A comparison between conventional and ultrasonic-assisted GFRP Machining

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2nd Conference on Composite Material Parts 2nd CIRP Conference Composite Parts Manufacturing (CIRP-CCMPM 2019) 2nd CIRP CIRP on Conference onMaterial Composite Material Parts Manufacturing Manufacturing

A between conventional and ultrasonic-assisted GFRP CIRP Design Conference, May 2018, Nantes, France A comparison comparison28th between conventional and ultrasonic-assisted GFRP Machining Machining A new methodology to analyze the functional and physical architecture of a a a a Mohammad Rabiey (2) a*, a, Stefan Richlea, Pascal Märchya *, Roman Roman Möckli Möckli , Stefan Richlefamily , Pascal identification Märchy existingMohammad productsRabiey for an(2) assembly oriented product a University a

of Applied Science East Switzerland, Rapperswil, Switzerland University of Applied Science East Switzerland, Rapperswil, Switzerland

Paul Stieffax: *, ++41-552-224-769. Jean-Yves Dantan, Alain Etienne, Ali Siadat * Corresponding author. Tel.: +41-552-224-058; E-mail address: [email protected] * Corresponding author. Tel.: +41-552-224-058; fax: ++41-552-224-769. E-mail address: [email protected] École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France *Abstract Corresponding author. Tel.: +33 3 87 37 54 30; E-mail address: [email protected] Abstract

GFRPs (Glass fiber reinforced polymers) are used in a wide range of applications because of their unique properties like high specific strength GFRPs (Glass fiber reinforced polymers) are used in a wide range of applications because of their unique properties like high specific strength and stiffness, low density, good durability and corrosion resistance as well as relatively low cost. Milling and grinding are mostly used for and stiffness, low density, good durability and corrosion resistance as well as relatively low cost. Milling and grinding are mostly used for machining of GFRP. Ultrasonic assisted machining is one of the hybrid processes often used to increase the productivity. This article is focused Abstract machining of GFRP. Ultrasonic assisted machining is one of the hybrid processes often used to increase the productivity. This article is focused on comparing milling and grinding in terms of cutting forces, surface roughness and tool wear for ultrasonic assisted and conventional machining. on comparing milling and grinding in terms of cutting forces, surface roughness and tool wear for ultrasonic assisted and conventional machining. work business describesenvironment, both technological (workpiece quality) and economical time) of machining. InThe today’s the trend towards more product variety and(processing customization is aspects unbroken. Due to this Additionally, development,an theempirical need of The work describes both technological (workpiece quality) and economical (processing time) aspects of machining. Additionally, an empirical analysis milling of GFRP based on the analysis of variances is presented. agile and for reconfigurable production systems emerged to cope with various products and product families. To design and optimize production analysis for milling of GFRP based on the analysis of variances is presented. systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to © 2019 2020aThe Authors. Published by Elsevier B.V. B.V. analyze product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 2nd on Composite Material Parts Manufacturing. nature of components. This fact impedes an efficient comparison and CIRP choiceConference of appropriate product family combinations for the production Peer-review under responsibility of the scientific committee of the 2nd CIRP Conference on Composite Material Parts Manufacturing. system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster Keywords: Composites, CFRP, GFRP, Grinding, Milling these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable Keywords: Composites, CFRP, GFRP, Grinding, Milling assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the similarity between product families by providing design support to both, production system planners and product designers. An illustrative 1. Introduction for the future [6]. This growth represents major challenge for example of a nail-clipper is used to explain the proposed methodology. Anfor industrial case [6]. study on two product families aaofmajor steering columns for of 1. Introduction the future This growth represents challenge tool and process development. thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach. tool and process development. TheThe industrial of lightweight construction materials such CFRP and GFRP components are often produced near net © 2017 Authors.use Published by Elsevier B.V. The industrial use of lightweight construction materials such CFRP and GFRP components are often produced near net Peer-review of the scientific committee of thefiber 28th CIRP Design 2018. as carbon under fiber responsibility reinforced polymers (CFRP) and glass shape by Conference forming processes, but do not meet the necessary

