Testing of the External Turning Tool Holder Equipped With Alternate Bars in Its Construction

Transcription

1 ISSN MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 21, No Testing of the External Turning Tool Holder Equipped With Alternate Bars in Its Constrution Marten MADISSOO 1, Argo ROSSNER 1, Jüri OLT 1, Viaheslav MAKSAROV 2 1 Estonian University of Life Sienes, Department of Agriultural and Prodution Engineering, Fr. R. Kreutzwaldi 56, Tartu 51014, Estonia 2 National Mineral Resoure University, Department of Mehanial Engineering, 2, Line 21, Vasilievsky Ostrov St. Petersburg Russia Reeived 16 June 2014; aepted 06 September 2014 The paper fouses on testing the utility effet of a utting tool holder with inserted alternate bars of different material struture in its onstrution. The different bars are made from rolled steel and have an anisotropi struture whih also has different mehanial properties. This onstrution is onsidered as a method to improve the utting stability of the utting tool and helps to redue the osillations in the finishing turning. A method for the onstrution of the damped instrument holder is proposed. A series of omparative experiments was arried out with both the standard and the modified tool holder. In the utting tests only the feed rate was hanged, utting speed and the depth of ud remained always the same. All tests were repeated three times. After eah utting the surfae roughness of all the treated surfaes were measured and ompared. Surfae roughness measurements revealed that the average surfae roughness improved 9 % with the use of the modified tool holder. Only in one ase, at the feed rate of 0.1 mm r -1 a poorer surfae roughness with the modified holder was ahieved. Keywords: hatter vibrations, metal utting, turning tool holder, damping of osillations. 1. INTRODUCTION There are a lot of different tool holders available for a large variety of speifi mahining requirements. At the same time also the material of the tool holders an be different. For example in mahining of hard workpiee materials very high stresses at on the tool holder through the utting tool. These stresses require the tool holder to have some speifi properties. Like high stiffness and it should be able to absorb the energy generated during the utting proess. It is also important that the material ost of the tool holder is taken into aount beause a lower ost provides a ompetitive advantage for manufaturers. In addition, the material properties of the tool holder an have also a large influene on both the surfae quality and dimensional auray of the mahined workpiee, and on the life of the utting tool [1]. The material properties of the workpiee and various other fators, an lead to exessive vibrations in the tool shaft, whih in turn auses undesirable hattering. From the standpoint of dynami stability we assume that the tool is the weakest subsystem of the turning proess [2]. The impat of periodial external fores ausing an osillatory proess with a frequeny equal to the exiting fores or omplex periodi proesses aused by nonlinear properties of the system, having its own damped and fored osillations of parametrially exited osillations and self-osillation system an violate the stability of the turning system [3]. The intensity of the fored vibrations is partiularly great in the resonant region, whih must be avoided in metal utting mahines as an operating method for finishing. One Corresponding author. Tel.: ; fax: address: marten.madissoo@emu.ee (M. Madissoo) aspet of improving the dynami stability of the subsystem "tool" is the reation of damped tools with inreased resistane, with elasti and damping elements that do not hange their appearane. It is important for the damped instruments to be haraterized by its adaptation to the variable toleranes, load balaning between the utting edges, as well as to prevent breakage of the utting edge. When examining the different tehniques that are known for enhaning dynami stiffness and hatterresistane of utting tools. We found that one possibility is to use different material bars with speifially assigned orientations to the stiffness axes in the tool. Aording to the studies made on boring bars by Eugene [4], the theory is that there is speifi orientation of stiffness axes relative to the utting fores, resulting in a signifiant inrease in dynami stability. It was deided to implement the same onept to external turning tool holders. Consequently the aim of the work was to reate a passive vibration damping tool with insert bars of anisotropi material properties integrated into the tool holder shank to improve the dynami behaviour of the utting tool. In the beginning the possible effets to external turning were questionable. Beause the holder is quite rigid and hatters vibration is a problem only in some diffiult situations where the utting fores are high. So in order to determine the effet of reduing the level of osillation when using the tool holder equipped with alternate bars in its onstrution we arried out a series of utting experiments ompared with an equivalent tool. 2. MATERIALS AND METHODS The term hatter is known as self-indued or regenerated vibrations, whih means that the movement of 391

