Material Stiffness and Cutting Parameters for Honeycomb Aluminum Sandwich Panel: a Comparison with Bulk Material

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Procedia Manufacturing 34 (2019) 385–392 Procedia Manufacturing 00 (2017) 000–000

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47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, USA

47thSME SME North North American Research Conference, NAMRC Pennsylvania, 47th AmericanManufacturing Manufacturing Research Conference, Penn47,State Behrend USA Erie, 47th SME North American Manufacturing Research Conference, NAMRC 47, Pennsylvania, USA Pennsylvania, 2019for Honeycomb Aluminum Material Stiffness and Cutting Parameters

Material Stiffness and Cutting Parameters for Honeycomb Aluminum Material Stiffness Cutting Parameterswith for Honeycomb Aluminum Sandwichand Panel: a Comparison Material Comparison with Bulk Bulk ManufacturingSandwich EngineeringPanel: Society aInternational Conference 2017, Material MESIC 2017, 28-30 June Sandwich Panel: Comparison 2017,aVigo (Pontevedra),with SpainBulk Material Derek Yip-Hoi*, David Gill, Jacob Gahan, Gavin Travis, Lukas Mackaay Derek Yip-Hoi*, David Gill, Jacob Gahan, Gavin Travis, Lukas Mackaay Derek Yip-Hoi*, David Gill, Jacob516Gahan, Gavin WA Travis, Lukas Mackaay Western Washington University, High St., Bellingham, 98225, USA

Western Washington University, 516 High St.,address: Bellingham, 98225, USA Costing models** Corresponding for capacity optimization inWA Industry 4.0: Trade-off author. +1-360-650-7236;. E-mail [email protected] Western Washington University, 516 High St.,address: Bellingham, WA 98225, USA Corresponding author. +1-360-650-7236;. E-mail [email protected] * Corresponding author. +1-360-650-7236;. E-mail address: [email protected] between used capacity and operational efficiency

