BURR FORMATION IN CONVENTIONAL MILLING

Introduction Classification Mechanism of formation Effects Of Cutting Conditions On Burr Formation

Jyoti Ranjan NayakBhubneswar,Orissa,India

INTRODUCTION 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.

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.

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

DEBURRING

The removal of burrs or deburring is often done by additional machining with abrasive or finishing tools. While manual deburring is a workable solution, it has several limitations. It is tedious, time consuming and yields unpredictable and

inconsistent results. Special precautions must be taken to ensure the safety of workers in a manual deburring cell. A manual deburring operation can also be a bottleneck in a production line. Deburring is necessary especially in the manufacturing of precision parts such as computer components or aircraft engine parts. For example,

In the computer storage manufacturing industry, an attached burr may inhibit assembly of the file, or a detached burr may later restrict file function or cause disk crash.

In the case of engine parts, producing a turbine blade with a specific contour is necessary to reduce the local stress intensity in the component and therefore to minimize the possibility of fracture

Deburring and edge finishing on precision components may constitute as much as 30% of the cost of the part and also can be the source of significant dimensional errors .Progress in manufacturing has led to the need for improved deburring technology. Increased understanding of burr formation can yield tremendous befits in reducing the production costs In evaluating the requirements for deburring, the properties and characteristics of burrs produced by manufacturing process must be understood. The shape of burrs is highly dependent upon the particular manufacturing operation used. A burr can be a sharp ragged projection or it can be a small swell of raised material. In orthogonal cutting either burr or edge breakout (negative burr) can be formed where the tool exits the workpiece, depending on exit geometry and material condition

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

BURR FORMATION MECHANISMSBurr 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 is shown in

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.

EFFECTS OF CUTTING CONDITIONS ON BURR FORMATION

The influence of in-plane exit angle on burr formation of Al 1100 is shown in Figure. Under the chosen cutting condition (depth of cut = 0.76 mm, feed rate = 0.03 mm/ tooth), only curl-type burrs, wave-type burrs and knife-type burrs were found according to different in-plane exit angles. The burr heights of a wave-type burr and a knife-type burr are essentially about the same as the depth of cut. The variation of burr height with respect to different in-plane exit angles and depths of cut is shown in Figs. 11 and 12, for Al 2024-T4 and Al 6061-T6, respectively. The types of burrs created are also shown in these figures. Feed rate for these tests was 0.03 mm/tooth. For a given in-plane exit angle, it can be seen that burr height increases proportionally with depth of cut and suddenly decreases at a critical depth of cut, dcr. This sudden change in burr height denotes the formation of the secondary burr.

It is also observed that dcr of Al 2024-T4 in Fig. 11 is less than that of Al 6061-T6 in Fig. 12 for a given in-plane exit angle. For example, dcr of Al 2024-T4 for an in-plane exit angle of 1201 is 1.1 mm, while dcr of Al 6061-T6 is 1.4 mm. From Table 2, the fracture strains of Al 2024-T4 and Al 6061-T6 are 0.13 and 0.5, respectively. Since the fracture strain is the maximum strain the material can sustain during plastic deformation without fracture, and can be used as an indication of ductility, it is believed that Al 6061-T6 is more ductile than Al 2024-T4. Therefore, Al 2024-T4 is more susceptible to the breaking away of the primary burr to form the secondary burr than Al 6061-T6. But this transition of burr formation has not occurred on edges with a 1501 in-plane exit angle in Al 6061-T6. For the same depth of cut, burr height increased with in-plane exit angle, the same as observed

in Fig. 10 for Al 1100. In Figs. 11 and 12, it can also be seen that dcr becomes larger with the increase of in-plane exit angle, which denotes that a smallin-plane exit angle is more susceptible to the formation of the secondary burr. In Fig. 11 for example, dcr of Al 2024- T4 for an in-plane exit angle of 120 is 1.1 mm, while dcr for an in-plane exit angle of 150 is 1.8 mm. The influence of feed rate on burr formation is also investigated, as can be seen in Fig. 13. In-plane exit angle was constant at 120 for these tests. The depths of cut were 1.02mm for Al 6061-T6 and 0.64mm for Al 2024-T4, respectively. Primary burrs of wave-type and knife-type were created at small feed rates (less than 0.1 mm/tooth). For a given depth of cut and in-plane exit angle, it can be seen that burr height remains almost the same with increasing feed rate until the secondary burr forms at the feed rate of 0.16 mm/tooth. Edge breakout is created for both Al 2024-T4 and Al 6061-T6 when the feed rate reaches 0.46 mm/tooth

BURR FORMATION IN MICRO-MILLING

Introduction Micro Cutting And Conventional Cutting Classification And Mechanism of formation Effects Of Cutting Conditions On Burr

