Evaluating the influence of cutting parameters on part’s accuracy and grinding wheel’s wear in grinding profile for ball bearing’s inner ring groove

This paper presents some research results to determine the impact of grinding parameters on grinding wheel’s wear and part’s accuracy in grinding profile for ball bearing''s inner ring groove.

EVALUATING THE INFLUENCE OF CUTTING PARAMETERS ON PART’S ACCURACY AND GRINDING WHEEL’S WEAR IN

GRINDING PROFILE FOR BALL BEARING’S INNER RING GROOVE

1

University of Economic and Technical Industries, 456 Minh Khai, Hai Ba Trung, Ha Noi 2

Hanoi University of Science and Technology, 1 Dai Co Viet, Hai Ba Trung, Ha Noi

*

Email:natuan.ck@uneti.edu.vn

Received: 14 November 2017; Accepted for publication: 5 July 2018

Abstract This paper presents some research results to determine the impact of grinding

parameters on grinding wheel’s wear and part’s accuracy in grinding profile for ball bearing's inner ring groove Firstly, the distribution diagrams of part’s tolerance zone and grinding wheel’s wear at different cutting conditions has been used to determine important outputs in the profile grinding process for the inner ring groove of the ball bearing Based on that, the experimental regression

functions that express the dependencies of the important outputs (grindstone wear Hz i, surface

roughness Ra, oval O p ) on the technical parameters (normal feed rate F n , speed of part V p, depth of

cutting t and the number of part in a grinding cycle N p) are determined by the least squares experimental planning method From those mathematical functions, the effect of grinding parameters on the grinding wheel’s wear and part’s accuracy in profile grinding for the inner ring groove is evaluated

Keywords: Profile grinding, surface roughness, cutting parameters

1 INTRODUCTION

Grinding is a popular finish processing method During the process, the accuracy of part, the durability of grinding wheel as well as the productivity of the process is highly dependent on the parameters of the cutting mode [1] Thus, one of the most important points to achieve the technical and economic efficiency of grinding process is to evaluate the influence of input factors

on the change of output factors Then, the output factors will be controlled as requirement

Among the output factors that need to be controlled, the part’s accuracy is the most important factor, especially for bearing items (roller bearing) [2] In term of structure, the roller bearing consists of four parts: outer ring, inner ring, roller and cage as the Figure 1 [3, 4] For the inner and outer ring parts of the bearing, the groove surface is the most important The profile grinding

Figure 1 Structure of roller [3, 4]

However, the previous researches have studied primarily on the influence of cutting

parameters on part’s surface roughness under surface grinding or cylindrical grinding [5, 6] The

influence of technical parameters on part’s accuracy under profile grinding for the inner ring

groove of the 6208 ball bearing has not been considered deeply

In this paper, the profile grinding operation for the inner ring groove of the 6208 ball

bearing was studied Thus, the theoretical background was analyzed to assure the accuracy in

profile grinding for the inner ring groove At the same time, experiments were conducted to

evaluate the influence of grinding parameters on the accuracy of the parts as well as the wear of

grinding wheel

2 RESEARCH CONTENT 2.1 Theoretical basis to assure processing accuracy in profile grinding for the inner ring

groove of ball bearing

For processing the groove surface, it requires not only dimension accuracy for groove

bottom’s diameter, groove’s radius and distance from groove central line to head surface, but also

the accuracy for correlative positions including the oval of groove bottom’s diameter, the circular

run-out of the groove central line in comparison to the head surface Especially, the surface

roughness of groove must be smaller 0.5 µm (Ra < 0.5 µm) [7] These technical requirements of

the finish grinding operation for the 6208 ball bearing’s inner ring groove are shown as Figure 2

Figure 2 Drawing shows the technical requirements of the finish grinding operation for the groove

of the 6208 ball bearing’s inner ring [7]

9 ±0.025

0.5 R6,17+0.05

However, the important issue is to find out a solution to assure the part’s accuracy during the process The processing standard errors and cause of these standard errors are necessary to

be determined to reduce them Thus, the following solutions would be applied to assure the accuracy in dimension and correlative position as well as surface roughness of the inner ring groove:

