Journal of
Manufacturing and Materials Processing
Article
Process Forces Analysis and a New Feed Control Strategy for Drilling of Unidirectional Carbon Fiber Reinforced Plastics (UD-CFRP) Konstantin Sauer 1, *, Martin Dix 1 and Matthias Putz 1,2 1 2
*
Institute for Machine Tools and Production Processes, Chemnitz University of Technology, 09111 Chemnitz, Germany;
[email protected] (M.D.);
[email protected] (M.P.) Fraunhofer Institute for Machine Tools and Forming Technology, 09126 Chemnitz, Germany Correspondence:
[email protected]; Tel.: +49-371-531-32900
Received: 17 May 2018; Accepted: 11 July 2018; Published: 13 July 2018
Abstract: Reliable machining of carbon fiber-reinforced plastics (CFRP) is the key for application of these lightweight materials. Due to its anisotropy, CFRP is a very difficult material to machine because of occurring delamination and fiber-pullouts. The tool design is especially crucial to minimize and to avoid these processing errors. In this paper a process analysis for drilling is shown for better understanding of the chip formation. Drilling of unidirectional CFRP enables the investigation of the effect of fiber orientation on the chip formation process. In theory, the amount of cut fibers and the cutting angle to the main cutting edge determine the cutting force. Experimental tests with varied macroscopic drill geometries verify this theory. Based on these detected relationships, the tool loads can be calculated for a successful tool design. Keywords: drilling; CFRP; tool design; fiber cutting angle
1. Introduction and State of the Art The implementation of consistent lightweight construction leads to an increased use of composite materials in the automotive industry. Carbon fiber-reinforced plastic (CFRP) in particular is often used for its good strength-to-weight ratio. Due to its structural anisotropy and inhomogeneity of material properties, CFRP is a material which is difficult to drill because of the occurring delamination, fiber-pullouts, and other damage [1]. Drilling in unidirectional (UD) CFRP enables the investigation of the effect of the fiber cutting angle θ (FCA, specified in Table 1) on the tool loads and bore quality. The main objective of this paper is to analyze the chip formation mechanism depending on the fiber cutting angle and to provide a basis for process control and tool design. The induced workpiece damage during drilling CFRP is the subject of many investigations. In this context, Pfeifroth examined the appearance of fiber-pullouts [2]. Eneyew and Ramulu observed the fiber-pullouts from θ = 135◦ to θ = 175◦ and from θ = 315◦ to θ = 355◦ [3]. Other researchers investigated the effect of delamination [4–8]. Chen introduced the delamination factor as a quotient of delaminated diameter to bore diameter for the qualitative delamination description [9]. Among others, Davim et al. [10] and Faraz et al. [11] have added the delamination area as a parameter to this factor. A key finding of the machining analysis is the increase of delamination with increasing feed force [12]. Heisel and Pfeifroth presented that an increasing point angle results in rising feed forces, improved quality at the entrance of the bore, but poorer drill exit quality [13]. Aurich et al. have confirmed these findings in their research investigations [14]. The influence of cutting speed on delamination is controversial, discussed in several scientific papers [15–21]. In general, the influence of the cutting speed on the machining forces and the machining result is negligible compared to the machining of metal. J. Manuf. Mater. Process. 2018, 2, 46; doi:10.3390/jmmp2030046
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Henerichs et al. showed that feed force can be lowered with a high clearance angle and a is controversial, discussed in several scientific general,delamination the influence [22]. smalldelamination wedge angle [22]. A sharp tool with a small cuttingpapers edge[15–21]. radius In reduces of the cutting speed on the machining forces and the machining result is negligible compared the [23]. Tsao and Hocheng showed that the feed force can be reduced with a pre-drilled tohole machining of metal. Sorrentino et al. described the decrease of push-out delamination using feed rate control to apply Henerichs et al. showed that feed force can be lowered with a high clearance angle and a small various feed rates during the drilling process [24]. They reduced the feed rate in the final phase of wedge angle [22]. A sharp tool with a small cutting edge radius reduces delamination [22]. Tsao and drilling to reduce the that thrust lower push-out delamination this[23]. way. Tsao andetHocheng Hocheng showed theforce feed and forcetocan be