Micromilling is one of the processes used to achieve miniaturized products, and which requires high dimensional and geometric quality [1] . This characteristic continues to be used until the present [2] . This process can be defined by its micromill diameter that must vary between 1 µm and 1000 µm [3, 4] , and is highlighted as it obtains lower surface roughness values, when dealing with small parts [5] .
One of the main features in microcutting is the size effect defined by the nonlinear increase of the specific energy associated with the reduction in the undeformed chip thickness [6] . Once that the cut occurs on the same scale as the material microstructure, the tool edge radius cannot be considered sharp, and the tool rake angle will be extremely negative [7, 8] .
With the negative rake angle and insufficient chip thickness, in light of the low values of feed rate, the workpiece material will slide in the tool flank face due to the elastic recovery, and as such there will exist a plowing effect rather than a cut [9] .
Another main characteristic of this processes is the minimum chip thickness, which is "the depth of cut or feed must be over a certain critical chip thickness before a chip will form" [6] . When this value is not reached, there will no longer exist a cut, but only a large amount of elastic deformation [6] . This can be present itself in two ways -rubbing or ploughing.
Rubbing occurs when the minimum chip thickness is not reached and then there occurs only friction between the workpiece and the tool flank face. When the chip thickness value arrives close to the minimum chip thickness the rubbing turns into ploughing and the material will become deformed, and as a result small amounts of material could become sheared. Once the minimum chip thickness is reached the material removal mechanism will become more like shearing, thus leading to microcutting [7, 10] .
All these accumulated peculiarities do not allow for the obtainment of the final required quality, while in some cases due to the existence of small dimensions, it is not possible to perform necessary additional finishing processes [3, 11] .
Micromilling is one of the processes used to achieve miniaturized products, and which requires high dimensional and geometric quality [1] . This characteristic continues to be used until the present [2] . This process can be defined by its micromill diameter that must vary between 1 µm and 1000 µm [3, 4] , and is highlighted as it obtains lower surface roughness values, when dealing with small parts [5] .
One of the main features in microcutting is the size effect defined by the nonlinear increase of the specific energy associated with the reduction in the undeformed chip thickness [6] . Once that the cut occurs on the same scale as the material microstructure, the tool edge radius cannot be considered sharp, and the tool rake angle will be extremely negative [7, 8] .
With the negative rake angle and insufficient chip thickness, in light of the low values of feed rate, the workpiece material will slide in the tool flank face due to the elastic recovery, and as such there will exist a plowing effect rather than a cut [9] .
Another main characteristic of this processes is the minimum chip thickness, which is "the depth of cut or feed must be over a certain critical chip thickness before a chip will form" [6] . When this value is not reached, there will no longer exist a cut, but only a large amount of elastic deformation [6] . This can be present itself in two ways -rubbing or ploughing.
Rubbing occurs when the minimum chip thickness is not reached and then there occurs only friction between the workpiece and the tool flank face. When the chip thickness value arrives close to the minimum chip thickness the rubbing turns into ploughing and the material will become deformed, and as a result small amounts of material could become sheared. Once the minimum chip thickness is reached the material removal mechanism will become more like shearing, thus leading to microcutting [7, 10] .
All these accumulated peculiarities do not allow for the obtainment of the final required quality, while in some cases due to the existence of small dimensions, it is not possible to perform necessary additional finishing processes [3, 11] . 
Micromilling is one of the processes used to achieve miniaturized products, and which requires high dimensional and geometric quality [1] . This characteristic continues to be used until the present [2] . This process can be defined by its micromill diameter that must vary between 1 µm and 1000 µm [3, 4] , and is highlighted as it obtains lower surface roughness values, when dealing with small parts [5] .
One of the main features in microcutting is the size effect defined by the nonlinear increase of the specific energy associated with the reduction in the undeformed chip thickness [6] . Once that the cut occurs on the same scale as the material microstructure, the tool edge radius cannot be considered sharp, and the tool rake angle will be extremely negative [7, 8] .
With the negative rake angle and insufficient chip thickness, in light of the low values of feed rate, the workpiece material will slide in the tool flank face due to the elastic recovery, and as such there will exist a plowing effect rather than a cut [9] .
