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Abstract

Interlayer burr formation in drilling of stacked aerospace materials is a common problem in aircraft assembly operations. Burrs formed at the interface of the stacked sheets need to be removed, and the deburring is a nonvalue but time and costs waste operation, particularly in automatic drilling and riveting assembly. This article presents an analytical model of the interlayer gap formation to predict the interlayer burr height, and drilling experiments were developed to understand the difference between the interlayer burr height and the interlayer gap. The impact of cutting force, spindle rational speed and feed rate was taken into consideration. Specific conclusions regarding the influence of the interlayer gap on burr formation were presented.

Keywords

Burr formation stacked aerospace materials drilling interlayer gap aircraft assembly

Introduction

Drilling is one of the most significant methods in mechanical manufacturing industry. Burr formation as one of the most common problems appears at both the entrance surface and the exit surface of the drill. Burr formation affects workpiece accuracy and performance in several ways: dimensional variation on part edge, increasing the complexity of the assembly, causing operator injuries and so on.^1,2 In aircraft riveting assembly, burr formation may cause adverse effects on fatigue reliability. Hence, deburring is an essential process in high-precision component manufacture; about 30% of the total manufacturing costs is attributed to deburring, especially in aircraft wing-box and fuselage assembly.³

Aluminum alloy due to its superior strength-to-weight ratio is widely used in aircraft structural component. According to some statistics, there are more than 1.3 million holes in a typical large aircraft.⁴ The typical drilling costs as a percentage of manufacturing cost varies up to 20%, and about 3%–10% of the holes are reworked in order to guarantee the component performance; deburring is essential before riveting, resulting in increase in time and cost.^4,5 In addition, it is difficult to achieve automation for deburring, as the automatic equipment is widely used in the aviation manufacturing corporation, and there has been a steadily increasing emphasis on to enhance the quality of drilled stacked workpieces and at the same time avoid the deburring operation.

Dornfeld and Min³ indicate that one of the future developments of comprehensive integrated strategies for burr minimization and prevention will depend on the continued development of predictive models with competent database for process specification. Many experimental studies have been conducted to reveal the mechanism of burr formation in drilling of metal materials. A lot of researches indicate that material properties, tool geometry and process parameters have significant influence on burr height. In addition, tool wear and temperature field distribution have a certain effect on burr size by changing the plasticity of the material.^6–12 Kuo et al.¹³ and Carvajal et al.¹⁴ have evaluated hole quality when single-shot drilling of metallic composite stacks and found feed rate has a strong influence on hole quality, the burr height in the metal layer is smaller at the lower feed rate level and tool coatings also have a significant effect on burr size.

Theoretical and finite element model (FEM) methods are also used to analyze the mechanism of burr formation. Segonds et al.¹⁵ proposed a simple analytical model to predict the type of burr at drilling exit based on the theory of slip-planes in drilling of ductile materials. An analytical model for drilling burr formation has been developed by Kim and Dornfeld;¹⁶ the model is based on the principle of energy conservation and metal cutting theory. Min et al.¹⁷ studied finite element simulation on the formation of uniform burr and crown burr when drilling different plastic materials; the FEM can evaluate the influence of process parameters on burr formation, and the simulation results are validated by drilling experiments. Kim and Dornfeld¹⁸ have built a mathematical model of drilling exit burr formation, and the theoretical model is in good agreement with the practical results; it is only applicable for good plastic material with low spindle speed, and the influence of spindle speed is not taken into account.