as carbon fiber reinforced polymers (CFRP) and glass fiber shape by forming processes, but do not meet the necessary reinforced polymers (GFRP) has increased strongly in recent quality requirements, which is why they have to be reworked reinforced polymers (GFRP) has increased strongly in recent quality requirements, which is why they have to be reworked Keywords: years [1].Assembly; Design method; Family identification by contour milling [7, 8]. However, the machining quality of years [1]. by contour milling [7, 8]. However, the machining quality of Fiber-reinforced composites have high specific stiffness, the reworked workpiece edges is often unacceptable due to Fiber-reinforced composites have high specific stiffness, the reworked workpiece edges is often unacceptable due to strength, toughness, fatigue and creep resistance, wear and matrix defects, fiber overhangs and delamination. [9, 10]. One strength, toughness, fatigue and creep resistance, wear and matrix defects, fiber overhangs and delamination. [9, 10]. One resistance as well as good damping properties [2-4]. of way raising range the manufacturing qualitymanufactured of these difficult-to1.corrosion Introduction theof and characteristics and/or corrosion resistance as well as good damping properties [2-4]. way ofproduct raising the manufacturing quality of these difficult-toDue to the exceptional material properties of composite assembled machine materials to a higher level could be challenge attained by in this system. this context, the main in Due to the exceptional material properties of composite machine materials to a In higher level could be attained by materials, a very wide in range applications, ultrasonic machining of these materials. Due to they thealso fasthave theof domain of modelling and analysis is now not only to cope with single materials, they also havedevelopment a very wide range of applications, ultrasonic machining of these materials. particularly in aerospace technology and of medical technology. Over the past 15product years, great has product been made in the communication and an ongoing trend digitization and products, a limited rangeprogress or existing families, particularly in aerospace technology and medical technology. Over the past 15 years, great progress has been made in the Compared to conventional structural materials, such as field of ultrasonic machining. In most cases, the tool is excited digitalization, facing important also be able to analyze and compare define Compared tomanufacturing conventional enterprises structural are materials, such as but field of to ultrasonic machining. In to most cases, products the tool istoexcited aluminum or fiber-reinforced composites allow up to 30% new by an ultrasonic vibration the machining process. This challenges insteel, today’s market environments: a continuing product families. It canduring be observed that classical existing aluminum or steel, fiber-reinforced composites allow up to 30% by an ultrasonic vibration during the machining process. This lighter constructions [5]. process achieved improved surface roughness, higher tendency towards reduction product are regrouped clients or features. lighter constructions [5]. of product development times and process families achieved improvedin function surface ofroughness, higher Despite the excellent material properties, these materials productivity, loweroriented cutting forcesfamilies and tool weartowhen shortened lifecycles. In addition, there isthese an increasing assembly are hardly find. Despiteproduct the excellent material properties, materials However, productivity, lower cuttingproduct forces and tool wear when have not spread rapidly in large series applications yet. The machining brittle and hard materials such as glass and ceramics demand customization, the same time in ayet. global On the product family products differ in two have notofspread rapidly inbeing largeatseries applications The machining brittle and hardlevel, materials such as glassmainly and ceramics production of fiber composites is difficult due to the lack of cost [11, 12]. competition with over thedue world. trend, characteristics: (i) the number of components and (ii) the production of fibercompetitors composites all is difficult to theThis lack of cost main [11, 12]. effective production technologies (huge amount of manual In the machining of composites, this ultrasonic-assisted which is inducing development fromamount macro ofto manual micro typeInofthe components (e.g. electronical). effective productionthetechnologies (huge machining of mechanical, composites,electrical, this ultrasonic-assisted works in the process), the complicated processing machining is still largely unexplored. This paper contains markets, results diminished the lot sizes due to augmenting Classical is methodologies works in thein process), complicated processing machining still largely considering unexplored.mainly This single paper products contains (heterogeneity of the material) and the high material price [5]. contour milling and grinding tests of GFRP materials with and product varietiesof(high-volume low-volume production) solitary, already existingtests product families analyze (heterogeneity the material) to and the high material price [1]. [5]. or contour milling and grinding of GFRP materials with the and Nevertheless, strong growth for GFRP and CFRP is forecasted without ultrasonic excitation of workpiece. To cope with this augmenting well as tois be able to product a physicaloflevel (components level) which Nevertheless, strong growth forvariety GFRPas and CFRP forecasted without structure ultrasoniconexcitation workpiece. identify possible optimization potentials in the existing causes difficulties regarding an efficient definition and 2212-8271 © 2019 The Authors. Published by Elsevier B.V. 2212-8271 ©system, 2019 TheitAuthors. Published by Elsevier B.V. knowledge production is important to have a precise comparison of different product families. Addressing this Peer-review under responsibility of the scientific committee of the 2nd CIRP Conference on Composite Material Parts Manufacturing. Peer-review under responsibility of the scientific committee of the 2nd CIRP Conference on Composite Material Parts Manufacturing. 2212-8271 © 2020 The Authors. Published by Elsevier B.V. This is an©open article Published under theby CC BY-NC-ND 2212-8271 2017access The Authors. Elsevier B.V. license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of scientific the scientific committee theCIRP 2nd Design CIRP Conference on Composite Material Parts Manufacturing. Peer-review under responsibility of the committee of the of 28th Conference 2018. 10.1016/j.procir.2019.09.040

