Seminar Report On Burr Formation

CONTENT • Introduction • Definition of burr • Effect of burrs and deburring • Burr formation in drilling Type of burr Burr formation mechanism

• Burr formation in grinding • Burr formation in milling Burr classification Burr formation mechanism

• Problem associated with burr •

Conclusion

• References

INTRODUCTION In engineering, a burr refers to the raised edge on a metal part. It may be present in the form of a fine wire on the edge of a freshly sharpened tool or as a raised portion on a surface, after being struck a blow from an equally hard or heavy object. More specifically, burrs are generally unwanted material remaining after a machining operation such as grinding, drilling, milling, or turning. These are undesirable projections of materials beyond the edge of the work-piece arising because of plastic deformation during machining Undesirable burrs are created in most machining processes. A burr is a plastically deformed material that remained on the work piece after machining. It is often in the form of a rough strip of metal at the edge of the work piece adjacent to the machined surface. The burrs produced on piece part edges in machining operations must be removed for most parts to function effectively.

DEFINITION OF BURR Burr is a thin ridge usually triangular in shape, developed along the edge of a workpiece during machining, shearing of sheet metals, forming and casting.

EFFECT OF BURRS AND DEBURRING The existence of burrs on a work piece may cause several problems, such as: • Decreasing the fit and ease of assembly of parts; • Damaging the dimensional accuracy and surface finish; • Increasing the cost and time of production due to deburring; • Jeopardizing the safety of workers and consumers; • Contributing to electrical short circuits; • Reducing cutting performance and tool life; and • Degrading the aesthetics of the components. • Automation is not possible

Burrs are injurious even during machining because they hit the cutting edge and cause groove wear. This groove wear, in turn, accelerates the burr growth. The past years have seen emphasis on increasing the quality of machined work pieces while at the same time reducing the cost per piece. Accompanying this is the decreasing size and increasing complexity of work pieces. Burr formation in machining accounts for a significant portion of machining costs for manufacturers throughout the world. As one could imagine, the cost and time needed to perform these deburring operations (process of removal of burr) is significant. Recently, because of miniaturization and increased precision of the machined parts, the size of burrs has been also reduced and deburring became even more difficult Burrs formed during machining are the cause of many industrial problems. Burrs can cause many problems during inspection, assembly and automated manufacturing of precision components. They usually reduce the quality of machined parts and can cause interference, jamming and misalignment of parts. Because of their sharpness, they can be a safety hazard to personnel. Burrs may reduce the fatigue life of components and can damage them. Burrs in machined work pieces are real “productivity killers”. The deburring processes are included in manufacturing, which increase the production cost and require a significant amount of time. The selection of an appropriate deburring method depends on the dimensions, type of work piece, and the location of the burr. Thus burr sizes must be controlled for the optimal choice of a deburring process. The costs associated with removing these burrs are substantial. The typical costs as a percentage of manufacturing cost varies up to 30% for high precision components such as aircraft engines, etc. In automotive components, the total amount of deburring cost for a part of medium complexity is approximately 14% of manufacturing expenses. The actual investment in deburring systems increases with part complexity and precision. To decrease deburring cost it is necessary to select mostly proper deburring way and to reduce burr size. These goals can be reached if burr formation mechanism is known, which make it possible to predict burr dimensions and to minimize their appearance by optimum choice of cutting conditions, tool and work piece geometry. A typical example drilling deburring examples from a particular firm is given in the table below

Small burr size has two immediate benefits, first, it eliminates the additional cost of deburring the component and the likelihood of damage during the deburring process and, second, in the

case burrs cannot be eliminated it improves the effectiveness of any deburring strategy due to reduced and more standard burr size and shape.

BURR FORMATION IN DRILLING: Type of Burr: Different types of burr can be classified according to their height and location of burr According to height of the burr it is classified into two classes · Uniform burr · Crown burr

Uniform burr: Uniform burrs are burrs whose height varies in between 1.1mm.Further it is classified into type 1 and type 2 .The height of type 1 burr varies in between 0.150 mm and that of type 2 is between 0.150 to1.1mm.The height of the burr is more uniform in this type of burr.

Crown burr: Crown burrs are burrs whose height varies in between 1.1 to 1.5 mm. The height of the burr is not uniform in this type of burr.

Crown burr

According to location of burr, there are three types of burr · Entrance burr · Interlayer burr · Exit burr

Entrance burr: The entrance burr is produced on the side of the work piece where the drill enters; this burr is usually considerably smaller than the exit burr and is usually of little concern as it may be removed easily by chamfering the hole.