Recently, there has been growing concern about the interlayer burr formation in drilling of stacked metal materials. Melkote et al.¹⁹ have found that tool point angle, workpiece clamping type and clamping distance are the significant factors influencing interlayer burr height while drilling of 2024-T3 and 7075-T6 stacked sheets. Jie²⁰ and Bi and Jie²¹ proposed a multi-objective optimization algorithm for process parameter optimization during drilling Ti6Al4v and 7075-T6 stacked plates and found the spindle speed of 2000 r/min, the feed rate of 0.075 mm/r, the pressure of 0.3 MPa and Ti-Al stacking sequence could achieve the smallest burr height. Choi et al.⁴ and Li et al.²² have identified that the different displacements due to elastic bending deformation of the two sheets cause the interlayer gap. The mechanical properties, drilling thrust and per-load pressure have impact on the size of the interlayer gap. Choi et al.⁴ proposed a FEM of interlayer gap formation when drilling stacked metal materials and found that the interlayer gap has great influence on the interlayer burr; however, there is not enough convincing evidence showing which factor affects the size of the interlayer gap and how to reduce the interlayer gap, and experimental verification is deficient either. Li et al.²² carried out a mathematical model of the interlayer gap based on the theory of plates and shells; drilling thrust and preload pressing force were taken into consideration. The model has, however, been made by several simplifications that may lead to failure when the pressing force is larger than the drilling force, and there is no experiment to validate the model.

Traditional experimental study of burr height prediction model has received limited industrial application. This limited use is due to the large number of parameters in the experimental design resulting in a large number of experiments and high costs. The purpose of this study is to propose an analytical model of the interlayer gap to predict the magnitude of interlayer burr height in drilling of stacked Al-7475 materials. The theoretical, simulation and experimental tools are combined to reduce the time and cost of parameter studies. Based on the theory of plates and shells, an interlayer gap formation model without per-load pressing force is presented to investigate the gap formation mechanism and the relationship between interlayer burr height and gap size. By FEM methods, a simulation model is conducted to predict the interlayer burr height in drilling stacked materials with per-load pressing force. Some drilling experiments were performed, and the interlayer burr height was measured to verify the validity of the interlayer burr height prediction model.

Interlayer burr height prediction model

Interlayer burr formation

Burr formation is a complex mechanical process; even though the final burrs have the similar geometrical morphology, the mechanism of formation process could be totally different.^8,15,16 The burr size depends mainly on the magnitude of the feed and cutting speed, when the feed rate and cutting speed are low, and the exit burr tends to have a uniform shape along the hole periphery.^16,18 For further analysis, the feed rate is the most significant parameter that influences the burr height, followed by the drill diameter, chisel edge, point angle and helix angle, while the influences of cutting speed could be neglected, and the feed rate acts on the burr height through drilling thrust.⁸ According to the mechanical modeling of drilling force, without taking tool wear into consideration, the thrust force clearly increases with the feed, and on contrary, it was not sensitive to the variation of the spindle rotational speed.^23,24 Hence, if the workpiece material and drill geometry are decided, feed rate is the most significant parameter that should be considered. As the drill approaches the exit surface of the workpiece, the material under the drill begins to deform. The distance between the initial fracture and the exit surface depends mainly on the magnitude of the thrust.^16,18 In a relatively small thrust, the plastic deformation zone expends from the center to the edge of the drill at the end of the drilling. And finally, the remaining material is bent and pushed out by the drill, and the initial fracture occurs at the edge of the hole. A uniform burr with a cap is formed along the hole periphery. As the thrust increases, the initial fracture will occur early at the center of the drill; by this time, efficient cutting continues and a uniform burr with no cap is formed. If the thrust grows larger, the plastic deformation occurs earlier at the center of the drill, the maximum strain will be larger and the material undergoing plastic deformation will also be thicker. Therefore, it is more likely that the initial fracture will occur at the center of the exit surface, leading to a crown burr. Since the point angle, clamp type and hole-to-clamp distance are being the most significant factors affecting interfacial burr formation, the influence of tool wear is slight;^1,19 when the clamp type and drill geometry are decided, the feed rate is most significant to interlayer burr height. In drilling of the stacked metal materials, no matter which kind of burr formation case occurs, the bottom plate could provide a supporting force to the top plate (see Figure 1). The supporting force may delay the occurring of the initial fracture, the remaining material at the final step of drilling the top plate could not be bent and pushed out until the supporting force disappear.

Figure 1.

