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Using FEA to design twist drill geometry Využití metody FEM pro konstrukci šroubovitého vrtáku 1 2 3 4 5

Roud, P. , Zetek, M. , Sklenička, J. , Česáková, I. , Kožmín, P. , FST, ZČU v Plzni, Czech Republic, 5HOFMEISTER s.r.o, Czech Republic e-mail: [email protected], [email protected], [email protected], [email protected], [email protected] 1,2,3,4

Abstrakt Cílem tohoto článku je představit možnosti využití metody FEM pro účely konstrukce monolitního šroubovitého vrtáku. Pro tento účel bylo provedeno několik numerických analýz zaměřených na vliv geometrie šroubovitého vrtáku na tvorbu třísky. Výsledky z numerické simulace byly dále konfrontovány s realitou pomocí experimentálního měření. Hlavními sledovanými veličinami byly tvar třísky a velikost řezných sil. Klíčová slova: FEM, Monolitní šroubovitý vrták, porovnání simulace-experiment Abstract The main aim of this study is to present the use of FEM for designing a solid twist drill. A set of numerical simulations were conducted. The main observed parameter was the influence of the twist drill’s geometry on chip formation. The results from numerical simulation were correlated with experimental measurements. The main observed parameters were chip shape and cutting forces. Keywords: FEM, solid twist drill, correlation simulation-experiment, 1 INTRODUCTION The increasing requirements of the market, such as higher productivity, machining new types of exotic materials and so on, pushes manufactures to improve their ingress to cutting tool design. The use of CAD systems is common nowadays. The main advantage of using CAD is well known, i.e. the associativity between a parametric model and drawings. The main disadvantage is that the designer has no information about the cutting tool’s performance until it is experimentally tested. In other words, the designer relies on a trial and error system, which is very time consuming. Therefore it would be desirable to have information about a cutting tool’s performance before it is physically made. The answer is numerical simulation, especially FEM. This method has proven its value in the automobile, aerospace and power industries, where it is used via software packages. The authors used the FEM found in the commercially available software package AdvantEdge FEM, for the design of solid twist drill geometry. In section two the results from the numerical simulations are presented. The comparison between results from simulations and experimental measurement are compared in the third section. The last section is a brief conclusion.

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz

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NUMERICAL SIMULATIONS

2.1 Influence of twist drill geometry on chip formation and cutting forces Two solid twist drills were created for this purpose, with different values of point and helix angle. The exact values of the point and helix angle are confidential. For this reason only the ranges of these parameters will be presented. The helix angle is between and the point angle is between . Other parameters, such as type and length of chisel edge, were the same for both geometries. Both drills have a diameter of 12mm. Models of the twist drill were imported into AdvantEdge FEM, where the boundary conditions and technological parameters were set up. The cutting speed was set to vc=50m/min and the feed to frev=0,25mm/rev, under dry conditions. The workpiece material was aluminum alloy Al 6061-T6, with Rm =310 MPa. Geometry with a smaller helix and point angle is denoted as geometry A, the other one is denoted as geometry B. The first observed parameter was chip shape see. Fig 1.

Fig.1a Geometry A

Fig.1b Geometry B¨ Fig.1 Chip shape comparison

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz

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As can be seen from Fig. 1, both tools tend to produce a segmented chip, which means we can focus on other parameters. If we have a closer look at the temperature field distribution over the chip, geometry B produces lower temperatures in the middle area of chip.

Fig.2a. Geometry A

Fig.2b. Geometry B Fig.2: Comparison of chip flow direction

Chip flow direction is compared in Fig. 2. From Figs. 2a and 2b it is clearly visible that the chip produced by geometry B has a steeper flow direction with respect to the drill’s axis and also the chip is more closed. This means that chips are better directed in to the flute. This

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz

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PAPER PROCEEDINGS

ISBN 978-80-7414-156-0

helps in chip removal and it also prevents contact between the chip and the wall of the hole, thus a lower surface roughness can be achieved. The last parameter to be compared was the magnitude of the cutting forces, see Fig 3.

Fig.3a Feed force Ff [N]

Fig.3b Cutting moment Mc [Nm] Fig.3 Cutting force comparison

In Fig. 3a it can be seen that the feed force which acts in the direction parallel to the drill’s axis is the same for both geometries. For Mc the geometry B produces lower torque. Exactly 5Nm as opposed to 5.5Nm which was produced by geometry A. This coincides with the temperature field distribution, where geometry B also has lower values for the temperature field. In conclusion, geometry B was chosen as more appropriate for drilling into Al6061 T6 alloy. 3 COMPARISON OF EXPERIMENTAL AND FEM SIMULATION RESULTS Based upon results from numerical simulations, the drill with geometry B was made. The geometry was experimentally tested on vertical milling machine MCV 750A. A 4-component piezo-electric dynamometer Kistler type 9272 was used to measure cutting forces. The data were post processed in LabView 8.2 software. The main observed parameters were chip shape, feed force Ff and cutting moment Mc. The technological parameters were the same as in the simulations: vc =50m/min, frev =0.25mm/rev and under dry conditions. The depth of holes was 15mm.

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz

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Comparison of chip shape

Fig.4a Chip produced by real drill

Fig.4b Chip predicted by AdvantEdge FEM Fig.4 Comparison of chip shape

Fig. 4a shows chips produced by the twist drill during the experiment. The chip in the blue ellipse was produced when the drill tip engaged into the workpiece. When the main cutting edges were fully engaged in the workpiece material, the drill started to produce segmented chips. These chips are in the red ellipses. When comparing with the chip predicted by AdvantEdge FEM it is clearly visible that chips from the experiment and the predicted model are identical. This is a very important conclusion, because it is possible to observe chip formation, before the physical prototype is made, which makes it possible to save time and costs for developing new drill geometries. Therefore, it can be said that the capabilities of the software can be used for predicting chip formation and thus the software should be a valuable tool for the cutting tool designer.

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz

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Comparison of cutting force

Fig. 5a Ff [N] comparison

Fig. 5b Mc [Nm] comparison Fig. 5 Cutting force comparison

Fig. 5a shows the graph for feed force Ff, which acts in the direction parallel to the drill’s axis. From the graph it can be seen that the trends of both forces are the same. The measured feed force was FfE=460N, while the force predicted by the software was about FfP = 350N. That is about 25% difference. But we must keep in mind that every numerical solution has some precision. What is more important is that the software fits in the trend of feed force Ff. In the case of cutting moment Mc from Fig. 5b the difference between predicted and measured Mc was about 30%. The precision of the solution is the same as in the case of feed force and it must be noted that if more precise cutting forces are required, a finer mesh should be used. 4 CONCLUSION The main conclusions of this paper are: • It is possible to use AdvantEdge FEM software for performance evaluation of different solid twist drill geometries

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz

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• •

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ISBN 978-80-7414-156-0

The chip formation predicted by the software 100% matches the real chip formation The difference in cutting forces were within a range of 25-30% but the trend is the same as in the real experiment

Acknowledgments The article is based on results from research project MPO FI-IM4/226 supported by the Ministry of Industry and Trade of the Czech Republic. The authors also would like to thank HOFMEISTER in the Czech Republic for manufacturing the solid twist drill prototype. We are also grateful to Mr. Bradley P. Ragozzino and Mr. Luis Zamorano from Third Wave Systems for their technical support.

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Date of Conference: 21st January 2010 Available Online on: http:\\casopis.strojirenskatechnologie.cz