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 lk,

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.

6

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.

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

Gri

nd

ing

co

st p

er p

rod

uct

Ct,

p (

VN

Đ)

Grinding wheel life- L (hour)

Cmt,p

Cgw,p

Ct,p

Ct,pmin; Lop

7

2. This study has proposed a model to improve the efficiency of internal

grinding process including: determinations of the appropriate lubricating-

cooling 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

No. Points

Code Uncode

Ra

(µm) Flow

rate

Concentration

Flow

rate

(l/m)

Concentration

(%)

1 P6 -1 -1 1 2 0,598

2 P8 1 -1 4 2 0,590

3 P3 0 1,4 2,5 5,6 0,518

4 P2 1 1 4 5 0,476

5 P9 0 0 2,5 3,5 0,418

6 P1 1,4 0 4,6 3,5 0,517

7 P9 0 0 2,5 3,5 0,414

8 P7 0 -1,4 2,5 1,3 0,618

9 P9 0 0 2,5 3,5 0,419

10 P4 -1 1 1 5 0,577

11 P5 -1,4 0 0,3 3,5 0,593

12 P9 0 0 2,5 3,5 0,423

13 P9 0 0 2,5 3,5 0,417

Results and Discussions

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 + 0,030x2

2 - 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

9

Table 2. Experimental results for Emulsion oil

No. Points

Code Uncode

Ra

(µm) Flow

rate

Concentra

tion

Flow rate

(l/m)

Concentr

ation

(%)

1 P6 -1 -1 1 3 0,303

2 P8 1 -1 4 3 0,42

3 P3 0 1,4 2,5 6,6 0,45

4 P2 1 1 4 6 0,435

5 P9 0 0 2,5 4,5 0,377

6 P1 1,4 0 4,6 4,5 0,445

7 P9 0 0 2,5 4,5 0,366

8 P7 0 -1,4 2,5 2,4 0,311

9 P9 0 0 2,5 4,5 0,371

10 P4 -1 1 1 6 0,487

11 P5 -1,4 0 0,38 4,5 0,452

12 P9 0 0 2,5 4,5 0,354

13 P9 0 0 2,5 4,5 0,356

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 + 0,004x2

2 (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

TT Factor Symbol Levels

1 2 3 4 5 6

1 Non-feeding

dressing times CK 0 1 2 3 4 5

2

Coarse

dressing

depth (mm)

ttho 0,02 0,025 0,03 - - -

3 Coarse

dressing times ntho 1 2 3 - - -

4 Fine dressing

depth (mm) ttinh 0,005 0,01 0,015 - - -

5 Fine dressing

times ntinh 1 2 3 - - -

6 Dressing feed

rate (m/p) Ssd 1 1,2 1,4 - - -

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.

12

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 non-

feeding 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:

Mea

n o

f R

a

13

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

1 2 1 2 3 3 5.toiuu ggRa A B C D E F T

Mea

n o

f S

/N r

atio

14

Substituting all of the parameters into equation

A confidence interval (CI) can be computed as:

1 1

1, , , 0,14e e

e

CI F f VN R

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 =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑒𝑠𝑡 𝑟𝑢𝑛𝑠

1 + 𝑠𝑢𝑚 𝑜𝑓 𝑑𝑒𝑔𝑟𝑒𝑒𝑠 𝑜𝑓 𝑓𝑟𝑒𝑒𝑑𝑜𝑚 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑎𝑐𝑡𝑜𝑟 𝑖𝑛 𝐸𝑞=

54

1 + 15= 3,375

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) ≤ 𝑅𝑎̅̅̅̅𝑜𝑝 ≤ (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,

0.404 0.481 0.48 0.484 0.493 5 0.504 0.318atoiuuR x m

15

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.

M

ean o

f M

RR

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).

5 2 1 1 3 3 5.toiuu ggMRR A B C D E F T

And we have: 33,42( / )toiuuMRR mm s

The CI confidence interval is calculated as follows:

1 1

1, , , 0, 415e e

e

CI F f VN R

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.

Figure 8. Effect of factors on S/N of MRR

𝑁𝑒 =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑠

1 + 𝑇ℎ𝑒 𝑡𝑜𝑡𝑎𝑙 𝑑𝑒𝑔𝑟𝑒𝑒𝑠 𝑜𝑓 𝑐𝑜𝑚𝑝𝑢𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑓𝑎𝑐𝑡𝑜𝑟𝑠 𝑖𝑛 𝑜𝑝𝑡𝑖𝑚𝑎𝑙 𝑓𝑜𝑟𝑚𝑢𝑙𝑎=

54

1 + 15

= 3,375

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:

M

ean

of

S/N

rat

io

17

32.973 3.803( / )opMRR mm 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:

, w , , , , ,( ). .t p a h m h t gw p mt h t gw pC C C t C C t C

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

, w ,

, gw

0 0 w

2(w ). 2(w ).. 1

60 ( ). ( ).

pd ed cw pd ed cm h a h dt p L s c

w e w e

a t a tC C tC t t t C

t D D t D D t

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 dw 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 Bgw 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

21

(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).

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,

5600

6100

6600

7100

7600

8100

13 14 15 16 17 18 19 20

Gri

nd

ing

cost

per

pro

du

ct C

t,p

(V

NĐ

)

Exchanged grinding wheel diameter - De (mm)

Cmin = 5.927 VNĐ

De,op = 17,5

C = 6.528VNĐ

Demin = 14

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.

3) 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, Tien-

Dung 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