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1 A. INTRODUCTION OF DISSERTATION 1. Dissertation title Optimization on determination of dressing parameters, lubricant conditions and exchanged grinding wheel diameter in internal cylindrical grinding process 2. Rationale of the study Nowadays, according to the great development of technologies, machining processes have to satisfy more and more requirements of mechanical products for quality as well as productivity. In reality, among machining processes, grinding is commonly used to obtain the high quality of surface finish. Especially, it predominates in machining annealed products with high hardness, high strength. It accounts for about 20-25% of the total expenditures for mechanical parts in industries. Because of these reasons, improvement of grinding performance and reduction of machining expenditure while remaining accuracy requirement have been interested in researchers. In comparison with other type of grinding, internal cylindrical grinding process is implemented in difficult conditions and tight spaces. For that reason, it is more difficult to study the process of internal grinding. Therefore, the research of the internal grinding process is less interested by
scientists than studying external grinding or surface grinding. In order to improve the internal grinding performance, many solutions have been proposed such as using high standard grinding wheels (diamond or CBN wheel), high speed grinding and optimizing grinding process parameters (cutting, dressing and lubricant parameters). Among these solutions, optimization of grinding process parameters has been considered in many studies. 90CrSi is steel alloy with high mechanic strength and abrasion resistance. It is commonly applied to make molds, low speed cutting tools and machine parts required high durable and abrasion resistance. In medical factories in the North of Vietnam, this type of steel is often used for making tablet punches and dies. Although, internal grinding process has been used in finished step for making tablet dies, its quality and productivity are still low. Therefore, the
2 results of this dissertation will provide suitable suggestions to improve the efficiency of the internal grinding process when making these parts. Based on previous studies, to increase the productivity and to reduce the grinding cost, there are three proposed solutions including using optimum lubricating-cooling condition, optimum dressing condition and using optimum exchanged grinding wheel diameter. 3. Research objects This study focuses on the internal grinding process for annealed 90CrSi steel alloy. 4. Research aims and objectives The aim of this study is to improve internal grinding process to reduce grinding cost and surface roughness and increase productivity. 5. Research methodology The proposed methodology is combined both theoretical and experimental studies. Theoretical study: internal grinding technologies and grinding cost calculation are analyzed and synthesized. Experimental study: the influence of lubricating-cooling parameters, dressing parameters and exchanged grinding wheel diameter on the internal grinding cost analyzed and optimized based in experiments. 6. Research contents Overview of internal grinding technologies; Study on effect of lubricating-cooling parameter, dressing parameter on surface roughness and
grinding productivity; Study on the calculation model of internal grinding cost and the influences of grinding process on grinding cost; Determination of optimal exchanged grinding wheel diameter. 7. New contributions This study has analyzed the internal grinding cost and the influence of grinding process parameters on the grinding cost. Determining model to calculate the optimal exchanged grinding wheel diameter (or optimum wheel lifetime) in internal grinding process and the influence of grinding process parameters on the optimal exchanged wheel diameter.
3 The lubricating-cooling parameters and dressing parameters have been analyzed and optimized based on grinding experiments of 90CrSi steel alloy. 8. Dissertation structure The dissertation includes the following parts: Introduction, 5 chapters, conclusions and appendix. Chapter 1: Overview of internal grinding process. Chapter 2: The model of efficiency improvement of internal grinding process and experimental system. Chapter 3: Experimental study on influence of lubricating-cooling parameter in internal grinding process Chapter 4: Experimental study on influence of dressing parameter in internal grinding process Chapter5: Determination of optimal exchanged grinding wheel diameter. 9. Significances Science significances This dissertation has studied the influence of lubricating-cooling parameters and dressing parameters on the surface roughness as well as the grinding productivity when internal grinding 90CrSi steel alloy. The model of grinding cost has been developed relating to the proposed formula of the optimal exchanged grinding wheel diameter. This research has provided a significant contribution in the reduction of grinding cost that is one of interested research directions in internal grinding process. Reality significances This study has determined solutions to improve the internal grinding efficiency to increase the grinding productivity and the reduction of the grinding cost when grinding 90CrSi tool steel. The results of this study can be applied for internal grinding tablet dies.
