Project on minimisation of Burr

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.

2Minimization of Burr

Effects 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

C. V. Raman college of Engineering


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

Item Hole range Approximate annualDeburring cost

Valves,housing,piston 1/16 to 1/18 $1,40,000

Combination valve 1/16 to 1/8 $30,000

Special valve various $8000Complex bracket 1/16 to 3/8 $8000

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 Measurement

Burr Measurement is one of the key elements of experiment and one of main challenges in burr research. There are many instruments. Profilometers are one of simplest and cheapest measuring instruments.

Meter M Horizontal Floor Profilometers :

Profilometers, otherwise known as profile projectors are highly sophisticated and versatile designed as per international standards. It is ideal for the rapid inspection and measurement (linear and angular) of small to medium size components such as watch parts,



gears, tools, rubber components, miniature electronic assemblies, and so on. Its best quality high resolution optics provide accurate, bright, clear and sharp image. The special front and back surface coated mirror are highly polished and lobbied distortion and reproduction. Three element condenser system and high intensity halogen lamps provide brilliant images even in daylight condition.

Technical specification

Projection Screen: Fully usable fine grain ground screens and rotatable screen has cross hair line and detachable clips to hold comparator charts.

Three Magnifications: 10 x lenses for general purpose, 20x lens for closer observation and 50x lens for higher precision works are used. All projection lens screw mounts type to project the work piece image on the screen at direct magnification, brightness adjustable for each magnification by condenser on turret. Magnification accuracy ±0.1% and surface ±0.15%.

Work Stage: Medium duty 200x200mm with standard micrometer heads 0-25 least count 0.1 mm, life long smooth movement assured by the incorporation of hard steel rolling both in longitudinal and transverse slide for accurate measurements.

Contour Illuminator: Equipped with helical type condenser built in cooling far for quick to match the respective objective so as to ensure bright and sharp image on the screen provided with 24v-150/250 halogen lamp with solid stage light control transformer.

Surface Illuminator: Specially designed surface illuminator system for shadow less light on optical axis for the inspection of deep cavities. Provided with 12v-100w halogen lamp with solid state light control transformer.



BURR IN DRILLINGDrilling is one of the most widely used machining processes making up of approximately 25% of all machining processes performed in production of goods. Drilling through a hole consists of three phases In the first phase the drill enters into the work piece. In the second phase a steady state torque and thrust is attained as the drill bit advances to the other end of the work piece. In the third phase the drill point breaks through the underside of the work piece and burrs may be produced.



Classification

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(type I and type II ) Crown burr

Uniform burr

Uniform burrs are burrs whose height varies in between 1.1mm.Further it is classified into type I and type II .The height of type I burr varies in between 0.150 mm and that of type II is between 0.150 to1.1mm.The height of the burr is more uniform in this type of burr. The drill cap can be seen in both

Uniform burr type I Uniform burr type II



Crown burrCrown 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.

According to location of burr, there are three types of burr

• Entrance burr• Interlayer burr• Exit burr



Entrance burrThe 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 burrThe 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 (only in case of multilayered material)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

The exit burr is usually larger than entry burr and is of prime importance owing to difficulty in removing it from work piece



Mechanism

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 and Crown Burr• Exit burr, Entrance Burr and Interlayer Burr

Uniform Burr and Crown Burr



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



Exit burr, Entrance Burr and Interlayer Burr

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.

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.

Literature Review



Burr Formation Variables

A relatively large amount of research has been conducted to understand the phenomenon of burr formation. Burr Height and thickness depends on the factors stated below in the diagram. Depending on these variables a cutting process may yield burr of different height and thickness. Optimizing all these variables will lead to burr with minimum dimensions

Methodology



Objectives: To study the variation heights of burr in drilling of Aluminum and Mild Steel work-pieces with different feed, spindle speed for and three different diameters

Experimental Parameters: From the literature review it is evident that burr height is depends on many variables shown in the figure. For this project the effect of feed, spindle speed, drill diameter and work material will be used as input parameters.

Materials: Aluminum alloy (bright aluminum) blocks of size 60mm x 30mm

x 16mm. Mild steel blocks of size 60mm x 30mm x 16mm.

Machine tool and Tools:



R. K. Engineering Radial Drilling machine with 2HP. The drilling machine allows three discrete variations in the spindle speed and three discrete variations in feed.

High speed steel (HSS) drill bits of diameter 6mm, 8mm and 10 mm.

