutting human dentin as low speeds, but dull rapidly at higher speed, when cutting the enamel. Once dulled the reduced cutting effectiveness creates increased heat and vibration. Carbide burs, now are used mainly replaced the steel bur for tooth preparation. Steel burs now mainly used to finishing procedures. Carbide burs better than the steel burs at all speeds and their efficiency is maximum at the higher speeds. All carbide burs have beads of cemented carbide in which microscopic carbide particles, usually tungsten carbide are held together ina matrix of cobalt or nickel. In most burs, the carbide head is attached to the steel shank and neck by welding or brazing. The substitution of steel for carbide in these portion of bur 6
where greater wear resistance is not required has several advantages. It permits manufacturers more freedom of design attaining the characteristic desired in the instrument and at the same time allows economy in the cost of material. In carbide burs the joint is located in the posterior part of head or have joint located in the shank. The carbide is stiffer and harder than steel bur brittle. If the carbide neck is subjected to sudden blow a shock will fracture but the steel neck will bent. A bur even slightly bent produces increased vibration and over cutting as a result of increased run out. Thus although the steel bur have reduced chances of breakage but if bent they produced more severe problem. â€œSteel burs are cut from blank steel stock by means of rotary cutting that cuts parallel to the long axis of the bur. The bin is then hardened and tempered until its Vickers hardens number is approximately 800. Tungsten carbide burs are the product of metalling i.e. a process of alloying in which complete fusion of the constituents does not occur. The tungsten carbide powder is mixed with powder of cobalt under pressure and heated in vacuum. A partial alloying or sintering of metal takes place. A blank is then formed and the bur is cut from it with a diamond tool. This cutting process is better controlled than the cutting of steel burs. The Vickers hardness rang for this type of bin is 1650-1700. Common design of the dental burs: Each dental bur consists of three parts â€“ a) shank, b) neck, c) head. Each has is own function, influencing its design and material used for its construction.
Shank design:- The shank is the part that fits into the hand piece, accepts the rotary motion from the hand piece and provides a bearing surface to control the alignment and concentricity of the instrument. The shank design and dimensions vary with the hand piece for which it is intended. The ADA specification no.23 for dental excavating burs include five classes of instrument shank. Three of these are:1) Straight hand piece shank 2) Latch type shank 3) Function grip angle hand piece shank Shank portion of the straight hand piece is a simple cylinder. It is held in the instrument by a metal chuck that accepts a range of shank diameter. Therefore precise control of the shank diameter is not as critical as for the other shank design. Not for excavation but for lab work. Latch type angle hand piece shank smaller in length but more complicated design can be used for the posterior teeth as for better visibility. Hand piece that use latch type burs normally have metal bin tubes within which the instrument fits as closely as possible. The posterior portion of the shank is flattened on one side so that the end of the instrument fits into a D shaped socket at the bottom of the bin tubes causing the instrument to be rotated. This type of instruments are used predominantly at low and medium speed ranges for finishing procedures. In these speeds the little amount of wobble inherent in the clearance between the instrument and the hand piece bur tube is controlled by lateral 8
pressure exerted during the cutting procedure. At higher speed latch type does not provide a true running instrument head. The friction grip shank design was developed for use with high speed hand pieces. This design is further smaller and thus gives the maximum access to the operating sight thus can be efficiently be used in the posterior areas. The shank is a simple cylinder and is held in the hand piece by friction between the shank and a plastic or a metal chuck. Neck Design: The neck is the intermediate portion that connects the head to the shanks. Except in the case of the larger, more massive instruments, the neck normally tapers from shank diameter to small size immediately adjacent to the head. The main function of the shank is to transmit rotational and transitional forces to the head. Head Design: The head is the working part of the instrument, the cutting edges or points that perform the desired shaping of the tooth structure. The heads of the burs shows the maximum variation in design and construction than either portions. Nomenclature for the dental brus: The dental bur is the small milling cutting instrument. A common design is displayed with the standard nomenclature.