2 the utting tool is affeted by existing fores and ourring movements. The fores resonate beause of the relatively little damping ability of the system and this leads to an unstable mahining proess [3, 5]. The removing of material from the workpiee with the lathe tool leaves a wave-ut surfae, whih is shown in Fig. 1. This is aused by the defletion in the diretion of the utting speed vetor. The defletion leads to the displaement of the utting edge in the feed diretion [6]. As a onsequene of this the spindle speed (n [rev/s]) and the hatter vibration frequeny ( ) ω have a relationship that affets the dynami hip thikness. Let us assume that the hatter vibration frequeny is ω [rad/s] or f [Hz]. The number of vibration waves left on the surfae of the workpiee is f ε f [ Hz] T[ s] = = k +, (1) n 2π where k is the integer number of waves and ε 2 π is the frational wave generated. The angle ε represents the phase differene between the inner and outer modulations. If the spindle and vibration frequenies have an integer ratio, the phase differene between the inner and outer waves on the hip surfae will be zero or 2 π ; hene the hip thikness will be onstant despite the presene of vibrations (Fig. 1). In this ase, the inner ( y ( t) ) and outer ( y( t T )) waves are parallel to eah other. Fig. 1. Wave generation [6] If the phase angle is not zero, the hip thikness hanges ontinuously. Consider k integer number of full vibration yles and phase shift 2, (2) π f T = 2kπ + ε where the phase shift between the inner and outer waves is ε = 3 π + 2ψ. The orresponding spindle period T[s] and speed n[rev/min] is found to be 2kπ + ε 60 T = n 2πf T. (3) In order to suppress the vibrations generated in suh a way the turning tool holder with alternate bars will be introdued. The peuliarity of the tool fitted with the proposed method is to inrease the damping ability and redue the level of self-osillations arising in the proess of utting. It is ahieved through the orderly disorientation of alternate bars with anisotropi material textures that form the holder s ore. This onstrution enables effetively dissipate the energy of the vibration waves at the boundaries of the transition points between the holder different bars. It also makes it more diffiult for the emergene of the resonane frequeny in the struture. The wave spread in different environments and materials are of a different nature. Flexible substrates osillation to our is alled the mehanial wave. Therefore, it is neessary to take a loser look at the environmental impat of wave diffusion and wave interferene phenomenon. By the transition from one material to another the parameters of the wave hange and it is resulting from the material properties suh as density and rigidity. In rigid materials is the wave diffusion speed signifiantly higher than in nonrigid materials and in high density materials is the rate of spread signifiantly lower than in low density materials. The wave dispersal speed is alulated by the following formula [7]. B v w =, (4) ρ where v w is the wave dispersal speed in the material m s -1, B is the ompressibility modulus Pa, ρ is the material density kg m -3. Another important feature of wave propagation from one medium to another is refration. Refration onsists in the hanges in wavelength by going from one medium to another [7]. This auses a hange in the wave propagation veloity. The transition to a softer material leads to a shortening of the wavelength and diretion hange. The hanges in diretion result from the material properties and are material-speifi [7]. The main vibration damping, however, is resulting from different defletions aused by vibrations in material pairs. The resulting situation an be desribed by the Hooke's law, aording to whih the elasti fore generated in the body is proportional to the hange of the body length x [7]. Hooke's law is written as follows: = k x, (5) F e where F e is the elasti fore generated in the body N, k is the material stiffness fator N m -1, x is the lengthening m. 3. EXPERIMENTAL DETAILS The basi priniple behind the modified tool holder lies in the vibration energy hange indued by the different materials struture and the emerging frition between the material pairs. An important role in the suppression of vibrations has the defletion of the material pairs resulting in different sized elasti fores. For further details, the material of the alternate bars is different from the tool holder material. The struture of the inserted bars is speified in the diretion of rolling, and the diretion of the struture is exatly known. In this way, the natural frequeny of the tool holder is hanged. This design should provide effetive vibration energy dissipation in the ontat surfaes between the different materials. To bring out the differenes in the struture of the tool holder and the additional omponents materials hardness tests and mirostruture examinations were made (Fig. 2). On the left-hand part of the figure is the steel with ferriteperlite struture, in whih the light part is ferrite and darker part is perlite. 392

