The effects of liquid-CO2 cooling, MQL and cutting parameters on drilling performance

An investigation is made into the effects of liquid carbon dioxide (LCO2) cooling, minimum-quantity lubrication (MQL) and cutting speed in drilling. Experimental measurements of torque, thrust force and temperature are made over a wide range of process and operating conditions. The resulting empirical models are used to quantify the individual contributions of the controlled parameters on drilling performance, and to facilitate temperature-based process optimization. Of particular interest is the need to carefully adjust the LCO2 flow rate for any combination of MQL flow rate and cutting speed. The optimization is validated both in simulation and actual drilling tests

The influence of single-channel liquid CO2 and MQL delivery on surface integrity in machining of Inconel 718

Sustainable machining of difficult-to-cut materials requires effective cooling and lubrication techniques. To substitute conventional flood cooling and lubrication, different techniques such as cryogenic cooling and/or minimum quantity lubrication (MQL) can be used. Liquid carbon dioxide (LCO2) can be pre-mixed with different lubricants before its delivery to the cutting zone. This article investigates the influence of this recently developed cooling and lubrication method on surface integrity characteristics in milling of Inconel 718. Surface roughness, surface topography and microstructure were evaluated for flood lubrication, dry cutting and LCO2 machining using a single-channel LCO2 and MQL strategy. Moreover, two different lubricants were evaluated for MQL: (i) conventional MQL oil and (ii) solid lubricant molybdenum di-sulphide (MoS2). In addition to being environmentally friendly, MoS2 lubricated LCO2 showed comparable surface characteristics to flood lubrication. Also, the use of lubricated LCO2 resulted in higher part surface cleanliness compared to flood lubrication

Tribology of solid-lubricated liquid carbon dioxide assisted machining

An investigation is made into the lubrication capabilities of solid-lubricated liquid carbon dioxide (LCO2) in comparison to flood lubrication, straight LCO2 and oil-lubricated LCO2 (MQL). The coefficient of friction is determined via tribological experiments, similar to machining, using an open tribometer which features an uncoated carbide insert sliding against a workpiece. Tribological experiments reveal superior performance of solid-lubricated LCO2. The milling experiments as well indicate that solid-lubricated LCO2 significantly reduces wear. The machined-surface topography is examined using high-magnification SEM, which shows no presence of adhered solid particles on the workpiece surface, providing a completely dry machining process

Functional literacy of preschool teachers

There was a time, when the ability to sign and do a bit of reading, was sufficient for a person to be able to successfully function within the society. Today however, that is not nearly enough. To be able to work, a person has to master flexible reading of diverse materials encountered on daily basis, be able to locate information in a variety of sources on her/his own, and poses sufficient knowledge for working with computers. In this thesis, I take a look at reading, functional, document, and computer literacy of preschool teachers, including their attitude towards reading, their ability to think critically, ability to work with computers, their opinion on their own grammar skills, and their ability to tackle new ways of composing preparations and analyses. The results, acquired through the means of a questioner, have been accordingly processed and interpreted. I have also compared the results of preschool teachers with the results of preschool assistants. Their attitude towards reading is mostly positive, they do contemplate the texts, but only a third of them do so critically. They consider their document literacy as adequate, but encounter a number of limitations when it comes to computers. Functional literacy enables the teachers to do their job smoothly and independently. In order for their work to be up to date, they have to follow innovations, regularly educate themselves, and take care of their own personal development

Fundamental characterization of cryogenic machining by applying solid lubricants

Kriogeno odrezovanje na osnovi kapljevitega ogljikovega dioksida (LCO2) predstavlja trajnostno in čisto obdelavo kovin in omogoča substitucijo konvencionalnim hladilno mazalnim sredstvom, katere so ponavadi emulzije vode in olja. LCO2 v odrezovalnih procesih predstavlja hladilni medij, ki mu dodajamo različna maziva za izboljšanje mazalnih lastnosti. Trdna maziva zmanjšujejo trenje tudi v za uporabo olj neprimernih pogojih. Zasnova implementacije trdnih maziv v LCO2 za doseganje hladilnih in mazalnih lastnosti tako predstavlja novost na področju kriogenega odrezovanja. V sklopu doktorske naloge je bil razvit enokanalni sistem za dovod mešanice LCO2 ter tekočega ali trdnega maziva. Izvedena je bila implementacija sistema za dovod mešanice skozi vreteno obdelovalnega stroja ter orodja direktno do rezalnega procesa. Z uporabo namensko razvitega tribometra so bile analizirane mazalne lastnosti različnih pristopov mazanja in hlajenja, vključujoč čisti LCO2 ter mešanice LCO2 ter tekočega ali trdnega maziva. Na procesih frezanja in vrtanja so bile določene osnovne karakteristike kriogenega odrezovanja na podlagi obstojnosti orodja, integritete obdelane površine, rezalnih sil ter rezalne temperature. Izdelani so bili napovedovalni modeli za optimizacijo kriogenega odrezovanja, kateri omogočajo trajnostno obdelavo preko zmanjšanja porabe hladilnega in mazalnega medija. Analiza obdelanih površin z elektronskim vrstičnim mikroskopom je dokazala smiselnost uporabe trdnih maziv iz vidika integritete obdelane površine ter končne čistosti izdelka.Cryogenic machining based on liquid carbon dioxide (LCO2) represents a sustainable and clean way of manufacturing and enables the substitution of conventional metalworking fluids such as water-based emulsions. In machining processes, LCO2 is a cooling medium to which various lubricants are added to improve lubrication properties. Solid lubricants can reduce friction even in the most demanding conditions. The concept of implementing solid lubricants in LCO2 to achieve cooling and lubricating properties is thus a novelty in the field of cryogenic machining. Within the scope of the doctoral thesis, a single-channel system for the supply of oil- and solid-lubricated LCO2 was developed. The system implementation for feeding the mixture through the spindle and the tool directly into the machining process was carried out. A special tribometer was used to analyse the lubrication properties of different cooling/lubrication strategies, including pure LCO2 and oil- and solid-lubricated LCO2. Milling and drilling processes were used to determine the basic properties of cryogenic machining based on tool life, integrity of the machined surface, cutting forces and cutting temperature. Predictive models were developed to optimise cryogenic machining for sustainable manufacturing by reducing the consumption of coolants and lubricants. Analysis of the machined surfaces with an electron beam microscope proved the benefit of using solid lubricants in terms of the integrity of the machined surface and the final cleanliness of the part