Drilling of stacked materials.

The gap around the hole periphery between the two plates is another important factor influencing an interlayer burr.⁴ The gap is a simple way to evaluate the effectiveness of supporting force mentioned above. When the drill breaks through the top plate, the gap between the two plates begins to grow, and the maximum gap appears when the plastic deformation of the top plate occurs.^4,22 The supporting force decreases rapidly at this moment; meanwhile, the material under the drill is bent and pushed out to form the interlayer burrs. The interlayer burr height $H_{I}$ equals to the sum of the burr height $H_{t}$ at the exit surface of the top layer and $H_{b}$ at the entrance surface of the bottom layer (see Figure 2). $H_{EA}$ and $H_{ET}$ are the burr heights at exit and entrance surfaces of the plate, respectively, when drilling a single plate. The interlayer gap is defined as the burr growth space $H_{s}$ . If $H_{s} < H_{EA} + H_{ET}$ , the bottom plate still has a supporting force to the top plate, which plays an adverse effect on burr formation; it is considered that $H_{I} \propto H_{s}$ . In order to explore the hypothesis that interlayer gap might have a positive correlation with the burr height, holes were drilled in four different feed rates. Figure 3 shows the comparison between theoretical interlayer gap and burr height in different feed rates without per-load pressing force. They are closed to each other and have the same developing trend. Hence, the size of the interlayer gap could be regarded as an approximate prediction of interlayer burr height, and decreasing the interlayer gap is an effective methods to control interlayer burr heights and improve the hole quality.

Figure 2.

Definition of interlayer burr height and gap.

Figure 3.

Difference between theoretical gap and interlayer burr height.

The model mainly includes two parts, and in each part, drilling thrust is considered as a uniformly distributed load on a circular area with hole diameter d (see Figure 4). Based on the theory of plates and shells, the interlayer gap formation model without per-load pressing force is conducted, while the gap formation model with per-load pressing force is calculated using FEM method.

Figure 4.

Mechanical analysis for a simply supported rectangular thin plate.

There are four assumptions in the model:

No initial gap as manufacture error occurs between the two plates;

The material has a liner behavior in terms of bending deflection;

The material is isotropic;

The hole drilled has no effect on the stiffness matrix of the plate.

Gap formation model without per-load pressing force

According to the theory of plates and shells,²⁵ if the ratio of the plate thickness and the minimal size of plate surface is less than 1/5 but greater than 1/80, the plate is called thin plate. If the maximum deflection of plate during bending does not exceed 1/5 of the plate thickness, it could be considered as bending with small deflection. Equation (1) is the differential equation for bending of thin plates²⁵

\frac{E δ^{3}}{12 (1 - μ^{2})} \nabla^{4} ω = q (x, y)

(1)

where E is the elasticity modulus of plate, $μ$ is Poisson’s ratio, $δ$ is the thickness of plate, $q (x, y)$ is the force distribution function, (x, y) is the coordinate in the plate surface, $ω$ is the deflection of plate and $\nabla^{4} ω$ can be given as equation (2)

\nabla^{4} ω = \frac{\partial (\frac{\partial^{2} ω}{\partial x^{2}} + \frac{\partial^{2} ω}{\partial y^{2}})}{\partial x} + \frac{\partial (\frac{\partial^{2} ω}{\partial x^{2}} + \frac{\partial^{2} ω}{\partial y^{2}})}{\partial y}

(2)

According to the theory of plates and shells, a simply supported rectangular thin plate, which is under a uniformly distributed load q on a circular area with diameter 2r, the deflection of the plate at position (x, y) is shown as equation (3)

\begin{array}{l} ω = \sum_{m = 1, 3, 5, \dots}^{\infty} \sum_{n = 1, 3, 5, \dots}^{\infty} \frac{8 q r J_{1} (C_{m n} r) s i n \frac{m π ξ}{a} s i n \frac{n π η}{b}}{π^{3} D C_{m n} a b {(\frac{m^{2}}{a^{2}} + \frac{n^{2}}{b^{2}})}^{2}} \\ \sin \frac{m π x}{a} s i n \frac{n π y}{b} \end{array}

(3)

where $D = E δ^{3} / (12 (1 - μ^{2}))$ , $C_{mn} = π \sqrt{(m^{2} / a^{2}) + (n^{2} / b^{2})}$ and $J_{1} (C_{mn} r)$ is the one-order Bessel function where the independent variable is $C_{mn} r$ .

