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Machining is the process of removing material in the form of chips by means of a wedge shaped tool. The need to manufacture high precision items and to machine difficult-to-cut materials economically leads to the development of improved machining processes. The feasibility of dry cutting in the manufacturing industries has received much attention due to high cost of cutting fluids, estimated at about 17% of the total manufacturing cost which is about twice the tooling cost itself.
Cutting fluid waste needs to be treated prior to disposal and furthermore, prolonged exposure to them is hazardous to the machine operators due to risk of skin cancer and breathing difficulties.
Dry cutting is desirable because not only it reduces manufacturing cost but also eliminates all the adverse negative effects associated with the usage of cutting fluids for cooling and lubricating purposes. In spite of the noble idea to implement the process of dry machining as mentioned above, the usage of cutting fluids in machining several types of materials which are difficult to machine (like super alloys, etc) offers several important advantages, especially to increase productivity and surface quality of the machined work piece and hence cannot be totally eliminated as of now.
This is because cutting fluid allows cutting processes to be carried out at much higher speed, higher feed rate and greater cutting depth due to increased lubricity and cooling at the chip-tool and chip-workpiece interface. When used effectively, cutting fluids not only improve surface finish and dimensional accuracy, but also decrease the amount of power consumptions.
Furthermore, cutting fluid also helps transport the excessive heat and chips produced during the cutting processes away from the cutting area, thus longer tool life may be achieved. Cutting fluid related costs and health concerns associated with exposure to cutting fluid mist and a growing desire to achieve environmental sustainability in manufacturing have caused industry and academia to re-examine the role of these fluids and quantify their benefits. In developed countries like USA stricter legislations have been enforced to minimize the use of cutting fluids in machining. Hence some of the noble approaches taken in this direction without compromising with the benefits achieved from flood application of cutting fluids are mentioned below:
1) Proper selection of cutting fluids is a very important, although complicated process as it includes various aspects of machining conditions and parameters. There has been a gradual shift from straight oils to soluble oils and further to vegetable and synthetic oils due to better cooling and lubricating properties and more importantly due to much safer handling and disposal as they are eco-friendly.
2) Proper application of cutting fluids is also an important aspect because in most cases, majority of the fluid applied goes waste without any benefit to the machining process. Thus areas like nozzle design, its placement, supply system and other machining conditions have to be considered effectively.
3) Cutting fluids also need to be managed meticulously after use to reduce their health and environmental effect and also to cut down disposal costs.
4) Gradual reduction of cutting fluid usage by increasing the use of near-dry and dry machining is the most promising step taken in this regard. Since past few decades many efforts are being undertaken to develop advanced machining processes using less or no cutting fluid. Although machining without the use of cutting fluids has become a popular avenue for eliminating the problems associated with their management and other advantages, but it has its associated drawbacks. The advantages of dry machining are obvious: cleaner parts, no waste generation, reduced cost of machining, reduced cost of chip recycling (no residual oil), etc.
However, these advantages do come at a cost. The most prohibitive part of switching to dry machining is the large capital expenditure required to start a dry machining operation. Machines and tools designed for cutting fluids cannot be readily adapted for dry cutting. New, more powerful machines must be purchased, and special tooling is often needed to withstand the high temperatures generated in dry cutting. The quality of machined parts may be affected significantly as the properties of the machined surface are significantly altered by dry machining in terms of its metallurgical properties and residual machining stresses. High cutting forces and temperatures in dry machining may cause the distortion of parts during machining. Moreover, parts are often rather hot after dry machining so their handling, inspection gauging, etc., may present a number of problems. Near-dry machining (NDM), also known as minimum quantity lubrication (MQL) machining, is in the process of development to provide at least partial solutions to the listed problems with dry machining.
In MQL machining, very small quantity of cutting fluid (CF) is supplied to the machining zone in very small (atomized) droplet size with high velocity. The amount of oil used is generally in the range of 10-100 ml/hr, which is about 1000 times smaller than that used in conventional flood application of oil. It was developed as an alternative to flood and internal high pressure coolant supply to reduce the CF consumption. The media is supplied as a mixture of air and oil in the form of an aerosol (often referred to as mist) with precise control over amount of oil and direction of spray to the cutting zone.
