The processes of abrasive machining with bonded abrasive tools can be discussed as a tribosystem. The interdependencies between the elements of this system (workpiece, abrasive grains, bonds, and machining environment) determine the tribological processes in the contact area. The combined effects of the processes strongly influence the course and results of abrasive machining. These processes can be divided into five basic groups: contact processes [1
], friction processes [2
], abrasive tool wear processes [2
], workpiece wear processes [3
], and lubrication processes [3
]. Among the abovementioned processes, lubrication processes of the tool–workpiece contact zone have a key influence on the conditions of abrasive machining, as they affect the physicochemical processes of the contact. In general, dry machining and machining with the use of a coolant, lubricant, and antiadhesive agent can be distinguished.
The literature describes a large number of methods of delivering coolants in the form of liquids, gases, solid lubricants, and antiadhesive mediums to the grinding zone. In recent years, growing environmental awareness has led to increasing attention being paid to developing methods to reduce coolant usage in machining processes, including grinding. Techniques are being developed to minimize the coolant expenditure, which can be described as hybrid methods, as they simultaneously deliver several types of coolant, lubricant, and antiadhesive agents. Most often, the transport medium is compressed air. The best-known hybrid methods of cooling and lubrication of the grinding zone are:
Minimum quantity lubrication (MQL) [8
Minimum quantity cooling (MQC) [1
Cold air MQL (CAMQL) [24
Using solid lubricants and antiadhesives [28
Due to its ease of use and versatility, as well as its good functionality, the CAOM method, which integrates the simultaneous use of MQL and cold air guns (CAG), appears to be particularly beneficial. The two methods are presented in more detail below.
The MQL method was developed in order to ensure the most favorable conditions for the implementation of machining, while minimizing expenditure on coolants. In the MQL systems, oil is sprayed under the influence of compressed air energy onto the grinding wheel active surface (GWAS) and the workpiece surface. In the literature sources [22
], a description is given of the MQL as a very important alternative to dry grinding, describing it as a near dry grinding (NDG) or minimum coolant grinding (MCG) technique. Even a small amount of liquid coolant entering the contact zone between the GWAS and workpiece surface can have a positive effect on the efficiency of the grinding process. Such an approach was the basis for the development of MQL systems aimed at reducing the environmental risk and disposal costs by limiting the amount of liquid coolant used [8
In the MQL method, the liquid coolant output of 7.2–97.2 mL/h (nearly 1000 times less than in the flood method) is used, while at the same time the liquid coolant is delivered precisely to the GWAS contact zone with the machined surface [21
]. The air–oil aerosol covers the GWAS and the workpiece surface, cooling them down and applying a lubricating oil film layer on the machined surface. This reduces the friction force between the abrasive grains and the workpiece surface, and also reduces the grinding force F
and the amount of heat generated [53
In the MQL method, the lubrication function is usually provided by oil, whereas the cooling function is provided mainly by compressed air. This very small amount of liquid coolant delivered to the grinding zone significantly reduces the friction in the GWAS contact zone with the machined surface and limits the adhesion of the grinding products to the grinding wheel [56
The amount of heat received by air is very limited and its ability to conduct heat is insufficient to efficiently cool the grinding zone [29
]. The research performed by Silva et al. [22
] shows that the MQL method provides lubrication at a higher level than the flood method, however the cooling function of the GWAS contact zone with the workpiece surface is much less effective. Table 1
presents the values of thermal capacity of the media used in the MQL method (air and oil) in comparison with the thermal capacity of water.
In the research described by Sadeghi et al. [19
], machined surfaces of steel in a hardened state after grinding with the MQL method were characterized by the lowest Ra
roughness parameter value among the considered cooling methods. Additionally, Tawakoli et al. [48
] showed that the use of the MQL method reduces the Ra
parameter value of the ground surface roughness in relation to flood method and dry grinding.
The application of the MQL method may contribute to the reduction of wear phenomena associated with active cutting vertices on the GWAS, causing them to maintain their sharpness over a longer period of time with respect to dry grinding, and sometimes also with respect to flood cooling (when grinding steel in the hardened state). This may have the effect of reducing the cross-sectional area of chips obtained in the process of grinding with this method and improving the morphology of the workpiece surface [19
Tawakoli et al. [24
] determined the influence of air–oil aerosol parameters obtained with the MQL method on the process of 100Cr6 steel grinding. The study showed that the angle of the air–oil aerosol delivery nozzle outlet has a significant effect on grinding conditions and results.
