Minimum Quantity Lubrication
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Metal cutting fluids changes the performance of machining operations because of their lubrication, cooling, and chip flushing functions. Typically, in the machining of hardened steel materials, no cutting fluid is applied in the interest of low cutting forces and low environmental impacts. Minimum quantity lubrication (MQL) presents itself as a viable alternative for hard machining with respect to tool wear, heat dissertation, and machined surface quality. This study compares the mechanical performance of minimum quantity lubrication to completely dry lubrication for the turning of hardened bearing-grade steel materials based on experimental measurement of cutting forces, tool temperature, white layer depth, and part finish. The results indicate that the use of minimum quantity lubrication leads to reduced surface roughness delayed tool flank wear, and lower cutting temperature, while also having a minimal effect on the cutting forces.
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The growing demand for higher productivity, product quality and overall economy in manufacturing by machining and grinding, particularly to meet the challenges thrown by liberalization and global cost competitiveness, insists high material removal rate and high stability and long life of the cutting tools. But high production machining and grinding with high cutting velocity, feed and depth of cut are inherently associated with generation of large amount of heat and high cutting temperature. Such high cutting temperature not only reduces dimensional accuracy and tool life but also impairs the surface integrity of the product.
In high speed machining conventional cutting fluid application fails to penetrate the chip–tool interface and thus cannot remove heat effectively. Addition of extreme pressure additives in the cutting fluids does not ensure penetration of coolant at the chip–tool interface to provide lubrication and cooling . However ,high-pressure jet of soluble oil, when applied at the chip–tool interface, could reduce cutting temperature and improve tool life to some extent .However, the advantages caused by the cutting fluids have been questioned lately, due to the several negative effectsthey cause. When inappropriately handled, cutting fluids may damage soil and water resources, causing serious loss to the environment. Therefore, the handling and disposal of cutting fluids must obey rigid rules of environmental protection. On the shop floor, the machine operators may be affected by thebad effects of cutting fluids, such as by skin and breathing problems For the companies, the costs related to cutting fluids represent a large amount of the total machining costs. Several research workers state that the costs related to cutting fluids are frequently higher than those related to cutting tools. Consequently, elimination on the use of cutting fluids, if possible, can be a significant economic incentive. Considering the high cost associated with the use of cutting fluids and project and implimentationed escalating costs when the stricter environmental laws are enforced, the choice seems obvious. Because of them some alternatives has been sought to minimize or even avoid the use of cutting fluid in machining operations. Some of these
alternatives are dry machining and machining with minimum quantity lubrication (MQL).Dry machining is now of great interest and actually, they meet with success in the field of environmentally friendly manufacturing . In reality, however, they are sometimes less effective when higher machining efficiency, better surface finish quality and severe cutting conditions are required. For these situations, semi-dry operations utilizing very small amount of cutting fluids are expected to become a powerful tool and, in fact, they already play a significant role in a number of practical applications .
minimum quantity lubrication (MQL) refers to the use of cutting fluids of only a minute amount—typically of a flow rate of 50–500 ml/h which is about three to four orders of magnitude lower than the amount commonly used in flood cooling condition. The concept of minimum quantity lubrication, sometimes referred to as near dry lubrication or micro-lubrication , has been suggested since a decade ago as a mean of addressing
the issues of environmental intrusiveness and occupational hazards associated with the airborne cutting fluid particles on factory shop floors. The minimization of cutting fluid also leads to economical benefits by way of saving lubricant costs and work piece/tool/machine cleaning cycle time. Significant progress has been made in dry and semidry machining recently, and minimum quantity lubrication(MQL) machining in particular has been accepted as a successful semi-dry application because of its environmentally friendly characteristics. Some good results have been obtained with this technique . Lugscheider et al. used this technique in reaming process of gray cast iron and aluminum alloy with coated carbide tools and concluded that it caused a reduction of tool wear when compared with the completely dry process and, consequently, an improvement in the surface quality of the holes.
The drilling of aluminum–silicon alloys is one of those processes where dry cutting is impossible due to the high ductility of the work piece material. Without cooling and lubrication, the chip sticks to the tool and breaks it in a very short cutting time. There fore, in this process a good alternative is the use of the MQL technique The present work experimentally investigates the role of minimum quantity lubrication on cutting temperature, chip reduction coefficient and dimensional deviation in plain turning of AISI-1040 steel at different speed-feed combinations by uncoated carbide insert and compares the effectiveness of MQL with that of dry machining and conventional cutting fluid.