Another main characteristic of this processes is the minimum chip thickness, which is "the depth of cut or feed must be over a certain critical chip thickness before a chip will form" [6] . When this value is not reached, there will no longer exist a cut, but only a large amount of elastic deformation [6] . This can be present itself in two ways -rubbing or ploughing.
Rubbing occurs when the minimum chip thickness is not reached and then there occurs only friction between the workpiece and the tool flank face. When the chip thickness value arrives close to the minimum chip thickness the rubbing turns into ploughing and the material will become deformed, and as a result small amounts of material could become sheared. Once the minimum chip thickness is reached the material removal mechanism will become more like shearing, thus leading to microcutting [7, 10] .
All these accumulated peculiarities do not allow for the obtainment of the final required quality, while in some cases due to the existence of small dimensions, it is not possible to perform necessary additional finishing processes [3, 11] .
Micromilling is one of the processes used to achieve miniaturized products, and which requires high dimensional and geometric quality [1] . This characteristic continues to be used until the present [2] . This process can be defined by its micromill diameter that must vary between 1 µm and 1000 µm [3, 4] , and is highlighted as it obtains lower surface roughness values, when dealing with small parts [5] .
One of the main features in microcutting is the size effect defined by the nonlinear increase of the specific energy associated with the reduction in the undeformed chip thickness [6] . Once that the cut occurs on the same scale as the material microstructure, the tool edge radius cannot be considered sharp, and the tool rake angle will be extremely negative [7, 8] .
With the negative rake angle and insufficient chip thickness, in light of the low values of feed rate, the workpiece material will slide in the tool flank face due to the elastic recovery, and as such there will exist a plowing effect rather than a cut [9] .
Another main characteristic of this processes is the minimum chip thickness, which is "the depth of cut or feed must be over a certain critical chip thickness before a chip will form" [6] . When this value is not reached, there will no longer exist a cut, but only a large amount of elastic deformation [6] . This can be present itself in two ways -rubbing or ploughing.
Rubbing occurs when the minimum chip thickness is not reached and then there occurs only friction between the workpiece and the tool flank face. When the chip thickness value arrives close to the minimum chip thickness the rubbing turns into ploughing and the material will become deformed, and as a result small amounts of material could become sheared. Once the minimum chip thickness is reached the material removal mechanism will become more like shearing, thus leading to microcutting [7, 10] .
All these accumulated peculiarities do not allow for the obtainment of the final required quality, while in some cases due to the existence of small dimensions, it is not possible to perform necessary additional finishing processes [3, 11] . 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to In order to improve the cut, an established method used is the application of cutting fluid. In general cutting fluid will allow for higher levels of heat removal from the cutting interface and less friction, which results in lower cutting forces [12] . The result will be a workpiece with a better surface finish and higher dimensional accuracy [13] .
The cutting fluid is usually applied in a conventional method, also called flooding, that delivers the fluid at low pressures and high rates (from 240 to 18 000 l/h) [14] .
For micromachining, the method that stands out is the minimum quantity of lubrication (MQL). In this method, the cutting fluid is applied at low rates, generally below 50 ml/h or 500 ml/h according to DIN 69090 [15] and Walker [16] respectively. In this method, the cutting fluid is delivered to the cutting zone by compressed air, this methodology is able to achieve similar or even better results than the conventional method [16] .
The authors in Ziberov, Da Silva, Jackson and Hung, [17] , developed tests using Ti6Al4V alloy, they used a 152.4 µm, with spindle speed 20 000 rpm, fz =0.1 µm. The cutting fluid used was the natural lubricant coolube, with a flow rate of 40.7 ml/h. The authors verified that the cutting fluid hinders the built-up edge (BUE) formation, which leads to a better surface finish. However, in their study, machining with cutting fluid leads to higher burr formation.