4 B. DISSERATION OUTLINES CHAPTER 1. OVERVIEW OF INTERNAL GRINDING PROCESSS 1.1. Internal grinding process: grinding schema, grinding shaft, position and role of internal grinding in machining process. 1.2. Properties of internal grinding process. - The properties of internal grinding process are grinding chord length l k, grinding depth az, equivalent grinding wheel diameter Dtd, shaving removal process of grinding grains, grinding productivity, grinding forces. - Wear of grinding wheel. - Grinding wheel life and method to determine it. - Surface roughness. - Topography of grinding wheel and method to measure Topography 1.3. Literature of internal grinding process. This section focuses on researches relating to the influences of lubricating-cooling, grinding and dressing parameter on the ground surface in the internal grinding process. In addition, the models to determine grinding cost such as Tarasow – Shaw, Field and Ebbrells – Rowe are reviewed and analyzed. 1.4. Proposal solution to improve grinding efficiency
Determination of appropriate lubricating-cooling conditions; Determination of optimal dressing parameters; Determination of optimal grinding wheel life (optimal exchanged grinding wheel diameter).
CHAPTER 2. MODEL TO IMPROVE THE EFFICIENCY OF INTERNAL GRINDING PROCESS AND DEVELOPMENT OF EXPERIMENTAL SYSTEM 2.1. Model to improve the efficiency of internal grinding process Normally, researches focus on the technical efficiency of the grinding process to improve the accuracy and the ground surface quality; reduce force, heat, vibration or increase productivity. In order to solve both directions, the dissertation develops a model to improve the efficiency of the internal cylindrical grinding process. This model has been proposed
5 including three parts to increase accuracy and machining surface quality and reduce grinding cost in internal cylindrical grinding process. Part 1: Input parameters. Internal grinding process is complex under the influences of many input parameters. These input parameters can be classified into five groups including: grinding machines and cutting parameters; workpieces; grinding wheels; dressing technologies and lubricating-cooling technologies. Among these five input groups, some significant groups can be chosen to be studied.
Figure 1. Model to improve the efficiency of internal grinding process Part 2: Solutions to improve the efficiency of internal grinding process Three solutions to improve the efficiency of internal grinding process including finding optimal lubricating-cooling parameters to reduce surface roughness and increase the grinding wheel life; finding optimal the dressing parameters to increase the grinding wheel life and the grinding productivity; finding the optimal grinding wheel life to reduce the grinding cost. These models will be presents in next chapter respectively. Part 3: The quality of the grinding process is increased and technical requirements are ensured, productivity is increased and grinding cost is decreased. All of the input lubricating-cooling parameters and dressing parameters affect the surface roughness.
Grinding cost per product Ct,p (VNĐ)
Based on these arguments, it can provide a model to improve the efficiency of the internal grinding process as shown in Figure 1. Three solutions are proposed for studying to improve the efficiency of the process when internal cylindrical grinding small holes. Among these solutions, the application of the optimal grinding wheel life (exchanged grinding wheel diameter) has not been considered in previous researches. Figure 2 presents the relationship between the grinding wheel life (L), grinding cost (Cgw,p) and cost of machine, labor and management (Cmt,p) in internal grinding process for one product.
Cmt,p Ct,pmin; Lop
Grinding wheel life- L (hour)
Figure 2. The relationship between the grinding wheel life and grinding cost The longer the grinding wheel life, the lower the cost of grinding wheel is. In contrast, the cost for machines, labors and management linearly increases with the machining time. The total machining cost for a part includes the expenditures of grinding wheel, machines, labors and management ... In Figure 2, a certain optimal grinding wheel life always exists. 2.2. Experimental system Experimental system includes technical system and measurement devices 2.3. Conclusion of chapter 2. 1. The input and output parameters have been analyzed and determined as the following: - Input parameters: Vđ, Vct, fa, fr, ae,tot, Cm,h, Cwa,h, dw, Rld tg, Srg, D0, Bgw, wpd, Cgw, tw, tsđ, Ssđ, nsđ, NĐ, LL. - Output: Ra, Ct,p and De,op
7 2. This study has proposed a model to improve the efficiency of internal grinding process including: determinations of the appropriate lubricatingcooling parameters, the optimal dressing parameters and the optimal exchanged grinding wheel diameter. These solutions will be presented in next chapters of this dissertation. 3. An experimental system has been developed to meet the requirements of experimental research. CHAPTER 3. EXPERIMENTAL STUDY ON INFLUENCE OF LUBRICATING-COOLING PARAMETER IN INTERNAL GRINDING PROCESS 3.1. Effect of cooling parameters on surface roughness Cooling flood is the most commonly used in grinding hole. Therefore, in this study, finding optimal coolant parameters is one of the directions to improve the grinding efficiency. 3.1.1. Experiment and results/ Experimental results a. Caltex Aquatex 3180 oil Table 1. Experimental results for Caltex Aquatex 3180 oil Code
8 The regression coefficients present the effect level of the flow rate, the concentration parameters and their interaction on the objective function Ra. They all have influence on the surface roughness, thus the regression function is a quadratic function as below: Y = 1,048 - 0,229x1 – 0,133x2 + 0,033x12 + 0,030x22 - 0,010x1x2 (1) When this parameter increases, the roughness Ra decreases to minimal value because friction is reduced. However, if the contribution is too high, the coolant is concentrated and then amount of chips sticking on the workpiece surface is increased. As a result, the surface roughness Ra is increased. Similarly, the flow rate also affects on the roughness Ra. There is an optimal flow rate to obtain a minimum roughness Ra. Increasing the flow rate, more coolant is in the cutting area and then the roughness Ra is reduced. However, the space of internal grinding is limited by the gring wheel dimension, increasing the flow rate does not increase amount of the coolant in cutting area. In addition, increasing the flow rate increases the concentraion of the coolant in the cutting area and also increases the chips on the workpiece surafce. That is an interaction effect between two parameters on the roughness Ra.