Abstract a a,* b b Abstract Abstract Aluminum sandwich panel provides a high-strength, light-weight structural material for use in aircraft and aerospace applications such as cabin Aluminum panel high-strength, material for use in has aircraft and aerospace applications such as cabin a University Minho,structural 4800-058 Guimarães, Portugal bulkheads, sandwich rotor blades, andprovides luggageaand cargo unit light-weight loadofdevices. This unique material also application in many other industries such as Aluminum sandwich panel provides aand high-strength, structural material for use in has aircraft and aerospace applications such as cabin bulkheads, rotor blades, and luggage cargo loadbulkheads, devices. This material also application in many industries such as bunit light-weight Unochapecó, 89809-000 Chapecó, SC, Brazil machine tool enclosures, museum exhibits, marine craft and unique hurricane panels for storm survivability. Thisother material, consisting of bulkheads, rotor blades, and luggage and cargo unit load devices. This unique material also has application in many other industries such as machine tool enclosures, museum exhibits, marine craft bulkheads, and hurricane panels for storm survivability. This material, consisting aluminum face sheets surrounding an aluminum honeycomb core, offers very high rigidity in a low density material. Machining this material of is machine tool museum exhibits, marine craft bulkheads, and very hurricane panels for survivability. This material,this consisting of aluminum faceenclosures, sheets aluminum honeycomb offers rigidity in astorm low density material. Machining quite challenging duesurrounding to variable an cutting conditions in the core, low density, lowhigh lateral stiffness honeycomb core. Machining often material requires isa aluminum face sheets surrounding an aluminum honeycomb core, offers very high rigidity in a low density material. Machining this material is quite challenging to variable cutting conditions in the low density, lowpartially lateral stiffness core. along Machining oftenedges. requires significant amountdue of post-processing in the form of manual removal of the released honeycomb core walls (flags) machined Thea quite challenging due to variable cutting conditions in the low density, lowpartially lateral stiffness honeycomb core. along Machining oftenedges. requires a significant amount of post-processing in the form of manual removal of the released core walls (flags) machined The purpose of this work is to reduce the flagging created in the machining of this material. The first step to improving the cutting conditions is to Abstract significant amount ofispost-processing in the form of manual removal of the partially released core walls (flags) along machined edges. is The purpose of this work to reduce the flagging created in the machining of this material. The first step to improving the cutting conditions to better understand the causes that impact them. In this pursuit, a series of experiments was conducted to measure, quantify, and study the cutting purpose of this work to reduce the flagging created in the machining this material. first step improving the cutting conditions is to better the is causes that impact them. In this pursuit, a series ofofexperiments wasThe conducted to to measure, andinstudy cutting forces understand during the machining aluminum sandwich panel. A force dynamometer was used to measure forces duringquantify, slot milling bulkthe aluminum Under the the concept of of "Industry 4.0",In this production processes will be toforces increasingly interconnected, better the causes that impact them. pursuit, a series of experiments waspushed conducted to be measure, quantify, and the cutting forces understand during of aluminum sandwich A force dynamometer was used to measure during slot milling instudy bulkmodel aluminum generating cuttingmachining coefficients for the force model.panel. Cutting results in bulk aluminum showed generally good agreement with the with forces during the machining aluminum sandwich panel. A force dynamometer was used to measure forcescontext, during slot milling inoptimization bulkmodel aluminum generating cutting coefficients the force model. Cutting results aluminum showed generally good agreement the with information based onfrom aofreal basis and, necessarily, more efficient. In then this capacity over-predictions ranging 5fortotime 20% depending on the feed rate.inmuch Abulk series of cutting tests was conducted on the with aluminum sandwich generating cuttingranging coefficients forto the force model. maximization, Cuttingfeed results bulk aluminum showed generally good agreement theand model with over-predictions from 20% depending rate.in Acontributing series of cutting tests was then conducted on the with aluminum sandwich goes beyond traditional aim of capacity also forcombinations organization’s value. panel in order tothe decouple the 5machining forces for on thethe face sheets, the honeycomb core, and of faceprofitability sheets and core. The data over-predictions ranging from 5machining to 20% depending on the feed rate. the A honeycomb series of cutting tests was then conducted on the aluminum sandwich panel in order to decouple the forces for the face sheets, core, and combinations of face sheets and core. The data revealed vibration in the honeycomb material that were significantly worseapproaches for shallower depths of cutcapacity involving the top face sheet and the upper Indeed, lean management and continuous improvement suggest optimization instead of panel in order to decouple the machining forces thesignificantly face sheets, worse the honeycomb core, and combinations of face sheets and Theupper data revealed vibration in the honeycomb material thatfor were for shallower depths ofspikes cut involving the top facespecific sheetcore. and the portion of the honeycomb structure despite efforts at stiffening the fixture. The data showed force that correlate with engagement maximization. The study of capacity optimization and costing models is an important research topic that deserves revealed in the honeycomb material that were significantly worse for shallower depths ofspikes cut involving the topwith facespecific sheet and the upper portion ofvibration the honeycomb structure despite efforts at were stiffening the fixture. data showed force thatclear correlate engagement conditions in the honeycomb structure. Peak forces measured as highThe as 400N though it is not entirely whether these peak values are portion of the honeycomb structure despite efforts stiffening theperspectives. fixture. The data showed force spikes that correlate withthese specific engagement contributions from both the practical andattheoretical This paper presents and discusses a mathematical conditions in the forces were measured highharmonic as 400N though it isThe not entirely clear whether peak values are representative of honeycomb actual peak structure. forces or aPeak combination of peak forceas with vibration. resulting cut walls showed signs of tearing, conditions incapacity the honeycomb structure. Peak forces were measured as highharmonic as 400N(ABC though it isThe not entirely clear whether these peakhas values are representative of actual peak forces or a combination of peak force with vibration. resulting cut walls showed signs of tearing, model for management based on different costing models and TDABC). A generic model been rubbing, and remaining cell walls indicating that for much of the cut, ideal shearing of the material did not occur. The research highlights the representative of actual peak forcesindicating or a combination of peak force with harmonic vibration. The resulting walls signs of tearing, rubbing, and and remaining walls that for much ofand the cut, ideal shearing of the material did maximization not cut occur. Theshowed research highlights the developed it was used to analyze idle capacity to strategies towards the of organization’s need for further study ofcell the actual mechanism of cell wall removal indesign this complex cutting environment. rubbing, and remaining walls indicating for much of the cut, ideal shearing of theenvironment. material did not occur. The research highlights the need for further study ofcell the actual mechanismthat of cell wall removal in this complex cutting value. trade-off capacity maximization vs removal operational is highlighted need forThe further study of the actual mechanism of cell wall in this efficiency complex cutting environment. and it is shown that capacity © 2019 The Authors. Published by Elsevier B.V. optimization might hide operational inefficiency. © 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. by Elsevier B.V. This is an open accessPublished article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) This an open access article under the BY-NC-ND CC (http://creativecommons.org/licenses/by-nc-nd/3.0/) © 2017 The Authors. Published by B.V. licenselicense © 2019 The Authors. Published by Elsevier B.V.BY-NC-ND This isisan open access article under the Elsevier CC Peer-review under responsibility of the Scientific Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. Peer-review under responsibility of the Scientific Committee NAMRI/SME. This is an open access article under CC BY-NC-ND license Peer-review under responsibility of the scientific committee ofofthe Manufacturing Engineering Society International Conference Peer-review under responsibility of the Scientific Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. Peer-review under responsibility of the Scientific Committee of NAMRI/SME. 2017.