The application of the MQL method avoids the clogging of intergranular spaces on the GWAS, while the lubrication is carried out on the whole circumference of the wheel, which ensures better slipping at the contact areas of active grains with the machined surface [22
]. However, the studies described by Hadad and Sharbati [16
] and Li et al. [58
] show that similarly to dry grinding, using the MQL method involves a high risk of grinding burns resulting on the workpiece.
The application of cooled compressed air is a constantly evolving field of scientific research and technical applications, and the nozzles used in this method are referred to as cold air guns (CAGs). The method is used in machining processes and in grinding [59
The minimization of the coolant and lubricant output, which takes part in the grinding process, may cause an increase in the workpiece temperature and the formation of thermal defects on the machined surface. Supporting the grinding processes with the application of a stream of compressed cooled air may significantly reduce the temperature in the grinding zone and reduce or completely eliminate the occurrence of thermal defects [41
The equipment used to obtain and supply compressed cooled air is characterized by uncomplicated construction, low purchase cost, and easy operation. The CAG nozzle is a device that creates a stream of compressed cooled air using vortex tubes. In CAG nozzles, the compressed air stream is rotated around its own axis in the vortex tube. This allows one to obtain streams of cold and hot air. The cooled compressed air obtained by means of CAG nozzles can reach temperatures 55 °C lower than those of the air supplying the device [40
The results of the tests described by Ramesh et al. [7
] showed a decrease in the value of the grinding force during the grinding process under the conditions of supplying a stream of CCA (with a pressure of 0.3 MPa, air temperature at the nozzle outlet in the range from −30 °C to −35 °C, flow rate of 0.4 m3
/min) in comparison with the grinding using the flood method, with specific material removal rates of Q’w
= 1.6 mm3
/s·mm for S45C steel and Q’w
/s·mm for SS304 steel.
Choi, Lee, and Jeong [61
] have shown that the application of 4% oil-in-water emulsion for cooling and lubrication of the grinding process allows one to obtain lower values for roughness parameters Ra
than in the case of delivery of CCA to the grinding zone. The higher value of roughness parameters of the ground surface Ra
in the case of cooling of the grinding zone with the use of a CAG nozzle may have been caused by the lack of lubrication and the limited possibility of cleaning the workpiece and grinding wheel surfaces, as compared to the process carried out with the use of oil-in-water emulsions.
Choi, Lee and Jeong [41
] carried out studies on the effect of CCA delivery on the process of internal cylindrical grinding with a cubic boron nitride (cBN) grinding wheel and Al2
abrasive grains. It was shown that with a decrease in the temperature value of the compressed cooled air stream and an increase in its discharge velocity, the Ra
roughness parameter values of the machined surface decreased and the occurrence of thermal defects of the workpiece was reduced. Moreover, it was observed that the value of tensile stress occurring on the surface layer of the workpiece after grinding decreased with an increase in the CCA flow rate from the nozzle [41
Lee and Lee [59
] optimized the microgrinding process with the use of compressed cooled air. The optimization of the process made it possible to reduce the specific grinding force F’
and to reduce the value of the Ra
roughness parameter of the ground surface, while at the same time maximizing the specific material removal rate Q’w
, thus indicating the great potential of this method.