Figure 3. Regression surface of Ra for Caltex Aquatex 3180 oil b. Emulsion
Results and Discussions Using Minitab software, analyzing the experiment results, we obtained the regression equation: Y= 0,218 – 0,006x1+0,038x2 - 0,016x1x2 + 0,016x12 + 0,004x22 (2)
Figure 4. Regression surface of Ra for Emulsion oil
10 In figure 4, when the flow rate is low, the concentration strongly affects to the roughness value. Increasing the concentration, surface roughness increase. When flow rate equal 4 l/min, the roughness value is almost constant for all concentration value. In general, the more Emulsion oil will increase the surface roughness. This is because Emulsion solution is high density, makes it difficult to escape chips and clean the machining surface. 3.1.2. Optimization of the concentration and the flow rate a. Caltex Aquatex 3180 oil Using response surface method, the relation between the concentration and the flow rate with the roughness Ra is shown. From the optimization plot, there exists an optimal set of these parameters to obtain a minimum roughness Ra. The solution of the optimal parameters are shown, the minimal roughness Ra is 0.4102µm with the concentration of 3.907% and the flow rate of 2.864 (l/min). b. Emulsion oil Similarly, we can determine the optimal cooling parameters when using Emulsion oil solution. The flow rate value is 1,38 l/min and the concentration value is 2,37%. The minimum of surface roughness Ramin = 0,3 µm 3.2. Conclusion of chapter 3. This chapter focuses on experimental study on influence of 2 types (Caltex Aquatex 3180 and Emulsion), lubricating-cooling parameter to surface roughness in internal grinding process. This study results show that: - When grinding and using Emulsion oil solution, surface roughness is better than using Caltex Aquatex 3180 oil solution. - The optimal Aquatex 3180, Emulsion oil solution lubricating-cooling parameter with was determined. + With Aquatex 3180 oil solution: the flow rate is 2,86 l/min and the concentration is 3,91% + With Emulsion solution: the flow rate is 1,38 l/min and the concentration is 2,37%.
11 CHAPTER 4. EXPERIMENTAL STUDY ON INFLUENCE OF DRESSING PARAMETER IN INTERNAL GRINDING PROCESS 4.1. Experimental setup Table 3. Dressing parameters and their values at different levels Levels TT Factor Symbol 1 2 3 4 5 6 Non-feeding 1 CK 0 1 2 3 4 5 dressing times Coarse 2 dressing ttho 0,02 0,025 0,03 depth (mm) Coarse 3 ntho 1 2 3 dressing times Fine dressing 4 ttinh 0,005 0,01 0,015 depth (mm) Fine dressing 5 ntinh 1 2 3 times Dressing feed 6 Ssd 1 1,2 1,4 rate (m/p) Dressing has 3 steps: coarse dressing, fine dressing and non-feeding dressing (Spark-out dressing). Dressing parameters included 6 factor: Coarse dressing depth, Coarse dressing times, Fine dressing depth, Fine dressing times and dressing feed rate. Using Minitab, the experiment setup was design. The experiment with the six dressing parameters including the dressing feed rate, the coarse dressing depth, the coarse dressing times, the fine dressing depth, the fine dressing times and the dressing number without depth of cut was conducted using Taguchi method. Table 2 shows the dressing parameters and their values at different levels. As it can be seen from the table, five three-level dressing parameters and one six-level dressing parameter are established for the experiment. The L18 (53x16) was used for the experiment work.