Instruments: Meter M Horizontal Floor Profilometers with 10 x lenses, 20x lens, 50x lens, Magnification accuracy ±0.1% and surface ±0.15%, with least count of 0.01 mm

Experimental Procedure



• To become familiar with burr formation in drilling• To determine the spindle speed and feed rate at different

settings of the drilling machine.• To drill holes in both Aluminum and Mild Steel work pieces with

all combinations of feed, spindle speed and diameter.• To determine the heights of burr using Profilometer.• To record the observations and plot graphically.

To determine the spindle speed and feed rate at different settings of the drilling machine.

The drilling machine allows three discrete variations in the spindle speed and three corresponding discrete variations in feed. Values of spindle speed are measured with help of Tachometer. Feed values are measured by recording the distance traversed using Graduations on the quill and recording the time by a stop watch and finally the feed is calculated.

The observations have been tabulated below.

RPM RPM value Feed Feed value(mm/sec)a 0.84



A 285 b 0.805c 1.59

B 554a 1.02b 1.53

c 2.87

C1022

a 2.81

b 2.87

c 4.87

To drill holes in both Aluminum and Mild Steel work pieces with all combinations of feed, spindle speed and diameter

Feed, spindle speed, drill diameter and work material will be used as input parameters. First we kept material constant i.e. aluminum.



Then we fix drill bit of 6 mm in the drilling machine. Now as we know that we can have three discreet values of spindle speed and corresponding three values of feed.

So three holes are drilled in each block with same spindle speed and three feed rates. This process was repeated for other two spindle speed. Then the drill bit was changed to 8 mm and we drilled 9 holes in three work pieces as described above by varying different spindle speed and feed rates. This process was repeated for 10 mm drill bit. Now the whole process was repeated taking mild steel as work material. While drilling the holes the corresponding spindle speed and feed rate were marked on the work piece.

To determine the heights of burr using Profilometer



After drilling the holes in work pieces they are now ready to be measured in profilometer. The work pieces were kept on the work stage and then surface illuminator was switched on. This creates an enlarged two dimensional shadow of work piece on the projection screen. From the shadow we can clearly make out the shadow of metal surface and the burr. Now using the rotatable screen the cross hair line was aligned to the metal surface shadow. Now the initial reading of the micrometer was noted down. The work stage was now moved by rotating the micrometer so that the cross hair line moved towards the shadow of the burr. The micrometer was rotated till the cross hair line touched the highest tip of the burr. At this position the micrometer reading was noted down. Now the final reading was subtracted from the initial reading and the burr height was obtained.This process was repeated for all holes and the burr height were calculated and noted down with all their corresponding spindle speed, feed rate and drill diameter.

Burr Observed in Drilling Process



The Results obtained from Profilometer are tabulated below are graphs are plotted thereafter. The tables shows the values of burr height at different RPM and feed.

Drill Diameter 8mm, Material Aluminum

RPM

FeedA(285) B(554) C(1022)

a 0.45 0.84 0.89

b 0.43 1.63 0.36

c 4 1.88 0.42

Drill Diameter 8mm, Material Mild steel

RPM

FeedA(285) B(554) C(1022)

a 0.51 0.62 0.46

b 0.84 1.23 0.53

c 1.01 0.64 1.05




RPM

FeedA B C

a 0.6 0.19 0.52

b 0.57 0.54 0.81

c 0.93 0.56 3.10


RPM

FeedA B C

a 1.08 0.45 3.00

b 0.79 3.65 2.99

c 4.12 4.06 3.06




RPM

FeedA B C

a 0.47 0.86 1.22

b 1.20 1.16 1.03

c 1.05 1.26 0.43


RPM

FeedA B C

a 0.82 0.31 0.99

b 1.25 0.46 0.93

c 1.09 0.52 0.44

Graphical Representation

The results are now plotted graphically for easier interpretation and understanding



Burr Height Vs Feed:

The first set of graphs plotted are burr Burr Height against Feed for different values of RPM for both Aluminum and Mild Steel for all three diameters.

Aluminum, 6mm diameter

Aluminum, 8mm diameter.



Aluminum, 10mm diameter

Mild Steel, 6mm diameter



Effect of Drill Diameter on Burr.



Aluminum

Mild Steel

Effect of Work Material on Burr Height



These graphs show the burr height against cutting conditions i.e. RPM and feed for aluminum and Mild steel. This helps us in comparing how burr height is influenced by work material and its properties.