- The groove surface would be lathed before grinding by hard turning method on CNC machine This decreases surplus stock, increases productivity for groove grinding and decreases the press error causing by bending deform occurring from previous heat treatment During the grinding process, the errors in the geometry of the work-piece cause the same type of errors in the part such as oval, cone, circular run-out, etc [2] If material hardness and surplus stock are inhomogeneous, the errors in the geometry of part will happen It is necessary to lathe the surface of groove before grinding

- Applying the profile grinding principle and using 3MK136B profile grinder to process the groove surface The Figure 3 presents the principle diagram of grinding profile for the inner ring groove of ball bearing with grinder 3MK136B

Figure 3 Diagram of profile grinding principle for the groove of the ball bearing’s inner ring [7]

1, 2: Work-piece holder; 3: Magnetic pole; 4: Grinding wheel; 5: Grinding work-piece

In this method, the profile of processing surface and working surface of grinding wheel are coincided Grinding process is proceeded by normal motion of machine (rotation motion of work-piece np, rotation motionof grinding wheel (nw) and normal feed motionof workbench Fn) Especially, rough grinding mode, finishing grinding mode and grinding mode without spark are implemented in the same process to improve productivity and ensure the part’s accuracy The normal motion process of the worktable for grinding one product is shown in Figure 4 It

is divided into the following stages: 1 The workbench turns rapidly; 2 The workbench approaches forward; 3 The workbench performs a normal motion for the rough grinding; 4 The machine grinds without normal feed motion in a period of 1 ÷ 2 seconds to rough bound;

5 The workbench performs a normal motion for the fine grinding; 6 The machine grinds without normal feed motion in a period of 1 ÷ 2 seconds to fine bound This is fine grinding period to grind without spark It helps to increase the polish and accuracy of the part; 7 The workbench turns back rapidly and returns its origin position; 8 Swing arm of grinder loads/unloads automatically

e

np

n w

s n

2

3 2

1 3

4

5

Work-piece holder

Grinding wheel Work-piece

F n

Figure 4 The radial feed motion process of the worktable to grind a part with profile grinding machine

3MK136B

- The surface of groove and the head surface of the inner ring are used as the positioning standard to minimize the mounting error and assure the correlative position between the groove central line and the head surface The grinding part is positioned at groove surface by two lock pins

or a short V shape block with 120 degree angle to control two degrees of freedom Therefore, the standard surface is coincident with the machining surface to minimize the mounting error Besides, the top face of the work-piece is positioned at the top of the magnetic pole 3 to control three degrees

of freedom and ensure parallelism between the groove center and the head The part is tightly clamped by magnet as shown in Figure 3 For this reason, the process of positioning and clamping part is done simply and quickly It improves the machining efficiency and minimizes installation errors

However, because profile grinding has a big contact area between the grinding wheel and the work-piece, the cutting force and cutting heat generated in this process are much larger than those

in other normal grinding process Thus, the grinding wheel is worn continuously and unequally at various points on the working surface As a result, its initial shape and accuracy change quickly, causing geometric error of the processing surface In addition, the grinding wheel’s wear decreases its cutting capability and durability, increases the cutting force, cutting heat, consumption power and vibration in the grinding process Therefore, grinding wheel’s wear influences directly on the precision elements of the grinding parts including groove’s surface roughness, groove bottom diameter’s oval and dimension accuracy, groove radius’s dimension accuracy For those reasons, the grinding wheel must be dressed frequently in the profile grinding process

However, the most important issue is to determine the suitable time to dress the grinding wheel This determines the machining accuracy and the durability of the grinding wheel Thus, it is necessary to determine the economical limitation wear value of the grinding wheel However, the accuracy of the processing part and the wear value of the grinding wheel depend on parameters of the grinding mode Therefore, it is very important to evaluate the effect of grinding parameters on the grinding wheel’s wear and part’s accuracy in profile grinding for the inner ring groove This is the basis to determine suitable cutting parameters

2.2 Experiment to determine the important outputs in profile grinding for the groove of the ball bearing’s inner ring

Normal cycle of grinding

Compensation

feed

Rapid jump back and return origin

Rough grinding feed

Fine grinding feed

Grasp head out Swing arm return Grasp head in.