The reason for the shifting trend towards MQL machining is that it is supposed to give the combined advantages of dry and conventional flood machining in an eco-friendly manner. The amount of oil used is so less that parts engaged in metal removal are practically dry, although the high velocity air jet carries the small but sufficient amount of oil precisely to the machining zone so as to provide the necessary cooling and lubrication, besides removal of the chips by the compressed air. Many research papers have been published which goes to show that MQL application involves much better machining parameters than dry machining which are also on a par or in some cases better than flood application of oil. Ueda et al. found that temperature reduction in MQL turning is approximately 5%, while in MQL end milling it is 10–15% and in MQL drilling it is 20–25% compared to the temperature in dry cutting. Khan and Dhar found that MQL with vegetable oil reduced the cutting forces by about 5–15% from that in dry cutting. The axial force decreased more predominantly than the power force.
They attributed this reduction as well as the improved tool life and finish of the machined surface to reduction of the cutting zone temperature as the major reason for the improved performance of machining operations. Similar results were obtained in machining 1040 steel. Li and Liang found cutting forces in machining 1045 steel lower in MQL compared with dry cutting. They also attributed this reduction to the cutting temperature difference. Other groups of researches have compared MQL with wet machining. Dhar et al. studied the effect of MQL in tuning of 4340 steel using external nozzle and aerosol supply to the tool. They found that the temperature at the tool–chip interface reduced by 5–10% (depending upon the particular combination of the cutting speed and feed) in MQL compared to wet machining. As a result, tool life and finish of the machined surface improved by 15– 20%. Filipovic and Stephenson found similarity in tool life in gun-drilling and cross-hole drilling of crankshafts between wet machining and MQL. Using MQL and a diamond-coated tool in the drilling of aluminium–silicon alloys, Braga et al. showed that the performance of the MQL process (in terms of forces, tool wear and quality of machined holes) was very similar to that obtained when using a large amount of water-soluble oil, with both coated and uncoated drills. Studying turning of brasses, Davim et al. concluded that, with proper selection of the MQL system, results similar to flood lubricant condition can be achieved. Although many research papers have attempted to give explanation for the distinctive results of MQL machining no direct conclusive evidence of most are provided due to the topic analysis being extremely difficult onto it.
One of the most feasible and sound explanation is provided by V.P.Astakhov in one of his literatures which accredits the reason to the embrittlement action of the cutting fluid, which reduces the strain at facture of the work material. This action is based on the Rebinder effect, directly concerned with the metal cutting process. He suggests that atomized oil possesses greater ability to enhance the embrittlement of the layer being removed and thus to reduce the work of plastic deformation done in the transformation of the layer being removed into the chip.
Aerosols used in MQL are generated using a process called atomization, which is the conversion of bulk liquid into a spray or mist (i.e., collection of tiny droplets), by passing the oil carried by pressurised air through a nozzle. The design of the atomizer is critical in MQL as it determines the concentration of the aerosol and the size of droplets. A distinction is drawn in the MQL technique between external supply via nozzles fitted separately in the machine area and internal supply of the medium via channels built into the tool. Each of these systems has specialized individual areas of application. In applications involving external supply, which is the aim of the current work in CNC turning, the aerosol is sprayed onto the tool from outside via a nozzle fitted close to the machining zone. This technique is used in sawing, end and face milling, and turning operations. Besides a well-designed nozzle, which is the primary inspiration for the current work and will be addressed soon, other equipments needed for the MQL set up in CNC turning are listed below: i. A compressor for sending pressurized air.
As also stated earlier the nozzle has the following tasks to accomplish in the current application:
It basically consists of an internal nozzle for introducing a very high velocity jet of air at its exit which is fitted into an external nozzle having a fine orifice for oil inlet very close to the exit point of internal nozzle. The internal and external nozzles are fitted together with the help of a cap tightened by screwed bolts. Some of the design parameters which are not so important in the current field of study have been kept in consistency with locally available designs which are easier to fabricate.
The inspiration for design of internal nozzle has been taken from a few fundamental books and other materials on fluid mechanics. It has been stated in a few papers that higher the air jet velocity higher will be its penetration capacity in the difficult-to-reach plastic area of contact between chip and tool which is the most challenging task in metal machining. Hence equipped with this knowledge, the design criterion of nozzle was studied thoroughly with an aim to achieve a very high velocity of air jet, comparable to or greater than the speed of a pressure wave (like sound) in air. It was observed that a simple converging nozzle could increase the velocity of the air jet at most to a value equal to speed of sound under the prevailing temperature condition, which is also referred to as a speed of Mach 1.