A novel cryogenic machining concept based on a lubricated liquid carbon dioxide

A novel single-channel supply of pre-mixed (a) liquid carbon dioxide (LCO2) and (b) oil – delivered via minimum quantity lubrication (MQL) – represents a significant advancement in cryogenic-machining technology. In this proof-of-concept study, an attempt is made to advance the understanding of the oil solubility in LCO2 and to analyze the oil-droplets and their impact on machining performance. The results indicate that the physical and chemical properties of oil distinctively affect its solubility in LCO2. The achieved solubility further influences the achievable oil-droplet size and distribution and tool life

Ocena trajnosti naprednih tehnologij odrezavanja

Efficient cooling and lubrication techniques are required to obtain sustainable machining of difficult-to-cut materials, which are the pillars of aerospace, automotive, medical and nuclear industries. Cryogenic machining with the assistance of lubricated Liquid Carbon Dioxide (LCO2) is a novel approach for sustainable manufacturing without the use of harmful water-based metalworking fluids (MWFs). In case of unavoidable use of MWFs under high pressure, such as turning finishing processes of difficult-to-cut materials, the pulsating high pressure delivery of MWFs prolongs the tool life and enables the control over chip length to prevent surface damage of high value-added parts. In this paper, sustainability assessment of both advanced principles was carried out, considering overall costs and operational safety. Experimental tests were executed on difficult-to-cut materials in comparison to conventional flood lubrication. For both techniques, longer tool life compared to flood lubrication was observed additional cleaner production and higher part quality led to reduced long-term overall costs. These advanced machining technologies are also operation safe, proving to be a sustainable alternative to conventional machining

A novel cryogenic machining concept based on a lubricated liquid carbon dioxide

A novel single-channel supply of pre-mixed (a) liquid carbon dioxide (LCO2) and (b) oil - delivered via minimum quantity lubrication (MQL) - represents a significant advancement in cryogenic-machining technology. In this proof-of-concept study, an attempt is made to advance the understanding of the oil solubility in LCO2 and to analyze the oil-droplets and their impact on machining performance. The results indicate that the physical and chemical properties of oil distinctively affect its solubility in LCO2. The achieved solubility further influences the achievable oil-droplet size and distribution and tool life

Analiza pretoka enokanalne mešanice kapljevitega CO2 in MQL pri trajnostnem odrezavanju

Single-channel supply of pre-mixed liquid carbon dioxide (LCO2) and minimum quantity lubrication (MQL) represents a state-of-the-art LCO2 assisted machining. However, to fully understand and optimize cooling and lubrication provided by the LCO2 + MQL, a fundamental media flow analysis is essential, yet not researched enough. Therefore, in this paper, media flow velocity and oil droplet size were analysed in supplying line and at the nozzle outlet using high-speed camera and proprietary single-channel system. Results indicate that pre-mixed media flow velocity is mainly influenced by the LCO2 expansion rate upon the nozzle outlet, wherein oil droplet size is largely dependent on the solubility between oil and LCO2. Media flow velocity increases significantly from an average of 40 m/s in the supplying line to the excess of 90 m/s at the nozzle outlet due to the pressure drop and LCO2 expansion. Furthermore, this volume expansion causes the oil droplet to increase to the point of critical, unstable droplet size. Afterward, the unstable oil droplet breaks up into smaller oil droplets. It was found, that nonpolar oil, with greater solubility in LCO2, compared to the polar oil, provides droplets as small as 2 [micro]m in diameter. Smaller oil droplets positively reflect on tool wear and tool life in LCO2 assisted machining, as the longest tool life was achieved by using the nonpolar oil for pre-mixed LCO2 + MQ