Influence the effect of balancing the grinding set on the accuracy and roughness of the machined surface

The article examines the effect of balancing the grinding wheel set on the accuracy and roughness of the machined surface during the production of cutting tools. Screw drills with a diameter of 12 mm were manufactured as well as test specimen in the shape of a recess with a width of 20 mm and a diameter of 18 mm. The workpiece material was sintered carbide (WC+Co). Properly balanced and unbalanced sets of cubic boron nitride (CBN) and synthetic diamond grinding wheels were used in the production of the tools and test specimens. A WZS 60 Reinecker tool grinder was used as the machine. The parameters of surface roughness and dimensional accuracy were measured in the experiment. Surface roughness was evaluated on a Zeiss Surfcom 5000 shape and contour measuring machine. Dimensional accuracy measurement was performed on an optical measuring machine Zoller Genius 3. In terms of dimensional accuracy, tools and test specimen s produced by wheel balancing to a lower level of balance quality G show greater dimensional inaccuracy than tools and test specimen s produced by wheel sets that were balanced to a higher quality level of balance G. From the point of view of evaluating the surface roughness, the results were not clear, but the tools and test specimens produced by CBN grinding wheels showed better surface roughness, regardless of the quality level of balance G.


Introduction
Grinding is a machining process characterized by the precision removal of material from a workpiece through the utilization of abrasive tools [1,2].Achieving precision and efficiency in grinding operations necessitates the proper balancing of grinding sets.Balancing grinding sets is important for several reasons.Balanced grinding sets reduce vibrations and irregularities, leading to a higher surface quality and precision of the ground surface.It also contributes to increased accuracy and repeatability of manufactured parts -cutting tools.The consequence of using an unbalanced tool assembly can be a reduction in the service life of grinding wheels and excessive wear of the bearings in the machine spindle [3][4][5][6].
A grinding wheel consists of abrasive grinding grains, binder and pores.The basic causes of grinding wheel imbalance include the tolerance of the clamping wheel, the inhomogeneity of the wheel, the parallelism and centering of the wheel.In the case of clamping the grinding wheel on the grinding mandrel, the imbalance of the initial imbalance of the clamping mandrel and the grinding wheel / wheels is created [7][8][9].The term unbalanced tool set means a rotating body whose central axis of inertia does not coincide with the axis of rotation [10].Based on the action, imbalance can be divided into 3 basic types: static, moment and dynamic imbalance [11].Balancing is a process in which the distribution of the mass of the tool assembly is detected and, if necessary, corrected so that the position of the main central axis of inertia of the tool assembly differs as little as possible from the position of the axis of rotation, i.e. so that the residual imbalance or oscillation in the bearings moves at operating speeds within the permissible differences [4,12].
Rotor balancing accuracy is assessed by JIS B 0905-1992 at the balancing quality level G.It is recommended to balance the grinding wheel to a value of G 1 [m.s 1 ] or less, for the maximum spindle rotation frequency.Grades of quality G are designed based on the interaction of the permissible specific residual unbalance (epor) and the maximum angular speed of the rotor (Ω).From the degree of balancing quality, it is possible to derive the permissible residual imbalance of Uzv, which is determined based on the relationship [13] where G is the level of balancing accuracy [m.s 1 ], m is the weight of the tool assembly [kg], and the n is the spindle rotation frequency [min 1 ].
The degree of balance quality G is a quantity dependent on the spindle rotation frequency.From figure 1 it follows that the higher the rotation frequency of the tool set, the lower the value of the permissible residual specific unbalance must be in order for the set to meet the required level of balancing quality G [13].In order to reduce unevenly distributed weight, balancers are used devices.The balancing device can measure the level of imbalance and subsequently reduce it to the permissible value for the required grinding parameters.In practice, they are most often used digital balancing devices.

Materials and methods
In order to investigate the influence of the balance of the tool assembly on the dimensional accuracy and roughness of the machined surfaces, a test specimen was designed a cylindrical test specimen with notches (test specimen) and a screw drill with a diameter of 12 mm.The experiment consists of the preparation of blanks for the production of test specimens and a screw drill, the design of operations and production strategies on a tool grinder, the production of test specimens and screw drills, and the control of dimensions and surface roughness.All activities necessary for the production and measurement of test specimens and screw drills are shown in figure 3.

