Influence of selected parameters of drag finishing on tool microgeometry

The aim of the experiment described in the paper was to determine how selected parameters of drag finishing such as process time, immersion depth and rotation frequency affect microgeometry of cutting edge. Four flute cemented carbide mills with diameter of 10 mm were used in this experiment. These mills were drag finished on prototype drag finishing machine developed on Faculty of Materials Science and Technology, Slovak University of Technology. Alicona Infinte Focus SL measuring machine was used to measure microgeometry of these mills. The main parameter of microgeometry of cutting edge was the size of the radius of cutting edge. In the article we compared which parameter has the biggest influence on size of cutting edge radius and also there was statistical evaluation carried out. The most significant parameter of drag finishing was determined to be the immersion depth.


Introduction
Cutting edge preparation is currently relevant topic with regard to increasing the tool life of cutting tools.There are many publications which deal with increasing tool life using some of the cutting edge preparation methods.In general we can divide these methods of cutting edge preparation into three groups -mechanical, thermal and chemical [1].The essence of mechanical methods is the removal of material by abrasive particles.These particles are carried by a suitable finishing medium.The medium can be constituted by air, liquid, fiber, etc.The resulting cutting edge radius or cutting edge chamfer is achieved by the speed of the process and the duration of the action of the abrasive particles on the cutting edge.Thermal methods are based on the action of thermal energy, which has a high intensity and thus melting or evaporation of the material from the cutting edge.Thermal energy can be in the form of a laser beam, a plasma beam, or as an electroerosion.A chemical reaction or an electrochemical reaction is the basis for chemical methods of modifying the microgeometry of cutting edges.However, most common cutting edge preparation methods are mechanical.
Abrasive jet machining is a method in which accelerated abrasive particles contained in the support medium, are brought to the cutting edge causing material removal.The support medium can be either air -dry abrasive jet machining, or water -wet abrasive jet maching [2,3].The advantage of the method is the precise direction of the flow and thus the creation of different geometry on the cutting edge.Wet abrasive jet machining creates a smoother surface due to the dampening effect of water.In terms of parameters, the lower the feed rate, the larger the radius of rounding.By increasing the pressure of the air/water flow, the cutting edge radius is increased [4].Brushing is another method used to finish the surface, remove defects and create the desired cutting edge radius.The mechanism of the method consists in the use of a rotary tool, which is usually in the shape of a disk or a wheel.The tool consists of a large number of wires or nylon brushes that contain abrasive particles [5].The brushing method makes it difficult to adjust the microgeometry of the cutting edge of complex cutting tools.On the other hand, it is possible to create asymmetric cutting edge radius by changing the brushing direction.The size of the cutting edge radius is influenced by the duration of the process, material of the abrasive particles, feed and the speed of rotation [6].The mechanism of magneto abrasive machining is the use of abrasive particles that are ferromagnetic.The cutting tool is immersed in a mixture of ferromagnetic abrasive particles.The movement of abrasive particles is ensured by a magnetic field generator.When adjusting the microgeometry of the cutting edge of cylindrical tools, two rotating disks are usually used, which are placed opposite each other.The distance of the device generating the magnetic field and the process time have the greatest influence on material removal [7][8][9].By using abrasive flow machining it is possible to obtain very good quality of the finish surface and to finish hard-to-reach surfaces.The medium used is a highly viscous polymer fluid that contains abrasive grains [1,10].During drag finishing, the cutting tools are placed in a container with an abrasive medium and the movement of the tools in this medium leads to the cutting edge radius.The device consists of a container in which the abrasive medium is freely placed and a suitable gearbox.By rotating the tools in several axes, the tool performs movement in the abrasive medium.Different methods of cutting edge preparation are illustrated in Figure 1.It can be said that the modification of the microgeometry of the cutting edge has a significant effect on the intensity of the wear of the cutting edge.In general by using any method of cutting edge modification, it is possible to achieve an increase in the durability of the cutting edges.How much is it possible to increase the durability of the cutting edge is influenced by the used parameters of individual methods [11][12][13].The modification of the microgeometry of the cutting edge has a negative influence on the components of the cutting force and material flow.By increasing the size of cutting edge radius, the cutting forces increase [14][15][16].In case of surface roughness in general, the larger the radius of the cutting edge, the more material is forced into the machined surface and the worse the surface roughness.The magnitude of the influence of the cutting edge radius decreases with increasing hardness of the material being processed.It is possible to say that a smaller radius of rounding of the cutting edge means less roughness of the surface [17][18][19].The residual stresses also behave on the same principle.Cutting edges that are rounded adequately affect the residual stresses near the surface by causing compressive stresses [20,21].
Most of the publications describe the effect of drag finishing on the machining process itself, or the researchers deal with the effect of the abrasive media on the cutting edge radius or surface roughness.Other process parameters are not sufficiently investigated.Since drag finishing is a type of grinding, the same mechanism apply to it as for conventional grinding.Therefore, it is possible to assume what effect the hardness and grain size of the abrasive medium will have.This is confirmed by research where it was found than the largest cutting edge radius was achieved with Al2O3 medium, then SiC + walnut shell medium and the smallest radius was achieved with walnut shell medium and corn medium.The difference was in the quality of the surface of the tools.The best surface quality was achieved with corn and walnut shell medium [21].We also know that the processing time will affect the size of the rounding radius.It is also known that the depth of immersion has a significant effect on the radius of rounding of the cutting edge [22].However, it is not sufficiently known what effect the combination of these parameters has.