reduced with a pre-drilledin hole Sorrentino al. investigated the effect of different drill bits applying twist drill, saw drill, candle stick step drill, described the decrease of push-out delamination using feed rate control to apply various drill, feed rates and during core drill Karpat[24]. andThey Bahtiyar used, for their research the [25,26]. drilling process reduced the feed rate in the finalwork, phase aofspecial drilling polycristalline to reduce the thrust force and to lower push-out delamination Tsao Hocheng investigated diamond (PCD) drill with double point angle [20]. In in thethis testway. series byand Grilo et al. [21], the spur drill the effect of different achieved the best results.drill bits applying twist drill, saw drill, candle stick drill, step drill, and core drill and of Bahtiyar used, for their edge research work, with a special polycristalline diamond The [25,26]. on-lineKarpat recording the current cutting position respect to the specimen during (PCD) drill with double point angle [20]. In the test series by Grilo et al. [21], the spur drill the machining process is described in Reference [27]. Schütte discovered that the cuttingachieved force causes the best results. the rise of feed forces while drilling with pilot bore [28]. Further, he described the minimum of feed The on-line recording of the current cutting edge position with respect to the specimen during force and passive force at θ = 45◦ . Eneyew and Ramulu showed different results in thrust force the machining process is described in Reference [27]. Schütte discovered that the cutting force causes progression [3]. They have determined thepilot minimum at Further, approximately 180◦ the . minimum of feed the rise of feed forces while drilling with bore [28]. he described Several have of showed FCA on the cutting process applying force and scientists passive force at θinvestigated = 45°. Eneyewthe andeffect Ramulu different results in thrust force orthogonal cut: [3]. They have determined the minimum at approximately 180°. progression ◦ and ◦ ≤ θ ≤ 180◦ , Several scientists havethe investigated the force effectatof on120 the cutting process applying ≤θ≤ Lopresto et al. determined largest cutting 90◦FCA the least at 150 orthogonal the greatest feedcut: forces appeared at θ = 30◦ [29]. Wang and Zhang measured the largest cutting force at ◦ Lopresto et al. determined theθ largest cutting force at 90° ≤ θ ≤ 120° and the least at 150° ≤ θ ≤ θ = 120 and the largest feed force at = 60◦ [30]. 180°, the greatest feed forces appeared at θ = 30° [29]. Wang and Zhang measured the largest cutting Chen et al. set up a model for predicting the cutting forces, dividing the process into the areas: force at θ = 120° and the largest feed force at θ = 60° [30]. 0◦ < θ < 90◦ , 90◦ ≤ θ < 180◦ , and θ = 0◦ [31]. Chen et al. set up a model for predicting the cutting forces, dividing the process into the areas: 0° < θ < 90°, 90° ≤ θ < 180°, and θ = 0° [31].
2. Experimental Setup
2. Experimental All experimentsSetup were performed on a conventional machining center. The forces and torque were
determined a rotating FCA was calculated determining the angle position All using experiments weredynamometer. performed on The a conventional machining by center. The forces and torque of the main spindle to the fiber orientation by using a light barrier (cf. Figure 1). The specimens were determined using a rotating dynamometer. The FCA was calculated by determining the angle of the main spindle the ×fiber orientation using light barrier (cf.= Figure 1).and The[mm] had position the dimensions [mm] 200 ×to200 5 for the boresby with thea diameter dtool 10 mm, specimens 200 × 200 ×d5tool for=the bores with diameter dmaterial tool = 10 mm, and 200 × 200 × 10had forthe thedimensions bores with[mm] the diameter 25.4 mm. Thethe workpiece is UD-CFPR [mm] 200 ×volume 10 for the bores (type with the diameter dtool = K63712) 25.4 mm.inThe workpiece material (100% 0◦ ), 200 60%× fiber content of fibers: Dialead epoxy resin. Each testisin this UD-CFPR (100% 0°), 60% fiber volume content (type of fibers: Dialead K63712) in epoxy resin. Each paper was carried out three times, that is, replicated twice. In relation to the state of the art, the cutting test in this paper was carried out three times, that is, replicated twice. In relation to the state of the speed for all tests was selected to be very low (vc = 0.63 m/min) in order to be able to apply the new art, the cutting speed for all tests was selected to be very low (vc = 0.63 m/min) in order to be able to strategies in Section 3.3, using the position control of the main spindle. apply the new strategies in Section 3.3, using the position control of the main spindle.