EXPERIMENTAL CONDITIONS AND PROCEDURE
For the present experimental studies, AISI-1040 steel rod of initial diameter 110mm and length 620mm was plain turned in a BMTF Lathe, Bangladesh, 4 hp by uncoated carbide insert of integrated chip breaker geometry at different speed-feed combinations under dry, wet and minimum quantity lubrication (MQL) conditions to study the role of MQL on the machinability characteristics of that work material mainly in respect of cutting temperature, chip reduction coefficient and dimensional deviation. The experimental conditions are given in Table 1.
Machine tool BMTF Lathe, Bangladesh, 4 hp
Work piece AISI-1040 steel (size: Ø110mm×620 mm) Cutting tool (insert)
Cutting insert Carbide, SNMM 120408 (P-30 ISOspecification), Drillco
Tool holder PSBNR 2525M12(ISO specification),
Working tool geometry −6◦, −6◦, 6◦, 6◦, 15◦, 75◦, 0.8 (mm)
Cutting velocity, Vc 64, 80, 110 and 130 m/min
Feed rate, So 0.10, 0.13, 0.16 and 0.20 mm/rev
Depth of cut, t 1.0mm
MQL supply: Air 7 bar, Lubricant: 60 ml/h (throughexternal nozzle)
Environment: Dry, wet (flood cooling) and minimum quantity lubrication (MQL)
Table2.1 Experimental conditions
The ranges of the cutting velocity (Vc) and feed rate (So) were selected based on the tool manufacturer’s recommendation and industrial practices. The depth of cut was kept constant since it has much less significant role on the machining characteristics excepting the magnitude of the cutting forces, which simply increase proportionally with the increase in depth of cut. The MQL needs to be supply at high pressure and impinged at high speed through the nozzle at the cutting zone.
Considering the conditions required for the present work and uninterrupted supply of MQL at constant pressure over a reasonably long cut, a MQL delivery system has been designed, fabricated and used. The schematic view of the MQL set up is shown in Fig. 1. The thin but high velocity stream of MQL was project and implimentationed along the auxiliary cutting edge of the insert, as indicated in a frame within Fig. 1, so that the coolant reaches as close to the chip–tool and the work–tool interfaces as possible .The photographic view of the experimental set-up is
Figure 2.1 Experimental setup
Figure2.1Block diagram of MQL
The MQL jet has been used mainly to target the rake and flank surface along the auxiliary cutting edge and to protect the auxiliary flank to enable better dimensional accuracy .MQL is expected to provide some favorable effects mainly through reduction in cutting temperature. The simple but reliable tool–work thermocouple technique has been employed to measure the average cutting temperature during turning at different Vc–So combinations by the uncoated carbide insert under dry, wet and MQL conditions.
For the present investigation, the calibration of the tool–work thermocouple has been carried out by external flame heating. The tool–work thermocouple junction was constructed using a long continuous chip of the concerned work material and a tungsten carbide insert to be used in actual cutting. To avoid generation of parasitic emf, a long carbide rod was used to extend the insert. A standard K-type thermocouple is mounted at the site of tool–work junction. The oxy-acetylenetorch simulated the heat generation phenomena in machining and raised the temperature at the chip–tool interface. Standard thermocouple directly monitored the junction temperature when a digital multimeter monitored the emf generated by the hot junction of the chip–tool. The effect of MQL on average chip–tool interface temperature at different Vc and So under dry, wet and MQL conditions is shown in Fig. 3.
The chip samples collected while turning the steel by the insert of configuration SNMM at different Vc–So combinations under dry, wet and MQL condition have been visually examined and categorized with respect to their shape and color. The result of such categorization of the chips produced at different conditions and environments by the AISI-1040 steel. The actual forms of the chips produced during machining the steel with a cutting velocity 110 m/min and feed 0.16 mm/rev under dry, wet and MQL conditions is shown in Fig. 4.
Another important machinability index is chip reduction coefficient, ζ (ratio of chip thickness after and before cut).For given tool geometry and cutting conditions, the value of ζ depends upon the nature of chip–tool interaction, chip contact length and chip form all of which are expected to be influenced by MQL in addition to the levels of Vc and So. The variation in value of ζ with Vc and So as well as machining environment evaluated for AISI-1040 steel have been plotted and shown in Fig. 5.]
The deviations in the job diameter before and after cuts were measured by a precision dial gauge with a least count of 1_m, which was traveled parallel to the axis of the job .MQL provided remarkable benefit in respect of controlling the increase in diameter of the finished job with machining time as can be seen in Fig. 6.