A) Bur tooth:- this terminates in the cutting edge or blade. It has two surfaces the tooth face which in the side of the tooth in the leading edge and back or flank of the tooth which is the side of the tooth in the trailing edge. B) Rake angle:- The rake angle is the angle that the face of the bur tooth makes with the radial line from the center of the bur to the blade. The rake angle can be negative, zero or positive. The positive rake angle is when the radial line is ahead of the face of the bur tooth. The negative rake angle is when the radial line is behind the face of the bur tooth. Zero rake angle is when the radial line and face cathodes. It is also called as radial rake angle. The more positive that the value angle more will be the cutting efficiency. Also radial rake angle is more efficient than the negative rake angle. Negative rake angle gives the better life expectancy of the bur. With negative rake angle the cut chip moves directly away from the blade edge and often fractures into small bits or dust. This in contrast to burs with the rake angle where the chips are larger and tend to dog the chip space. The steel burs have the rake angle because positivity of the rake angle decreases the size of the bur tooth and its tooth angle, thus decreasing its bulk. As a result, there is a great possibility that bur teeth will be curved, flattened or even fracture during cutting. The positive rake angle can be used with TC where the greater hardness and strength of the material allow sacrifice of bulk to obtain a more efficient cutting edge. Land:- The plane surface immediately following the cutting edge.
Clearance angle: The angle between the back of the tooth and the work. If a land is present on the bur, the clearance angle is divided into: primary clearance angle which, the angle the land will make with work and secondary clearance which the angle between the back of the bin tooth and work. When the back surface of tooth is curved, the clearance is called radial clearance. Tooth angle:- This is measured between face and back. Flute or chip space:- The space between the successive teeth. Classification of Burs: According to the mode of the attachment to the hand piece, they can be, -
friction grip type
According to the hand piece they are designed for, -
contra angle bin
straight hand piece bin
According to revolution they can be class, Right â€“ Revolve clockwise most of the burs revolve clockwise Left â€“ Revolve anticlockwise According to the length of the head, -
According to use, -
Cutting bur (6 bladed)
Finishing and polishing burs (12 to 40 to 60 bladed)
According to the orientation of the bur teeth:-
They can be straight or special
When the bur teeth are cut parallel to the long axis of the bur they are designated as straight
When the bur teeth are cut obliquely to the long axis of the teeth they are special (for better unclogging)
According to the type of the flutes:1) Plain fissure burs / Non-crosscut. 2) Cross cut burs Cross cut burs have notches in the blade edges or cuts across the blades to increase cutting effectiveness at low and medium speeds. Cross cut burs are not used at the high speeds because at the high speeds they produce underlying rough surfaces. A certain amount of perpendicular force is required to make a blade grasp the surface and start cutting as it passes across the surface. The harder the surface duller is the blade, and the greater its length, the more force that is required to initiate the cutting. By reducing the total length of the bur blade that is actively cutting at any one time, the cross cut effectively increases both cutting pressure resulting from rotation of the bur and the pressure holding the blade edge, the
tooth. As each cross out blade cuts, it leaves small ridges of the tooth structure standing behind the notches. Because the notches in the two succeeding blades do not line up with the each other, the ridges left by one blade are moved by the following one as the low or medium speeds. However at the high speeds allowed with air turbine hand pieces, the contact of the bur with the tooth is not continuous and usually only one blades cut effectively. Under these circumstances, although high cutting rate of the cross cut bin is maintained, the ridges are not removed and thus much rougher surface results. According to shapes and sizes of the bur:In United States, dental burs traditionally have been described the arbitrary numerical code for head size and shape. For e.g. No.2 = 1 mm of diameter round bur No.57 = 1 mm diameter straight fissure bur No.34 = 1 mm diameter inverted cone bur Newer classification as given by FDI and ISO tends to use separate designation for shape (usually shape name) and size (usually a number giving the head diameter in tenth of millimeter (e.g. round straight fissure plain 010, inverted core 008). The bur shapes referred to contour or silhouette of the head. The basic head shapes are round, inverted cone, pear, straight fissure and tapered fissure. A) Round bur:- Round in shape and customarily has been used for the purposes such as initial entry into the tooth, extension of the preparation.