Light barrier Extraction system Rotating dynamometer
Clamped specimen
Figure 1. Experimental setup.
Figure 1. Experimental setup.
All bores were pre-drilled to avoid chisel edge effects (cf. Table 1). The delamination factor according Chenpre-drilled [9] of peel-up push-out of the hole did notfactor All borestowere to delamination avoid chiseland edge effectsdelamination (cf. Table 1). Thepilot delamination exceed 1.01. All instances of delamination were measured using a measuring light microscope, according to Chen [9] of peel-up delamination and push-out delamination of the pilot hole did
not exceed 1.01. All instances of delamination were measured using a measuring light microscope,
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Makrolite from Vision Engineering. The illustration of a drill rotation in Section 3 is carried out under 3 of 7 drill full engagement. The following parameters have been implemented for this purpose.
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Makrolite from Vision Engineering. The illustration of a drill rotation in Section 3 is carried out under drill full engagement. Table 1. Parameters and experimental setup for the test series. J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 3 of 7 following parameters have been implemented forand this purpose. ToolThe Diameter Pilot Hole Machine No. T1 Tool No.
T2
T1
T2
T3
T3
dtool
Angles
Tool Engineering. The illustration of a drill rotation Fiber Cutting (FCA) θ Makrolite from Vision in Section Angle 3 is carried out under Measurement d pilot drill full engagement.
σ = 85° Table 1. Parameters and experimental setup for the testpurpose. series. The following parameters have been implemented for this 10 mm γ ≈ 4° Pilot Hole and setup for the test series. Diameter β ≈ 76° Table 1. ParametersMachine and experimental Tool Fiber Cutting Angle (FCA) θ Angles 1.2 mm dpilot Measurement dtool Tool Diameter Pilot Hole Machine and DMGTool Mori σ = 118° Angles Fiber Cutting Angle (FCA) θ No. dtoolTwist drill, dpilot σ = 85◦ DMC 850 V, Measurement 10 mm γ ≈ 38° σ = 85° ◦ solid carbide, 10 mm γ≈4 Kistler rotating T1 10 mm γ ≈ 4° β ≈ 40° uncoated, β ≈ 76◦ β ≈ 76°dry dynamometer DMG Mori 1.2 mm σ = 118° 1.2 mm machining TwistDMG drill, Mori σ = 118◦ Twist drill, 9170A DMC 850 V, T2 10 mm γ ≈ 38° σ = 118° DMC 850 V, ◦ solid carbide, 10 mm γ ≈ 38 Kistler rotating β ≈ solid 40° carbide, uncoated, dry 25.4 mm γ ≈ 38° ◦ 3.05 mm Kistler rotating dynamometer β ≈ 40 uncoated, dry machining 9170A dynamometer σ = 118° β ≈ 40° ◦ 25.4 mm
3. Results
σ = 118 γ ≈ 38◦ β ≈ 40◦
T3
25.4 mm
3.05 mm
machining 3.05 mm
γ ≈ 38° β ≈ 40°
9170A
3. Results
3. Results 3.1. Comparison of the Different Drill Geometries 3.1. Comparison of the Different Drill Geometries The feed force and torque of the drilling tests with different rake angles are compared with each 3.1. Comparison of the Differentother Drill in Geometries 2. Due to the different tool geometries it can be determined that different The feed force and torque of theFigure drilling tests with different rake angles are compared with each characteristics occur and that the fiber orientation has a strong influence on the progression of the otherThe in feed Figure Due to tool the different it rake can angles be determined that with different force2.and torque ofloads. the drillingtool testsgeometries with different are compared each characteristics occur that the fiber a strong influencethat on different the progression of the other in Figure 2. Dueand to the different toolorientation geometrieshas it can be determined characteristics tool loads. occur and that the fiber orientation has a strong influence on the progression of the tool loads.
Figure 2. Comparison of feed force (left) and torque (right) during drilling with pilot bore using two different tool geometries.