A. Santana , P. Afonso , A. Zanin , R. Wernke

Keywords: Aluminum sandwich panel, machining forces, slot milling, honeycomb core, cutting coefficients, composite ply sandwich Keywords: Aluminum sandwich panel, machining forces, slot milling, honeycomb core, cutting coefficients, composite ply sandwich Keywords: Aluminum sandwich panel, machining forces, slot milling, honeycomb core, cutting coefficients, composite ply sandwich

Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency

1. Introduction step in improving the machining process for this difficult 1. Introduction step in improving the machining processprocess for this difficult material is to understand the machining better. In 1. Introduction 1. Introduction step in improving the machining processprocess for this difficult material is this to understand the machining better. In Aluminum honeycomb sandwich panel, often sold under pursuit of understanding, a series of experiments was material is to understand the machining process better. In Aluminum honeycomb sandwich panel, often sold under pursuit of this understanding, a series of experiments was trade names likehoneycomb Teklam™ and Nordam™, is often useful sold for aircraft conducted tothis attempt to compare atheseries machining of this material Aluminum sandwich panel, under pursuit of understanding, of experiments was The cost of idle capacity is a fundamental information for companies and their management of extreme importance trade names likeasTeklam™ and Nordam™, is useful for aircraft conducted to attempt to compare machining of this material manufacturing well as other transportation manufacturing to the well-studied machining of the solid metal. The purpose of trade namesproduction likeasTeklam™ and Nordam™, is useful for aircraft conducted to attempt to compare the machining of material in modern In density. general, itMachining is defined as unused capacity or production potential and canThe bethis measured manufacturing well systems. as and other transportation manufacturing to the well-studied machining ofthe solid metal. purpose of due to its high strength low this this comparison is to identify similarities and difference manufacturing as well as other transportation manufacturing to the well-studied machining of solid metal. The purpose of due to its high strength and low density. Machining this this comparison is to identify the similarities and difference in several ways: tons of production, available hours of manufacturing, etc. The management of the idle capacity material is high quitestrength challenging duedensity. to the Machining highly varying between honeycomb machining and bulk machining so that due to its and low this this comparison is to identify the similarities and difference material isconditions quite due to +351 the varying between honeycomb machining and address bulk machining so that * Paulo Afonso. Tel.:challenging +351 253 510encounters 761; fax: 253 604 741 machining as the tool the highly different angles strategies can be developed to better the challenges of material isconditions [email protected] challenging due to the highly varying between honeycomb machining and address bulk machining so that machining as the tool encounters the different angles strategies can be developed to better the challenges of E-mail address: of the honeycomb wall structure bonded at its top and bottom machining low density, low lateral stiffness material with machining conditions as structure the tool encounters the different angles strategies can be density, developed to better address thematerial challenges of of the honeycomb wall bonded at its top and bottom machining low low lateral stiffness with edges the more rigid top and bottom face first varying angles presentation cutter stiffness workpiece engagement. of the to honeycomb wall structure bonded at itssheets. top andThe bottom machining lowof low of with edges to the moreThe rigid top and bottom sheets. varying angles ofdensity, presentation oflateral cutter workpiecematerial engagement. 2351-9789 © 2017 Authors. Published byface Elsevier B.V. The first edges to the more rigidAuthors. top and bottom face sheets. of The varying angles of Society presentation of cutter workpiece engagement. Peer-review under responsibility of the scientific the first Manufacturing Engineering International Conference 2017. 2351-9789 © 2019 The Published bycommittee Elsevier B.V. 2351-9789 © 2019 The Authors. Published by Elsevier B.V. 2351-9789 ©open 2019 The Authors. Published by Elsevier B.V. Thisisisanan access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) This open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/) 2351-9789 ©under 2019 Thearticle Authors. Published by Elsevier B.V. This is an open access under the Scientific CC BY-NC-ND license Peer-review under responsibility of the Scientific Committee of NAMRI/SME. Peer-review responsibility of the Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. This is an open access article under CC BY-NC-ND license Peer-review under responsibility of the Scientific Committee of (http://creativecommons.org/licenses/by-nc-nd/3.0/) NAMRI/SME. 10.1016/j.promfg.2019.06.182 Peer-review under responsibility of the Scientific Committee of NAMRI/SME.

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2. Background Research into machinability of materials and the modeling of the underlying processes has been the subject of significant attention. This work is novel in that it lies at the intersection of a number of these areas though it cannot claim to truly be rooted fully in the fundamentals of any. The areas of intersection are the machinability of metals, the machinability of composites, and the machinability of non-rigid or compliant structures. The oldest of these areas is the first. The study of the mechanics of metal cutting goes back to Merchant [1] who developed the orthogonal cutting model and has advanced to the point where sophisticated models are available to predict cutting forces and vibrations such as the work by Budak and Altintas [2]. This work will utilize as a starting point one of these models. The honeycomb sandwich panel in this work is a composite structure albeit one that is completely aluminum. The face sheets and plies that are bonded and expanded are often different material grades with different tempers and rolling-induced directional properties. There is limited reported research on machinability studies of this material though several studies have been performed on aluminum honeycomb core alone. Most of the studies have utilized some method of stabilizing the honeycomb structure, whether by freezing water in the cells or utilizing iron powder in the presence of a strong magnetic field. Wang [3] studied the effect of cryogenically freezing water in the cells of the honeycomb to both increase compressive strength and to maintain rigidity in cutting. This study found average machining forces to increase considerably with the ice, along with a reduction in flagging. However, this method is only feasible for open cell honeycomb and not for sandwich ply material. Qui [4] conducted an in-depth study of the toolworkpiece interaction for individual cell walls of aluminium honeycomb and the resulting cutting forces as related to the positions of the tool and the angle of the wall. The study included 1mm axial depth of cut and ice-stabilization of the walls along with an assumption that the walls did not buckle, collapse, or wrap around the tool, but instead sheared nicely. The assumptions used in the model appear to cause the resulting force predictions to smooth out all high frequency force variations for reasons that are not immediately evident from the text. The machining of aluminium sandwich panel, i.e. honeycomb core with face sheets attached, appears to have little published and the studies of honeycomb core alone lack some relevance due to the use of methods to constrain the honeycomb core during cutting. The structure of sandwich panel is quite challenging with deformable core constrained only at the top and bottom surfaces. Additionally, some of the tools that have been developed for core machining have their performance negatively impacted by the presence of the face sheets. Additionally, the presence of the face sheets brings significant challenges to the study and analysis of the cutting process. When looking for studies of low-rigidity materials, one can look to studies of composite ply sandwich material. A study on the use of finite element modeling to predict the impact of cell structure and cutting tools geometry on the machining of