Formation INTRODUCTION

Micro-machining is the most fundamental technology for production of miniaturized parts and components. Micro-machining is a material removal process by means of mechanical force. Many products have been reduced in dimension and weight so as to increase their handiness and to reduce their cost, such as in the semiconductor and biomedical fields Micro-machining must be precision machining in order to manufacture miniature components within very close tolerances. It hinges on the progress of the machine that is able to carry out micro-machining operations and on the development of micro- tools. The very limited availability of micro-tools is always a major concern in micro-machining. Unreliable tool life and early tool failure are the most

important problems when employing micro-tools for micro-machining applications. In micro-machining, vibration and chip flow characteristics are almost unnoticeable without the use of special equipment. Micro-tools can break before the cutting edge of the tool gets damaged

Most machining operations do not often leave behind smooth or well-defined edges on the part. Instead, parts will most likely end up exhibiting ragged, protruding, sometimes hardened, material along edges, known as burrs. Kim reported several problems affecting form and function of parts in the manufacturing processes due to burrs. Therefore, burrs must generally be removed in subsequent deburring processes to allow the part to meet specified tolerances

MICRO CUTTING AND CONVENTIONAL CUTTING The micro-machining mechanism in this study is quite different from that of traditional metal cutting operation. To achieve recommended cutting speeds for conventional machining in micro machining the rotational spindle speed required is far above the limit of commercially available spindles and that micro tools used for cutting stainless steel are easily fractured at high cutting speeds. Hence, a lower range of cutting speed was used which is very low compared to those employed in traditional metal cuttings. In this situation the cutting edges of micro-tools fail to remove materials by the formation of cutting chips, indicating that grinding and plowing processes prevail. Material is removed with the advancement of micro-tool by the mechanisms of both plastic deformation and fracture.

In the micro-machining process, the burr is usually very difficult to remove and, more importantly, burr removal can seriously damage the work piece. Conventional deburring operations cannot be easily applied to micro-burrs due to the small size of parts. In addition, deburring may introduce dimensional errors and residual stresses in the component. These problems are highly dependent on burr size and type. Hence, the best solution is to prevent burr formation in the first place

In

conventional processes, top or entrance type burrs are substantially smaller than exit type burrs, and usually no deburring process is necessary. However, micro-top or entrance type burrs are comparatively large because the radius of the cutting edge is large compared to the feed per tooth

CLASSIFICATION AND MECHANISM OF FORMATION OF BURR

According to Gwo-Lianq Chern and Ying-Jeng Engin

Basically there are four types of burrs produced in micro- milling

Primary burr; Needle-like burr; Feathery burr; Minor burr

Fig. 10. Photos of burrs (100×) in micro-machining: (a) primary burr; (b) needle-like burr; (c) feathery burr; (d) minor burr. N is in rpm, S in mm/min and d in µm. The formation process of primary burr can be illustrated in Figure When the micro-tool fails to produce a chip after the engagement, rubbing takes place instead. Material ahead of the tool path is pushed and deformed plastically, Figure(a). Then it is fractured near the middle, Figure(b). The primary burr is turned over, Figure(c) and is formed with a

width of about half of the tool diameter, Figure(d). If fracture occurs in the primary burr during the micro-machining process, feathery burr or needle-like burr is formed.

Minor burr is created when both the axial engagement and the feed rate are very small. d is only 1 µm and Sz is as low as 0.025 µm for the minor burr formation in Figure

It is noted that the primary burr, the feathery burr and the needle-like burr are all produced on the side of micro-slot where up milling occurs. The cutting force increases after initial tool engagement. Those burrs produced on that side remain attached on the slot edge. On the opposite side where down milling exists, the burrs tend to be removed with the chip formation and thus few burrs can be found.

When the axial engagement is large (5 µm), fracture is hard to occur due to the large burr thickness produced. Several points can be summarized in our micro-machining experiments. When a small tool width is employed, i.e. when rake angle is a small negative value, burr size decreases noticeably. The formation of minor burr is dominated by the depth of cut, with some influence of increase in feed per tooth. Burr formation is retarded when depth of cut is 1 µm where minor burr is produced.

EFFECTS OF CUTTING CONDITIONS ON BURR FORMATION

The size and type of burr are a function of machining variables to better understand micro-burr formation mechanisms. The machining variables are stated below.

Cutting speed: The first cutting parameter is cutting speed. As stated earlier micro tools are easily fractured at high cutting speeds. Hence, a lower range of cutting speed was used is considered.

Feed: The second parameter is feed, which plays an important role in determining chip thickness and the resulting cutting force. However, there is no available reference to determine feed in micro-cutting. The smallest tool diameter referred to in typical machining handbooks is in the sub-millimeter range.