As shown in Figure 2, the gap between the two layers is the deflection of the bottom plate $ω$ , which could be achieved through equation (3).

Gap formation model with per-load pressing force

According to the analysis results in gap formation model without per-load pressing force, in the case of a constant thrust, the deflection of the top plate will exceed the bottom plate’s with the increase in the per-load pressing force; by this time, the elastic contact between the two layers may occur, and the contact area grows with the increase in the pressing force. It is difficult to estimate the value of the interlayer compression force and contacted area through a numerical calculation method. To simplify the model, a three-dimensional FEM of a quarter of the plates was conducted using ABAQUS 6.12 computer-aided engineering (CAE), and it is shown in Figure 5. The plates were meshed using hexahedra nodes and three-dimensional solid elements (reduced integration C3D8R elements) with a higher mesh density in the top plate than the bottom plate. Fixed constrained conditions were applied around the plates to restrict any movement on the top and bottom plates on the three orthogonal axes in order to represent the simple support condition. The preload pressing force was transferred to the top plate through the interaction between the press foot’s lower surface and the plate’s upper surface. A rigid cylinder instead of the drill applied thrust force through the contact between the drill’s lower surface and the bottom plate’s upper surface. A prescribed load was then applied to each plate through drill and press foot, and the subsequent deflection of the plates was calculated.

Figure 5.

FEM method of stacked plates with preload pressing force.

Experiment procedure

Experiment system

Drilling platform

A LGmazak430Al computer numerical control (CNC) machine center was used in drilling of stacked Al-7475 sheets; it would allow spindle speed in the range of 40–12,000 r/min and feed rate in a wide range of frequency conversion infinitely adjustable speed. The sheets were positioned in the CNC center by special fixtures, and there were four fastened bolts at the edge of the stacked workpieces to avoid the dislocation occurred between the two workpieces during drilling process. In addition, the four fastened bolts at the edge of the workpieces made them regard as a simply supported thin plate, just the same as the boundary condition assumed in the interlayer gap formation model (see Figure 6). The drilling thrust forces were detected by a machining force dynamometer and data acquisition card during drilling process, after drilling the interlayer burr heights were measured.

Figure 6.

Drilling platform.

Thrust force monitor platform

Thrust force is the most significant parameter that induced the interlayer gap, and the values of the thrust participated in the interlayer gap formation calculation model were the actual measured value during drilling. An Interface 1216 machining force dynamometer was mounted to the fixture to monitor and collect the practical drilling thrust (see Figure 7). A linear stage module was used to keep the proper alignment between drill and dynamometer. The achieved waveform could be seen in Figure 8, and it contains the initial drill entry, cutting edge entering the top plate and drill starts, the stable value of thrust when drilling top layer, the stable value of thrust when drilling bottom layer, tool exit through the back side of the plate and drill exit. The thrust at the moment that the drill broke through the top layer and just began to cut the bottom layer induced the interlayer gap. The actual magnitude of drilling thrust was the arithmetic mean value of the collected factors during stable drilling process.

Figure 7.

Diagram of drilling force monitor platform.

Figure 8.

Thrust when drilling Al-7475 stacks. Diameter d = 5 mm, spindle rotational speed n = 2700 r/min, feed rate f = 0.05 mm/r and workpiece thickness t = 3 mm.