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Nomenclature ae ap CFRP D Fy Fx f fz GFRP n t vC vf z δ ϑ ψ

width of cut depth of cut carbon fiber reinforced polymers cutter diameter [mm] force perpendicular to the feed direction [N] force in feed direction [N] frequency [Hz] feed per tooth [mm/tooth] glass fiber reinforced polymers revolutions per minute [rev/min] time [s] cutting speed [m/min] or [m/s] feed rate [mm/min] or [m/s] number of teeth phase shift fiber separation angle fiber orientation angle

1.1. Fiber Orientation The necessity of contour milling fibre-reinforced composites is due to the shaping process of the components. The correct choice of the feed rate and the speed of the milling and grinding tool have major influence on the quality of the machining of GFRP [6, 13] In most of the milling process, the delamination often occurs on the outer laminate layers, while the cut surfaces usually show no visible damage. This is due to the fact that the outer laminate layers are not supported by adjacent laminate layers [14]. In order to counteract this effect, there is a special milling tool which was developed for contour milling of composites. The most common cause of delamination at the component edges is the increasing tool wear, since a large cutting edge radius makes it more difficult to locally bend the fibers and the cutting forces increase. The fiber orientation of the workpiece during the milling process also has a very large influence on delamination, cutting forces and temperatures [9, 16]. This is shown schematically in Figure 1 during up milling.