Nomex® honeycomb core has been report by Jaafar et al. [5]. However, Nomex® is a material with significantly different mechanical properties to aluminum, and this application focused on free-standing structures i.e. no face sheets. Research on the challenges faced when machining freestanding Nomex®, and when bonded as core material to fiber-glass face sheets, has been reported by the authors (Gill et al. [6], Yip-Hoi and Gill [7]). Some of the studies in [7] that suggest mechanisms for how the cells rupture and collapse during cutting are helpful to this work and will be introduced in a later section. Numerous patents have been filed on cutters designed specifically for machining honeycomb structures [8], [9]. Though variation of the cutter geometry is not a focus of this work, this is likely a critical factor in the efficiency of cutting and should be borne in mind as the impact of this work is considered. It should be mentioned in passing that there is a growing body of research in the area of machining of composites such as Carbon Fiber Reinforce Polymers (CFRP). However, there are major differences in machining ply layup laminar structures over honeycomb sandwich panel that makes this body of research largely irrelevant. 2.1. Mechanics of Milling As mentioned in the previous section we utilize as a starting point in this work a force prediction model reported in Altintas [10]. This was previously developed by Altintas and Spence [11]. This model has been used extensively including in the work by Campatelli and Scippa [12] upon which the machining tests for bulk aluminum in this work have been calibrated. The relevance of force prediction to understanding and modeling the mechanics of machining this material was an open question when this research was initiated. Given the expected challenges due to compliance of the structure our goals to start were less ambitious and focused on whether it was even possible to obtain meaningful force measurements and whether any correlation with a model was likely feasible. To predict the cutting forces acting on a helical end mill, the chip load, cutter geometry and cutting force components (radial, tangential and axial) must be found at each point along the helical cutting edges. At a given location and orientation of the cutter as it rotates, the differential tangential (dFt), radial (dFr) and axial (dFa) cutting forces acting on each horizontal element are given are given by equation 1, [10].

[

]

dFt , j (φ j ( z ), z ) = K tc h j (φ j ( z )) + K te dz

[

]

dFr , j (φ j ( z ), z ) = K rc h j (φ j ( z )) + K re dz

[

]

(1)

dFa , j (φ j ( z ), z ) = K ac h j (φ j ( z ) + K ae dz

Where, • φj(z) ∈[φst, φet]zis the immersion angle for flute j at axial location z. φst, φet are the entry and exit angles at each height measured clockwise from the feed direction. • Ktc, Krc, and Kac are cutting coefficients for the shearing forces in the tangential, radial and axial directions. • Kte, Kre, and Kae are cutting coefficients for edge ploughing forces in the tangential, radial and axial directions. • hj(φj, z) is the uncut chip thickness at each height z and



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angular orientation φj of the cutting edge. Its value is given by equation (2) with c being the feed per tooth. (2) h j (φ j , z ) = c sin φ j ( z ) The differential forces given in equations (1) can be transformed to a Cartesian reference frame where the X-axis is along the feed direction and the Z-axis along the tool axis. These are obtained using the following transformation,  dFx , j (φ j , z ) − cos φ j − sin φ j 0  dFt , j (φ j , z ) (3)      − cos φ j 0 ×  dFr , j (φ j , z ) dFy , j (φ j , z ) =  sin φ j  dFz , j (φ j , z )  0 0 1 dFa , j (φ j , z )   These are summed to obtain the feed, axial and normal forces acting on the cutter using, N

d

Fi (φ ) = ∑∑ dFi , j (φ j , z )