B) Inverted cone:- Rapidly tapered cone with apex of the cone directed towards the shank. Head length similar to the diameter. The shape is suitable to provide undercuts in the tooth preparation. C) Pear shaped:- Portion of the slight tapered cone with the small end of the cone directed toward bur shank. The end of the head is continuously curved or is flat with mounted corners. A normal length pear shaped bur for gold foil (Class I) (length slightly more than the width) long length pear (length three times more than width for the tooth preparation for the amalgam). D) Straight fissure bur:- Elongated cylinder for the extension of the cavity can be straight. Crosscut cylindrical fissure bur. E) Tapered fissure bur;- Slightly tapered with the end of the cone directed away from the shank. Tooth used for the tooth preparation of the indirect restorations. F) End cutting burs:- Cylindrical in shape with just end carrying the blades extending the preparation apically without the axial reduction. The numbering system was given by S.S. White dental manufacturing company in 1891 for their first machine made burs. The original numbering system grouped burs by 9 shapes and 11 sizes. Cross cut bur efficient in slow speed 5000 prefix end cutting bur, 900 prefix. Modifications of Bur: 1) Crosscut 2) Extended head lengths:- Carbide fissure burs with the extended head lengths two to three times those of tapered fissure bur of similar diameter have been
introduced. Such a design is never used in slow speed with such a brittle material. The applied force required to make a bur cut at speeds of 500 to 600 rpm would normally be sufficient to fracture such an attenuated head. For these burs extremely light pressure and high speed. 3) Rounding of the sharp tip corners:- Introduced by Markley and Sockwell. Because the teeth are relatively brittle the sharp angles produced by the conventional burs can result in high stress concentration and increase in tendency to tooth to fracture. Bur heads with the rounded corners resulted in lower stresses in the restored teeth, enhance the strength of the tooth by pressuring the vital dentin, and facilitates the adaptation of the restorative material. These type of bur (tungsten carbide or diamond) last longer because there are no sharp corners to chip and wear off. Characteristics of Rotary Instrumentation: 1) Speed:Speed refers to not only revolutions per minute but also surface feet per unit line of contact that the tool has with the work to be cut. According to the industrial investigation, the maximum cutting efficiency of the cutting tool of uniform width ranges between 5000-6000 surfaces feet per unit time. Since the surface feet per min is controlled mainly by rpm and the size of the tool, it is important to consider the size of one working tool in relative to the speed of the operating. A rotary tool should be large in diameter when used with low speed to obtain an optimum surface feet per unit time and vice versa.
The various speed in dentistry are classified as:The advantages of the use of the increased speeds â€“ 1) Small rotary instruments could be used with the increased speeds 2) Less fatigue is resulted with both the operator and patient 3) Control of the instrument by the operator is better 4) Due to high speed very light pressure is required 5) Less vibrations is felt by the patient 6) The efficiency and the life of the cutting tool is increased 7) Removal of old filling is simplified The disadvantages of higher speeds are:1) With increased speed increased temperature of the tooth which might damage the tooth pulp. 2) High speeds results in greater wear of the working parts of the hand piece therefore frequent repairs and replacements in addition to more exacting and properly used. 3) Unless properly used high speeds have a tendency to create striations on the tooth structure. Biologic principles of the tooth preparation:1) Removes least amount of the tooth tissue 2) No injurious effects to the periodontal tissue and pulp 3) Least discomfort to the patient and fatigue to operator 4) No pathologic reactions initiated in the pulp
Pressure:- Pressure is the resultant effect of the two factors under the control. 1) Force: The gripping of the headpiece and its positioning and application to the tooth. 2) Area: The amount of surface area of the cutting tool in contact with the tooth surface during the cutting operation. Using the same force the smaller cutting tool apply the larger pressure than the larger cutting tools. To have both small and large tools cut at the same pressure it is necessary to reduce the force applied with smaller one. If the constant force is reduced on the smaller tools so that there is equal pressure on the tooth. Obviously the larger tools remove more. Tooth structure since there is more single feet per minute contact. To have smaller tool remove the same amount of the tooth structure the rpm has to be increased, thus increasing surface per minute contact. It has been observed that low speed requires 2-4 pounds force, high speed requires less force (1 pound) and ultra high speed still less force (1-4 ounces) for efficient cutting. Desirable feature of the higher speed is â€“ better control and less fatigue on part of the operator and greater patient comfort. Heat Production: Heat generated is directly proportional to the pressure, rpm and area of tooth in contact with the tool. Therefore any factor increased heat production is increased heat production up to 130Â°F causes permanent damage to the pulps. Even temperature up to 113Â°F within the pulp can produce the inflammatory responses that could result to pulpus and pulpal necrosis 17