3 Fig. 2. Material struture of inserts (Hydax 25) and the tool holder (DCLNR2020K12) The gray shades of material partiles represent martensiti struture [8, 9]. The steel has a Rokwell hardness of 81.8 HRB. On the right-hand part of the figure is the tool holder steel whih is very dense and finegrained. The struture is haraterized by high homogeneity and it has a tempered martensiti steel struture. The steel has Rokwell hardness of 114 HRB. As disussed above we an assume that the ombination of the strutures with high hardness and medium plastiity will help to improve the vibration damping properties of the new tool holder. In the researh we tested the modified utting tool holders (Fig. 3) for finishing turning at the optimum utting depth at high utting speeds. The turning tests were arried out on a Haas SL10-THE lathe. No oolant or other lubriants were used amplifier. The amplified signal was send to the omputer for reording via an analogue-to-digital onverter BMCM AD16f USB. Fig. 3. The Modified utting tool holder Both the original and the modified holders were equipped with the Mitsubishi insert CNMG SH UE6110, whih is well suited for today inreasingly growing dry proessing. The inserts utting edge radius was 0.4 mm and it is designed for finishing proessing. The reommended utting parameters were: a) utting depth ap = mm; b) feed rate f = mm r -1 ; ) utting speed v = m min -1 [10]. During the tests, vibrations were measured with the piezoeletri aeleration sensor PCB 603C01 (Fig. 4) whih meets the standard ISO In addition, the aeleration sensor signal was amplified with GUNT PT Fig. 4. Aeleration sensor PCB 603C01 The roughness measuring devie MarSurf PS1 was used for surfae roughness measurement. The devie is used to ontrol the quality of produts and the determined roughness Ra meets standard ISO 4287:1997 [11]. The turning test was performed with the original holder and then with the material Hydax 25 modified holder. During these tests the same workpiee of the material S335J2+AR and utting parameters were used. Only the spindle speed inreased beause of the derease of the workpiee diameter but the utting speed reminded the same. Four different feed rates were used and with eah feed rate three runs were made with both the modified and the original tool holder. This allowed obtaining omparable results by proessing the workpiee. A total of 24 utting test were made. In addition two tests were made to analyze the utting tool holder s natural frequeny. For this purpose the tool holder whih was attahed to the lathe was hit with a hammer for a single time and the resulting vibration pulse was saved. Also a free running test was performed to investigate the value of the natural frequenies of the lathe. This operating state allows the analysis of the natural frequeny of the lathe spindle, the workpiee and of the whole system to failitate further analysis of the signals. The test setup is shown in the Fig

4 the FFT transform of the signal was performed. In the following Fig. 6, the test results are given. For all analysis the features are identified in the figures as follows: the thin line is for the modified tool holder equipped with Hydax 25 steel and the filled area is for the original tool holder. Fig. 5. The test setup: 1 aelerometer; 2 signal amplifier, 3 analog-to-digital onverter; 4 data reorder (PC); L tool holder protrusion The vibration parameters obtained from the turning experiments were stored in a omputer for future analysis. For evaluating the effiieny of the damping method the proessed surfae roughness is measured after eah feed rate series. The obtained vibration sensor signals were reorded and analyzed with the program Nextview RESULTS After the turning experiments the surfae roughness of all the treated surfaes were measured. In one feed rate series the surfae roughness reated with both the holders was measured and their average values were alulated. Then the average value differene between them was alulated. A negative differene indiates a worse surfae roughness rated with the modified tool holder ompared to the original holder and positive differene means a better surfae roughness rated with the modified tool holder. The average values of the surfae roughness are listed in the following Table 1. Table 1. Roughness measurements after all feed rate sequenes Fig. 6. Power pulses aused by the vibrations of the original holder and with the Hydax 25 steel modified holder Aording to the Fig. 6 it an be seen that the installation of additional omponents from Hydax 25 steel made it possible to signifiantly redue the vibrations in the region of 1000 to 2500 and 3200 to 4500 Hz, while reating new, low-amplitude vibration frequeny in the region 500 to 1000 Hz and 2500 to 3200 Hz. The best vibrations damping was reahed at the feed rate of 0.3 mm r -1. Also the above-mentioned surfae roughness variation was the best at this feed rate. The frequeny representation of the vibrations is shown in the Fig. 7. feed rate f,mm r -1 original Hydax 25 variation surfae roughness Ra, μm % Table 1 indiates that only in one ase the obtained surfae roughness with the modified utting tool holders was worse than the surfae roughness obtained with the original holder. For the vibration analyze the omputer software NextView was used, whih allows the data reading from an analogue-to-digital onverter and fast Fourier transform (FFT) onversion. In this way, the frequeny of presentation of the resulting data enables the omprehensive analysis of the vibrations generated. For all the vibration signals a set of data points from one-seond is used for the analysis. The data set is seleted after the visual unifiation of the vibration signal. The amount of data inludes 15,000 measurements, aording to the Nyquist-Shannon theorem this is the minimum measurement points to ahieve a measuring range of 7500 Hz [12]. First, to determine the natural frequenies of the different tool holders they were hit with a rubber hammer and the generated vibrations were measured and Fig. 7. The frequeny representation of vibrations at the feed rate of 0.3 mm r -1 In the region 2500 to 3500 Hz relatively large vibrations are notied in the ase of the modified tool holder. However in the higher frequeny spetrum an attenuation of vibrations from 4800 Hz is notieable with the utting tool holder with additional steel bars. Considering the ahieved surfae roughness and ourring vibrations it is possible to onlude that the most important influene in these experiments had high-frequeny vibrations, whih are above 4500 Hz. 5. DISCUSSION Aording to the test results the modified tool ompared to an analogue utter improves the quality of treatment. The vibration data analysis onludes that the 394