Research by Dos Santos, Da Silva and Jackson [18] studied the cutting fluid application in the micromilling of UNS S32205 duplex stainless steel, with a 381 µm diameter micromill, with a spindle speed of 20 000 rpm and 30 000 rpm, with fz =1,5 µm. The cutting fluid and flow rate were the same as those used by Ziberov, Da Silva, Jackson and Hung [17] . The authors in [18] found similar results, where the slot made with cutting fluid had lower surface roughness and less adhered material.
The study performed by Zhang [19] compared four fluid conditions, which they called: (a) ultrasonically atomized water, (b) nebulized canola oil, (c) water and oil mixture, and (d) nebulized 5% TRIM solution. The authors adopted two different materials Al6061 and Steel 1018. Full immersion slots were milled on both materials. Their cutting parameters were: fz =0.3 and 1.0 µm with a spindle speed of 60 000 rpm and 50 000 rpm. The tests indicated that the oil water mixture is more effective for improving the surface, the authors indicated that the improvement was major from the cooling property of the cutting fluid.
Due to the importance of using cutting fluid in micromachining to manufacture a workpiece with a better surface finish, this research aims at analyzing the influence of cutting fluid application frequency on the surface quality of the workpiece, since this parameter has not yet been investigated by studies that focus on the effect of cutting fluid application on micromachining. 
The tests were performed in age-hardened nickel-based superalloy Inconel 718, considered as a hard to cut material.
Some of Inconel 718 mechanical properties are shown on Table 1 . The material was prepared in order to obtain 15 mm X 20 mm X 15 mm blocks. The Micromilling tests were carried out on a Mini-mill / GX CNC, manufactured by Minitech Machinery Corporation®. The Mini-mill has a maximum spindle speed (n) of 60000 rpm and 4 axes with position resolution of 0.1 μm. The tool is a ultra-refined cemented carbide micro end mill (MS2MSD0040) coated with (Al, Ti)N, 400 µm diameter, 2 flutes, manufactured by Mitsubishi.
The geometry of the micromill is shown in Figure 1 , before the tests, all the micromills were observed under the scanning electron microscope (SEM) in order to verify their integrity. The experiment consisted of machining micro slots of 15 mm in length on the workpiece, for each test a new microtool was used. The cutting parameters are shown on Table 2 . The cutting fluid used was Coolube 2210EP, manufactured by UNIST. The nozzle position for the lubricant application is seen in Figure 2 . The magnetic holder of the nozzle was gripped near the spindle in order to keep the distance between the nozzle and the cutting tool constant. The system was set to deliver the fluid at an air pressure of 33 psi (0.23MPa), similar to that found in the study by Ziberov, Da Silva, Jackson and Hung [17] . According to [21] at this air pressure, the jet velocity exceeds the velocity of the tool and as such ensures proper lubricant wetting.
Two flow rates were evaluated, the first 40.7 ml/h, which was also used by Dos Santos, Da Silva and Jackson [18] and Ziberov, Da Silva, Jackson and Hung [17] for the micromilling process. The second was 270.0 ml/h. Considering that the delivery system is based on fluid pulses, the number of pulses per minute for these rates were 30 and 200, respectively. Dry tests were also performed for comparison reasons and replicas were made for all the conditions. The slots and the micromills were evaluated via SEM images using the TM3000 Hitachi microscope. An optical microscope BX51M, Olympus, was also used to obtain images of an entire slot.
A Form Talysurf Intra Taylor Hobson® profilometer with a measurement range of 1.0 mm and 16 nm resolution was used to measure the surface roughness, the parameter was the arithmetic average roughness (Ra). A cut off of 0.8 mm and a gaussian filter was used. The results will be presented through the mean values and standard deviation obtained by three measurements in the trial and three in their respective replica, for all the conditions.
After machining and evaluating the slots via SEM, there was a noted pattern on the surfaces. The dry test presented a rougher surface, with workpiece material adhered to the surface, irregular feed marks and burrs, as shown in Figure 3 . The material adhered onto the surface was confirmed to be workpiece material through use of an energy dispersive x-ray spectroscopy (EDS), which is the same SEM used to obtain the images.