6mm diameter

8mm diameter



10 mm diameter

Conclusions



As we know that machining does not yield unique results. So no matter how many parameters we keep constant the result of a particular machining operation yields different results at different attempts. But we can know how the result varies with change of different parameters.

After recording the burr height with different parameter we plotted graphs to know how the burr height varies with different parameters.

Feed vs. burr height: The first three graphs are of aluminum material. We observed that at low rpm as soon as we increase feed rate the burr height increases. We further observed that at medium rpm as soon as we increase feed burr height tends to a constant value. At highest rpm there is no such pattern. The same pattern can be observed in mild steel also with a few exceptions.

Effect of Drill Diameter on Burr: In aluminum we observe that average burr height is maximum with 8mm drill diameter whereas in mild steel 6mm drill gives the largest burr.

Effect of Work Material on Burr Height: There are three graphs one for each drill diameter. We observed that the two graphs of 8 mm and 10 mm drill diameter the burr height of aluminum is less compare to mild steel. Which is true as far as theory is concerned but the graph of 6 mm there is an exception where burr height is more in mild steel than aluminum.

BURR IN MILLING



In face milling operations, the largest burrs are formed when radial tool engagement is beyond a critical value. The height of these burrs approach, and may even exceed, the depth of cut. Tool path planning can be used to minimize burr formation by machining the work piece at low radial engagement conditions. In more practical terms, tool paths are drawn within the limits of a feasible offset region as determined by production tolerances and empirical burr size data from a given material. This approach, however, becomes too restrictive when the geometry of the work piece reaches certain level of complexity

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



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.



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 formationTypes of burr are

Backward or entrance burr Sideward burr Forward or exit burr Leaned burr

Mechanisms

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, xy, 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 curl-up. However, the burr root 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



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.

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.

Literature Review

Burr Formation Variables

A relatively large amount of research has been conducted to understand the phenomenon of burr formation. Burr Height and thickness depends on the many factors such as cutter geometry and its sharpness, work material, machine vibration and rigidity, machining condition i.e. feed, depth of cut, in plane exit angle. Cutting fluid, its flow rate and method of application also have effect on the size of the burr. Depending on these variables a cutting process may yield burr of different height and thickness. Optimizing all these variables will lead to burr with minimum dimensions



Methodology

Objectives: To study the variation heights of burr in Face milling of Aluminum work-pieces with three different in-plane exit angle combined with four values of axial depth of cut.

Experimental Parameters: From the literature review it is evident that burr height is depends on many variables shown in the figure. For this project the effect of in-plane exit angle and depth of cut and work material will be used as input parameters.

Materials:



Aluminum alloy (bright aluminum) blocks of size 50mm x 50mm x 25mm.

Machine tool and Tools:

Dynamill Vertical Milling machine with 3HP. The milling machine is fitted with a face milling cutter.

High speed steel (HSS) milling cutter with 2mm diameter.



Instruments: Meter M Horizontal Floor Profilometers with 10 x lenses, 20x lens, 50x lens, Magnification accuracy ±0.1% and surface ±0.15%, with least count of 0.01 mm

Experimental Procedure

• To become familiar with burr formation in milling• To determine the Radial depth of cut for each in plane exit

angle.• To do single pass face milling in both Aluminum work pieces

with all combinations of axial depth of cut, and in plane exit angle.

• To determine the heights of burr using Profilometer.• To record the observations and plot graphically.

To determine the Radial depth of cut for each in plane exit angle.

From literature review we know that burr height depends a lot on in plane exit angle but during machining has to done using the radial



depth of cut. So we need determine the corresponding radial depth of cut for all the in plane exit angle values using geometry.

If angle XAY is 154.1 o then XAB is 180 o - 154.1 o = 25.9 o

And angle OAB should be 90 o – 25.9 o = 64.1o

OB = OA. sin (OAB) = OA*sin (64.1 o)

Now OA = 10 mm, so OB = 10 * 0.899OB = 8.99 ≈ 9mm

Radial depth of cut = BC = OC + OBBC = 10 + 9= 19mm

Radial depth of cut= 19 mm

Similarly we can calculate Radial depths of cut for other in plane exit angles are calculated and the observations have been tabulated below.

In plane Exit Angle Radial depth of cut44.4 o 3mm90 o 10mm



154.1 o 19mm

So there would twelve cutting conditions as we keep the speed and feed constant for all the cutting conditions.

To do single pass face milling in both Aluminum and Mild Steel work pieces with all combinations of axial depth of cut, and in plane exit angle.