Mean of Ra
4.2. Effect of dressing parameter to surface roughness (Ra) and material remove rate (MRR) in internal grinding. 4.2.1. Experiment results and single-objective optimization. a, Influence of dressing parameter to Ra From the analysis of variance – ANOVA, it is clearly seen that the nonfeeding dressing times has the largest effect on surface roughness Ra, The other parameters, which have the effect on Ra, sequence: coarse dressing depth, coarse dressing times, fine dressing depth, fine dressing times and dressing feed rate. Table 4. The effect of dressing parameters on Ra at their levels Level CK ttho ntho ttinh ntinh Ssd 1 0,4043 0,4929 0,4797 0,5193 0,5146 0,5023 2 0,4407 0,4808 0,5034 0,4836 0,5059 0,5144 3 0,4542 0,5396 0,5302 0,5104 0,4929 0,4967 4 0,5453 5 0,6193 6 0,5629 Delta 0,2150 0,0588 0,0505 0,0357 0,0217 0,0177 Rank 1 2 3 4 5 6
Figure 5. Effect of dressing parameters on Ra Discussion:
Mean of S/N ratio
If dressing has not the non-feeding dressing times, topography of grinding will become rougher. The space for escaping, containing chip is larger, so cutting heat, force and roughness decrease. The more non-feeding dressing times reduce, the more ridiculous peaks will be reduced and thus increasing Ra Dressing depth increases, surface is rougher, grinding wheel time life and MRR increase (suitable for rough grinding). Coarse dressing times increase, thus Ra increase. The reason is that coarse dressing times increase, number of undulating peaks in grinding increases and Ra increase. Fine dressing depth is too small, leading to the undulating height on surface grinding small, so that difficult to contain and escape chips, leading to Ra increase. In other way, fine dressing depth increase, the undulating height on surface grinding is higher but quickly flattened, so that grinding wheel is worn out rapidly and Ra increase. b. Optimum Surface roughness
Figure 6. Effect of dressing parameters on S/N The optimal value of Ra is determined by the parameter level (circle) in figure 6: CK = 0 time (A1); ttho = 0,025 mm (B2); ntho = 1 time (C1); ttinh = 0,01mm (D2); ntinh = 3 time (E3); Ssd = 1,4 m/min (F3). Optimum value of Ra Ratoiuu A1 B2 C1 D2 E3 F3 5.Tgg
14 Substituting all of the parameters into equation Ratoiuu 0.404 0.481 0.48 0.484 0.493 5x0.504 0.318m
A confidence interval (CI) can be computed as: 1 1 CI F 1, f e , Ve , 0,14 N R e
Where, 𝐹∝ (1, 𝑓𝑒 ) = 8,5262 is a coefficient for the confidence level %=90%, fe =2 is the degree of freedom of error, Ve = 0,003822 is the mean of error, R =3 is the number of trials in each experiment 𝑁𝑒 =
Based on = 90% the predicted optimum material removal rate with the optimum level of dressing parameters including nCK1/ttho2/ntho1/ttint2/ntinh3/S3: (0,318 − 0,14) ≤ ̅̅̅̅ 𝑅𝑎𝑜𝑝 ≤ (0,318 + 0,14) or 0,178 ≤ Raop ≤0,458 µm c. Influence of dressing parameter to MRR. Material removal rate MRR (mm3/s) is determined by the volume of material removal per unit time. The volume of material removal during a grinding process is determined by testing the hole diameter before and after grinding. Grinding wheel life is determined by worker experience, grinding force Py. Table 5. The effect of dressing parameters on MRR Level CK ttho ntho ttinh ntinh Ssd 1 2,109 2,446 2,450 2,577 2,253 2,355 2 2,033 2,463 2,384 2,314 2,382 2,426 3 2,475 2,318 2,393 2,336 2,591 2,445 4 2,462 5 2,438 6 2,937 Delta 0,905 0,146 0,066 0,264 0,338 0,090 Rank 1 4 6 3 2 5 From the analysis of variance – ANOVA, it is clearly seen that the effect on MRR, sequence: non-feeding dressing times, fine dressing times,
Mean of MRR
fine dressing depth, coarse dressing depth, dressing feed rate and coarse dressing times. Non-feeding dressing times has the largest effect on the material removal rate. When number of non-feeding dressing times increase, MRR will increase (op The number of non-feeding dressing greatly affects the grinding productivity. The higher the values of it, the higher productivity (as opposed to affecting Ra). When increasing the number of non-feeding dressing, the finer the surface of the grinding, the more blade density and the number of slots for keeping chips are high.
Figure 7. Effect of dressing parameters on MRR The increase of the dressing depth of cut leads to the reduction of the productivity. The dressing depth of cut from 0.02mm to 0.025mm hardly changes the MRR and when the roughness equals 0.03mm, the MRR decreases. When the dressing depth of cut increases from 0.005mm to 0.01mm, the MRR decreases and when the ttinh increased to 0.015mm, the MRR did not increase much. This is because with the increase of the dressing depth of cut, MRR reduces. The number of rough dressing has almost no effect on MRR. The number of fine dressing is the second most powerful factor on MRR after the number of superfine dressing. MRR is proportional to the number of fine dressing.