It is known that, in general, increased feed increases the thrust force. A correlation between feed and thrust force with varying tool diameters can be approximated by applying the Ernst-Merchant’s shear plane model to the cutting process. Figure shows the shear plane model applied to a section of the cutting edge of a tool. Shear force can be calculated as follows

where, k is the shear strength of material and d is the tool diameter. With Merchant’s equation, we can calculate Ft, the thrust force exerted on the cutting edge:

Since stress directly influences burr formation and tool wear, an effective stress is considered, and can be represented as follows:

Here, the same tool geometry and material are being considered, so it can be assumed that the effective stress is determined only by ft/d. As the tool diameter decreases to the micro-scale, to prevent tool breakage due to high stress, feed should also be decreased linearly. With this concept, an extrapolated feed for a micro tool can be calculated However, this can only be a starting range for the experiments. Therefore, an optimal feed for burr formation and tool life should be determined.

Feed / radius of cutting edge: The third parameter is ft/R, or feed divided by the radius of a cutting edge, which affects rake angle, chip thickness and, consequently, specific energy. This parameter shows how much the cutting edge radius plays a role in the cutting process with respect to tool diameter. For

example, for a 19 mm tool diameter, the cutting edge radius is about 14µm. If the recommended feed of 0.13 mm is used, ft/R is about 9, and the cutting edge radius effect is insignificant. For a micro tool, the radius of the cutting edge cannot be decreased to the same extent as a decrease in diameter. This is because there is a limit to how sharp the tool can be to avoid fractures of the cutting edge. For instance, for a 254 µm tool diameter and cutting edge radius of 2.2 µm, if a 2.2 µm feed is used, the ratio is about 1. For this case, the rake angle becomes negative and consequently the chip thickness increases. To investigate this effect, three different values of ft/R were used, as shown in Figure above. Table shows the corresponding cutting conditions

Figure (a) shows results of burr height versus feed. Feed has a strong effect on burr height and burr height is linearly proportional to feed. Figure (b) shows burr height as cutting speed increases. At high feeds per tooth, 2.2 and 3.2 µm, burr height increases as cutting speed increases.

However, the opposite result was obtained at low

feeds. This result, which can be related to tool wear, will be explained later in the report. In general, it is difficult to measure tool wear, and even more so in micro cutting, due to the small size of the tools. It was observed that burr size is related to the amount of tool wear. Figure 9 shows burr height versus the number of holes machined for a typical test. A big jump in burr height at point A can be seen due to fatal tool wear. A SEM of a worn tool after point A can be seen in Figure below. As a tool becomes worn, ft /R decreases because of an increase in cutting edge radius. As ft /R decreases, the rake angle becomes more negative and chip thickness increases. Consequently, burr size increases. Therefore, the tool should be changed before the limit amount of cut, point A, in order to avoid large burr formation, and also to prevent severe tool deformation. Herein, tool life is defined as the number of holes created until rapid increase of burr height occurs. To investigate the effect of cutting parameters on tool life, 3 iterations for each of the 9 conditions have been tested. This resulted in 3000 holes created, and the burr size of each was measured.

Figure (a) below shows tool life versus feed. As feed increases tool life decreases due to an increase of cutting force. But tool life also decreases at feed = 1.3 µm, which is smaller than the radius of the cutting edge. This result can be explained by the increase of specific energy required to form a chip, as the feed is decreased below the cutting edge radius [Backer 1952]. At the lower feed, defined as ft/R<1, the rake angle becomes negative so that the sliding and the plowing processes dominate instead

of the cutting process. Figure (b) aboveshows tool life versus

cutting speed. Except for the lowest feed per tooth, tool life decreases as cutting speed increases. At the lowest feed, tool life increases as cutting speed increases. This can be explained by the built-up edge observed in several SEM images of tools at this particular condition. If the part of the built-up edge remains on the tool, the tool can continue to cut for a long time without wear. Since metal flow around the tool edge tends to become more uniform and laminar as cutting speed is increased, the built-up edge persists when using WC-Co tools and the rate of wear decreases as cutting speed increases [Trent and Wright, 2000]. This uniform metal flow can explain why burr height decreased as cutting speed increased at this particular feed

METHODS OF CONTROL, PREVENTION AND

REMOVAL OF BURRS

Five level integration required for burr minimization

Contour Chart Of Burr Formation

CONTROL AND PREVENTION

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

Figure Five level integration required for burr minimization

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.

Design for Burr Minimization

Burr 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

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

exit burr side burr

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-planningCutting 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 controling exit burr formation is the in-plane exit angle . Adjusting the width of cut can effective ly 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 formtaion 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 Planning

Tool 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 workpiece: workpiece geometry, tool geometry and tool path. At this stage, workpiece 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 workpiece 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 ft2 +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 ft2 -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,