The various factors which causes heat production are:a) Speed of the rotating tool â€“ Temperature rise occurs within 10 to 20 secs after the cutting operation has begun b) Size of the cutting instrument c) Pressure The heat production ill effects can be minimized by using coolants, such as flowing water a water-air spray or air. Coolant must be employed which, to be effective should be applied at a point of contact between the cutting instrument and tooth tissue. Peyton found that even with water coolant, excessive temperature developed when large diameter instruments or excessive forces were applied with increased operating speed. This indicates that mere the use coolants, per sec, does not eliminate the excessive temperature rise. The minimum water volume to be applied was 1.5 mm per min. Another advantage of the water coolant that tooth debris from cutting is removed rapidly, preventing the clogging of the burs. This results in greater efficiency and prolongs the life of the cutting tool. D) Vibration:Vibration is not only major annoying factor for the patient but is also causes fatigue for the operator, excessive wear of the instrument and most importantly, a destruction reaction in tooth and supporting tissues. Vibration is the product of the equipment used and the speed of rotation. The deleterious effects of vibration are two fold in origin:1) Amplitude 2) Undesirable modulating frequencies 18
Amplitude:- A wave of vibration consists of the frequency and amplitude. At low speeds the amplitude is more but frequency is less. At higher speeds the reverse is true. The greater destruction is caused by large amplitude cause destruction to instrument, apprehension in patient but also great fatigue to the client. Vibration waves are measured by cycles per second. 6000 rpm sets up fundamental vibrational wave of 100 cycles per sec. At 100600 rpm there is 1600 cycles per second at this amplitude the vibration are practically imperceptible to patient. This is concluded that higher rpm produces less amplitude and greater frequency. Undesirable modulating frequencies: The second deleterious effects of vibration is caused by improperly designed, or poorly maintained equipment. Although there must be a fundamental vibrational wave, improper equipment use or care allows modulatory frequency to be established so that series of the vibration (in different directions) are perceived by the patient and dentist. The end result is again apprehension in the patient, fatigue for dentist and accelerated wear for cutting. To reduce this the equipment should be free from such defects. E) Patient reactions:The factors that cause patient apprehension consists primarily of heat production, vibrational sensation, length and operating time and number of visits. The proper understanding of the instrument, the speed, use of coolants, intermittent application of tool, sharp instruments etc in greatly minimizing both
patient discomfort and unnecessary irritation to oral stricture. Patient can be anaesthetized. F) Operator fatigue:The major causes of operator fatigue are duration of operation, vibration produced in hand piece, forces, needed to control the rotating instrument, apprehension on the part of dentist. High speed rotary instrumentation minimizes fatigue by decreasing time of operation. Proper balancing also reduces the forces needed to control the instrument. FACTORS INFLUENCING THE CUTTING EFFICIENCY OF BUR: A) Influence of design and manufacturing:1) Rake angle:The more positive that the rake angle is, the greater is the burâ€™s cutting efficiency. All burs with radial positive angle cut more effectively than designs with negative rake angles. In negative rake angle the cut chip moves directly away from the blade edge and often fractures into small bits or dust. This in contrast to burs with a positive rake angle where chips are large tends to clog the chip space. Thus there is a practical objection to use of the positive rake angles in dental burs, particularly in steel burs because as the positivity of the make angle decreases the size, size of the bur tooth and its tooth angle, thus decreasing its bulk. As a result there is great possibility that bur teeth will be curved, flattened or even fractured during cutting.
Tungsten carbide burs can positive rake angle because the hardness and strength of the material compensates the sacrifice of the bulk to obtain a more efficient cutting edge. The burs with a negative rake angle or radial clearance with short bur tooth height is employed to contribute to a longer bur life. 2) Clearance Angle:This angle provides clearance between the work and the cutting edge to prevent tooth back from rubbing on the work. There is always a component of the frictional force on one cutting edge as it rubs against the surface following the dislodgement of the chips. Any slight wear of the cutting edge will increase the dulling perceptibility. Large clearance angle may result in less rapid dulling of the bur. 3) Number of teeth or blades and their distribution:The number of teeth in a dental bur is usually limited to 6-8. Since the external load is distributed among the blades actively cutting, as the number of blades is decreased, the magnitude of forces at each blade increases and the thickness of the chip removed by each flute correspondingly increases. Under certain conditions, really the same amount of material can be removed by either 8, 7 or 6 fluted burs i.e. the product of chip thickness removed by each tooth and number of flutes may be nearly constant. The burs with fewer number of the teeth, gives increased space between the bur teeth which reduces their clogging tendency. If each bur tooth is removing more material the tendency for bur tooth wear should be greater and cutting life reduced. 21