Workpiece
The material of the blank for the production of test specimen and the tool is made of sintered carbide marked F10 supplied by the German company PCG Hardmetall.Basic material properties are listed in table 1 [5].
Table 1.Basic properties of material F10 [5].The blank supplied by the company had dimensions of 20 × 6 × 330 mm.Since the semi-finished product for the production of the test specimens have dimensions of 20 × 165 mm, a semi-finished product supplied by the manufacturer was necessary divide into two pieces.An electroerosive machine (Charmill robofill 310) was used.The semi-finished product for the production of the screw drill had dimensions of 12 × 6× 85 mm.

Grinding set
To produce the test specimen and the tool, peripheral grinding wheels (1A1) from Urdiamant were used.The grinding wheel from CBN (figure 4a) was used for the production of test specimen No. 1.A synthetic diamond grinding wheel (figure 4b) was used to produce test specimen No. 2, 3 and for the production of screw drills.Both grinding wheels are the same size, shape and thickness of the abrasive layer.Grinding sets must be balanced before use.Balancing is carried out on Haimer ToolDynamic Comfort plus 2009 balancing device.Each grinding set was balanced to a 20 000 min 1 before experiments start.

Design of the NC program for the production of test specimen and screw drill
The design of the NC program for the production of the test specimen and the tool was carried out in the software NUMROTOplus.Creation of an NC program consists in entering values into predefined ones dialog boxes.After generating and optimizing the NC program, a 3D simulation took place production of a test specimen and a tool for the purpose of verifying the functionality and checking the collisions of the semi-finished products grinding set, or other elements of the MTWP system.The result of the simulation of the production of the test specimen is shown in figure 5a.The result of the screw drill simulation is shown in figure 5b.

Production of the test specimen and screw drill
The test specimens and the screw drills were made on a universal tool grinder WZS 60 REINECKER The values of the cutting speeds were the same during the production of the test specimen and the screw drill.The values of the feed rates were the same for everyone when making the indentations on the test specimen recess, but in the production of a screw drill, they depended on the operation being performed.The manufactured test specimen is shown in figure 6. Due to uneven transitions between the individual notches is the corner radius of the grinding wheel (0.2 mm).Overall, they were produced 3 test specimens.Test specimen No. 1 was made with a CBN grinding wheel and test specimen No.The produced screw drill is shown in figure 7. A total of 2 were produced screw drills using a synthetic diamond grinding wheel.The first drill was made with a new balanced grinding wheel.The second drill was made only after the experimental one use of the disc for the production of test specimen No. 2 and 3.

Measurement of test specimens and screw drills
The produced test specimens and screw drills had to be measured.On the Zoller Genius 3 device was used to measure dimensions of the created surfaces.For detection quality of roughness, the Zeiss SURFCOM 5000 device was used.
The measurement of test specimens consisted of 2 steps.In the first step, the Zoller Genius 3 device was used to check the dimensions of the finished surfaces.A measuring program was created in the device, which checked the diameter of the test specimen.Each indentation area was measured 3 times (figure.8a).From the obtained data, an average was calculated for each area.
In the second step, the roughness profile on the individual surfaces was also created using the Zeiss SURFCOM 5000 device.Each indentation area was measured from 3 sides (figure.8b).From the obtained data, an average was calculated for each area.The measurement of screw drills was carried out using the device Zoller Genius 3. Since the device has a program for measuring screw drills, manual measurement of selected parameters was not necessary.Parameters such as apex angle, diameter, helix pitch angle, core diameter, helix pitch length and transverse cutting edge angle were measured on the screw drills produced.

Results and discussion
The tables below show the measurement results for test specimens and screw drills.In addition to measurement results, the tables also contain balance values of the grinding set after the production of the given element.

Evaluation of data obtained during test specimens measurement using Zoller Genius 3
The results of measuring the dimensional deviations of the test specimens are shown in Table .2. The table contains data on the measured dimensional deviation from the required reference dimension of 18 mm.Since the rotation frequency of the grinding wheels was 4200 min 1 during the entire experiment, it can be concluded that surfaces 5, 6 and 7 on test specimen No. 1 were made with a grinding wheel that was not balanced.It is clear from the values that when using the CBN grinding wheel, a lower dimensional accuracy was achieved than when using the synthetic diamond grinding wheel.Relatively large variance of deviations for test specimen No. 1 is the cause of inappropriate use of a CBN wheel for grinding a test specimen made of sintered carbide.Using the wrong abrasive material results in higher wear of the grinding wheel, excessive clogging of the pores, loss of self-sharpening ability and reducing the cutting ability.The manifestation is an increased strain on the machine and a reduction the roughness of the produced surfaces (the grinding wheel loses its cutting properties after filling the pores and the machined material "strokes").The course of the first 3 deviations has a decreasing tendency in the graph, which can be attributed to the clogging of the pores of the grinding wheel.The values of the other 2 deviations show upward trend, which can be attributed to the revival of cutting properties and wear grinding wheel.
The course of deviations on the test specimen No. 2 and 3 show an approximately linear character, which can be explained by the constant wear of the grinding wheel.Due to the fact that the graphs do not distinguish the deviation caused by the wear of the disc and deviation caused by balancing, it is not possible to say absolutely clearly that balancing of the grinding wheel has or does not affect the dimensional accuracy of the produced surfaces.For confirming or refuting the influence of grinding wheel balance on dimensional deviation is it is necessary to filter out the deviation caused by the wear of the disc.