Magnetization

Grinding time

Displacement

distance of

workbench

02

Megnetic coil

Feed step

motor

Dynamic

profile of

workbench

Annotation

Mechanical

arm

Demagnetization

Fast approach feed Rapid feed

No spark grinding

No spark grinding

Compensation

movement of

workbench

2.2.1 Tools and experimental equipment

- The grinding wheel with white fused alumina grains was used to grind the inner ring groove

of 6208 ball bearing made from SUJ2 alloy steel Table 1 and table 2 show the specifications of the grinding conditions and grinding wheels

Table 1 Specifications of grinding conditions

Work-piece: SUJ2 alloy steel

Hardness: 60 ÷ 65 HRC

Grinding method: Profile grinding

Table 2 Specifications of grinding wheel

Code 500x8x203WA100xLV60 Grade Soft

Grain White fused alumina Bond Vitrified

- Equipment for experiment: Profile grinding machine 3MK136B (Figure 5)

- Roughness measuring device: SJ400 Roughness Tester (Figure 6)

Figure 5 Profile grinding machine 3MK136B Figure 6 SJ400 Roughness Tester.

- Equipment for measuring radius of the inner ring groove: In the experiment, the roughness and contour meter CL-1A of Shanghai Taile (Figure 7) was used to measure radius of the inner ring groove

Figure 7 The roughness and contour meter CL-1A

of Shanghai Taile

Figure 8 The pneumatic measuring probe systems

to measure grinding wheel’s wear [8, 9]

- Equipment for measuring wear of the grinding wheel: In this experiment, the wear value of grinding wheel was measured by applying pneumatic probe system [8-10] (Figure 8) The pneumatic probe system measures the wear value at two different points on the working surface of

the grinding wheel However, this study only considers the wear value at the margin of the curving edge surface of the grinding wheel where the wear value is maximum

- Equipment for measuring the diameter of the groove bottom, the oval of the groove bottom diameter, the distance from central line of groove to its head surface and the circular run-out of the groove central line with its head surface In this experiment, the Chinese measurement equipment D022 was used to determine position and the diameter of inner ring groove of the ball bearing (Figure 9)

Figure 9 Images and diagram for structural principle of the measurement equipment D022 This equipment applies the comparative method to measure tolerance of groove bottom’s diameter, the oval of the groove bottom diameter, the circular run-out of its groove central line with its head surface and tolerance of distance from its groove central line to its head surface Before measuring, it is necessary to select standard sample bearing In measuring process, the first step is to press the lever of bracket that is used to fix the groove of bearing (part No 7) to take out the sample bearing After that, the ball bearing’s inner ring No 3 (part need to be measured) is put in the measurement equipment Then, the lever No 7 is released to let the pin No 4 to fix the groove’s position Next, the groove surface that need to be measured will be located on two lock pins No 1 and

No 2 In this position, the ball bearing’s inner ring will be circled one round The measuring meter

No 5 indicates the maximum and minimum values of the groove bottom’s diameter The difference

05

03

02

01

07

08

09 4

°

04

06

01

02

05

03

07

06

04

between these maximum and minimum values is its oval value Meanwhile, the measuring meter No

6 indicates the maximum and minimum values of the distance from its groove’s central line to its head surface Based on that, the circular run-out of its groove central line with its head surface is easily determined The value of this tolerance is exactly equal to the difference between the maximum and minimum values of the distance from its groove’s central line to its head surface Therefore, each measurement can simultaneously determine four tolerances including groove bottom diameter’s oval level, the circular run-out of its groove central line to its head surface, groove bottom diameter’s dimension tolerance, the dimension tolerance of distance from its groove central line to its head surface