Even if the mass flow rate of air is kept on increasing, the speed at the outlet does not increase further, which is referred to as choked condition. But a nozzle having an initial converging section followed by a diverging section and separated by a minimum cross-section throat (also referred to as converging-diverging nozzle) has the capacity to increase the air jet velocity at exit to a value way above Mach1. Depending on the inlet pressure and temperature conditions, mass flow rate of air and crosssectional area at the throat and exit, the air jet takes a speed of Mach 1 at the throat and further increases to higher values which give rise to a shock wave (due to an abrupt change in pressure and temperature conditions) at any particular cross section of the diverging section. If all the parameters are controlled carefully then the shock wave occurs just at the exit of the nozzle. This will favourably cause turbulence in that region, where the cutting fluid will also enter from the orifice in the exterior nozzle and this turbulent high speed air jet will have better capacity to finely atomize the oil particles. Following is a summary of the equations used in the determination of the throat and exit cross-sectional areas of the internal nozzle, considering the flow behaviour of the air stream to be in an idealized adiabatic condition. This approximation is quite valid because the fluid flow speed is considerably high. The inlet air stream is considered to be stagnant, i.e. with zero velocity. Let the inlet or stagnation properties be defined by the subscript zero, the throat properties by subscript‘t’, and exit properties by subscript‘e’ and other symbols have usual meanings. • h 0 = h t + (v2/2) — Steady Flow Energy Equation (SFEE) Treating air as a perfect gas, we may also write, C p T 0 = C p T t + (v t 2/2) But, C p = (γ*R/(γ-1)), hence (T 0 /T t ) = 1+ ((γ-1)/2)*M t 2, where M t = Mach No. at throat = v t /(γRT t )0.5 • • (P 0/ P t ) = (T 0 /T t )(γ/γ-1) and (ρ 0/ ρ t ) = (T 0 /T t )(1/γ-1) —- adiabatic relations For air, γ = 1.4, hence for achieving Mach 1 at the throat, we have (T 0 /T t ) = 1.2 (P 0/ P t ) = 1.893 (ρ 0/ ρ t ) = 1.577 • Mass flow rate of air is given by m = ρ t A t v t where A t is the cross-sectional area at the throat or m = (γ/RT 0 )0.5 * (P 0 A t /((γ+1)/2)^( (γ+1)/2(γ-1))).
Using SFEE and adiabatic relations between throat and exit similarly, we have (T e /T t ) = ((γ+1)/2)/(1+( (γ-1)/2)* M e 2) = (1.2T e /T 0 ) (P 0/ P e ) = (T 0 /T e )(γ/γ-1) (ρ 0/ ρ e ) = (T 0 /T e )(1/γ-1) V e = (γRT e )0.5 * M e m = ρeAeve where A e is the cross-sectional area at the throat.
Calculation It is desired that at exit of internal nozzle a Mach. No. of 1.5 is achieved. Hence, the throat will invariably have air speed equal to Mach 1. Considering inlet conditions as follows: P 0 = 6 bar, T 0 = 27oC = 300K and Diameter at the throat, d t = 1mm (for convenience of fabricating), we have m = 1.1 g/s v e = 432.5 m/s and, diameter at the throat, d e =1.5 mm approximately Due to unavoidable factors of irreversibility the flow will not be perfectly adiabatic and there will be some energy loss from air flowing through the nozzle which is expected to show up as a loss in the kinetic energy, thereby decreasing the velocity at the exit than the desired one. Considering this factor and also the ease of fabrication, the diameter at the nozzle exit has been slightly increased from the one calculated above. The new adopted value is, d e = 2 mm All the other dimensions as shown in fig.3and table.1 are considered from the viewpoints of strength of material used, ease of fabrication and the locally available standards.
Basically the design concept of the external nozzle has been taken from the standard ones available in the market with some modifications to meet the requirements of the current work. It has the following important tasks to perform:
Length of straight section at exit Length of straight section at inlet Thickness of the nozzle ID of oil inlet orifice Distance between exit of internal nozzle and start of converging section
L=10mm L=28mm t=8mm Φ 1mm L=3mm
Calculations for the velocity of the aerosol at the exit of the external nozzle To find the velocity with which the aerosol is expected to come out from the exit of the external nozzle, we consider the following criteria with some suitable approximations:
The cap has been designed such that it precisely connects the external and internal nozzles and convenient enough for engaging and disengaging the two as and when required. The internal nozzle is made to tight fit within the cap which is further fitted into the external nozzle with the help of threaded nuts.
The reliability, durability, performance and wear life of the nozzle depends on proper material selection. Hence this is a very important aspect that should be considered with proper attention. Among other factors the most important in determining the selection of the material are corrosion and erosion resistance, apart from being economical too. It should also have good strength to weight ratio and have the ability to handle low temperature fluid without any shape distortion, because the temperature of the air stream inside the nozzle can go as low as -500C. With the above considerations, it was found that Brass has a very good combination of the desirable properties. It has good strength and ductility combined with excellent corrosion resistance and superb machinability. It is also available in a very wide variety of product forms and sizes to allow minimum machining to finished dimensions.