16 The dressing feed does not affect much to the MRR (similar to the affecting to Ra). d. Optimization of MRR The value MRR max is determined by the following equation at levels: CK (A6); ttho (B2); ntho (C1); ttinh (D1); ntinh (E3); Ssđ (F3). MRRtoiuu A5 B2 C1 D1 E3 F3 5.Tgg
And we have: MRRtoiuu 3, 42(mm3 / s) The CI confidence interval is calculated as follows: 1 1 CI F 1, f e ,Ve , 0, 415 Ne R
Mean of S/N ratio
Where, 𝐹∝ (1, 𝑓𝑒 ) = 8,5262 is a coefficient with significance level %=90%, fe =2 is the degree of freedom of error, Ve = 0,032125 is the average error, neff is the number of effective iterations, R = 3 is the number of iterations of an experiment.
Accordingly, with significance level = 90% the surface roughness is predicted with the optimum level of input parameters nCK6/ttho2/ntho1/ttint1/ntinh3/S3 such as:
17 2.973 MRRop 3.803(mm3 / s)
4.3. Multi-objective optimization In this study, a combination of Taguchi and GRA methods was used to optimize the negotiation of two outputs of the dressing mode when internal grinding: MRR and Ra. The greatest value of gray relation of each factor is the optimal level of that factor. Therefore, according to Figure 9, the optimal parameters of the dressing process when internal grinding meet both surface roughness minimum and MRR maximum are: ttho1/ntho1/CK6/ntinh3/ttinh1/Ssđ3 corresponding to ttho=0,02mm, ntho = 1 times, CK = 5 times, ntinh = 3 times, ttinh = 0,005 mm, Ssđ = 1,4 m/ph. Whereby: (𝑅𝑎) 𝑇𝑜𝑖𝑢𝑢 = 0,4929 + 0,4797 + 0,563 + 0,5193 + 0,4929 + 0,4966 − 5 ∗ 0,5045 = 0,522 µ𝑚 (𝑀𝑅𝑅) 𝑇𝑜𝑖𝑢𝑢 = 2,446 + 2,45 + 2,937 + 2,577 + 2,591 + 2,445 − 5 ∗ 2,4089 = 3,402 𝑚𝑚3 /𝑠
Figure 9. Main effect plot for means 4.4. Conclusions of chapter 4 1. The process of dressing should follow rough, fine and super fine dressing steps to help stabilize the topography of the wheel. The number of times the super fine dressing has the greatest effect on the surface roughness and the grinding performance. The super fine dressing can reduce the surface roughness but it can help to increase the grinding productivity
18 significantly. The greater the depth of rough dressing and fine dressing can increase the surface roughness and reduce the MRR. Therefore, it is advisable to choose a suitable depth of dressing. The larger of the number of rough dressing also increase the surface roughness and reduce the MRR. Also, the more fine dressing times will help reduce the surface roughness and increase MRR. 2. The results of the study help to choose the optimum dressing mode when internal grinding 90CrSi tool: +) For minimum surface roughness (fine grinding) the optimum dressing parameters are: (CK = 0; ttho = 0,025mm; ntho = 1; ttinh = 0,01mm; ntinh = 3; Ssđ = 1,4m/p) Ramin = 0,318µm +) For maximum grinding productivities (rough grinding) the optimum dressing parameters are (CK = 5; ttho = 0,025mm; ntho = 1; ttinh = 0,005mm; ntinh = 3; Ssđ = 1,4m/p) MRRmax = 3,42 mm3/s) +) For multi-objective optimization CK = 5, ttho =0,02mm; ntho = 1, ttinh = 0,005mm, ntinh = 3, Ssđ = 1,4m/p and MRR = 3,402mm3/s, Ra = 0,522µm. CHƯƠNG 5. OPTIMIZATION OF EXCHANGED GRINDING WHEEL DIAMETER This chapter will investigate the determination of the optimum exchanged grinding wheel diameter and the effect of the parameters on the optimum exchanged diameter based on the analysis of grinding costs. In addition, the effectiveness of applying the optimum diameter in internal grinding is also indicated. 5.1. Cost analysis Based on previous researches on cost models for machining process, a new cost model that calculates the cost of the internal grinding process has been proposed. As follows:
Ct , p (Cwa ,h Cm ,h ).tt Cgw, p Cmt ,h .tt C gw, p Where, Cmt,h machine cost (VNĐ/h) Cwa,h administration cost and labor cost (VNĐ/h) Cgw,p grinding wheel cost (VNĐ) tt is total time for grinding one part (h)
19 Ct , p
2(w pd aed ).tcw 2(w pd aed ).tc Cm,h Cwa ,h t . t L t s t c 1 d Cgw 60 t ( D D ). t ( D D ). t w 0 e w 0 e w