The values of radial depths of cut are 3mm, 10mm and 19mm whereas values chosen for axial depths of cut are 0.5mm, 0.75mm, 1.00mm and 1.25mm. So that gives us a combination of tweleve different milling conditions. We do two of them in each as shown in the figure. So we will six blocks two cuts on each side.



To determine the heights of burr using Profilometer

After milling operation the work pieces they are now ready to be measured in profilometer. The work pieces were kept on the work stage and then surface illuminator was



switched on. This creates an enlarged two dimensional shadow of work piece on the projection screen. From the shadow we can clearly make out the shadow of metal surface and the burr. Now using the rotatable screen the cross hair line was aligned to the metal surface shadow. Now the initial reading of the micrometer was noted down. The work stage was now moved by rotating the micrometer so that the cross hair line moved towards the shadow of the burr. The micrometer was rotated till the cross hair line touched the highest tip of the burr. At this position the micrometer reading was noted down. Now the final reading was subtracted from the initial reading and the burr height was obtained.

This process was repeated for all workpieces the burr height were calculated and noted down.

METHODS OF CONTROL OF BURRS

To effectively address burr prevention, the entire „process chain “from design to manufacturing must be considered, Figure 2. Here we see the importance of integrating all the elements affecting burrs, from the part design, including material selection, to the machining process

Since milling (specially face milling) figures so prominently in the manufacture of so many parts, for example, automotive engines and transmission components, it has been a major focus for burr reduction and prevention for many years. In milling, the kinematics of tool exits from the work piece is a dominant factor in burr formation and, as a result, substantial success has been realized by adjusting the tool path over the work piece.



Figure: Five level integration required for burr minimization

Design for Burr MinimizationBurr formation is significantly affected by how a product is designed . This section introduces several rules that allow designers to enhance edge quality at the early design stage. If the part material and its surface treatment do not change, then the part geometry and its edge features are the main factors determining burr formation. Four design rules are presented and can be embedded into CAD systems, as shown in Figure

1. Avoid through holes

Drilling exit burr often occurs along the exit side of a hole. On the contrary, entrance burrs are usually small and considered burr-free. Therefore, it is beneficial to avoid design of through holes. As shown



schematically in Figure, through holes should be replaced with blind holes for reducing exit burr when this design change does not affect the original product function

Exit burr

2. Avoid 90º counter sinks

Figure shows that simple counter sinks with angles greater than 90º creates an exit angle of 135º or greater . In this case, no burrs will form at the bottom intersection for certain materials. Hence it is favorable not to adopt 90º counter sinks in a product design if edge quality is an important concern

Exit burr

3. Select appropriate step/slot depths



A curly side burr will form along the side edges of a slot/step when the axial depth of cut is smaller than a critical value. Increasing the depth of cut is likely to produce less detrimental side burrs that loosely attach to the work part. This type of burr is easy to remove in the deburring operation. Therefore, a slot/step depth should be appropriately selected so that the curly side burr will not produce

Exit burr

Side burr

4. Add appropriate chamfers

Since the tool path cannot be adjusted, it is difficult to prevent exit burr formation in a slot/step milling operation. However, adding chamfers along the part edges can, to a large extent, reduce exit burr size. At the design stage, a simple look-up table can effectively help designers select both the chamfer size and angle in order to meet specified edge quality. Notice that chamfering requires a secondary finishing operation. An interesting issue thus arises: a chamfering operation prior to a face milling operation may reduce the total manufacturing cost compared to the reverse operation sequence, which is prevalently used in industry

Chamfer



Macro-planning

Machining sequences determine what types of burr will form. Each exit burr, side burr, or top burr requires different deburring work as well as the associated deburring cost. Therefore machining operation sequencing can be a feasible way to reduce deburring cost. The edge-precision macro-planner contains a set of machining feature decomposition and re-mapping rules that result in less detrimental machining burrs.

(1) Milling prior to drilling because drilling burrs are easy to form compared to milling burrs



(2) Shallow slot/step prior to deep slot/step for trade-offs between top burr and exit burr formation along the edge A

1. 2.

(3) depth first in the slot/step milling to avoid the formation of curly side along the edge B

(4) width first in the pocket milling for trade-offs between top burr and exit burr formation along the island edges

3. 4.


A


B

Micro-planning

Cutting parameters including feed per tooth, the radial depth of cut and the axial depth of cut are determined at the micro-planning stage. A series of milling experiments have been conducted for Al 6061 to select appropriate parameters.