Exit burr The exit burr is formed on the opposite side of the work piece as the drill breaks through. These burrs are usually more substantial than the entrance burr, and as they are on the opposite side of the work piece to the machine, they are more challenging to remove. They can also be located within a cavity in the work piece where there is no access to the exit side of the hole

Interlayer burr When drilling through a number of layers, interlayer burrs are often formed between the layers. In certain circumstances these burrs must be removed. In order to do this, it may be necessary to dismantle the parts to remove the burrs

Two burrs are generally produced in the drilling of every hole; entrance and exit burr. The exit burr is usually larger than entry burr and is of prime importance owing to difficulty in removing it from work piece

Mechanism of burr formation: As we see earlier, burrs can be classified according to their height and location. Now we will go through the mechanism of formation of burr for each type.

Uniform Burr: As the drill approaches the exit surface, the material under the chisel edge begins to deform. The distance from the exit surface to the point where the deformation starts depends on the thrust force of the drill. As the drill advances, the plastic deformation zone expands from the center to the edge of the drill. At the final steps, the remaining material is bent and pushed out to form a uniform burr with a drill cap. Here the fracture takes place at the chisel edge. Type 1 and type 2 uniform burr formation has same mechanism. Under small feed force condition this type of burr is formed.

Crown Burr: A larger thrust force induces plastic deformation earlier in the process, making the thicker material layer ahead of the drill undergo plastic deformation, inducing a larger maximum stress on the exit surface. As a result, initial rupture will occur at the center. The remaining material is then bent and pushed out without being cut to form a relatively large burr

Stage in burr formation

Exit burr: As it can be said that uniform burr and crown burr are two types of exit burr so the mechanism of formation of exit burr is same as that of uniform burr or crown burr depending on feed condition.

Entrance Burr: At the beginning of drilling process, material bumped up on the perimeter of the drill engaging region because of the indenting action of the drill bit. As more part of cutting edge engages in machining, hole diameter becomes larger and before all the cutting edge is engaged in the machining, small amount of bumps of material is always observed on the perimeter of the hole (t = 1.5 ~ 1.875 sec in). When almost all cutting edges are engaged, the shape of bump on the perimeter starts to change from a smooth, round shape to a biased, shaper shape (t = 2.125). Finally, as all drill bit engages, i.e. drill completely penetrates through the first layer; entrance burr remains on the surface. (t = 2.25 ~ 2.625 sec). The shape of remaining burr is uniform around the hole and slightly bent outward of the hole.

Interlayer Burr: The formation of inter-layer burr is shown in Figure. They depend on the combination of the upper and the lower materials, the thickness of the sealant between layers and the process parameters. Here two plates are being drilled is shown in the figure at different instant. Until around 0.7 second, two plates bend elastically and no gap between the two layers is observed. Axial displacement of the observing nodes increases almost linearly up to 0.34 mm. No material failure occurs in this stage. Plastic deformation is limited near the area around the drill tip engaged to the drilling. At around 0.7 second, the displacement of the observing nodes falls down to 0.13 mm and a small gap starts to form and increases as the drill advances. Material starts to fail around the drill tip area and element elimination starts to occur at this stage. Drill bit starts to engage the second layer at around t = 2.6 second. Interlayer gap starts to increase significantly as the first layer starts to spring back and the second layer is pushed toward feed direction by the drill bit. Inter-layer gap reaches the maximum at around t = 4.5second and starts to decrease as the second layer starts to spring back as the plastic deformation of outer surface increases. From around t = 5.5 second, the displacement of the node on the entrance surface of

the second layer (node 2) starts to go below that of the node on the exit surface of the first layer (node 1).This means that the second layer touches previously formed inter-layer burr.

Typical inter-layer burrs are shown in Figure. In general, as the drill moves downwards, a large exit burr forms at the exit surface of the upper material and a small entrance burr forms at the entrance surface of the lower material. When the sealant is thick enough, as in Figure (a), (b), and (c), the exit burr of the upper material is fully developed. If the upper material is ductile or the process conditions are in a specific range which results in a large uniform burr , the exit burr reaches to the top of the entrance burr, Figure (b), and sometimes it is deformed by hitting the entrance burr, Figure (c). Depending on the profile of the entrance burr and material properties such as hardness, the exit burr changes its growing path inwards, Figure (d), or in the worst case, outwards, Figure (c). When the sealant is thin, Figure (d), (e), and (f), the interference between the exit burr and the entrance burr occurs before the exit burr is fully developed.