Burr measurements

The ISO 13715:2000²⁶ standard defines burr size as a deviation from the ideal geometrical edge. Due to the geometric irregularity of the burr, the random cross section has been used for describing basic parameters, but the measurement is a difficult and time-consuming work. Hence, the most often and easy parameters to be measured are burr height and burr thickness. In this article, the main purpose was to investigate the relationship between the interlayer gap and the burr height; hence, the burr height was the most significant parameter while others were not taken into consideration.

There are many different methods of burr detection and measurement; in this article, the RETC three-dimensional (3D) white light interferometer (see Figure 9(a)) was chosen to observe the burr morphology and measure the burr height. The measuring accuracy of the white light interferometer is ±0.01 mm, and each hole was measured at four different locations spaced at 90° intervals around the hole circumference for entrance and exit surfaces (see Figure 9(b)); the arithmetic mean of the four values was regarded as the value of the burr height for each set of the drilling condition. The minimum interlayer burr height is 22.4 µm in the feed rate of 0.04 mm/r with the pressing force of 330 N, while the maximum interlayer burr height is 204.3 µm in the feed rate of 0.10 mm/r with the pressing force of 82.5 N.

Figure 9.

Burr measurement platform: (a) white light interferometer and (b) burr measurement position.

Workpiece materials and cutting tool

In this work, some Al-7475 sheets of 300 × 100 × 3 mm³ were used as the workpiece material. The mechanical and physical properties are given in Table 1.

Table 1.

Mechanical–physical properties of Al-7475.

Mechanical–physical properties	Material
	Al-7475
Tensile strength (MPa)	572
Elongation (%)	11
Density (g/cm³)	2.81
Elasticity modules (GPa)	72
Thermal conductivity (W/mK)	130
Specific heat capacity (J/kg K)	960
Poisson’s ratio	0.33

Solid carbide twist drills with diamond films coated are widely used in dry drilling of metal materials due to their superior hot hardness. In this test, solid carbide twist drills with diamond films coated were used; the drills have cylindrical shanks, and geometric parameters were 5 mm in diameter, 118° point angle and 30° helix angle (see Figure 10).

Figure 10.

Drill geometry.

Drilling procedure

The drilling experiments consisted of two parts. The first part intended to evaluate the contribution of processing parameters to drilling thrust. Spindle rational speed and feed rate were selected as control factors, and each factor had five levels (see Table 2). A 5 × 5 orthogonal experiment designed using Taguchi method was conducted to investigate the relationship between processing parameters and thrust, and each set of parameters was repeated for three times. The statistical analysis of the data was performed by analysis of variance (ANOVA) to achieve the contribution of each factor.

Table 2.

Factor settings for analyzing drilling thrust.

Factors	Levels	Units	Values
			Level 1	Level 2	Level 3	Level 4	Level 5
Spindle rotational speed	5	r/min	2700	3500	4300	5100	5900
Feed rate	5	mm/r	0.03	0.04	0.05	0.06	0.07

The second part was to investigate the relationship between the interlayer gap and burr height. A series of drilling experiments were necessary. First, drilling of stacked Al-7475 sheets without per-load pressing force was carried out, and burr heights were measured; the interlayer gap was calculated through the interlayer gap formation model. Second, the drilling tests with the per-load pressing force were implemented. The per-load pressing force consists of five levels, and the processing parameters are kept consistent with the previous experiments. The processing parameters are shown in Table 3.

Table 3.

Factor settings for analyzing interlayer burr height.

Factors	Levels	Units	Values
Spindle rotational speed	1	r/min	2000
Feed rate	4	mm/r	0.04	0.06	0.08	0.10
Pressing force	5	N	82.5	165	330	660	990

The per-load pressing force was applied on the plates through per-load pressing force drive devices (see Figure 11). The device consists of two AIRIAC 50 × 50 cylinders, pressure regulating valve and press foot. The press foot is fixed with the piston, and the five levels of pressing force could be achieved by changing the air pressure; the correspondence between air pressures and pressing forces is shown in Table 4.

Figure 11.

(a) Per-load pressing force drive device and (b) press foot.

Table 4.

Correspondence between air pressure and pressing force.