2.2.2 Experiment method

In profile grinding operation, the parameters of the grinding regime include the velocity of cutting (Vw), the velocity of part (Vp), the rate of normal feed (Sn) for rough grinding and fine grinding, the depth of cut (t) for rough grinding and fine grinding, the number of parts

in a grinding cycle (Np) However, for grinding on a CNC grinder with a specific grinding wheel, the velocity of grinding wheel is usually chosen according to the specifications of the grinding wheel that has been given by the manufacturer For example, the grinding wheel of 500x8x203WA100xLV60 has the grinding wheel’s velocity (Vw) of 60 m/s Thus, some grinders are manufactured with fixed spindle speed value Therefore, in order to simplify the study, this paper considers only four input parameters which are the rate of normal feed (Sn),

the velocity of part (Vp), the depth of cut (t) and the number of parts in a grinding cycle (Np) In addition, the cutting regime for rough grinding has insignificant effect on the quality of

grinding parts This article considers only cutting regime parameters for fine grinding to evaluate

the influence of its on the machining accuracy and the grinding wheel wear Therefore, the four parameters of the cutting mode selected in this study are the normal feed rate for fine grinding

parts in one grinding cycle (Np) The values of other parameters are kept constant throughout the experiment Those input parameters are selected according to basis experiments and a mechanical notebook [10] The specific values are chosen for the experiment including three sets

of cutting mode parameters as shown in Table 3

Table 3 Three sets of cutting parameters in the experiment

Sn fine

(µm/s)

Vp

(m/min)

tfine

(µm)

Np

(part)

Vw

(m/s)

Sn rough

(µm/s)

trough

(µm) The first set of

cutting parameters 12.5 12 15 30 60 30 120 The second set of

cutting parameters 12.5 12 10 30 60 30 120 The third set of

cutting parameters 5 18 20 30 60 30 120

2.2.3 Experiment results

After carrying out experiments and collecting results, data is analyzed and processed From the measurement results and based on the technical drawing of the grinding operation (Figure 2), using an application of Matlab software, the diagrams for distribution of the part accuracy tolerances zone and the grinding wheel wear were built as shown in Figure 10, Figure 11 and Figure 12

Figure 10 Diagram for distribution of the part’s accuracy tolerances zone and the grinding wheel’s

wear according to the number of parts with the first set of cutting mode parameters

Figure 11 Diagram for distribution of the part’s accuracy tolerances zone and the grinding wheel’s

wear according to the number of parts with the second set of cutting mode parameter

Part’s accuracy and grinding wheel’s wear (µm)

Hz limitation = 9.6

Suitable time to dress grinding wheel to assure part’s accuracy since at this time

Ra 12 = 0.5 = Ra Re

The number of parts

Part’s accuracy and grinding wheel’s wear (µm)

Hz limitation = 9.9

Suitable time to dress grinding wheel to assure part’s accuracy since at this time

Ra 15 = 0.5 = Ra re

The number of parts

Figure 12 Diagram for distribution of the part’s accuracy tolerances zone and the grinding wheel’s

wear according to the number of parts with the third set of cutting mode parameters

In Figure 10, Figure 11, Figure 12:

+ D: Deviation of groove diameter dimension

* O: Oval level of groove diameter

∆ A: Dimension deviation of distance from groove’s central line to head surface

MD: Circular run-out of groove central line to head surface

o R: Dimension deviation of groove radius

• Ra: Surface roughness of groove

x Hz: Wear value at the edge of the curving edge surface of grinding wheel

(All the values showing in diagram with plotting scale of 1, exception for Ra value using plotting scale of 10)

From the above diagrams, some findings can be presented as follows:

- For a certain set of cutting parameters, when the number of parts in one grinding cycle increases, the wear of grinding wheel and the surface roughness of groove also increase Therefore, according to time progress for each cutting mode, there is a moment at which the processing accuracy of part will exceed the required accuracy With the first set of cutting parameters, when the 12th part is grinded its surface roughness of groove is equal to the required surface roughness value (Ra12 = Rare = 0.5 µm) For this reason, from the 12th part onward, the surface roughness in particular and the processing accuracy in general are not guaranteed as the requirement This is the appropriate time to dress grinding wheel Thus, the wear value at the