Interlayer burrs in drilling a multilayer material

BURR FORMATION IN GRINDING: Under common operating conditions grinding burrs are comparatively small and can often be found only at the microscopic level. Depending on the field of application even these microscopic burrs can affect the functionality of a workpiece. Furthermore, the increased application of high performance grinding processes, which cause an increase in burr size with a resulting effect on the functionality of the workpiece, lead to the investigation of burr formation mechanisms in grinding. Resulting burrs are located at the edges of the workpiece and can be classified as entrance burr, side burr and exit burr (Figure la). These burrs vary size and shape and are created through different formation mechanisms.

Figure 1: Burrs at workpiece edges

Burr formation in grinding that is small burrs that generated in conventional grinding and the spiral burrs, generated under high performance grinding conditions. Similar to the burr formation in processes with defined cutting edges, the exit edge burr formation process in grinding can be subdivided into the steps continuous grinding, pre-initiation, burr initiation, burr development and final burr formation. Continuous Grinding

During the grinding process a quasi-stationary process status is established as long as there are no anticipating outer influences like chattering or strong tool wear. Normal and tangential forces act from the grinding wheel onto the workpiece. Multiple superpositioned microscopic single grit cutting processes lead to a material removal with microscopic chips. High cutting velocities cause a strong outer and inner friction on and in the workpiece. The continuous heat flux through friction ensures a persisting flow-behavior in the contact zone. The restricted thermal conductivity of the workpiece produces a thin area of strongly increased temperature with a high temperature gradient in the contact zone between grinding wheel and workpiece -the plastic zone. Along the plastic contact zone these forces lead to a compression in the normal direction and to an elongation in the tangential direction. In a larger distance from the contact zone a small elastic deformation zone with an increased temperature surrounds the plastic zone in the workpiece. Beyond the plastic layer, the resisting power of the workpiece material against the acting forces is very high. Therefore, during the continuous grinding phase no macroscopic deformation occurs outside the contact zone.

Pre-initiation

When the grinding wheel approaches the exit edge, at first the elastic deformation zone reaches the exit edge. Affected by the geometry of the workpiece edge, the elastic zone either intersects the workpiece edge - small exit edge angle - or appears at the exit edge as elastic bending – big exit edge angle. The plastic deformation zone is also considered to be extended toward the workpiece edge. Burr Initiation

The plastic deformation zone occurs at the workpiece edge. Continuous heat flux and reduced conduction because of the reduced surrounding material volume lead to a local temperature increase. Burr Development

The increasing temperature leads to a reduced stiffness of the remaining workpiece material at the exit edge. Therefore, the normal and tangential forces increase the compression in the normal and the elongation in the tangential direction of the remaining material - the plastic zone increases. More and more material will remain deformed at the edge instead of getting separated. This leads to a bending of the edge and to the formation of the burr under conventional grinding conditions. If the depth of cut, the cutting velocity and therefore the temperature is increasing further, as for high efficiency grinding processes, plastic flow of the remaining material will increase. The material to be removed is getting pushed in the direction of the burr and drives the already existing burr further out of the workpiece. The higher the cutting velocity and therefore the friction force and the temperatures, the faster the material is driven out of the workpiece. At the same time the burr thickness decreases. Through this mechanism the spiral burrs can also be explained. An area of increased thickness at the top of the burr, which could be detected on most metallographic sections of spiral burrs, can be ascribed to the hesitant start of the plastic flow process. At the end of the grinding process, the burr root also gets ground.

Final Burr Formation

The shape and size of the final burr is influenced by various parameters and predetermined through the cutting conditions.

BURR FORMATION IN MILLING: Burr classification: Classification based on mechanism of formation Gillespie and Blotter classified the machining burrs into four specific types: · Poisson burr: The Poisson burr is a result of a material’s tendency to bulge at the sides when it is compressed until permanent plastic deformation occurs · Roll-over burr: The rollover burr is essentially a chip that is bent rather than sheared, resulting in a comparatively larger burr. This type of burr is also known as an exit burr because it is usually formed at the end of a cut in face milling. · Tear burr: A tear burr is the result of material tearing loose from the work piece rather than shearing. · Cut-off burr: The cut-off burr is a projection of material left when the work piece falls from the stock before the separating cut has been completed

Schematic of tear and roll-over burr formation.

Classification according to Kishimoto et al · Primary burr · Secondary burr

Classification according to Gwo-Lianq Chern Gwo-Lianq Chern classified burr according shape of burr and the type of burr formed is highly dependent on the in plane exit angle; five types of burrs are: · Knife-type burr · Wave-type burr · Curl-type burr · Edge breakout and · Secondary burr, as shown schematically in figure .The first three types of burrs can be classified into the primary burrs as Kishimoto et al defined.