Contrary to the research results of other researchers [13], the higher feed force was measured at the drill with lower point angle of σ = 85°. The chisel edge cannot influence the feed force progression because the bores were drilled with a pilot bore of dpilot = 1.2 mm. The basic level of feed force of the drill is higher with a smaller rake angle. The general course of the feed force and torque graphs can be explained as follows: With a (left) large rake angle γ, such(right) as on tool T2, the major cutting edgepilot grips under fiberstwo at θ Figure 2. 2. Comparison of feed feed force force and torque during drilling with bore the using Figure Comparison of (left) and torque (right) during drilling with pilot bore using two = 0°, creating a small leading crack. The fibers run over the rake face and cause a negative feed force. different tool tool geometries. geometries. This mechanism can be used to reduce delamination when the drill bit exits at a defined fiber cutting different angle. Thus, the feed force would orient into the composite material that would support itself. Due the described properties, the large rake angle is difficult to handle at the bore entrance. The cutting Contrary to the researchto results of other researchers [13], the higher feed force was measured at peelsof upother the first layer and encourages delamination at the feed drill entrance. Contrary to the researchwedge results researchers [13], the higher force was measured at In σ the=2nd quarter the fibers edge are preloaded according to Pfeifroth [2], thatforce is, theprogression fibers are the drill with lower point angle of 85°. The chisel cannot influence the feed the drill with lower point angle of σ =clamped. 85◦ . The chiselstiffness edge across cannot forcespring progression bilaterally The reduced the influence fiber directionthe and feed the resulting back because the bores were drilled with a pilot bore of dincrease pilot = 1.2 mm. The basic level of feed force of the effect of the ensure feed force The up to θbasic < 135°.level In this range, the preload because the bores were drilled with a composite pilot bore ofandpilot = in1.2 mm. of feed force isof the by cutting on one side. The course preloaded of fibers canfeed be separated precisely by the cutting can drill is higher with a smaller dissolved rake angle. The general the forcemore and torque graphs drill is higher with a smaller rake angle.they The general the infeed torque can be edge because are fixed at bothcourse sides. Thisof results a highforce torqueand and good surfacegraphs quality. The
be explained as follows: explained as follows: With a large rake angle γ, such as on tool T2, the major cutting edge grips under the fibers at θ With a large rake angle γ, such as on tool T2, the major cutting edge grips under the fibers at = 0°, ◦creating a small leading crack. The fibers run over the rake face and cause a negative feed force. θ = 0 , creating a small leading crack. The fibers run over the rake face and cause a negative feed force. This mechanism can be used to reduce delamination when the drill bit exits at a defined fiber cutting This mechanism can be used to reduce delamination when the drill bit exits at a defined fiber cutting angle. Thus, the feed force would orient into the composite material that would support itself. Due angle. Thus, the feed force would orient into the composite material that would support itself. Due to to the described properties, the large rake angle is difficult to handle at the bore entrance. The cutting the described properties, the large rake angle is difficult to handle at the bore entrance. The cutting wedge peels up the first layer and encourages delamination at the drill entrance. wedge peels up the first layer and encourages delamination at the drill entrance. In the 2nd quarter the fibers are preloaded according to Pfeifroth [2], that is, the fibers are In the 2nd quarter the fibers are preloaded according to Pfeifroth [2], that is, the fibers are bilaterally clamped. The reduced stiffness across the fiber direction and the resulting spring back bilaterally clamped. The reduced stiffness across the fiber direction and the resulting spring back effect of the composite ensure an increase in feed force up to θ < 135°. In this range, the preload is effect of the composite ensure an increase in feed force up to θ < 135◦ . In this range, the preload is dissolved by cutting on one side. The preloaded fibers can be separated more precisely by the cutting dissolved by cutting on one side. The preloaded fibers can be separated more precisely by the cutting edge because they are fixed at both sides. This results in a high torque and good surface quality. The edge because they are fixed at both sides. This results in a high torque and good surface quality. The fiber cutting angle θ = 135◦ is equal to angle of 45◦ between the major cutting edge and the fibers.
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fiber cutting angle θ = 135° is equal to angle of 45° between the major cutting edge and the fibers. Here, most uncut fibers are located underneath the major cutting edge due to the three-dimensional its maximum maximum due due to to the the spring spring back back effect effect of of the the composite. composite. cutting process. The feed force reaches its ◦ θ == 180 180° because the maximum amount of fibers in The second torque maximum appears at θ radial direction is in contact in this cutting edge position. In the 3rd quarter there is no more preload ◦ a crack AsAs just described at 0° a crack appears that on the fibers. fibers. They Theyare areclamped clampedonly onlyon onone oneside. side. just described at