Factors	Units
Air pressure	MPa	0.05	0.1	0.2	0.4	0.6
Pressing force	N	82.5	165	330	660	990

Results and discussion

Taguchi method results

According to burr height prediction model, drilling thrust is the main factor that induces the interlayer gap. A series of tests were completed to assess the influence of drilling parameters on thrust force; the results could be seen in Table 5, and the ANOVA was used to analyze the contribution of each parameter.

Table 5.

Experimental results of Taguchi method.

Experiment no.	Level of factors		Experimental results
	Spindle rotational speed (r/min)	Feed rate (mm/r)	Thrust (N)
1	1	1	71.05
2	1	2	83.6
3	1	3	94.69
4	1	4	98.82
5	1	5	99.87
6	2	1	69.90
7	2	2	72.7
8	2	3	81.43
9	2	4	91.87
10	2	5	97.57
11	3	1	71.7
12	3	2	76.55
13	3	3	78.15
14	3	4	87.89
15	3	5	94.00
16	4	1	68.68
17	4	2	72.00
18	4	3	76.02
19	4	4	83.32
20	4	5	95.84
21	5	1	66.01
22	5	2	67.96
23	5	3	76.03
24	5	4	85.00
25	5	5	94.65

The result of the ANOVA indicated that the contribution of the feed rate to thrust reached 60.33%. In comparison, the influence of the spindle rotational speed could be neglected (see Table 6). Hence, feed rate is the significant parameter to thrust force in drilling of Al-7475 stacked sheets.

Table 6.

ANOVA results of the drilling thrust of Al-7475 sheet.

Source	SS	DF	MS	F	Contribution (%)
Spindle rotational speed (r/min)	412.938	4	103.235	2.956	10.97
Feed rate (mm/r)	2271.81	4	567.953	16.265	60.33
Error	139.67	4	34.918
Total	2824.42	24

SS: sum of squares; DF: degrees of freedom; MS: mean square; F: F-value.

Drilling experiment results

Many researches suggest that the drilling thrust increases with the increase in the feed rate. According to drilling experimental results, the thrusts corresponding to the feed rates are shown in Figure 12. From this figure, we could see that the relationship between feed rate and thrust is an approximate linear relationship, and a higher feed rate induces a greater thrust.

Figure 12.

Correlation between feed rate and drilling thrust.

In order to prove the correlation between interlayer gap and burr height, holes were drilled with continuous position coordinates (see Figure 13), and the contrast between interlayer burr height and gap is shown in Figure 14. Drilling with per-load pressing force is analyzed later in this article.

Figure 13.

Drilling holes with continuous coordinates.

Figure 14.

Difference between theoretically calculated results of the interlayer gap and measured burr height.

The difference between theoretically calculated result of the interlayer gap and measured burr height reflects that the point in the plate close to the location holes has a smaller deflection and leads to a minor size of bur height. The point in the center of the plates has the maximal deflection, and the burr height also reaches the largest size at the point. Hence, we could conclude that interlayer gap has a significant impact on the burr height size. Both of them have the same developing trend, while the burr height size is greater than the magnitude of the interlayer gap.

The two-dimensional (2D) morphology of interlayer burr due to different drilling thrust is compared in Table 7; the hole presented in Table 7 was drilled at the center of the plate. As expected, when feed rate is small, the burrs in the exit surface of the top plate are small uniform burr and the burrs in the entrance surface of the bottom plate are slight. With feed rate increasing, the thickness and height of burrs become larger. At feed rate of 0.10 mm/r, exit interlayer burr $H_{t}$ becomes crown burr, and entrance interlayer burr becomes large uniform burr, which could be attributed to greater thrust force.

Table 7.

2D morphology of interlayer burr.