5 modified utting tool holders has been able to suppress vibrations at high, frequenies above 4000 Hz. Pratial issues related to the ontrol seletion of the utting proess, are depending on the requirements of dimensional and geometri auray of mahined surfaes [13], whih an be solved using an instrument equipped with alternate bars. Aording to the results of experimental studies using a frational experiment obtained the funtional dependene of the atual margin of auray of the proessed surfae. The funtional dependene of the atual margin of auray in roundness ϕ 06 (Eq. 6), ovality ϕ Nk (Eq. 7) and oneϕ k (Eq. 8) are obtained as the ratio of tolerane to proess parts to the field of geometrial deviations of the form of dispersion as a funtion of utting speed, depth of ut and feed by using a modified instrument t s = v 6 t = o s 1 = t s ϕ ; (6) Nk ϕ ; (7) 41 v ϕ. (8) k v = The alulated values ϕ f ( v, s, t) of the funtions allow you to predit the atual value of the stok on the auray of the deviations of geometri shapes, mahined surfaes, depending on the set of the utting tools, as well as provide an opportunity to make a omparative analysis of the dependene of the atual margin of auray of the utting onditions for finish turning utting tools equipped with alternate bars in the holder and tools ounterpart. The atual margin of auray in the proessing with the proposed utter holder design is higher than when operating with an analogue utter on the same utting onditions. 6. CONCLUSIONS 0.74 The developed method for inreasing the stability of the utting proess by finishing turning is based on the use of anisotropi properties of plastially deformed strutural steel and the effets of dissipation of vibration waves in the transition setion between the bars whih form the texture of the modified tool holder. Based on the studied physial properties of Hydax 25 steel and the original tool holder the modified utting tool holder onstrution was reated. First experimental researh examining the influene of the damping properties of the holder to the parameters of utting and quality of the proessed surfae were arried out. Surfae roughness measurements of the test results revealed that only in one ase a poorer surfae roughness was ahieved with the modified holder at low feed rate of 0.1 mm r -1. However, in other ases the results showed that the surfae quality is better at higher feed rates. This means that the solution meets its objetive to inrease produtivity without any loss of quality. Materials analysis and hardness results onfirmed the suitability of the material properties for the modifiation of the tool holder. At the same time it must be onsider that the passive vibration damping method is very speifi to the utting parameters and material properties. Based on this the making of further analyzes would need a larger and more voluminous experimental group. It would be important to arry out tests on turning of various materials and the use of additional details in the turning tool holder s onstrution whih would require arrying out a greater volume of material-speifi analysis. In addition, analysis of vibrations ourring at higher frequenies should be further analyzed and test with a variety of different utting inserts should be arried out. For omparison, it would be useful to arry out tests on various lathe types for identifying their effet of properties. In addition future work is planned to be made on the identifying of the effets of wear resistane properties. It is also important to investigate the influene of different parameters to the dimensional and geometri auray of the workpiee using a modified instrument to find the best onstrution for vibration damping and produt quality. REFERENCES 1. Brandon, C., Gegg, C., Steve, S., Albert, C. J. Luo. Mahine Tool Vibrations and Cutting Dynamis. Springer, New York, 2011: pp Tony, L., Shmitz, K. Smith, S. Mahining Dynamis. Springer, New York, 2009: pp Astakhov, V. P. Tribolology of Metal Cutting. Elsevier, London, 2006: pp Eugene, I., Rivin Hongling, K. Enhanement of Dynami Stability of Cantilever Tooling Strutures International Journal of Mahine Tools and Manufature 32 (4) 1992: pp Siddhpura, M., Paurobally, R. A review of hatter vibration researh in turning International Journal of Mahine Tools and Manufature : pp Tobias, S. A. Mahine Tool Vibrations. University of Birmingham, Blakie, William, C., Elmore, M., Heald, A. Physis of Waves. Dover edition, New York, 1985: pp William D., Callister, D., Rethwish, G. Fundamentals of Materials Siene and Engineering: An Integrated Approah, Publisher: Wiley, 2012: pp Brue, L., Bramfitt, A., Bensoter, O. Metallographer`s Guide. ASM International, 2002: pp Mitsubishi CVD Coated Grade for Steel turning UE6110, produt/pdf/b/b089g.pdf, Mahr. Marsurf PS , duts/metrology/by-mahr-produt- Group/MarSurf/Systems/Portable-Instruments/ MarSurf-PS- 1/, Shmitz, T. L., Smith, K., S. Mehanial Vibrations, Springer, New York, 2012: pp Maksarov, V. V., Olt, J., Leemet, T. Inrease in the stability of tehnologial system with ontrol of the proess of utting International Siene Conferene and Manufaturing Tehnology : pp