Five type of burr in face milling.

Classification according to Nakayama and Arai Nakayama and Arai approached the classification of machining burrs by: (1) the cutting edge which is directly associated with the burr formation; and (2) the mode and direction of the burr formation. Types of burr are · Backward or entrance burr · Sideward burr · Forward or exit burr · Leaned burr

Burr formation mechanism: Burr Formation Mechanisms according to Gwo-Lianq Chern:

Type of burr formed is highly dependent on the in plane exit angle, ψ. In-plane exit angle, ψ, is defined as the angle in the machining plane between the cutting velocity vector, V, and the edge of the work piece to be machined, x y, as shown in figure below.

This definition is consistent with the definition of the in-plane exit angle in orthogonal cutting and is different from the exit angle given by Kishimoto et al (explained later).

Knife-Type Burr: The knife-type burr is created by the pushing out of the uncut part, AB, near the transition machined surface when ψ reaches 150, as shown in Figure below The burr height (h) is about the same as the depth of cut, d. The burr is formed by the plastic bending moment, Mf, which is caused by the feed force Ff, exerted on AB as the cutter advances.

Photo of the knife-type burr

Wave type Burr: For ψ approximately 90, wave-type burrs are created. These types of burrs are produced from the machined surface periodically. Figure illustrates the formation of the wave-type burrs A roll-over burr is formed along the edge CD after tool exit from the transition-machined surface. This roll-over burr is removed by the next tool passage except for the material very close to the corner D. This can be understood by the ‘‘minimum undeformed chip thickness’’ in that there is a minimum undeformed chip thickness below which a chip will not be formed. When this occurs rubbing takes place instead. Applying this idea to the secondary cutting edge on the clearance surface of a milling tool in this case (where the in- plane exit angle approximates 90), it is found that a small triangular portion of the material that should be removed will be left behind, as can be seen in figure is the corner angle of the tool. The small triangular portion left behind has been called a ‘‘Spanzipfel’’. The Spanzipfel, S, will be plastically deformed as it comes in contact with the clearance surface of the tool. Thus, as the tool moves by an increment of the feed, the Spanzipfel near the corner D in Figure is bent by an angle of 90-ζ, and the material in S on AD is compressed to BD. Eventually, as the cutter advances, the roll-over burr on the edge CD is bent and compressed to the edge DE. The length of CD is L1 and the length of DE is L2, where L2 approximates L1sin z. This indicates that the roll-over burr along CD is ‘‘squeezed’’ to the shorter length DE, which contributes to the formation of the wave-type burr as schematically illustrated in figure above. Thickness of the wave-type burr also increases under volume-constancy relationship during this plastic deformation. The interval of the wave-type burrs, p in Figure above is found to be of the order of several millimeters and to increase with feed rate. Close-up photo (×90) of the wave-type burr is shown in Figure. The formation of a wave-type burr will increase the difficulty of deburring due to its complexity of geometric shape and larger thickness, and thus should be avoided. Actually, an in-plane exit angle about 90 is very unfavorable from the view point of tool life. The cutter enters the work piece and takes a chip that gets progressively thicker as the cutter tooth rotates. Since the center of the cutter lies along the work piece edge in this case, as shown in Figure the tool exits the work piece where the undeformed chip thickness reaches a maximum. The cutting force increases with the undeformed chip thickness which is the feed per revolution in single-tooth face milling. Since the variation of cutting force is the most in this situation, tool life will be reduced accordingly

Curl-Type Burrs: For in-plane exit angles less than 45, curl-type burrs are found after machining, as shown in figure below .A roll-over burr along the edge KJ is created after the tool exits from the transition machined surface.

This roll-over burr curls up as a result of the next tool passage due to rubbing. The burr along KJ is then bent and compressed to the edge KL. The burr height is reduced because of the curlup. However, the burr roots thickness, hr, is larger than those of knife-type burrs. Photo of the curl-type burr is shown in Figure below. In plane exit angles less than 90 are not suggested in real applications since the cutter center is outside the work piece and the material removal rate is low in this case.

