Thermal aspects, material considerations and cooling strategies in cryogenic machining

 Conventional machining prolongs tool life by using cutting oils to cool the metal cutting process. Unfortunately, the cutting fluid contaminates the environment, and endangers the health of humans. Cryogenic machining offers an environmentally safe alternative to conventional machining by using liquid nitrogen, which can be naturally recycled. However, for the cryogenic machining process to be effective and economical, manufacturers must select the correct cooling approach. This paper describes our experimental study to investigate the cryogenic properties of some common cutting tool materials and five workpiece materials of industrial interest: low carbon steel, AISI 1010, high carbon steel AISI 1070, bearing steel AISI 52100, titanium alloy Ti-6Al-4V, and cast aluminum alloy A390. The paper addresses the major aspects of heat generated in metal cutting in terms of its effects on chip formation, tool wear, and on the functional integrity of the machined component. The paper then discusses the cooling strategies for cryogenic machining each material based on the thermal effects and material properties. The investigators conclude that the cooling approach must be finely adjusted for different materials to obtain the optimum effectiveness in cryogenic machining. The goal of our study is to provide a basis for designing the cryogenic machining system. Received: 25 November 1998 / Accepted: 12 February 1999     

Material selection for the tool holder working under hard milling conditions using different multi criteria decision making methods

Nowadays machining of materials in their hardened state, also called hard machining, is a challenge in production of tools and molds. It has some advantages such as lower process time and lower manufacturing cost when compared to conventional machining. In machining of hard workpiece materials, however, very high stresses act on the tool holder through the cutting tool. These stresses necessitate the tool holder to have some specific properties. Especially in hard milling, the tool holder should have high stiffness and should be able to dissipate the energy generated during interrupted cutting. Material cost of the tool holder is also important since lower costs provide a competitive advantage for manufacturers. The material selection for the tool holder should be conducted considering aforementioned requirements. To tackle the difficulty of the material selection with specific properties from a large number of alternatives, multi-criteria decision-making (MCDM) methods have been used. In this paper a decision model including extended PROMETHEE II (EXPROM2) (preference ranking organization method for enrichment evaluation), TOPSIS (technique for order performance by similarity to ideal solution) and VIKOR (VIšekriterijumsko KOmpromisno Rangiranje) methods were used for the selection of the best material for the tool holder used in hard milling. The criteria weighting was performed by compromised weighting method composed of AHP (analytic hierarchy process) and Entropy methods. The candidate materials were ranked by using these methods and the results obtained by each method were compared. It was confirmed that MCDM methods can be used for the solution of real time material selection problems. Tungsten carbide–cobalt and Fe–5Cr–Mo–V aircraft steel were found as the best materials for the tool holder production. The obtained results are found to be rather satisfactory and can be used in design stage of hard machining operations.     

Cost-effective way of hard turning with newly developed HSN2-coated tool

EN-31 (AISI 52100, hardness 55 HRC) is one of the difficult-to-cut steel alloys and it is commonly used in shafts and bearings. Nowadays, it is becoming a challenge to the cutting tool material for economical machining of extremely tough and hard steels. In general, CBN and PCBN tools are used for machining hardened steel. However, machining cost using these tools becomes higher due to high tool cost. For this purpose, carbide tool using selective coatings is the best substitute having comparable tool life, while its cost is approximately one-tenth of CBN tool. In this work, the newly developed second-generation TiAlxN super nitride (i.e., HSN²) is selected for PVD coating on carbide tool insert and further characterized using thermogravimetric analysis and differential scanning calorimetry for oxidation and thermal stability at high temperature. Later, HSN²-coated carbide inserts are successfully tested for their sustainability to expected tool life for turning of AISI 52100 steel. In the present study, forces, surface finish, and tool wear are used as a measure to appraise the performance of hard turning process. Experimentally, it is found that speed, feed rate, and depth of cut have considerable impact on forces, insert wear, and surface roughness of the machined surface.     

Investigating the Machinability of AISI 420 Stainless Steel Using Factorial Design

This work aims at studying the machining characteristics of high-strength materials using carbide cutting tool inserts at different cutting conditions. This is an essential step in building up an accurate machining information system. The tested material is high-strength stainless steel of the AISI 420 type. Machining tests were carried out using orthogonal cutting conducted to investigate the machining characteristics for high-strength stainless steel AISI 420 at different cutting conditions and tool rake angles. This assessment is achieved by investigating the effect of cutting parameters (cutting speed, feed, depth of cut, and tool geometry) on cutting forces, specific cutting energy, shear angle, coefficient of friction, shear stress, shear strain, and shear strain rate. Empirical equations and a correlation for the behavior of each of the output responses were investigated as a function of the independent variables. Main effect and interaction plot were presented for the most influential factors affecting the main cutting force and the power consumed.     