Similar to the look-up table used in the conventional micro-planner, it is also beneficial to set up a series of machining tables for reducing burr formation. However, since mostly geometric factors dominate burr formation behavior, a set of simple rules is established from the



experimental data, instead of complete burr formation databases. These rules provide useful “handles” for burr control in the micro-planning:

(1) Exit burr in the cutting direction: one important factor controlling exit burr formation is the in-plane exit angle. Adjusting the width of cut can effectively control the burr height.

(2) Exit burr in the feed direction: in the slotting operation using end milling cutters, typically exit burr height along the edge is described as a bell-shape curve, i.e. larger burr height in the middle and smaller burr height close to the ends [10]. In addition, burr height is approximately proportional to the depth of cut. Feed per tooth also affects exit burr formation but not significantly.

(3) Side burr: the depth of cut is the most important factor determining the occurrence of curly side burr. Our experimental data shows that a depth of cut smaller than 0.25R favors the formation of curly side burr.

(4) Edge breakout: the width of cut (equivalently, the exit angle) dominates the occurrence of edge breakout. Given tool geometry, this edge defect only takes place within the exit angle range from 120o to 135o, not depending on other cutting parameters.



The strategies used in the micro-planer for enhancing edge quality are summarized in Table 1.

Tool Path PlanningTool engagement, to a large extent, determines machining burr formation. Therefore, burr minimization can be achieved by controlling tool engagement conditions. Three main factors affecting how a tool cutting edge leaves the work piece: work piece geometry, tool geometry and tool path. At this stage, work piece design and tool geometry are usually fixed, so only tool path can be used for reducing burr formation. A set of five tool path planning algorithms are developed:



(1) Avoiding tool exits for 2D polygonal contours by adjusting the radial depth of cut,

(2) Avoiding tool exits for 2D contours with circular edges by adjusting the radial depth of cut,

(3) Avoiding tool exits for 2D polygonal contours by adjusting tool position,

(4) Avoiding tool exits for 2D contours with circular edges by adjusting tool position, and

(5) Avoiding tool exits for a 2D free-form contour by adjusting tool position.

To avoid tool exits is the crucial point for reducing exit burr formation, because tool exit is the necessary condition for exit burr to occur. When the tool always enters the workpiece, only a secondary burr can be produced, which is generally considered to be burr-free.

Examples of application of burr minimization strategies

Tool path planning in milling:

One of the most successful areas of application of burr minimization strategies is in tool path planning for face milling. To a great extent, burr formation in milling can be prevented by adjusting the path of the milling cutter over the work piece face. Specific cases have been evaluated in automotive engine manufacturing with major



automobile companies. This can be extended to optimization of the process to insure that surface quality, including flatness, specifications are met or exceeded. Figure 10 shows a conventional tool path for face milling a surface on a cast AlSi alloy automotive engine block. The presence of substantial burrs at critical locations required frequent tool changes as well as additional deburring operations.

The optimized tool path using the criteria described above is shown in Figure 11 and, in Figure 12, shows the resulting burr free work piece. Although the tool path is substantially longer in this example, it was possible to increase the feed rate without loss of surface finish to maintain the required 5 second cycle time for the process. The tool life (as a result of dramatically reduced burr formation) was increased by a factor of 3 and the resulting savings per machine/year were estimated at approximately $50,000



Contour Chart Of Burr Formation

An empirical model described by least squares and a contour chart describing the results are proposed for use to minimize burr formation and improve tool life. An empirical model of burr formation obtained by lease squares method is shown below. Here, y is burr height [µm].

Figure 12. Contour chart of burr formation.

y = 7.5 - 3.5 Vc + 5.3 ft + 0.2Vc2 + 0.8 ft

2 +1.0Vc ft

Where, Vc is cutting speed and ft is feed per tooth. Figure 12 shows the contour chart based on Equation (4).



The following equation is an empirical model for tool life:

y = - 387+ 44Vc + 443 ft- Vc2 - 89 ft

2 -18Vc ft (5)

where, y is the number of holes created before the failure. Figure 13 shows a contour chart based on Eqation 5 . With these two charts, burr formation and tool life can be controlled and optimized. For example, Figure 14 shows the combined contour chart of equations 4 and 5. Using this chart, a confirmation test was conducted to compare burr formation and tool life at two cutting conditions, A and B.

Table 2 shows burr height, tool life and material removal rate



Figure Contour chart of burr formation and tool life



References

BURR FORMATION IN DRILLING

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”

BURR FORMATION IN MILLING

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,


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