Secondary Burr: The secondary burr, as shown in Fig. 4(e), is formed when fracture causing separation of the primary burr occurs near the root of the burr. It takes place when the plastic strain at the root of the primary burrs becomes too high for the material to sustain. The burr height is thus reduced notably. The existence of secondary burrs is sometime hard to recognize by naked eyes. But some small protrusions can still be felt by the fingers when rubbing the machined edge. Photo of the secondary burr is shown in Figure

Secondary burr

Edge Breakout Burr: The edge breakout, as shown in Figure is found when the metal removal rate becomes very high. A rough chamfer and sharp burrs are then created along the tool exit edge. The interval of the sharp burrs on the breakout edge, p, is the same as the feed rate. Photo of the edge breakout is shown below. Edge breakout occurred when very high feed rate was chosen. Such high feed rate is rarely seen in aluminum alloys under normal cutting conditions because of the ease of machining this relatively ductile material. Surface finish deteriorates with increasing feed rate, and thus feed rate seldom reaches a level high enough to cause edge breakout

Burr Formation Mechanisms according to Gillespie and Blotter · Poisson burr: Poisson burr is formed as a result of lateral bulging of material along the work edge when it is compressed under a passing cutting tool. · Roll-over burr: Roll-over burr is essentially a chip that remains attached to the work and pushed ahead of the cutting tool's path on exit from the work rather than being broken in formation. · Tear burr: Tear burr is usually formed in a punching operation as the result of material deforming basically due to the tool/die clearance and, like the roll- over burr, adhering to the work piece edge when the tool exits the part. · Cut-off burr: Cut-off burr the cut-off burr is a projection of material left when the work piece falls from the stock before the separating cut has been completed A combination of the Poisson and tear burr can end up as a so-called top burr or entrance burr [Lee 2001], along the top edge of a machined slot, or along the periphery of a hole when a tool enters it.

Schematic of tear and roll-over burr formation.

Burr Formation Mechanisms according to Kishimoto et al · Primary burr · Secondary burr Primary burr is the roll-over burr produced on the tool exit edge. The burr thickness was found to vary from minimum to maximum burr thickness along the length of the burr. They claimed that through proper selection of parameters- depth of cut, the exit angle, and the corner angle (inclination angle) of the tool, the roll-over burr produced during the face milling process will be separated at its thinnest portion and only a small burr will remain on the edge of the machined part. They named the former normal roll-over burr a ‘‘primary burr’’ and the latter one a ‘‘secondary burr’’ which is the material remaining after the breakage of the primary burr.

Exit angle and corner angle

PROBLEM ASSOCIATED WITH BURRS: •

Noisy, unsafe, operation in assembled machine parts.

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Produce friction and wear in parts moving relative to each other.

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Short circuit in electrical component.

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May reduce fatigue life in components.

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There may be an edge crack during heat treatment.

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Increase leakage in hydro – pneumatic systems.

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Safety hazardous to personal as the burrs is usually sharp.

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Burrs in drilling occupy more than 40% of total machining time

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Reduce production efficiency, increased cost.

CONCLUSION The primary problems associated with the machining such as burr formation was discussed in this report. Different type of burr, produce in conventional cutting and their mechanism also discussed here. Burr formation in drilling is a common problem in conventional machining and also micro-machining. Burr formation in grinding that is small burrs that generated in conventional grinding and the spiral burrs, generated under high performance grinding conditions. The existence of burrs on a work piece may cause several problems such as Decreasing the fit and ease of assembly of parts, Damaging the dimensional accuracy and surface finish, Increasing the cost and time of production due to deburring, Jeopardizing the safety of workers and consumers, Contributing to electrical short circuits, Reducing cutting performance and tool life, and Degrading the aesthetics of the components.

REFERENCES Sung-lim ko1 , Jae-eun chang1, “Development of drill geometry for Burr minimization in drilling.” P. Stringer, G. Byrne and E. Ahearne, “Tool Design for Burr Removal In Drilling Operations.” Jinsoo Kim, “Development of a drilling burr control chart for stainless steel.” Jihong Choi, Sangkee Min, David A. Dornfeld, “Finite Element Modeling of Burr Formation in Drilling of a Multi-Layered Material” J.C. Aurich (2), H. Sudermann, H. Bil, “Characterisation of Burr Formation in Grinding and Prospects for Modelling” David Dornfeld, “Strategies for Preventing and Minimizing Burr Formation” K. Nakayama, M. Arai, “Burr formation in metal cutting, Annals of the CIRP 36 (1) (1987) 33– 36”. D.A. Dornfeld, E. Erickson, “Robotic deburring with real time acoustic emission feedback control, in: Proceedings of the Symposium on Mechanics of Deburring and Surface Finishing Processes.” L.K. Gillespie, The academic challenge of burr technology, Allied Bendix Aerospace, KC division, REFERENCE WEBSITES http://www.pdfcoke.com http://www.wikipedia.com http://www.google.com http://www.sciencedirect.com