Feed rate (mm/r)	$H_{t}$	$H_{b}$
0.04
0.06
0.08
0.10

In order to reduce the influence of the interlayer gap to burr height, drilling experiments with five levels of preload pressing force were implemented. The gap size was calculated through the proposed model, and the results are shown in Figure 15. The curves of the interlayer gap and interlayer burr were achieved through the power function fitting method. In a certain feed rate, the interlayer gap decreases with the per-load pressing force increasing. The interlayer gap decreases rapidly, when the per-load pressing force changes from 82.5 to 165 N. The downward trend of the interlayer gap flattens out between 165 and 330 N. Between 330 and 990 N, the effect of preload pressing force on the interlayer gap becomes very weak.

Figure 15.

Interlayer gap in different levels of preload pressing force.

The relationship between the measured burr height and the interlayer gap with per-load pressing force is shown in Figure 16. The burr height decreases with the decrease in the interlayer gap, and the changing tendency of the burr height remains the same with the interlayer gap. It could also be seen that pressing force equals to 330 N and is a critical point. This means that when the pressing force is less than the critical point, the effect of the pressing force is obvious, and burr height decreases rapidly, while the pressing force exceeds the critical point, the effect becomes weak, and the burr height tends to be a constant value (see Figure 17).

Figure 16.

Effect of pressing force on the burr height: (a) feed rate = 0.04, (b) feed rate = 0.06, (c) feed rate = 0.08 and (d) feed rate = 0.10.

Figure 17.

Effect of pressing force on the burr height changing tendency: (a) feed rate = 0.04, (b) feed rate = 0.06, (c) feed rate = 0.08 and (d) feed rate = 0.10.

Figure 18 shows the difference of burr height and interlayer gap with a certain pressing force. The burr height increases with the increase in drilling thrust, which has the same tendency with the interlayer gap value. The critical effect performance is obvious when the pressing force equals to 330 N. When the pressing force reaches 330 N, both the burr height and the interlayer gap tend to be stable and almost remain in a constant value.

Figure 18.

Correlation between interlayer gap and burr height in different levels of pressing force: (a) 82.5 N, (b) 165 N, (c) 330 N, (d) 660 N and (e) 990 N.

Discussion

Stacked Al-7475 materials are finding large number of applications in aircraft manufacture, drilling of such material is unavoidable. The objective of this article was to understand the correlation between interlayer gap and burr height. As expected, we found that interlayer burr height between the two plates is positively associated with the gap size. In addition, this study also extends prior evidence that feed rate is the main factor that induces the drilling thrust. The interlayer gap was calculated through analytical and FEM methods, and the errors between the theoretical prediction model and the experimental result attribute to the following three reasons.

First, the feature of metallic flow in the interlayer gap was not taken into consideration, and the material flows into the gap and at the same time the chisel edge breaks through the top plate. If the interlayer gap is small enough, the exit burr of the top plate may be deformed by hitting the entrance burr and make the exit burr overlap with the entrance burr. When drilling with pressing force, the gap between the two layers is very small, and the overlapping of burrs may occur. The exit burr of the top plate and entrance burr of the bottom plate were measured separately, and the interlayer burrs equal to the sum of them; hence, the interlayer burr measured is larger than the actual burr formed due to the interlayer gap. Besides, depending on the profile of burrs and materials hardness, the exit burr may change its growing path inward or outward, and the material flowing into the interlayer gap is also a part of the interlayer burrs. Second, an assumption is proposed that no initial gap due to manufacture error occurs between the two plates, but in fact, the initial gap is inevitable. Another phenomenon is that when drilling the stacked metal materials, the gap between the two layers begins to occur as soon as the drill touches the top plate, due to different values of the two plates. It could be concluded that the thrust is not the only factor that causes interlayer gap, and the contacting force between the two layers is also the inducement. As a result, when drilling with or without pressing force, the actual interlayer gap is larger than the gap size calculated through the burr height prediction model. Therefore, there is more growing space for interlayer burr to form due to the initial gap. Third, when drilling without per-load pressing force, the holes next to the location points have a smaller burr height than the holes at the middle of the plate. It could attribute to the better rigidity and less initial gap, and the clamp type and clamping distance are the most significant factors in interfacial burr formation. Besides, when the drill tip is moving through the workpiece, the motion of the drill’s helix slightly pulls the plate upward. In the drilling of stacked metal materials, as the drill is exiting the first sheet and entering the second sheet, the first sheet is being pulled upward, and the second is being pushed downward; therefore, the actual gap between the two layers is greater than the deflection induced by the thrust force, and a larger burr height is also achieved.