(4)

j =1 z =0

where i ∈ (x, y, z). N is the number of flutes on the cutter. This model assumes no runout in the cutting tool, a simplification chosen due to the already complex cutting conditions in the honeycomb material. Models including runout have been developed including Li [13] which indicated that there was some change of shape in the cutter workpiece engagement, but the effect of runout is shown to be less than 10% at certain peak values for a deep flank cutting operation. Our testing data showed inequality of cutting forces between the two flutes of the tool indicative of runout, however, the effects were less than 20% of the signal amplitude. Given the varied cutting conditions of honeycomb, this simplification is reasonable for this research. 3. Method To apply the force model above for the measurement of machining forces in aluminum sandwich panels, the cutting coefficients were first measured for bulk aluminum to calibrate the equipment and to validate the implementation of the model. The model was then used to predict and measure the forces generated in cutting the face sheets alone, the face sheets with some percentage of the honeycomb, and the honeycomb alone. 3.1. Machining Force Testing Instrumentation A set of tests was conducted to understand the machining forces present while machining aluminum sandwich panel. These tests were performed on a Haas VF-1 vertical milling machine and force measurement utilized a Kistler 9257b 3-axis machining dynamometer connected to a Kistler 5010 3-channel charge amplifier. Each charge amplifier had 180kHz timing crystals. Data collection from the charge amp included a National Instruments 9125 analog input module in a cDAQ 9124 backplane. Connections between the dynamometer and charge amplifier are in an armored cable with BNC connectors to the charge amps. The charge amp output is a +/-10V signal utilizing BNC connection to the data acquisition hardware. A Labview virtual instrument (VI) was created to format, scale, and record data. The VI allows data collection rates to be set

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and the 9215 analog input has the capability of collecting 100,000 samples per second per channel at 16 bit data resolution. For the study, much of the data was acquired at 36,000 samples per second per channel. 3.2. Selection of Time Constants The Kistler 9257b utilizes high impedance piezo-electric sensors, thus requiring the 5010 charge amplifiers (one for each sensor channel.) These charge amplifiers have a selectable time constant which has three options – short, medium, or long time constant (STC, MTC, LTC respectively). The long time constant has the lowest decay rate but can suffer from signal drift and is provided primarily for quasi-static calibration. Conversely, the short time constant experiences little drift but has a high signal decay rate essentially acting as a high pass filter. When measuring with the dynamometer, the proper time constant must be selected that matches the characteristics of the dynamometer and the force profile being measured. Machining forces from this milling process have a medium rate of force input, in this case a 2-flute tool rotating at 6000rpm giving a force input rate of 200Hz. Testing showed the STC to produce non-linear results for the development of the cutting coefficients. This resulted in poor agreement between the model and the experimentally measured data. The medium and long time constant produced linear data that was easily fitted for developing cutting constants. The models produced by this data were much closer to measured data with the LTC data lagging behind the model in phase. The medium time constant had the best fit of the model to the measured data and was therefore used for the honeycomb sandwich panel work under the assumption that the cutting frequency (the driving frequency) is equal to the bulk metal tests, but understanding that there may be some error due to the higher rate of vibrational oscillations seen in the honeycomb data. 3.3. Machining Fixture Honeycomb’s lack of transverse rigidity is one of the challenges of machining the sandwich panel material. In order to be able to compare machining passes at different layers of the honeycomb and to isolate the cutting forces from as many vibrational effects as possible, the material was held on the top of the dynamometer using a special fixture. The fixture, shown in Figure 1 uses a 16mm thick clamping top plate on top of the material which applies an out-of-plane (vertical) compressive load to the sandwich panel while also limiting in-plane (horizontal) motion through the fixture bolts and horseshoe fixture. The frame has window cutouts that allow the slot cutting process to occur just as if the fixture were not in place. The cutter plunges to the side of the panel and feeds into the panel, following the slot in the top of the fixture. While the mass of the fixture will reduce the natural frequency of the dynamometer somewhat, the rigidity of the fixture is much

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4

needed in the honeycomb for results to be compared to bulk aluminum.

Figure 1. A slotted fixture used to reduce in-plane vibration in the honeycomb sandwich panel during slot cutting

* Teklam Face sheet Material

Figure 2. Measured and modelled cutting forces for slot milling of bulk Al 6061-T6

4. Results and Analysis The purpose of the initial cutting tests was to validate and calibrate the model with the identification of cutting coefficients. A set of further cutting tests was designed to decouple the machining forces for the face sheets independently, the honeycomb core independently, and then to measure the cutting forces with combinations of face sheets and core.

The cutting tool used to obtain these coefficients was a two fluted, carbide, ¼” flat end mill with a 30° helix angle. As can be seen in both images, the model amplitude is somewhat larger than the actual data – approximately 8% for the Y axis and 34% for the X axis. The exact source of this overestimate remains elusive, however it is believed to be due to the very small depth of cut and the geometry irregularities that are present in the grind at the very end of the cutting tool.