5.2. Effect of parameters on the cost of the internal grinding process As mentioned in section 5.1, the grinding cost when internal grinding is affected by many parameters. These parameters include 18 grinding parameters such as the original diameter of the stone, the width of the grinding wheel, the hole diameter, the number of vertical feed speed, the diameter of the grinding wheel, etc., and the cost components such as the machine cost, the labor cost, the grinding wheel cost, etc. Factors that significantly affect the cost of grinding include Rld, tw, ae,tot, Cgw, Cm,h, D0, and . In addition, factors D0, tg, td, wpd, aed, Cwa,h, Srg, Bgw are the small impacting factors on the cost. Especially, factors Srg, Bgw, aed, wpd, Cwa,h are negligible impact on internal grinding cost. Among the influencing parameters, the ratio between the length and hole diameter Rld (J) is the most powerful factor affecting internal grinding cost. This is because the deeper the hole, the harder it is to grind and requires more complex grinding technology. The cost of machine Cm,h, the labor cost and the hourly management of Cwa,h and the cost of grinding wheel Cgw a positive effect on the processing cost. This means that when these values increase, the grinding cost increases. The increase in the dressing depth will lead to the increase of the cost but its impact was not significant. Also, the dressing time td is a factor that does not significantly affect the cost. If td increases, the cost increases. This is because the longer of the dressing time, the more time it takes to sharpen and lead to increased grinding costs. Therefore, to reduce the cost of grinding, it is necessary to study the dressing process such as automating the dressing process, reducing the time for replacing dressing tools ... Besides, the total depth of grinding cut ae,tot is the most influential factor on the grinding cost (ranked 3rd in the level of influence). The larger the amount of the depth of cut, the more the time of grinding will increase and lead to an increase in the cost of grinding. Therefore, the optimum depth of
20 cut should be selected appropriately, in accordance with machining requirements in order to reduce the cost. The most powerful impact on the grinding cost is the ratio of the hole length to hole diameter Rld and the part diameter. When this ratio is larger the grinding cost will increase. Meanwhile, the processing conditions will be harsher and the horizontal feed speed cannot be large. Also, the amount of removal material is also large, thus increasing the grinding cost. In addition, the greater the surface roughness grade Srg will increase the grinding cost. So to reduce grinding costs should not choose the large Rld (if possible). is a factor closely related to the original part diameter d w and the initial grinding diameter D0. Increasing the ratio will increase the grinding cost. As analyzed above, the increase in grinding wheel cost will increase the grinding cost. However, the higher the wheel lifetime will reduce the cost. Also, the impact of wheel lifetime is greater than the impact of wheel cost. In addition, the amount of wheel wear wpd and the width of wheel B gw do not affect the grinding cost much. Therefore, if we use high quality grinding wheel (expensive, durable) we can reduce the grinding cost. In addition, optimizing grinding parameters to increase the wheel lifetime also helps to reduce the grinding cost. The initial wheel diameter D0 and the hole diameter are two parameters depending on the coefficient . Therefore, increasing D0 can increase the average cutting speed and reduce the machining time. However, in this case, the processing conditions are also changed. Therefore, the amount of removal material increases and increases the cost of grinding. Besides, the exchanged wheel diameter De also affects the grinding cost. When delta (De / D0) decreases (or De decreases), it will reduce the grinding cost. 5.3. Optimal exchanged wheel diameter 5.3.1. Determining optimal exchanged wheel diameter Figure 10 describes the relationship between the cost of grinding a part (VND / h) and the exchanged wheel diameter (mm). This relationship is built based on the calculation according to the formula in Section 4.1 with the following data: D0=20 (mm); Bgw=25 (mm); aed=0,12 (mm); Cm,h=70.000 (VNĐ/h); Cwa,h=46.000 (VNĐ/h); Cgw=70.000 (VNĐ); tw=20
Grinding cost per product Ct,p (VNĐ)
(min); wpd=0,02 (mm); tg=7; Rld=2, td=0,3 (min), tcw=2,4 (phút), tL=0,54 (min), ts=0,3(min), Srg = 7, dw=25 (mm), ae,tot=0,1 (mm). From this figure, it can be seen that the grinding cost depends heavily on the exchanged wheel diameter (or the wheel lifetime). In addition, there exists an optimal exchanged wheel diameter at which the grinding cost is minimum (Cmin = 5,927 VND; De,op = 17,5mm). The value of this optimal exchanged wheel diameter is much larger than that of traditional exchanged wheel diameter (in this case, about 14 mm).