It appears that drilling with preload pressing force at one side of the workpiece is an efficient way to control the interlayer burr height. It has been proven that the interlayer gap decreases with the increase in the pressing force, as well as the burr height. In drilling of Ti-6Al-4V and Al-7075-T6 stacks, the interlayer thrust force and the flexural rigidity of each layer are significant factors on eliminating the interlayer gap, and the two levels of pressing force were also used to control the interlayer burr.²⁰ The burr size was smaller when using a larger pressing force. In optimizing of processing parameters for burrs in drilling of Ti-6Al-4V and Al-7075-T6 stacks, a larger pressing force also has a contribution to decrease the interlayer burr size.²¹ As the levels of pressing force are limited, the burr size developing tendency is not found. Our results also indicate that there is a critical pressing force, when the magnitude of pressing force is less than the critical value, and the burr height decreases rapidly. On contrary, when the pressing force exceeds the critical value, the burr height becomes stable and the increasing of the pressing force has little influence on burr height, and the burr height tends to be a constant value. Based on the experimental results, a curve that describes the effects of the pressing force could be presented (see Figure 19), where point A is the critical point. In a certain thrust value, the pressing force in point A leads to the best interlayer burr height. The critical point could be calculated through the interlayer burr height prediction model mentioned in this article.

Figure 19.

Effect of pressing force on burr height.

Conclusion

This article has successfully demonstrated that the interlayer burr height prediction model was an efficient way to predict the drilling burr height between the two plates. In particular, it has been shown that the interlayer burr height is positively associated with the gap size in drilling of stacked Al-7475. Furthermore, the following observations were made:

The interlayer burr height has the same changing tendency with the interlayer gap, and an approximate method to predict the interlayer burr height is to calculate the gap between the two layers through analytical model.

Decreasing the interlayer gap is an efficient way to control the interlayer burr height size to the satisfying value in drilling of stacked metal materials. The interlayer gap is mainly induced by the drilling thrust; in addition, a higher thrust induces a greater interlayer gap, and the thickness and height of burrs also become larger. At the exaggerated thrust, exit interlayer burr becomes crown burr, and entrance interlayer burr becomes large uniform burr, which need to be deburred by special tools.

Preload pressing force is a simple and practical way to minimize the interlayer gap. Experiment results obtained by applying Taguchi method have shown that the pressing force has a critical value which provides the best correlation between burr height and the magnitude of pressing force in a special cutting condition.

Knowing the feature of metal flow in the interlayer gap may be a better understanding of the mechanism of the interlayer burr formation and is worth of further research.

Footnotes

Appendix 1 Acknowledgements

The authors would like to sincerely thank Nanjing University of Aeronautics and Astronautics for their material resources support and technical input.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was supported by Aeronautical Science Foundation of China (nos 2013ZE52067 and 2014ZE52057), the Fundamental Research Funds for the Central Universities (no. NS2015052) and the National Natural Science Foundation of China (no. 51575273).

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Investigation of correlation between interlayer gap and burr height in drilling of stacked Al-7475 materials

Abstract

Keywords

Introduction

Interlayer burr height prediction model

Interlayer burr formation

Gap formation model without per-load pressing force

Gap formation model with per-load pressing force

Experiment procedure

Experiment system

Drilling platform

Thrust force monitor platform

Burr measurements

Workpiece materials and cutting tool

Drilling procedure

Results and discussion

Taguchi method results

Drilling experiment results

Discussion

Conclusion

Footnotes

Appendix 1

Acknowledgements

Declaration of Conflicting Interests

Funding

References