Materials and Methods
In order to verify the influence of selected parameters of drag finishing, it was necessary at first to select the cutting tools.Solid carbide end mills were chosen to be used in the experiments.These mills were at first grinded on tool grinder Reinecker WZS 60.Macrogeometry of these tools was measured on Zoller measuring machine.After these step the drag finishing experiments were carried out.The selected parameters are rotation frequency, immersion depth and process time.The reason why these parameters where chosen is that these parameters are the most important when drag finishing and they have the biggest influence on size of cutting edge radius.There are not many publications that deal with the influence of drag finishing parameters.Size of cutting edge radius was measured on Alicona optical measuring machine.

Cutting tools
To verify the influence of parameters of drag finishing, it necessary to have cutting tools.In this case we used four flute solid carbide end mills.The macrogeometry of these tools is specially designed for cutting stainless steel AISI 316L.Stainless steel AISI 316L belongs to difficult-to-cut materials category.Therefore, it is assumed that cutting edge radius will have significant effect on tool wear, when milling this type of material.
Sintered carbide from manufacturer CERATIZIT marked CTS20D was used as the material for the cutting tools.The average grain size of this sintered carbide is 0.5 to 0.8 μm, which ranks it among sintered carbides with a sub-micron grain size.Its chemical composition consists of tungsten carbide together with 10% cobalt, so it ranks among single-component sintered carbides.According to the ISO classification, it belongs to the group K20 -K40.Tools were ground from sintered rod on a Reincker WZS 60 CNC grinder.The geometry of these tools is shown in Table 1.The program for the grinder was created in the NUMROTOplus software.After grinding, the tool geometry was measured on ZOLLER Genius 3+, for checking the macrogeometry of the tool.

Drag finishing machine
Drag finishing of cutting tools took place on a prototype drag finishing machine.This machine was designed and manufactured on faculty of Materials Science and Technology in Trnava, Slovak University of Technology.During drag finishing, the influence of all drag finishing parameters that can be set on this device was monitored.The same cutting tool and the same abrasive media were used in all experiments.Technological parameters whose influence was monitored were rotation frequency, immersion depth and process time (Table 2).An abrasive medium made of aluminium oxide Al203 with an average grain size of 1-3 mm was used for drag finishing.This type of medium is especially intended for creating cutting edge radius more than 30 μm.From the point of view of rotation frequency, the minimum, maximum and average value of rotation frequency were selected.The cutting part of the solid carbide end mill is 23 mm.Based on this value, the minimum immersion depth value of 30 mm was set.Other immersion depth values were chosen as a multiple of the minimum immersion depth.Process time was chosen based on previously carried out experiments.