8100 7600 7100
C = 6.528VNĐ Demin = 14
Cmin = 5.927 VNĐ De,op = 17,5
Exchanged grinding wheel diameter - De (mm)
Figue10. Exchanged wheel diameter versus grinding cost As mentioned above, because the exchanged wheel diameter greatly affects the cost of grinding, finding the value of the optimal exchanged wheel diameter will help to reduce grinding cost significantly. When comparing the cost of grinding when changing the wheel at the optimum exchanged diameter De, op = 17.5mm with the cost of replacing the wheel at the traditional exchanged diameter, min = 14mm, it is found that the cost reduced from 6,528 VND/ part to only 5,927 VND / part (down 9.02%). The average total grinding time decreased from 192 (seconds) to 164 (seconds) (down 14.7%). 5.3.2. Effect of process parameters on the exchanged wheel diameter D0 greatest impact on the exchanged wheel diameter De,op, next are D0*tw, D0*Cgw, Cgw, tw, D0*Cmh, Cm,h, Cwa,h, tw*Cgw, D0*Cwah, D0*aed,
22 Cmh*Cwah, Cmh*Cgw, aed and Bgw*aed. The ratio Rld, the wheel wear wpd, the accuracy grade tg does not affect De,op. 5.3.3. Modeling optimal exchanged wheel diameter The relation equation between De,op and the main influencing parameters can be written 5 with the correlation coefficient r2 = 99,63%. De,op = -2.614 + 0.6620 D0 + 0.0408 Bgw + 7.45 aed - 0.0304 tw + 0.000003 Cm,h + 0.000010 Cwa,h - 0.000011 Cgw - 0.5421 D0*aed + 0.008034 D0*tw + 0.000001 D0*Cwa,h - 0.000001 D0*Cgw - 0.416 Bgw*aed 5.4. Conclusions of chapter 5 1. A model for calculating the grinding cost when internal grinding with a number of parameters has been built. From this model the effect of grinding process parameters and several cost components on the grinding cost was investigated. Thereby some conclusions were given: - The ratio between the length and diameter of the hole has the strongest impact on grinding cost; - The cost of machine, the labor cost and the grinding wheel cost have a significant effect on the grinding cost. The cost of grinding will increase when these costs increase; - Some solutions to reduce the cost of grinding have been proposed, such as reducing the cost of machines, the cost of grinding wheel, the labor costs (workers, management ...); Using abrasive wheel with high durability and studying methods to improve the wheel lifetime and determine the appropriate amount of dressing depth of cut; Finding methods to reduce the dressing time and the time for changing dressing tools... 2. The exchanged wheel diameter greatly affects the cost of grinding. Also, there exists an optimal value of the exchanged wheel diameter at which the grinding cost is minimal. In addition, a formula to determine the optimal exchanged grinding wheel diameter De, op has been proposed. 3. The influence of these factors on the optimum exchanged wheel diameter is as follows: The initial diameter of the grinding wheel D0 has the strongest influence on the exchanged grinding wheel diameter De,op, next is the grinding wheel cost Cgw, the wheel lifetime tw, the machine cost Cm,h, the labor cost Cwa,h. Also, the dressing depth of cut aeđ. The ratio Rld, the wheel wear wpd, the accuracy grade tg do not affect De,op. The quadratic
23 factors influence De,op are D0*tw, D0*Cgw, D0*Cmh, tw*Cgw, D0*Cwah, D0*aed, Cmh*Cwah, Cmh*Cgw and the final is Bgw*aed. - The economic efficiency of applying the optimum exchanged wheel diameter helps to reduce the cost of grinding per part by 9.02%, the total grinding time decreases by 14.7%. CONCLUSIONS AND RECOMMENDATION Conclusions The objective of this thesis is to improve the efficiency of internal grinding process. In order to do that, it is necessary to solve the following problems: Determining a reasonable cooling lubrication mode, determining a reasonable dressing parameters and determining the optimal exchanged wheel diameter. The main results and new contributions of the thesis can be summarized as follows: 1. Proposing models to improve efficiency when internal grinding. Since then propose solutions to improve the efficiency when grinding. 2. Experimental study of the effects of the flow rate, the concentration of coolant solutions of the two types of coolants including Aquatex 3180 and Emulsion on the surface roughness and proposed the optimal coolant method for the two types of solutions when internal grinding of 90CrSi tool steel. 3. Researched the effect of the dressing parameters on the surface roughness and the grinding performance. The proposed dressing process is divided into 03 steps: rough dressing, fine dressing and super fine dressing. In particular, the number of times the super fine dressing is most strongly influenced by the surface roughness and the grinding performance. The optimal dressing parameters when grinding 90CrSi tool steel has helped improve the surface quality and increase the productivity significantly. 4. Develop a model to calculate the cost of internal grinding and investigate the impact of factors on the grinding cost. In this model, the impact of 18 factors of grinding cost is included. These factors include component costs such as grinding costs, human costs (including labor, management, etc.), grinding wheel costs, etc. and grinding process parameters such as the initial wheel diameter, the wheel width, the wheel
24 wear, the total dressing depth of cut, the dressing time have taken into investigation. 5. Building a method of determining the optimal exchanged wheel diameter when internal grinding to achieve the lowest grinding cost based on building and solving a cost optimization problem. By applying the formula of optimal exchanged grinding wheel diameter, the grinding cost can be reduced by 9.02%, the total grinding time is reduced by 14.7%. This method is applicable in cases where the grinder is unable to change the spindle rotation speed. Recommendation Although this research has found a number of solutions to improve the efficiency of internal grinding process, there are still issues that need further investment in research. Specifically include the following research directions: 1) Research on the method to supply of the coolant into deep areas of grinding. 2)
Cutting conditions when grinding small and deep holes with the diameter less than 10 mm are very fierce. Therefore, it is needed further researches.