4.1. Comparison of Force Prediction with Experimental Results for Cutting of Face Sheets To better gage the applicability of the force model, and the limitations of the experimental setup to the study of Teklam®, experimentation was conducted for comparison with the model. A control test using extruded Al 6061-T6 was first performed, followed by testing on the Al 2024-T3 face sheets of the Teklam® sandwich panel. Cutting coefficients were first determined for both materials using a slot cut that put the cutting tool into full engagement, with the results recorded in Table 1. The results in Figure 2 show good correlation between the model and coefficients calculated for Al 6061-T6 with the top face sheet of the aluminium sandwich panel. It is important to note that the cutting feed direction is the Y direction for these tests. Figure 3 shows a similar measurement for a cut made in the face sheet of the honeycomb. Table 1. Experimentally derived cutting coefficients for Al 6061 and Al 5056 Cutting Coefficient Al 6061-T6 Al 2024-T3* Extruded Face Sheet Ktc

1.164 MPa

0.760 MPa

Kte

0.827 N/mm

1.654 N/mm

Krc

0.609 MPa

0.862 MPa

Kre

0.630 N/mm

1.92 N/mm

Process Parameters

Al 6061-T6

Al 2024-T3*

Depth of Cut

1.27 mm (0.05in)

0.61 mm (0.024in)

Spindle Speed

6000 rpm

6000 rpm

feedrates

500-2000 mm/m

500-2000 mm/m

Figure 3. Measured and modelled cutting force results for Al 2024-T3 as is used in the Teklam face sheets

4.2. Observations from Experimental Results The comparison of the cuts in similar materials reveals a number of interesting results. First, the magnitude of the force for cutting the face sheets is approximately 40% lower than the force required to cut the bulk material. This is a reasonable result as the thickness of the face sheet is less than half that of the bulk material. The tempering of the alloy (a T3 treatment softens with aging after heat treatment) means a less hard material which should reduce the cutting force. The most striking difference between the two figures is the higher



Derek Yip-Hoi et al. / Procedia Manufacturing 34 (2019) 385–392

Author name / Procedia Manufacturing 00 (2019) 000–000

frequency oscillation that is overlaid on the force signal of the honeycomb sandwich panel material. This oscillation was seen in the data across the range of feed rates tested 0.127 – 0.212 mm/tooth (60-100ipm). The source of this oscillation is not fully understood and efforts at definitively identifying potential causes were unsuccessful. Differences between the sandwich panel tests and the bulk material tests included the presence of honeycomb panel and the fixture which was slightly different than that used for the bulk material testing. The bulk material appears to also have a high frequency signal imposed on the primary force measurement, but the frequency of the two oscillations does not appear to be the same. Efforts to eliminate this oscillation included increasing the stiffness of the horseshoe fixture and the clamping top plate, and adding a damping material below the sandwich panel material. Neither of these changes made a significant difference on the oscillation.

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4.4. Force Measurement of Honeycomb Panel Cutting Only the Honeycomb Core A set of cutting tests was performed in which a full-

4.3. Analyzing Oscillations in the Data The natural frequency (λ) of the X and Y axes of the dynamometer, as reported by the manufacturer, are λ=2200Hz and λ=2400Hz respectively. As stated earlier, the tool rotation imparts a force oscillation at 200Hz. The guidelines from the dynamometer’s manufacturer state that the dynamometer is expected to have less than 1% error up to 0.1λ (220Hz) and less than 5% error up to 0.2λ (440Hz). This shows that the machining process is easily within the design capability of the dynamometer. These results would suggest that there is an oscillation occurring in the aluminum sandwich panel that is driving a natural frequency in the dynamometer. The highly interrupted nature of cutting as the cutter edges enters and exits different parts of the honeycomb structure could be producing this oscillation. Because the efforts at removing the oscillation mechanically were unsuccessfully, a filter was used to remove the oscillation frequency from the signal. In order to analyze the signal, a fast Fourier transform of the signal was performed which identified the primary signal peak between 2500-3000Hz. A KaiserBessel window low pass filter using a finite impulse response filter was also designed in the MATLAB Filter Designer and applied. The filter utilized a passband ripple frequency of 1000Hz and a stopband ripple frequency of 1500Hz. Figure 4 shows a portion of the cutting force signal, the FFT, and the signal reconstructed from the FFT using 23 selected frequencies below 500Hz. Figure 5 shows the filtered signal from the low-pass filter. While the filtering of data can lead to undesired results, the close proximity of this frequency to the dynamometer’s natural frequency, as found in literature, suggests that there would be little usable data in the frequency regimes removed by the filter. Of passing interest is the alternating difference in peak value that could be attributable to runout.