Cooling the grinding zone with the use of CAG nozzles in a very effective way minimizes the occurrence of grinding defects on the treated surface, and moreover minimizes their application and reduces the negative impact on the PCS environment, which is consistent with the current trend of development of manufacturing techniques [61
Both of these described methods are combined in the CAOM method, in which the lubrication function realized by the MQL method is additionally supported by the CCA stream generated by the CAG nozzle. As a result, it is possible to avoid the occurrence of unfavorable changes in the structure of the workpiece surface layer in the form of burns [41
]. The described features of the MQL method and CAG nozzles were the basis for undertaking research, aiming to implement this CAOM method in the internal cylindrical grinding process. To date, such cooling and lubrication methods had been developed only for surface grinding, cylindrical grinding, and shape grinding, processes. As a result, a new hybrid method was developed, in which a patented method of centrifugal air–oil aerosol delivery through the grinding wheel was combined with a CAG nozzle generating CCA [62
]. The aim of the presented research was to determine the influence of the application of the hybrid cooling and lubrication method in the grinding zone on the course and results of internal cylindrical grinding of 100Cr6 steel in comparison with other methods of cooling and lubrication, as well as dry grinding. In particular, the study sought to determine the effects of the use of the centrifugal MQL-CCA method on the life of a grinding wheel, the grinding power P
, volumetric wear of the grinding wheel Vs
, material removal Vw
, grinding ratio G
, thermal conditions of the grinding process, roughness and residual stress of the machined surface, and clogging of the GWAS in comparison to the four other varieties of cooling and lubrication conditions. This paper presents a description of a hybrid method of cooling and lubrication of the grinding zone, integrating centrifugal (through a grinding wheel) lubrication with a minimum quantity of lubricant and cooling with a compressed cooled air stream generated by a cold air gun (CAG) (Section 2
). Then, the methodology (Section 3.1
) and results (Section 3.2
) of experimental studies are presented in detail, with the aim of determining the influence of the application of the hybrid method of cooling and lubrication of the machining zone on the course and results of the internal cylindrical grinding process of 100Cr6 steel in comparison with other methods of cooling and lubrication, as well as dry grinding. In the last part of the manuscript (Section 4
), detailed conclusions are given in relation to a number of criteria to assess the grinding process.
2. Setup of Centrifugal MQL-CCA Method of Coolant Delivery during Internal Cylindrical Grinding Process
While developing the described hybrid method of cooling and lubrication of the grinding zone, the need to support the cooling function (implemented in the MQL method to a very limited extent) was taken into account, along with the problem of collecting chips on the workpiece. The minimum flow of the oil delivery in the form of an air–oil aerosol in the MQL method means that chips and other grinding products are not washed out of the machining zone, which can cause them to end up in the GWAS–workpiece contact zone again, interfering with the machining process. Accumulation of chips close to the grinding zone also increases the risk of clogging intergranular spaces of the GWAS. Accordingly, the described method uses a dual-outlet CCA supply line from the CAG nozzle to the machining zone. One outlet of the line was directed ahead of the grinding zone to cool it, while the other outlet was located directly behind the grinding wheel–workpiece contact zone to blow out the machining products (mainly chips).
The following components of the centrifugal air–oil aerosol delivery system were used to configure the test stand:
A ZR-K 360° six-nozzle, omnidirectional, minimum quantity lubrication head;
MQL head supply system, with compressed air and oil from the workpiece spindle side;
A specially designed grinding wheel arbor;
The system used to fix the ZR-K 360° head inside the rotating grinding wheel arbor;
A special ceramic grinding wheel with dimensions of 40 mm × 20 mm × 26 mm, adapted to work with the hollow grinding arbor;
As a lubricant, an oil was used called Cimtech® MQL from CIMCOOL® Fluid Technology, part of Milacron LLC.
In addition, a Vortec 610 CAG nozzle equipped with a dual-outlet supply line was placed in near the grinding zone, allowing for precise directing of the CCA jet to the desired areas of the grinding zone [63
]. The outlets of the CAG nozzle supply line had openings with a diameter of 6.3 mm and the nozzle itself was supplied with compressed air at a pressure of 0.6 MPa. As a result, the CAG reduced the air temperature at the outlet from the supply line to about −5 °C. The Vortec 610 nozzle configured in this way gave CCA with a total output (calculated for two outlets) QCCA
= 49.8 dm3
/min (0.00083 m3
shows the view of the machining zone of the RUP 28P universal grinding machine (Mechanical Works Tarnów SA, Tarnów, Poland) [64
], with all components necessary for the grinding process according to the assumptions of the described hybrid cooling and lubrication method.
The research was divided into two parts. In the first stage, simulation tests were carried out, using the computational fluid dynamics (CFD) supported by the finite element method. The aim was to determine the most favorable geometrical and kinematic parameters of the process in terms of the flow of cooling and lubricating media (air–oil aerosol and CCA), as well as heat exchange.
The conditions and results of these simulations were described in detail in the paper [65
]. The most advantageous of the variants analyzed in the simulation studies was then selected for experimental research, in which the service life of the grinding wheels was determined with reference to four methods of cooling and lubricating the machining zone.