Burr Formation in Conventional Drilling

Introduction Effects of Burr Types of Burrs Mechanism of burr formation Minimization of Burr Methods Of Deburring

By:Soumya mahapatra

(soumya_00457@yahoo.co.in)

Jyoti Ranjan Nayak(jyotiranjan_541@yahoo.com)

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.

EFFECTS OF BURR

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. Drilling burrs, for example, are common when drilling almost any material. As one could imagine, the cost and time needed to perform these drilling and 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.

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.

TYPES OF BURRSDifferent types of burr can be classified according to their

height and location of burrAccording to height of the burr it is classified into two classes

Uniform burr Crown burr

Uniform burrUniform 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.

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 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 burrWhen 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 produced in the drilling of every hole;

entrance and exit burr

MECHANISM OF BURR FORMATIONAs 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

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 thesecond 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.

MINIMIZATION OF BURR

As we see that the presence of burr in metals has many ill effects so different methods are used to minimize the burr. Some of them are being discussed.

2-Axis Drilling Burr Control Charts

Drilling Burr Control Charts (DBCC) can be used to predict what type of burr will be formed given certain drilling parameters, this can be used to minimize burr or to alter the type of burr produced. The first parameter of the chart is related to the feed rate and drill diameter. The thrust force directly affects the amount of the plastic deformation of the material at the final stage of the drilling process, and as a result, influences burr formation. It is known that increase in the feed rate in drilling tends to increase the thrust force. Correlation between feed rate and thrust force with varying drill diameters can be approximated by applying the shear plane model to the drilling process

Figure shows the shear plane model applied to a fraction of the cutting edge of a drill. Together with the Merchant equation,

φ = π / 4 + (α-l) / 2, and the expression for the rake angle varying with relative radius, R. we can calculate the thrust force that is exerted on

a fraction of the cutting edge. Finally total thrust force can be expressed as:Where, k is the shear strength of material, f is feed rate (mm/rev), h is the helix angle, 2κ is the point angle of the drill and t is the ratio of web thickness to the drill diameter. Since it is stress that directly influences the burr formation, an effective stress is considered and can be represented as the following.

Since the same drill geometry and material are being considered, it is believed that the effective stress is determined only by f /d. As a result, f /d influence the final burr formation and can be used as a parameter for the chart.

Another important parameter is cutting speed, defined as the product of drill diameter and spindle speed, N. Depending on the cutting speed, the amount of heat generated at the cutting edge changes greatly, influencing some properties of the work piece material. It also affects the rate of tool wear, especially for the corner wear which is believed to have a large influence in drilling burr formation

Finally, the parameters for the chart are:

A DBCC is formed using experimental results; they can be used to relate feed rate (f), drill diameter (d), tool rake angle (α) and spindle speed (N) to the type of burr produced. An example of a DBCC is shown in Fig, this is DBCC is for Stainless Steel AISI304L.

Fn is a non dimensionalised feed parameter (=f/d), S is a cutting parameter (=α*d*N).

Drilling Burr Control Chart for stainless steel (AISI304L) [8] Note: Type I are Uniform burrs, Type II are transient burrs, Type III are crown burrs

3-Axis Drilling Burr Control Charts

It is possible to add a further axis to the DBCC. This axis allows the graphs to be applied to different work piece materials. In order to add the third axis it is necessary to develop a parameter (G) which will relate the tendency of the material to produce a burr to some of its material properties. As the G value increases the tendency to form large burrs moves towards lower feed and speed rates. An example of a 3-Axis DBCC as developed by Reich-Weise et al is shown in Fig. 3. Fn = f /d, S = (N) (d) 105, G = K−1 (σt−σy) (A+Z) Where σt = Material tensile strength, σy = Material yield strength, A = Percent area reduction at fracture, Z = Percent elongation at fracture, K = A constant used to create a dimensionless number.

Pre-drilling and pre-chamfering

A study carried out into the effect of pre-drilling and chamfering the predrilled hole was carried out. This study found that by pre-drilling it was possible to reduce the size of burr formed, however it has been shown that chamfering the predrilled hole can eliminate bothentrance and exit burrs. In order to eliminate the burr the hole must be chamfered to the final diameter, as illustrated in fig. In this figure the hole was predrilled (dp) to 2.75mm, the final hole diameter (D) was 6.1mm.