When the micromachining was performed with the higher cutting fluid rate, the surface improved and a slot with lower burrs, no adhered material and smooth feed marks was obtained, as seen in Figure 4 . This result was expected, since cutting fluid is a well-known method for improving surface quality. The surface roughness values are also reduced with fluid application. Table 3 contains the mean values with standard deviation for 3 measurements. The cutting fluid improved (reduction in value) the surface roughness in approximately 60%. Research by Ucun, Aslantas and Bedir [22] also studied the micromilling of Inconel 718 comparing both dry and MQL conditions. The authors applied Coolube 2210, the same as that used in this research, with a flow rate of 150 ml/h, the micromill used was a 760 µm diameter, two flutes and a spindle speed of 20.000 rpm. Their results for fz = 3.75 (µm/tooth) and 100 µm depth of cut, were similar to the results on Table 2 . The Ra values were approximately 0.15 µm and 0.22 µm for MQL and dry, respectively.
The difference in the roughness values for the slots machined with MQL (40.7 ml/h and 270.0 ml/h) is lower when compared to the dry condition. However, one notes that roughness increases with the decrease of fluid rate (or frequency of fluid application). If one analyzes the slot machined at 40.7 ml/h, as shown in Figure 5 , there are some noted regions that are similar to the condition encountered when no fluid is applied.
By analyzing Figure 5 , one notes that there is an increase in the feed marks, burs and the appearance of adhered material. This variation in quality was initially associated with the microtool wear and in order to verify this correlation new slots were machined onto the workpiece. With the increase of machined length, it was expected an increase in tool wear and therefore a deterioration in machined surface quality. The result exhibited exactly this scenario, the slots continued to present regions with high quality followed by regions with lower quality.
These differences became larger with the micromilled length. The quality of the slot surface (first slot, machining length of 15 mm) presents a subtle deterioration, as seen in Figure 6 . The two regions can clearly be seen in the slot surface, the red squares indicate the regions with lower quality, for a machining length of 135 mm. The difference between the two regions was due the cutting fluid delivery rate. Once the cutting fluid is applied by pulses, the use of lower fluid rates means not only less cutting fluid (40.7 ml/h) on the tool-workpiece interface, but also a fewer number of pulses per minute (30). This means that the machined length between the pulses is larger than the one at the higher rate (270.0 ml/h).
In view of the cutting parameters, the machining time for one slot was 4.5 s, for the lower rate (40.7 ml/h) only 2.25 pulses were delivered during machining, while for the higher rate (270.0 ml/h) the number of pulses was 15. This means that the machined lengths between pulses were: 6.7 mm and 1.0 mm, respectively.
Further evidence concerning the influence exerted by the number of pulses is found in the number of different regions per slot. For the lower flow rate, a pulse will occur every 2 s approximately, as such each slot will receive at least 2 pulses, and between the pulses there will be a worsened surface. In Figure 7 , two entire slots can be seen, the one on the left with the lower flow rate, and the one on the right with the higher flow rate. This in fact is an important practical result, since the literature focuses on the flow rate and some authors do not mention if the cutting fluid was applied continually or by pulses. Even though some authors have already used the same flow rate, the occurrence of this phenomenon has never been reported in micromilling.
In addition to the fluid delivery system, the low machinability of the Inconel 718, might have enhanced the difference between the regions. Inconel 718 is a refractory alloy, susceptible to cold work, high temperatures, high deformation rates, as well as hindered through the forming of chips, during micromilling [23] .
In this way, the contribution of this work is to add the fluid application frequency as an important variable during micromilling. It influences the surface quality and burr formation, therefore, it is important for high quality micro components. 
After micromilling tests under dry conditions, with 40.7 ml/h (30 pulses per minute) and with 270.0 ml/h (200 pulses per minute) the following conclusions can be drawn:
The cutting fluid can improve surface integrity: reducing surface roughness values, avoid adherence of material on the surface, while reducing burr formation for Inconel 718.
The reduction in surface roughness value with the cutting fluid at the higher flow reached 60%.
The condition that is most appropriate for a good slot quality is 200 pulses per minute (270.0 ml/h). The lower flow rate (40.7 ml/h in 30 pulses per minute) is not suitable for micromilling Inconel 718.