Figure 4. High frequency oscillations matching the natural frequency of the dynamometer (top), a FFT of those frequencies (middle), and a reconstructed signal from some of the frequencies

Figure 5. The cutting force signature illustrating the signal before filtering (blue) and after filtering (red)

engagement slot cut was made into the honeycomb core material only. The endmill was plunged to depth outside of the part and then a slot milled into the part, proceeding across the entire width of the part exiting on the other side. To enable this cut, the top face sheet was first milled off of the slot area using a tool of slightly larger diameter than the test end mill. This eliminated any direct interaction between the top sheet of the material and the cutting tool. The bottom face sheet was left in place to reproduce the minimal support condition encountered in practice when machining this material. As such, the effects of the top and bottom sheets are not completely eliminated as they act to stabilize the core structure at the bottom and all around the slot being cut. But this is necessary as fully unrestrained honeycomb is not rigid enough to resist. In any slot cut, there is a “climb” side of the cut and a “conventional” side of the cut. The conventional side is the side

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upon which the rotational velocity of the tool tip is in the same direction as the feeding of the tool. The climb side is where the rotational velocity the cutting edge is opposite the feed direction of the tool. And at the front edge of the tool, the rotational velocity of the cutting edge is perpendicular to the feed direction. These three areas are of interest as the cutting action appears to be quite different at each. Figure 6 illustrates the force signature obtained from cutting the honeycomb structure aligned with an image of the structure prior to cutting. The ribbon direction shows the orientation of the bonded double walls where plies were joined before the structure was expanded. Four cells labelled 1-4 have their boundaries highlighted along with the first contact point of that cell with the advancing cutter labelled a-d respectively. The connected wall of an adjacent cell at these first contact points is also accentuated. It is clear that the interaction of the cutter with each of the cells is going to be different. The attack direction of the cutting edge through node ‘a’ in cell 1 has the effect of pulling downwards on the cell putting it under tension. At node ‘b’ in cell 2, the cutting forces are acting directly into and along a bonded double wall. This is a compressive force. And in cell 4, the feed forces of the cutter are also acting to compress this cell at node ‘d’. Force spikes are evident across the pass with the largest, almost 400N occurring when the cutter is engaging with cell 2.

Of particular interest are multi-wall flag formation (case (c)) and collapsing cells (d) – (f)). Referring back to Figure 7 when the cutter passes through node b′ a wall with a free end, bb′ is generated. This wall would fold over the front of the cutter similar to what is depicted in Figure 9(b). As the cutter advances to node b, this leads to an overlapping wall from bb′ being pushed back onto bc. When the cutter penetrates cell 2 at

Figure 7. An enlarged view of cell 2 with the cutter positioned at a 400N force spike in the X direction (200N spike in the Y direction.) At right are images of the slot walls generated on the conventional (top) and climb (bottom) sides of the slot.

Figure 8. The cell cutting and collapse mechanisms as identified for the cutting of composite ply sandwich as reported previously by the authors [2] is indicative of the collapse in aluminum sandwich panel as well Figure 6. Slot milling through 100% honeycomb core with no face sheets in cut shows the material to be cut (above) and the X and Y cutting forces. The green circles represent the tool at different times during the cut.

An enlarged view of cell 2 is shown in Figure 7 with the cutter location aligned with the 400N force spike. Also shown on the right are the conditions of the cut side of the slots at this location, the top showing the side generated from conventional milling and the both from climb. The cause for this force spike is unclear at this time. However, some possibilities can be suggested. We first note that the engagement condition when the spike occurs is not exactly repeated again in the pass studied and no other spikes of this magnitude are present. Prior studies of the authors on the machinability of composite sandwich panel have identified flag formation and cell collapsing mechanisms at work. These are shown in Figure 9. While those studies focused on a different material and cutter trajectories through the cell than those in this work, it is reasonable to assume that these mechanisms are also at work when machining aluminum honeycomb.

node b, the free wall loses its last fixed support. Instead of being restricted to pivot about node b, the entire wall structure is now free to move. We speculate that as the cutter advances further towards node c, the overlapping wall gets dragged into the cutter flute and is torn rather than sheared away from the uncut structure. Evidence that tearing is a mechanism that is at work can be seen from Figure 8. The climb side of all the slots generated during experimentation show significant destruction to the cell wall edges that remain making them difficult to identify. This is in sharp contrast to the conventional side where clear edges are present. These conventional side edges are on wall flags that are bent out of the way as the cutter advances. The serrations that are visible suggest vibrations of the flag against the cutting edge.



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100N is consistent with the forces obtained when cutting just the face sheet described earlier and represented in Figure 3.

Figure 10. A machining cut of 100% honeycomb depth plus the bottom sheet is shown with an image of the core before the cut aligned with the X and Y forces. The top detail shows the cell being engaged that coincides with a force spike of 200N.

4.6. Future Work Figure 9. Conventional (left) and climb (right) honeycomb slot edges generated in cutting. Fraying edges on climb side are evidence of tearing mechanism in the cutting process.