12th part is the economic limitation wear value of the grinding wheel corresponding the above cutting mode Therefore, with the first set of cutting parameters the economic limitation wear value is equal to the wear value at the 12th part, i.e.Hzlimitation = Hz19 = 9.6 µm

- With different cutting parameters, the economic limitation wear values of the grinding wheel are different With the second set of cutting parameters, the economic limitation wear

Part’s accuracy and grinding wheel’s wear (µm)

Hz limitation = 10.3

The number of parts

Suitable time to repair grinding wheel to assure part’s accuracy since at this time

Ra 16 = 0.5 = Ra re

value is equal to the wear value at the 15th part, i.e.Hzlimitation = Hz15 = 9.9 µm Meanwhile, with the third set of cutting parameters, the economic limitation wear value is equal to the wear value

at the 16th part, i.e.Hzlimitation = Hz16 = 10.3 µm

- When the parameters of cutting mode (Sn, Vp, t) changes, the grinding wheel’s wear value (Hz1, Hz2, etc.), the part’s surface roughness (Ra1, Ra2, etc.) and the oval of groove bottom’s diameter (O1, O2, etc.) will also change For the first set of cutting parameters (figure 10), those values after grinding part no 1, 2, 3 and 4 will be respectively Hz1 = 3.9 µm; Hz2 = 5.0 µm; Hz3 = 5.9 µm; Hz4 = 6.5 µm; Ra1 = 0.37 µm; Ra2 = 0.40 µm; Ra3 = 0.42 µm; Ra4 = 0.44 µm; O1 = O2 =

O3 = O4 = 2.83 µm Meanwhile, with the second set of cutting parameters (Figure 11) those values after grinding part no 1, 2, 3 and 4 will be respectively Hz1 = 3.8 µm; Hz2 = 4.9 µm; Hz3 = 5.7 µm; Hz4 = 6.3 µm; Ra1 = 0.35 µm; Ra2 = 0.39 µm; Ra3 = 0.41 µm; Ra4 = 0.42 µm; O1 = O2 =

O3 = O4 = 2.67 µm Thus, the cutting mode parameters have influence on the grinding wheel’s wear, the groove’s surface roughness and the oval level of groove bottom’s diameter While the other precision elements of part including groove bottom’s diameter, groove’s radius, distance from its groove center line to its head surface and the circular run-out-of its groove center line to its head surface can be impacted but at very low level This proves that the cutting parameters has influence on the part’s accuracy but mainly on the grinding wheel’s wear, the part’s surface roughness and the groove bottom diameter’s oval This can be explained that in the profile grinding process for the groove of the 6208 ball bearing’s inner ring, the deviation of distance from the groove center line to the head surface and the circular run-out of the groove central line to the head surface are mainly due to the standard error of jigs and fixtures The input parameters of the cutting mode influence insignificantly on the two precision factors mentioned previously Meanwhile, the grinding wheel’s wear has impact on deviation of groove radius and groove bottom diameter However, the error is insignificant in comparison with the requested standard accuracy because the wear value of grinding wheel is very small

In order to estimate the degree of this influence, the experiment planning methods should

be applied However, the above research results are the base to set up a suitable experiment planning issue in the following section To decrease the number of experiments but still guarantee the expected requirement, it normally only considers the influence of grinding parameters on three output factors which are the most affected They are the part’s surface roughness, groove bottom diameter’s oval and grinding wheel’s wear

2.3 Experiment to determine mathematical functions for the important outputs over technical parameters in profile grinding for the ball bearing’s inner ring groove

The experimental conditions here are similar to the experimental conditions in the previous section However, these experiments are implemented with the cutting mode as shown in table 4

Table 4 Experimental conditions

Parameters/Factors Experimental levels

Low level (1) Base level (2) High level (3)