Burr minimization through drill tip design

Gillespie carried out research into the effects of drill geometry on burr, he found that increasing helix angle will reduce burr height by up to 50%, thickness by up to 20% and radius by 6%.Sofronas presented method to round the drill cutting edge, increase helix angle and harden the exit surface. But surface hardening makes a problem since this smaller burr causes more difficulty to be removed by exit surface hardening. It was also shown that burr height increases with drill diameter, however the most important characteristic of a drill bit in terms of burr minimization is its sharpness. It has been shown that by changing the tip of the drill it is possible

to minimize the burr which is formed. Lee has also used method of cutting condition changing, and controlled the thrust force by changing the feed rate during drilling.

During cutting process of drill, according to point angle, material's property and cutting condition, plastic deformation region is formed by the stress caused by cutting resistance in the slant section remained when drill exits hole as shown in fig.1, and then burr is formed simultaneously with cutting. Therefore, as the volume for bending deformation in drilling is larger, larger exit burr is formed at the same time

Fig.1 shows cutting process in step drilling. Unlike conventional drill, after the front cutting edge drills completely, the second cutting process starts at the step cutting edge. During second cutting process, cutting can be prolonged until plastic bending deformation occurs at the remained part. The position at which bending deformation starts is determined by the rigidity of the remained part shown in Fig.1. That’s why, y1, R1 in Fig display critical point where ‘cutting’ is discontinued and ‘bending’ starts and the ‘remains’ is changed to burr. This critical position is decided by the shape of the remains. When the stiffness of the part which is bent by thrust force imposed in drill cutting edge is large enough, no bending occurs and cutting is continued. As a result, burr formation is delayed and small burr is formed.

To find out effects by various geometrical factors, 5 kinds of drills are designed and manufactured. Usual High Speed Steel (HSS) drill and carbide drill are used, which are commercialized. Chamfer drill is designed to have chamfer at the corner of edge, which is specified as chamfer length, L, and chamfer angle, θ2, as in Fig.2. Round drill with radius, R, at corner and step drill with step angle, θ2, and step diameter, D2.

Observation of burr formation in chamfered drill

Chamfer drills with 60° and 40° chamfer angle at the corner of cutting edge are designed for burr formation. In Fig (a), the burrs formed in drilling by conventional High Speed Steel drill and carbide drill are measured and compared with the burr by chamfer drill. The burr height from conventional drills is larger than that from chamfered drill. The chamfered drill with 60° chamfer angle produces larger burr than in drill with 40° chamfer angle. Fig.4 shows the drilling process at the moment after cutting by main cutting edge with 140° point angle. The chamfered edge starts cutting and burr is formed by bending deformation. Considering the same normal stress on the chamfered edge, it can be predicted intuitively that the stiffness of the remained part which is represented as hatched area in Fig.4 is larger in the case with 40° chamfer angle than in the case with 60° chamfer angle. The remained part will be cut if this part keeps enough stiffness not to be bended into burrs. This is the key concept for the drills for burr minimization

Observation of burr formation in round drill

In round drill, the drills with corner radii R1.5 and R2.5 at drill cutting edge have been used. It is observed from Fig.(b) that larger burrs are formed in drill with R2.5 corner radius which is larger that R1.5. The uncut volume in R1.5 drill is much smaller than in R2.5 drill as the hatched area in Fig.5 represents the stiffness roughly, which means that the stiffness of the remained part in R1.5 drill is much larger than in R2.5 drill when drill exits. More bending deformation in R2.5 drill produces larger burr. Therefore it can be insisted that the larger cutting resistance in drill with larger corner radius at the moment of drill exit will induce larger burr.

Observation of burr formation in step drill

In step drill, the drills with various step angle and step diameter have been used. 5 drills with different step angles, 130°, 100°, 75°, 60° and 40°, are manufactured with same step diameter, 9mm. In Fig.(c), the burr height in drills with 130° and 100° step angle is almost same as that in conventional drill as shown in Fig.(a).When the step angle reduced to °, 60° and 40°, the burr height reduced to less than 0.1mm. The mechanism of burr formation in step drill can be explained by Fig.6. The remained part just before step edge exits hole keeps very thin shape,

which keeps very small stiffness to bending deformation. Then it can be easily predicted that the thin part will be bent to large burr. During this process, step edge does not machine the part;

instead it pushes the remained part into burr. From Fig.6, the cap which produced in first cutting by front cutting edge with 140° point angle and attached to the burr by bending deformation is a clear evidence of this mechanism. The cap formed at the first cutting is pushed out along with burr at the second cutting by step drill with 100° and 130° step angle because stiffness is not enough to support thrust force in uncut volume. So, the burst type burr is largely formed according to the size of difference between drill diameter and step diameter. Burr can be reduced when step cutting edge has small step angle less than 75°, in which case the remained part keeps larger stiffness and small burr will be produced as a result of less deformation. In Fig. the effect of step diameter on burr formation can be observed. By reducing the step size from 1mm to 0.5mm, the burr height decreases drastically and more stable size according to the change of feed rate