Measuring of microgeometry
The measurement of the microgeometry of the solid carbide end mills was carried out on an Alicona InfiniteFocusSL optical 3D measuring device.The device also includes the EdgeMaster software module, which is used to measure the microgeometry of cutting tools.It is possible to measure all types of mills, drills or cutting inserts.In addition to the cutting edge radius (from 2 µm), the device is able to measure the symmetry factor (K-factor) of the cutting edge and other parameters characterizing the cutting edge.In Figure 2, it is possible to see the cutting edge radius development during drag finishing, while the figure on the left is the cutting edge before cutting edge preparation, in the figure in the middle, the size of cutting edge radius is 20 µm, and in the figure on the right, the size of cutting edge radius is 60 µm.A reference measurement type was created for measurement purposes.The necessary information about the cutting tool has been entered.A 20x magnification lens was selected for the measurement, with a measurement distance of 16 mm, a vertical resolution of 50 nm, and a maximum measurement area of 2500 mm 2 .The minimum size of cutting edge radius for this lens is 3 µm.During the measurement, a section of the cutting edge was scanned, which was divided into 50 sections, and in each section the device calculated all the specified characteristics of the cutting edge.The resulting cutting edge radius is calculated by the software based on an algorithm from all 50 cuts.One of the important measurement settings is the illumination of the measured object, in this case the cutting tool.Appropriate lighting conditions were chosen based on testing.After the reference type of measurement was correctly set, this type was used for all measurements of the cutting tools before and after drag finishing.Before each measurement, the tools were cleaned in an ultrasonic cleaner Elmasonic P. The cleaning took place in order to remove the oil residues that got on the tools during grinding and the remains of abrasive particles.99.9% isopropyl alcohol was used as a cleaning agent.In order to ensure the same position of the tool, a fixture was used that was designed specifically for the solid caribe end mills that were measured in this paper.The designed fixture can be seen in Figure 3.The fixture consists of two holes through which the fixture was clamped to the Alicona Infinite Focus SL device, a cylindrical part that ensured the same position of the solid carbide end mills, while this part is inclined at the same angle as helix of the solid carbide end mills.There is a protrusion (marked in black in Figure 3) on the fixture, on which the cutting edge of the solid carbide end mill were fixed during the measurement.Since the measured solid carbide end mills had four cutting edges and their spacing was 90°, the same rotation of the tool during measurement was always ensured.The cutting edge on the helix was measured.The cutting edge was measured at a distance of 3 mm from the end of the cutting tool.

Results and Discussion
In order to analyse the influence of the technological parameters of drag finishing, it is appropriate to use statistical methods.The DOE method is the most used method for experiment planning.Three factors were chosen -the rotation frequency of the rotor/holder, the immersion depth and the process time, while each factor was varied at three levels (Table 2).The radius of rounding of the cutting edge was monitored as an output.The measured data is shown in Table 3.The analysis of variance (ANOVA) was used to evaluate the influence of individual factors.The evaluation of the responses was carried out using the Discriminant Function Analysis (DFA).The DFA method helps us determine the suitability of the input parameters for the desired response value.When evaluating the radius of rounding of the cutting edge, in this case it is required that its value be as large as possible, because then it is possible to assess the influence of factors on the value of the cutting edge radius.Based on the calculations, a total suitability index for individual levels was obtained, which is shown in Table 4.The values obtained from the DFA analysis were used to calculate the influence of the input technological parameters of drag finishing process on the observed response -cutting edge radius.Based on the calculations, Table 5 was obtained, presenting an evaluation of variance analysis and elucidating the degree of variability among the levels of individual factors.1.5597 Subsequently, it was possible to evaluate the ANOVA analysis using the p-value, which is used to express the influence of the parameters as a percentage.P -values were shown in Table 6.From these values, it can be concluded that the depth of immersion has the greatest influence on the radius of rounding of the cutting edge.Rotational frequency and processing time have almost the same effect, but less than the immersion depth.At the same time, it is possible to see that all factors are statistically significant.The p-value error was as high as 50.12 %.This was probably due to the fact that the cutting edge radius value was calculated from three tools, each with 4 cutting edges, for a total of 12 values.Table 6.ANOVA evaluation according to Pvalue.

Conclusion
The article deals with drag finishing as one of the methods for modifying the microgeometry of the cutting edge.In the article, it was assessed what influence individual parameters of drag finishing have on the cutting edge radius.In general, it is known what influence the abrasive medium has, and it is therefore possible to state that the hardness and size of the individual abrasive grains are significant parameters from the point of view of the drag finishing medium.However, it is not sufficiently researched what effect the technological parameters of drag finishing have, such as process time, immersion depth and rotation frequency.Experiments were carried out on a prototype drag finishing machine where it was found that all parameters affect the size of the cutting edge radius.From the measured data it is apparent that by increasing the immersion depth, the rotation frequency or the processing time, the cutting edge radius increases.An experiment was conducted based on DOE and the influence of individual parameters was assessed.Immersion depth was found to have the greatest effect.This parameter has three times bigger influence as the other monitored parameters, moreover process time and rotation frequency have roughly comparable influence.

Figure 2 .
Figure 2. Developemenet of cutting edge radius during drag finishing.

Figure 3 .
Figure 3. Fixture for measuring cutting edge radius of the cutting tools.

Table 2 .
Values of selected drag finishing parameters.

Table 3 .
Measured data of cutting edge radius.

Table 4 .
Overall suitability index for individual factor levels.