Fig. 3. Variations in average chip–tool interface temperature with cutting
velocity and feed rate during turning under dry, wet and MQL conditions
EXPERIMENTAL RESULTS AND DISCUSSION
During machining any ductile materials, heat is generated at the primary deformation zone, secondary deformation zone and the flank (clearance) surfaces, but the temperature becomes maximum at the chip–tool interface. The cutting temperature measured in the present work refers mainly to the average chip–tool interface temperature.
Any cutting fluid applied conventionally cannot reduce this chip–tool interface temperature effectively because the fluid can hardly penetrate into that the interface where the chip–tool contact is mostly plastic in nature particularly at higher cutting velocity and feed. However, MQL jet could have reduced the cutting temperature quite significantly though in different degrees for different cutting velocity and feed combinations as can be seen in Fig. 3.
The presence of the grooves along the cutting edges and the hills on the tool rake surface and reduced chip–tool contact length may have helped the MQL jet to come closer to the chip–tool interface and thus effectively cool that interface. The difference in the effectiveness of MQL observed under different Vc and So can be reasonably attributed to variation in the nature and extent of chip–tool contact with the changes in Vc and So. The pattern of chips in machining ductile metals generally depend upon the mechanical properties of the work material, tool geometry particularly rake angle, levels of Vc and So, nature of chip–tool interaction and the cutting environment .In absence of chip breaker, length and uniformity of chips increase with the increase in ductility and softness of the work material, tool rake angle and cutting velocity unless thechip–tool interaction is adverse causing intensive friction and built-up edge formation.
Table 3.1Shape and co lour of chips at different vc and so condition
It shows that the steel when machined under dry and wet conditions produced spiral type chips and the color of the chips become blue. The geometry of the insert is such that the chips first came out continuously got curled along normal plane and then hitting at the principal flank of the insert broke into pieces with regular size and shape.
When machined under MQL the form of these ductile chips change appreciably into more or less half turn and their back surface appeared much brighter and smoother. This indicates that the amount of reduction of temperature and presence of MQL enabled favorable chip–tool interaction and elimination of even trace of built-up edge formation.
The color of the chips have also become much lighter, i.e. metallic from blue depending upon Vc and So due to reduction in cutting temperature by MQL. The actual forms of chips produced during turning at cutting velocity 110 m/min and feed 0.16 mm/rev under dry, wet and MQL condition as can be seen in Fig. 4.
Figure 3.1 shape of chips at different condition
Fig. 4. Actual forms of chips produced during turning at cutting velocity 110 m/min and feed 0.16 mm/rev under (a) dry, (b) wet and © MQL conditions.
Almost all the parameters involved in machining have direct and indirect influence on the thickness of the chips during deformation. The degree of chip thickening which is assessed by chip reduction coefficient, ζ plays sizeable role on cutting forces and hence on cutting energy requirements and cutting temperature. Fig. 5 shows that MQL has reduced the value of ζ particularly at lower values of Vc and So.
Figure. 3.2. Variation in chip reduction coefficient, ζ, with cutting velocity and feed rate during turning under dry, wet and MQL conditions.
By MQL application, ζ is reasonably expected to decrease for reduction in friction at the chip–tool interface and reduction in deterioration of effective rake angle by built-up edge formation and wear of the cutting edge mainly due to reduction in cutting temperature .MQL provided remarkable benefit in respect of controlling the increase in diameter of the finished job with machining time as can be seen in Fig. 6.
Figure. 3.3. Dimensional deviations observed after one full pass under dry, wet and MQL conditions
In straight turning, the finished job diameter generally deviates from its desired value with the progress of machining, i.e. along the job-length mainly for change in the effective depth of cut due to several reasons which include wear of the tool nose, over all compliance of the machine–fixture–tool–work (M–F–T–W) system and thermal expansion of the job during machining followed by cooling. Therefore, if the M–F–T–W system is rigid, variation in diameter would be governed mainly by the heat and cutting temperature .With the increase in temperature the rate of growth of auxiliary flank wear and thermal expansion of the job will increase. MQL takes away the major portion of heat and reduces the temperature yielding reduction in dimensional deviation desirably
Based on the results of the present experimental investigation the following conclusions can be drawn:
• The cutting performance of MQL machining is better than that of conventional machining with flood cutting fluid supply.
• MQL provides the benefits mainly by reducing the cutting temperature, which improves the chip–tool interaction and maintains sharpness of the cutting edges.
• Due to MQL, the form and color of the steel chips became favorable for more effective cooling and improvements in nature of interaction at the chip–tool interface.
• Dimensional accuracy improved mainly due to reduction of wear and damage at the tool tip by the application of MQL.
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