For all step drills with 0.5 mm step size and step angle less than 75°, the burr height is less than 0.1mm. When the step size reduced with smaller step angle, the uncut volume which is remained when drill exits becomes very small as shown in Fig.6, which guarantees small and stable burr formation. However too small step size will induce burnishing effect rather than cutting operation in step edges. Therefore it will

be necessary to determine optimal step size for minimization of burr formation

METHODS OF DEBURRINGUltrasonic vibration in drilling

Fig shows the experimental device. For the experiment, the work piece and the ultrasonic horn placed in water tank and water tank with abrasive. The water tank was made of rectangular pieces of acryl. Therefore, the distance between the work piece and the ultrasonic horn as well as the distance between the bottom of the water tank and the work piece could be adjusted. Table 1 shows the specifications of the ultrasonic transducer amplifier and the actuator. The resonance frequency, the maximum amplitude of the horn, and the maximum output power of the amplifier were 20 kHz, 70㎛ , 750W, respectively. The resonance frequency could be automatically adjusted depending on the load. The horn was inserted into the water by 10mm.

The strength of the ultrasonic vibration differs depending on the efficiency of actuator and the output power of the amplifier. The vertical vibration of the tip was transferred through water or slurry between the horn and the work piece. The transferred energy creates a cavitations and explosive power in the water that cause deburring. Therefore, it is necessary to calculate the strength of the frequency transferred through water.

The distribution of the frequency strength in three dimensional medium after the ultrasonic frequency is transduced is given by the equation.

The distribution of the ultrasonic vibration pressure depends on the properties of the medium, initial speed, density, and frequency. However, the pressure is reduced greatly with the distance. As a result, the pressure at the horn tip is about 20Pa. The term cavitation threshold is used to describe the minimum conditions necessary to initiate cavitations.

Experimental results are shown in Fig below. Fig shows the shapes of burrs before and after the experiment. Burr has been completely removed as shown in Fig. The amount of deburring is not uniform on circumstance of the hole. This is because the initial burr shape and size are different. Therefore, appropriate selection of deburring conditions is necessary. In addition, surface was damaged during deburring around the hole in the part. This is because that ultrasonic is strong enough to deburring.

Figure: Shapes Of Burrs Before And After The Experiment

Before The Experiment

After The Experiment

Integrated drilling and deburring tools

A drill bit has been developed by Kubota et al. [21] which is capable of deburring. This tool operates by utilizing two cutting edges on the reverse of the tip to remove the burr once the hole has been drilled

A number of tool companies have developed integrated drilling and deburring tools which are capable of first drilling a hole and then removing any burr which is formed. These tools work for holes with a diameter greater than 9.5 mm . An example of one of these tools can be seen in Fig below.

Burroff tools: One-Pass Hole-Deburring tool which can remove both Entrance and exit burr

Simple and Rugged One-Piece Construction Allows cutting chips to clear easily

How it works

1. Integral cutting edges remove the burr from the front of the hole as the tool enters the hole.

2 .The slotted design allows the tool to 'collapse’ under load as the tool feeds through the work piece. The crowned and polished top surface of the cutting edges will not mar the inside surface of the hole.

3. The back of the hole is deburred on the return stroke

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 chartFor stainless steel”.

H. Z. Choi, s. W. Lee, y. J. Choi, s. L. Ko “Study on deburring technology using Ultrasonic Cavitations”

Jihong Choi ,Sangkee Min, David A. Dornfeld “Finite Element Modeling of Burr Formation in Drilling of a Multi-Layered Material”

Choi, Sangkee Min ,David Dornfeld ,Mahboob Alam, T. Tzong “ Modeling of Inter-Layer Gap Formation in Drilling of a Multi-Layered MaterialJihong ”