The yield strength of brass varies within a wide range, although for safety purpose we can consider the minimum value which is around 124 MPa. The nozzle may be considered to be of cylindrical shape with insignificant deviation for the calculation of hoop stress, which is the most important aspect in the consideration of its strength against failure. The Circumferential stress (or hoop stress) in the nozzle is given by σ h = pD/2t where p is the air pressure, D is the internal diameter of the nozzle and t is the thickness of the nozzle. The maximum pressure expected by the nozzle to encounter, p= 8bar = 8*105 Pa. After air expands in the internal nozzle, pressure will decrease in the annular space of external nozzle. So it is sufficient to check the material strength of the interior nozzle at the section having maximum external diameter, D= 8mm.
The thickness is same throughout, t= 2mm. Hence, the maximum hoop stress inside the nozzle is determined to be σ h = 1.6 MPa As the hoop stress is much lesser than the yield stress of brass, hence the design is in safe mode from the viewpoint of failure against tensile stress.
The entire parts of the nozzle were manufactured by an industry in New Delhi, equipped with many modern sophisticated machines. The accuracy of the dimensions could be achieved up to the micron scale.
As discussed earlier, several components make up the complete MQL system. For a convenient simple setup the following have been chosen: • Fluid supply system – a burette for oil storage, a small pump for continuously supplying oil from the burette and an IV set for maintaining oil level in the infusion set at a particular height throughout so as to overcome the back pressure from the oil inlet to the nozzle by means of gravitational head. This ensures continuous oil supply to the nozzle at the desired small quantity which is also simultaneously measured by a control valve. • Air supply system – a compressor of maximum capacity of 8 bar and a pressure gauge fitted close to the air inlet to the nozzle to measure the delivery pressure to the nozzle connected by hose pipes.
The components and parameters adopted in the analysis of MQL machining are as shown below in table 4. Table 4: Components and parameters of MQL machining Sl. No. 1 2 Component/parameter Workpiece Cutting Tool Description Bearing Steel Coated carbide insert Kennametal – CNMG120408MS 3 4 Tool holder Machine Tool PCLNL 2020 K12 CNC Lathe Machine Leadwell-Fanuc Series Oi Mate-TD 5 6 Cutting Fluid Nozzle Water Soluble Oil (1: 80 of oil by volume) Internally mixed micro-nozzle with twin fluid atomization 7 8 9 10 11 12 13 14 Oil flow rate Air pressure Cutting velocity Feed Depth of cut Dynamometer Surface roughness Pump 250 ml/hr 3 bar 60, 80, 100 m/min 0.14, 0.18, 0.22 mm/rev 0.5 mm KISTLER dynoWare (model no.-9129AA). Talysurf (Taylor Hobson) Solenoid operated diaphragm dosing pump
Bearing steel was chosen as the workpiece for the current experiment primarily because of its high hardness (greater than 58 RC) and hence posing difficulty in machining. The objective is to achieve better machinability of the material by using Minimum Quantity Lubrication as compared to dry machining and also preferably wet machining with flood application of lubricant. Bearing Steel also has very high fatigue and bending strength but low corrosion resistance generally.
The insert was chosen based upon compatibility with the mechanical properties of the workpiece. It is a fine grained tungsten carbide (WC) insert with a single layer of Ti-Al-N (PVD) coating bonded by 6% Co substrate. The insert is highly wear resistant and can withstand integrity under high temperature conditions. It also has good toughness and deformation resistance and is suitable for medium speed machining. The edge is sharp with a nose radius of 0.8mm, a negative rake angle (-6o) and double sided chip breakers.
The cutting parameters were decided based on the general recommendations of the tool-work combination. The turning operation was done for all combinations of 3 different values of speed (V) and feed (f), keeping the depth of cut (d) constant in dry, wet and MQL machining. The values of V, f and d are given in table 3. In MQL machining, the oil flow rate was kept fixed at 250 ml/hr to get a continuous flow of oil to the cutting zone through the nozzle. Also the air inlet pressure to the nozzle was maintained at 3 bar, as an increase in it was posing hindrance to the continuous flow of oil through the oil inlet orifice by giving a back pressure and thereby causing oil leakage from sideways of the nozzle and connecting pipes.