Investigation of the effects of coolant parameters and dressing parameters on the mechanical and physical properties of the workpiece surface.
25 LIST OF PUBLISHED WORKS RELATED TO THE THESIS * Internal journal papers 1. Banh Tien Long, Vu Ngoc Pi, Le Xuan Hung, Ta Viet Cuong, A study on the effects of coolant regimes to surfaceroughness in in ternalgrinding of steel 9XC, VietNam Mechanical Engineering Journal, Vol 5, 2016, pp 71 – 76 (In Vietnamese) 2. Banh Tien Long, Vu Ngoc Pi, Le Xuan Hung, Luu Anh Tung, Buiding cutting regime formulas for internal grinding, TNU Journal of Science and Technology, Vol 9, 2016, page 15 – 18 (In Vietnamese). * Internatinonal journal papers 3. Vu Ngoc Pi, Le Xuan Hung, Luu Anh Tung and Banh Tien Long, “Cost Optimization of Internal Grinding”, Journal of Materials Science and Engineering B 6 (11-12) (2016) page 291 – 296. 4. Le Xuan Hung, Tran Thi Hong, Le Hong Ky, Luu Anh Tung, Nguyen Thi Thanh Nga, Vu Ngoc Pi, “Optimum dressing parameters for maximum material removal rate when internal cylindrical grinding using Taguchi method”, International Journal of Mechanical Engineering and Technology (IJMET), Volume 9, Issue 12, December 2018, pp. 123–129. Scopus 5. Le Xuan Hung, Vu Ngoc Pi, Tran Thi Hong, Le Hong Ky, Vu Thi Lien, Luu Anh Tung, Banh Tien Long, “Multi-objective Optimization of Dressing Parameters of Internal Cylindrical Grinding for 90CrSi Alloy Steel Using Taguchi Method and Grey Relational Analysis”, 9th International Conference on Materials Processing and Characterization, 8th – 10th March 2019, Materials Today: Proceedings, Available online at www.sciencedirect.com. Scopus (Accepted) 6. Le Xuan Hung, Tran Thi Hong, Le Hong Ky, Nguyen Quoc Tuan, Luu Anh Tung, Banh Tien Long, Vu Ngoc Pi, A study on calculation of optimum exchanged grinding wheel diameter when internal grinding, 9th International Conference on Materials Processing and Characterization, 8th – 10th March 2019, Materials Today: Proceedings, Available online at www.sciencedirect.com. Scopus (Accepted) 7. Le Xuan Hung, Vu Thi Lien, Luu Anh Tung, Vu Ngoc Pi, Le Hong Ky, Tran Thi Hong, Hoang Tien Dung, Banh Tien Long, “A study on cost optimization of internal cylindrical grinding”, International Journal of Mechanical Engineering and Technology (IJMET), Volume 10, Issue 1, January 2019, pp. 414 – 423. Scopus 8. Thi-Hong Tran, Xuan-Hung Le, Quoc-Tuan Nguyen, Hong-Ky Le, TienDung Hoang, Anh-Tung Luu, Tien-Long Banh and Ngoc-Pi Vu, “Optimization of Replaced Grinding Wheel Diameter for Minimum Grinding Cost in Internal Grinding”, Applied Sciences, 9(7), March, 2019, pp. 1363. SCIE 9. Le Xuan Hung, Vu Thi Lien, Vu Ngoc Pi, Banh Tien Long, “A Study on Coolant Parameters in Internal Grinding of 90CrSi Steel”, Materials Science Forum, Vol. 950, pp 24-31, Apirl, 2019 Trans Tech Publications, Switzerland. Scopus