Fissure bur with straight flutes produces less temperature rise than one with spinal flutes. This is due to the fact that the straight fluted bur produces large chips. These chips carries some heat with it. Thus a bur with the fewer flute will be cooler in operating. It has been seen that the fewer number of bur tooth, the greater the tendency for vibration however if there are two or more teeth in contact with work at onetime, this effect would be at greater importance, particularly if the bur is 8 or 6 fluted. If the bur teeth is crosscut, the number of the bur teeth can be increased, based on the assumption that cross cutting reduces the friction in cutting and provides more chip space. Some burs will have 10-12 or even up to 40 blades. They are not designed for cutting. They are used only for finishing and polishing of a dental restoration. 4) Run out concentricity:Concentricity is a direct measurement of the symmetry of the bur head itself. It measures how closely a single circle can be passed through the tips of all blade. Thus concentricity is an indication of whether one blade is longer or shorter than the others. It is a static measurement not directly related to function. Run out:- It is a dynamic test measuring the accuracy with which all blade tips pass through a single point when the instrument is rotated. It measures not only the concentricity of the head, but also accuracy with which the centre of rotation passes through the center of the head or run out refers to the eccentricity or
maximum displacement of the bur head from its axis of rotation while the bur turns. The average value of clinically acceptable run out is about 0.023 mm. Run out will depend not only on the design of the bur itself, but also on the precision of the dental hand piece. Even a perfectly concentric head will exhibit substantial amount if the head is off longitudinal on the axis of bur, the bur is bent, the bur is not held straight on the hand piece. The run out is clinically more significant term because it is the primary cause of vibration during the cutting and the factor that determines the minimum diameter of the hole that can be prepared by the bur. It is because of the run out error that burs normally cut holes measurably larger than the head diameter. As run out tell the maximum displacement of the bur head from its long axis while rotation. If the bur moves away from the tooth periodically, blades will not cut equally. If the operator senses this lack, he will probably exerts more pressure. The result will be that one stage revolution the bur and the work tend to be pushed apart. Only to be driven together at the next half revolution, resulting in disagreeable vibration. These vibrations causes removal of tissue by shattering than cutting action. 5) Finish of one flutes:The dental bur is formed by cutting each flute into the bur blank with a rotating cutter while it progresses nearly parallel to the axis of the bur. During the first cut or pass of the cutter the flute is roughly formed. The second cut places
cutting edge as the bur flute. However considerable roughness along the flute will remain. This roughness may be removed by making subsequent passes or cuts on the bur flute. Test for cutting efficiency were done on different types of burs undergoing two, four and six flute cuts. Those cut size most efficient while those cut two times well the least efficient. 6) Heat Treatment:Heat treatment is used to harden the bur that is made up of soft steel. This operation presents the edge placed on the flute by utter and hardens the bur to increase the cutting life. 7) Design of flute ends:The dental burs have two types of flute ends â€“ a) Revelation cut, b) Star cut. a) Revelation cut, where the flute comes together at two junction near a diametrical cutting edge. b) Star cut, where the end flutes come together in a common junction at the axis of the bur. Revelation cut shows some of greater cutting efficiency in direct cutting. In lateral cutting both types prove to be of equal efficiency. 8) Bur diameter:Generally the force on the each bur look from the external load do not depend on the diameter of the bur, but rather on number of flutes or teeth and their
rotational position. The average linear displacement per revolution and length of cut does not depend upon the diameter of the bur. It follows that because the length of the cut is constant, the material removed will vary directly with the bur diameter as well as the tongue and mechanical energy that the power source required to supply. 9) Depth of engagement:As the depth of engagement decreases, the force intensity on each small portion of the bur tooth still cutting correspondingly increased according to the average displacement per flute should be increased. This increase is so great that the volume of material removed from by the shallow cut is more than the deeper cut. 10) Influence of load:Load signifies the force exerted by dentist on the tool head and not pressure or stress induced in the tooth during cutting. The force or the load is related to the rotational speed of bur of a given design. The exact amount of force exerted is not known but maximum of 1000 gm (2 pounds) for low rotational speed and from 60120 gm (2-4 ounces) at high rotational speed. 11) Influence of speed: The rate of increase in cutting at rotational speed above 30,000 rpm is greater than below this speed. However it has been found that at very high speeds 150,000 and above, the time required for the removal of the same weight of tooth structure is nearly same at still higher speed.
There is also a minimum rotation speed for a given load below which the tool will not cut. The greater the load lower the minimum rotational speed. The correlation between load and minimum rotational speed depends upon whether enamel or dentin is being cut, the design and composition of bur and similar factors.