Fig. 1. Fiber orientation and fiber separation angle

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In milling, the cutting edge direction to the fiber orientation angle ψ is permanently changed depending on the pressure angle. For these reasons, the fiber separation angle ϑ (angle between the current cutting edge direction of the cutting point and the fiber) would be defined. Depending on the fiber separation angle ϑ, the load condition caused by the cutting edge motion performs different failure forms on the material. With a fiber separation angle of ϑ = 0°, the laminate fails essentially due to compressive loads which cause the fibers to buckle. At a fiber separation angle ϑ = 90°, the fibers are separated by bending. If the fiber separation angle is between 0° ˂ ϑ ˂ 90°, a combination of the two separation mechanisms mentioned above takes place [5]. When milling fiber-reinforced polymers, up milling machining is generally recommended. The reason for this is perhaps the impact force on the workpiece during down milling when the tool is penetrating to the workpiece at maximum chip thickness. This penetration of the tool cutting edge into the material can lead to damage in the form of cracks in the matrix. With up milling on the other hand, the tool cutting edge enters the workpiece to be machined more smoothly and fewer cracks occur [5, 9, 16 [9][16]. 1.2. Ultrasonic assisted milling In ultrasonic assisted milling, the vibration speed of the workpiece must be faster than the speed of the milling tool so that the tool cutting edge is released from the component and a discontinuous cut can be generated [2]. During the milling process, the relative speed of the tool and the chip thickness change continuously [11]. Since the size and weight of workpieces excited by ultrasonic change as the machining process increases, the natural frequency of the system also changes, and the sonotrode would have to be adjusted with different frequencies. 2. Experimental Setup Figure 2 shows the experimental setup of the experimental facility. All milling and grinding tests were carried out in dry condition on an ELB high precision surface and profile grinding machine with an extra milling spindle. Each test was performed three times to make sure the repeatability of the investigation. The cutting forces during the milling process were recorded with a dynamometer 9255 C from Kistler. The amplifier was also from Kistler, type 5070A and the tests carried out with a sampling rate of 8 kHz. The amplitude of the vibration (5 µm) was measured with an eddy current measurement system eddy NCDT 3100/3100S. The block sonotrode is a self-construction and the sonotrode Table is made of aluminium. The GFRP parts were clamped directly on to the Table with a steel clamp. The sonotrode is excited with a vibration of 20 kHz. A MarSurf UD 130 roughness tester and the Keyence VHX-600 digital microscope were used to evaluate the surface roughness.

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3

Fig. 2. Experimental setup for the milling of GFRP

2.1. Tools A milling tool for all tests is from the company “Sandvik Coromant”. The special feature of these milling tools is, that the cutting edges change the spiral direction to create a pressure against the centre of the work part during milling, thus preventing delamination as far as possible. The carbide tool has a PVD (Ti, Al, N2) coating with the type designation 2P4600600-NA 1630. The radial rake angle of the cutting edge is 5°. The diameter of the tool is 6 mm within 6 teeth, length of 76 mm and with the helix angle of 40°. The infeed in the milling depth ap and the depth of cut in the milling ae are 4 mm and 0.5 mm respectively for all test runs. An end mill holder with the HSK63 for machine connection was applied. All tests are carried out with new brand tools. A diamond grinding tool with electroplated nickel bonding was used to carry out the grinding tests. The diameter of the grinding head is 10 mm and the grains have a grain size of D126. The depth of cut ae is 0.01 mm and the infeed of cut ap is equal to the GFRP thickness which is 4 mm. Various tests were carried out with different cutting speed vc , different feed rate vf, and up grinding as well as down grinding. 2.2. Materials Bi-directional GFRP plates with a layer configuration of 0/90° with canvas fabric embedded in epoxy resin matrix were used as workpieces for these experiments. The plates are made of prepreg, but no quality assessment was carried out before or after manufacture. The dimensions of the workpiece with length, width and thickness are 180 mm x 25 mm x 4 mm respectively. Each layer was 0.5 mm. The physical and mechanical properties of the GFRP used in this study are given in Table 1.

Fig. 3. Schematic of tests parameters as well as tools used in the experiments, milling (left) and grinding (right) Table 1. Material properties of used GFRP (at room Temperature) Material

Tensile Strength (MPa)

Impact strength (Charpy) kJ/m2

Elasticity Module (GPa)

Density (g/cm3)

GFRP

240

50

22

2

The cutting force Fx is in feed direction and the cutting force Fy is perpendicular to the feed direction. The resulting cutting force Fr is the calculated hypotenuse of force Fx and force Fy. As the depth of cut is rather low the Fx and Fy can represent the Ft (tangential force) and Fn (normal force) respectively for both milling and grinding. The schematic diagram for milling experiments is shown in Figure 3. 2.3. Machining parameters The machining parameter is shown in Table 2. The cutting parameters of tools were based on recommendation of the supplier and also with consideration of the machine spindle speed limitation. Table 2. Grinding and milling parameters Tool

Cutting speed

Feed speed

Carbide milling tool

151 – 264 m/min

0.02 – 0.08 mm/tooth

Diamond grinding tool

471 – 707 m/min

3500 – 8750 mm/min

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3. Results 3.1. Ultrasonic vibration With the vibration frequency of 20 kHz, the sonotrode with a clamping system are excited with a maximum amplitude of 5 µm based on our measurement with the eddy current measurement system (Figure 4).