The nozzle designed for the current experiment was found to have several manufacturing defects. Most importantly, its oil inlet orifice was not fabricated properly and also misplaced from the originally designed location. Hence it was causing oil leakage from the surrounding of the orifice and even after several attempts of minor adjustments, the leakage failed to stop completely. This lead to a discontinuous flow of oil to the cutting zone and consequently failed to serve the purpose of the current work. Following all these problems with the newly fabricated nozzle, another available nozzle (fig.11) similar to it was used to verify the effect of MQL machining as compared to dry and wet machining. The primary difference between the two nozzles is in the design of its internal nozzle used for obtaining a high velocity jet of air. Contrary to the nozzle deigned for the current work which has a convergent-divergent type of internal nozzle (sec 4.1.1), the older nozzle finally used in MQL machining has a simple convergent type of internal nozzle. It was able to provide a continuous flow of oil to the cutting zone. A water-soluble oil with excellent cooling property but low lubrication property was selected as the media for MQL machining. The factors behind this choice are its easy availability and low cost, thus keeping the process economical and simple.
The cutting forces were measured and recorded in all three Cartesian coordinate directions for each combination of cutting velocity and feed with a constant depth of cut throughout maintained and performed for dry, wet and MQL machining serially. The cylindrical workpiece, 200mm long and 50mm diameter, was first skin turned to a diameter of 49.5 mm using a worn out tool to remove the surface defects that may have been present. There were a total of 27 machining operations performed (9 each in dry, wet and MQL) each on a cutting length of 5 mm. The main cutting force (F c ) (in direction of cutting velocity) shows the following trend (graph.1 and 2) with change of feed (f) and cutting velocity (V c ) respectively in all 3 types of machining. 350 300 250 Cutting Force (N) 200 150 100 50 0 0.14 0.18 0.22 Feed (mm/rev) Dry Wet MQL
Graph 1: Variation of Fc with f keeping constant value of Vc=60m/min and depth of cut=0.5mm. 350 300 Cutting Force (N) 250 200 150 100 50 0 60 80 100 Cutting Velocity (m/min) Dry Wet MQL
Graph 2: Variation of Fc with Vc keeping constant value of f =0.22mm/rev and depth of cut=0.5mm. From the graphs shown above, it is observed that the cutting force maintains an increasing trend with an increasing feed and a decreasing trend with an increase in cutting velocity in all 3 types of machining conducted, but the lowest values have been obtained with the MQL setup. An average reduction of about 13% from dry machining and about 6% from wet machining has been achieved in the cutting force with respect to MQL machining.
This verifies the reduction in cutting force with MQL machining compared to dry and wet machining as has already been found in several experiments conducted till date. The surface roughness of the workpiece was also examined for all the machined surfaces under a Talysurf. The average roughness parameter (R a ) shows the following trend (graph.3 and 4) with change of feed (f) and cutting velocity (V c ) respectively in all 3 types of machining. 2.5 2 1.5 Ra (µm) 1 0.5 0 0.14 0.18 Feed (mm/rev) 0.22 Dry Wet MQL
Graph 3: Variation of Ra with f keeping constant value of Vc=80m/min and depth of cut=0.5mm. 2.5 2 1.5 Ra (µm) 1 0.5 0 60 80 100 Cutting Velocity (m/min) Dry Wet MQL
Graph 4: Variation of Ra with Vc keeping constant value of f =0.22mm/rev and depth of cut=0.5mm. The graphs shown above reveal that surface finish shows a deteriorating trend with an increase in feed and an improving trend with an increase in cutting velocity and the best results have been achieved for both dry and MQL machining, which are almost similar and quite better than with wet machining. This verifies that the MQL setup has been successful in providing a good surface finish to the workpiece as compared to the conventional methods of wet and dry machining which is in agreement with several experiments conducted till date.
In the course of the current work, a nozzle was designed for the purpose of application in MQL machining with an intention of getting better results in terms of less tool forces, improved surface roughness of workpiece, etc. Unfortunately the newly designed nozzle suffered from some manufacturing defects and hence posed some problems during application and consequently the machining experiments had to be conducted with a different nozzle which was already available. Nevertheless, machining with the other nozzle in the MQL setup produced very desirable results in accordance with many recent literatures in the field of MQL machining.
A substantial decrease in cutting force was achieved with the MQL setup compared to dry and wet machining. Also the surface finish was best with MQL machining which was quite close to dry but much better than wet machining. It may be concluded that the utility of Minimum Quantity Lubrication in machining has been verified, although much better results can be achieved by properly optimising all the parameters related to machining conditions like cutting velocity, feed, oil composition and its consumption rate, inlet pressure of compressed air and so on, which has been skipped in the current work due to paucity of time.
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