4.5. Force Measurement of Machining Honeycomb with the Top or Bottom Panel Tests were also performed that measured the forces of cutting a face sheet at the same time as a portion of the honeycomb core material. These tests were run with the top face sheet and the top 50% of the honeycomb, the bottom face sheet with the bottom 50% honeycomb, and the bottom face sheet with 100% honeycomb. These are typical axial engagements used in practice. In all tests, the tool was cutting a fully-engaged slot. For those instances when the bottom face sheet is machined, the top face sheet had been cut away from the slot with a larger diameter tool. Figure 10 shows the original structure, the X and Y force measurements, and a close up of the cutter about to engage cell 1 where a force spike is observed. Similar reasoning can be applied to the behavior of the cell walls labelled bb′ and cb as the cutter passes through the end nodes as was presented in previous section, as a plausible explanation for the force spike observed. Also evident from comparing this force signature with the one from machining only the honeycomb, is the superposition of the force’s generated from cutting the bottom face sheet over those from the honeycomb. The magnitude of this band of less than

There is a lot of work yet to do before this cutting process is well understood. The nature of the cutting process described here seems to be strongly affected by tearing of cell walls. Therefore, additional experiments need to be performed where the machining force is collected in addition to high speed videography of the cutting process, sound recording (preferably with frequency analysis), and chip collection for each of the processes. This testing was performed with a commercially available tool that is being used by an industrial research partner, however, literature and tool catalogs present a much broader spectrum of available tools. Study of different tool geometries and the effects on machining force in honeycomb would be a useful endeavor. Finally, because the force signature indicates that the cell walls are undergoing a tearing process rather than a clean shearing process, an effort to move the machining to shear might be advantageous. One proposed method would be to increase the spindle speed to impart more energy which would cause a transition to shearing. 5. Conclusions A machining model was utilized with cutting coefficients obtained from the cutting of bulk aluminum. Further testing was completed to measure the cutting forces produced in slot milling bulk aluminum and aluminum sandwich panel. The machining process was decoupled into individual tests of face sheets, honeycomb core, and combinations of face sheets with core. The testing showed significant vibration for cutting in the

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upper half of the material for face sheets alone, honeycomb alone, and combinations. Machining forces overall showed resonance at the natural frequency of the dynamometer which was removed using a filter. Typical force measurements were in the 100N range for the bottom sheet with the full depth of honeycomb. These forces are higher than much of literature though the literature tends to have very shallow depths of cut. The measurements also contained force spikes as large as 400N that appear to correlate with tangential engagement with multiple wall sections simultaneously. The resulting surfaces of the slots appear to indicate that the material was torn rather than sheared. The results highlight the need for further testing with high speed video and audio recording to further characterize the machining of aluminum sandwich panel. Acknowledgements The authors wish to thank Zodiac Aircraft Cabin Interiors of Bellingham, WA for providing the aluminum sandwich panel used in this research. A machining References [1] Merchant, M.E. (1945). Mechanics of the Metal Cutting Process, ii. Plasticity Conditions in Orthogonal Cutting, Journal of Applied Physics, vol. 16:318-324. [2] Budak, E. and Altintas, Y., (1998).Analytical prediction of chatter stability in milling—part I: general formulation. Journal of dynamic systems, measurement, and control, 120(1), pp.22-30. [3] Wang, F., Liu, J., Li, L., & Shu, Q. (2017). Green machining of aluminum honeycomb treated using ice fixation in cryogenic. International Journal of Advanced Manufacturing Technology, 92(1–4), 943–952. [4] Qiu, K., Ming, W., Shen, L., An, Q., & Chen, M. (2017). Study on the cutting force in machining of aluminum honeycomb core material. Composite Structures, 164, 58–67. [5] Jaafar M, Atlati S, Makich H (2017) A 3D FE modeling of machining process of Nomex® honeycomb core:influence of the cell structure behaviour and specific tool geometry. Procedia CIRP 58:505–510. [6] Gill, D, Yip-Hoi, D., Meaker, M., Boni, T., Eggeman, E., Brennan, A., Anderson, A., (2017). Studying the Mechanisms of High Rates of Tool Wear in the Machining of Aramid Honeycomb Composites., Proceedings of the ASME 2017 20th International Manufacturing Science and Engineering Conference, MSEC2017, June 4-8, 2017, Los Angeles, CA, USA [7] Yip-Hoi, D., Gill, D. (2018). Investigation and Modeling of Flag Generation in Honeycomb Sandwich Panel Machining. Proceedings of the ASME 2018 International Mechanical Engineering Congress and Exposition, November 9-15, 2018, Pittsburgh, PA, USA. [8] Lund, W.C. and Pedersen, D.E., Boeing Co, 1990. Milling cutter for honeycomb core material. U.S. Patent 4,907,920. [9] Van De Bogart, L.J., Boeing Co, 1984. Combination opposed helix router for routing composite material face sheets having honeycomb core. U.S. Patent 4,480,949. [10] Altintus, Y. (2013) Manufacturing Automation: Metal Cutting Mechanics, Machine Tool Vibrations, and CNC Design. Cambridge University Press, NY [11] Spence, A. D., Altintas, Y., & Kirkpatrick, D. G. (1990). Direct Calculation of Machining Parameters from a Solid Model. Computers in Industry, 14(4), 280. [12] Campatelli, G. and Scippa, A., 2012. Prediction of milling cutting force coefficients for Aluminum 6082-T4. Procedia CIRP, 1, pp.563-568. [13] Li XD, Zhou CP, Qiu JH (2014) Mechanical considerations for unfolded honeycomb milling. Fiber Reinf Plast &Compos 6:70–73.