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After a chip removal volume of 159.3 cm3, the average tangential cutting force Fx increased to 32 N and the average normal cutting force Fy to 34 N. The increase in tangential force Fx was 28%, while the increase in normal force Fy was 88%. It is clear that the perpendicular force to the feed direction Fy increases much more than the force in feed direction Fx. This is mainly due to the fact that the cutting edge of the tool becomes blunted with increasing machining time. This wear of the tool is shown in Figure 5. It can be seen that the cutting edge of the tool becomes very blunt and that the clearance surface shows signs of wear. In addition, burnt matrix residues adhered to the milling tool after the test, which demonstrates that the thermal effect during the milling process was high.

Fig. 4. Vibration frequency and amplitude

The oscillation of the sonotrode moves approximately equally in all three spatial directions. It is assumed that the deviation in frequency is caused by the superposition of vibrations from the sonotrode, the clamping system and the work part. It is noticeable that the fiber-reinforced polymers excited by the vibration heat up during the vibration process. If polymers are excited with an oscillation, the force or strain amplitude reacts with a phase shift δ and damping. The resulting hysteresis shows the work converted per period (conversion into thermal energy) as shown in Figure 5 [17].

Fig. 5. Tool wear a) new tool and b) after material removal of 159.3 cm3

To compare the up milling with the down milling, all tests were carried out with the cutting speed of vc 188 m/min and ae of 0.5 mm with different feed rates. Figure 6 shows the average resulting cutting forces during the machining of bidirectional GFRP. It can be seen that the resulting cutting forces are smaller at higher feed rates during the up milling compared to the down milling.

Fig. 5. Phase shift in hysteresis [17]

This may lead to an additional increase in the temperature at the interface generated by the milling tool and the risk of thermal damage to the workpiece increases [18]. Fig. 6. Down milling vs up milling of GFRP

3.2. Cutting forces A wear test was first carried out with the milling tool during the up milling. The cutting speed vc was 188 m/min and the feed per tooth fz was 0.03 mm/tooth. During the first milling pass with the not yet worn milling tool, the average tangential force Fx was 25 N and the average normal force Fy was 18 N.

The main difference between the milling methods is that in down milling the normal cutting force Fy is considerably higher than the tangential cutting force Fx. In up milling it is the other way around and the tangential cutting force Fx is higher than the normal cutting force Fy. Since the wear test shows that the force Fy increases considerably more than the force Fx, it can be said that up

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milling is more suitable. This is due to the fact that the largest cutting forces during up milling act in the feed direction and in the direction of the material to be removed. Since the greatest force component in down milling is perpendicular to the direction of the material to be removed and the force Fy increases more strongly with increasing wear, it is possible that the force Fy adds up even more strongly with increasing tool wear and thus more cracks develop in the matrix. In addition, the same experiments were carried out with a constant cutting speed of vc 188 m/min and different feed rates in up milling with ultrasonic support. Figure 7 shows that ultrasonic machining of GFRP produces higher cutting forces on average than conventional milling without ultrasonic assistance milling except at high feed rate.

Fig. 7. Conventional vs ultrasonic assisted milling of GFRP

This shows that ultrasonic milling has disadvantageous in terms of the cutting force when machining GFRP for the applied test condition. Experiments with conventional grinding without ultrasonic support were carried out with cutting speed vc of 550 m/min. The feed rates were 3500 mm/min, 5250 mm/min, 7000 mm/min and 8750 mm/min. Since the depth of cut ae during the grinding tests is very low at 0.01 mm, correspondingly low cutting forces were generated during the grinding tests.