Evaluation of data obtained during test specimens measurement using Zeiss SURFCOM 5000
The results of measuring the roughness of the test specimens are shown in Table 3.The table contains data on the measured mean arithmetic deviation of the profile (Ra) and of the highest profile height (Rz) of individual surfaces on the manufactured test specimens.
It is clear from the graphs that both types of roughness (Ra, Rz) have an increasing tendency.The lowest value of roughness Ra and Rz is achieved when using the CBN grinding wheel.Balancing the grinding set before machining (min 1 ) 20 000 10 651

Conclusion
From the obtained data, it can be concluded that the test specimen produced using a CBN grinding wheel shows a lower surface roughness than the test specimen produced using a synthetic diamond grinding wheel.The reason for the lower surface roughness is the inappropriate use of the CBN grinding wheel for grinding test specimen made of sintered carbide.When grinding, there was a loss of cutting ability due to clogging of the pores.Due to the loss of cutting ability, the grinding wheel was unable to remove the required amount of material from the semi-finished product and so the material only "stroked".The lowest roughness value Ra is 0.865 μm.
From the inspection of the dimensions, the manufactured surfaces on test specimen No. 1 show a higher dimensional deviation than test specimens made with a synthetic diamond wheel.The deviation of the size on the test specimen ranges from 0.238 mm to 0.259 mm, while for the test specimens of manufactured grinding wheel aids with synthetic diamond from 0.069 mm to 0.119 mm.Test specimen No. 2 and 3 show a higher roughness value from 0.987 μm to 1.050 μm.From the point of view of the effect of balancing on the dimensional accuracy and surface roughness, it can be argued that the deviations of the dimensions and roughness of the produced test specimens are independent of the degree of balancing of the grinding wheel.
From the values of the dimensional deviations of the screw drills, the apex angle on the screw drill No. 1 shows a greater deviation from the required dimension of 118° (by 4.47°) than screw drill No. 2. The greatest deviation is shown by the angle of the transverse cutting edge on screw drill No. 2 (9.26°).When assessing the impact of balancing on the dimensional accuracy of the produced drills, it can be said that, except for the value of the diameter and the apex angle, the produced drill No. 1.This tool was made with a grinding wheel balanced to a higher rotation frequency (20 000 min 1 ) than tool number 2 (10 651 min 1 ).Therefore, it can be said that in the production of screw drills, there is an obvious connection between the quality of the balance of the grinding wheel and dimensional deviations.

Figure 1 .
Figure 1.Permissible residual unbalance composed of the degree of balance quality G and the frequency of rotation of the rotor [13].

Figure 3 .
Figure 3. Stages and activities necessary for the implementation of the experiment.

Figure 5 .
Figure 5.The result of the simulation a) test specimen and b) screw drill.

a b Figure 8 .
Measurement of test specimens.a) measuring the specimen size, b) measuring the roughness of the test specimen.

Table 2 .
The result of measuring the dimensional accuracy of the test specimens.

Table 3 .
Values of the mean average arithmetic deviation Ra and the highest height of the profile Rz.Evaluation of the data obtained during the measurement of screw drills with Zoller Genius 3 The results of measuring the dimensional deviations of screw drills are shown in table4.It is obvious that, in addition to the values of diameter and apex angle, it is more accurately produced screw drill No. 1.Thus, it is possible to partially confirm the hypothesis that what is a grinding set the more accurately balanced, the more accurate the dimensions of the tool can be achieved.It is a paradox a measured diameter value that is greater than the blank diameter value.The reason this phenomenon is the creation of a subtle increase in the production of grooves.After further modifications to the tool (polishing, rectification) the diameter value of the screw drill would correspond to the diameter semifinished product.

Table 4 .
Deviations of selected parameters of screw drills.