Fig. 8. Cutting force by grinding of GFRP

5

The resulting cutting forces when grinding GFRP were from 3.5 N to 4.6 N. The forces in down grinding have turned out to be greater than the forces in up grinding (Figure 8). 3.3. Surface quality The surface roughness for all the tests were very near to each other with the consideration of typical accuracy encompassed with surface roughness testing. In all milling tests with GFRP, it was shown that an average roughness value Ra of 1.00 μm was obtained for up milling with the feeds per tooth of 0.02 mm/rev, 0.04 mm/rev, 0.06 mm/rev and 0.08 mm/rev and a cutting speed of 188 m/min. In the same test with down milling, the average surface roughness value Ra increases to 1.20 μm. The roughness measurement was performed perpendicular to the feed direction for all cases. All these roughness values are low it and indicates that the tools used in these experiments are well suited for processing of GFRPs. However, it is observed that up milling produces lower surface roughness values than down milling. In addition, the average roughness values for up milling with ultrasonic support increase slightly compared to conventional up milling. Besides it can also be observed that the surface roughness of grinding in contrast to milling are rather higher, although the roughness achieved by grinding is generally acceptable for industrial components. This is rather against the expectation of the grinding for other materials, which shows generally better surface roughness. It is assumed that the roughness values become worse during grinding due to the unfavorable fiber separation angle. The delamination depth measured on the surface of the material for all experiments and it never exceeds 0.176 mm which is generally negligible for most industrial application. 3.4. Empirical modelling of milling forces The material composition of composites and particularly GFRPs is so that the general models of milling process used in metals cannot be applied for. The main reason is that the chip thickness depends on not only the interaction of tool edge with polymer matrix but also with glass fiber. That is why the models based on chip thickness is not valid. As there is no other physical model to predict the milling forces for machining of GFRPs, the empirical method can be useful although the model is constraint by the applied tests parameters windows. Within the parameters applied for this investigation for the up milling and down milling condition by conventional strategy, the forces in x and y directions (which are approximately tangential and normal forces correspondently) are modeled using the analysis of variance (ANOVA) by regression method. Table 3 shows the milling result used for regression analysis. As the depth of cut was constant the forces are only correlated to cutting speed as well as feed rate. The results are demonstrated in equation 1 and 2. 𝐹𝐹𝐹𝐹 = 3.838 𝑣𝑣𝑓𝑓1.12 ∙ 𝑣𝑣𝑐𝑐−0.314

𝐹𝐹𝐹𝐹 = 9.189 𝑣𝑣𝑓𝑓0.661 ∙ 𝑣𝑣𝑐𝑐−0.463

(1) (2)

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[2]

Where the F is force both in x and y direction [N], vc is cutting speed [m/s] and vf is feed rate [m/s].

[3]

Table 3. Milling parameters and results used for the regression analysis

[4]

Cutting speed

Feed speed

Fx

Fy

vc (m/min)

f (mm/tooth)

(N)

(N)

Process

188

0.02

12.8

9.2

Up-milling

188

0.04

16.8

9.8

Up-milling

188

0.06

18.3

13.3

Up-milling

188

0.08

30.6

15.5

Up-milling

151

0.04

14.3

9.0

Up-milling

188

0.04

15.3

9.7

Up-milling

226

0.04

20.0

10.0

Up-milling

264

0.04

16.8

13.8

Up-milling

188

0.02

4.0

12.0

Down milling

188

0.04

9.5

25.0

Down milling

188

0.06

15.3

31.5

Down milling

188

0.08

20.1

35.0

Down milling

[5]

[6]

[7]

[8]

[9]

4. Conclusion 

If GFRPs are excited with an oscillation by a sonotrode, the workpieces heat up. In the case of heavy machining operations, the natural frequency of the system also changes as the mass of the component decreases.



By milling the bi-directional GFRP, the normal force increases considerably more than the force in the feed direction with increasing tool wear.



By milling with ultrasonic support, the average roughness value increases slightly compared to conventional milling.

 



The cutting forces in ultrasonic assisted up milling of GFRP increases slightly at lower feed rate. The surface roughness Ra achieved after grinding in the frame of the investigation parameters shows a rougher surface than milling. No considerable delamination was found in all tests

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