Effects of vegetable-based cutting fluids on the wear in drilling

This study focuses on both formulation of vegetable-based cutting fluids (VBCFs) and machining with these cutting fluids. For this purpose, characterizations of chemical and physical analyses of these formulated cutting fluids are carried out. In this study, performances of three VBCFs developed from crude sunflower oil, refined sunflower oil, refined canola oil and commercial semi-synthetic cutting fluid are compared in terms of tool wear, thrust force and surface roughness during drilling of AISI 304 austenitic stainless steel with HSSE tool. Experimental results show that canola-based cutting fluid gives the best performance due to its higher lubricant properties with respect to other cutting fluids at the constant cutting conditions (spindle speed of 750 rpm and feed rate of 0.1 mm/rev).     

Economical and ecological cryogenic machining of AISI 304 austenitic stainless steel

 While it is a clean alternative to conventional machining using environmentally polluting cutting oils and emulsions, cryogenic machining using liquid nitrogen has been reported to increase cutting forces and shorten tool life when cutting AISI 304 austenitic stainless steel. This paper presents improved results by using an economical cryogenic cooling approach designed after studying the cryogenic properties of the stainless steel material. By injecting a small amount of liquid nitrogen to the chip–tool interface, but not to the workpiece, this approach yielded a 67% tool-life improvement at 3.82 m/s and a 43% improvement at the medium speed of 3.40 m/s when compared with conventional emulsion cooling. It improved machining productivity and reduced production cost. In this study, different cryogenic machining approaches were compared in the machining test using commercial carbide inserts. The results show the cooling approach is crucial in attaining the benefits of cryogenic machining in cutting stainless steel. Received: 7 February 2000 / Accepted: 30 April 2000     

Numerical simulation of finish hard turning for AISI H13 die steel

A coupled thermo-mechanical model of plane-strain orthogonal turning of hardened steel was presented. In general, the flow stress models used in computer simulation of machining processes are a function of effective strain, effective strain rate and temperature developed during the cutting process. However, these models do not adequately describe the material behavior in hard machining, where the workpiece material is machined in its hardened condition. This hardness modifies the strength and work hardening characteristics of the material being cut. So, the flow stress of the work-material was taken with literature [H. Yan, J. Hua, R. Shivpuri, Development of flow stress model for hard machining of AISI H13 work tool steel. The Fourth International Conference on Physical and Numerical Simulation of Materials Processing, Shanghui in China, 2004, p. 5] in order to take into account the effect of the large strain, strain-rate, temperature and initial workpiece hardness. Then a series of numerical simulations had been done to investigate the effect of machining parameters on the machinability of hardened steel AISI H13 in finish turning process. The results obtained are helpful for optimizing process parameters and improving the design of cutting inserts in finish turning of hardened steel AISI H13.     

Effect of a nanoparticle-filled lubricant in turning of AISI 316L stainless steel (SS)

Release of heat and generation of friction associated with machining operation ever posture a problem which not only reduce the tool life but also impair the quality of the product. Nano cutting fluids play a significant role in machining operations and impact tool life and quality of work. In the present work, tool flank wear is analyzed during turning AISI 316L Stainless steel (SS) under a nano cutting environment. Experiments are conducted by turning of AISI 316L SS under wet machining with and without multiwalled carbon nanotube (MWCNT) inclusions in the conventional lubricant. The second order quadratic models were developed to predict tool wear using response surface methodology (RSM) based D-optimal design. Machining parameters such as speed, feed rate, and depth of cut are chosen as numerical factors and the type of lubricant is considered as the categorical factor. The results show that the influence of the feed rate is more significant while machining the AISI 316L SS with a whisker reinforced ceramic insert. The addition of MWCNTs in SAE20W40 enhances the tool performance with their enhanced penetration. After turning experiment, a scanning electron microscope (SEM) with energy dispersive X-ray (EDS) was used to investigate the tool wear.     

Investigations on hard machining of Impax Hi Hard tool steel

In this paper, experimental investigations are carried out by end milling process on hardened tool steel, Impax Hi Hard (Hardness 55 HRC) a newly developed tool steel material used by tool and die making industries. Experiments are performed with an aim to study performance investigations of machining parameters such as cutting speed, feed, depth of cut and width of cut with consideration of multiple responses viz. volume of material removed, tool wear, tool life and surface finish to evaluate the performance of PVD coated carbide inserts and ball end mill cutters. It has been observed through scanning electron microscope, X-ray diffraction technique (EDX) that chipping and adhesion are active tool wear mechanisms and saw-toothed chips are formed while machining of Impax Hi Hard steel. It is also noticed out that tool life is not enhanced while machining with minimum quantity lubricant than dry machining. From the investigations, it is observed that hard machining can be considered as an alternative to grinding and EDM, traditional methods of machining difficult-to-machine materials i.e. hardened steel with hardness greater than 50 HRC with a scope of improved productivity, increased flexibility, decreased capital expenses and reduced environmental waste.     

Tool wear and tool life of PCBN,binderless cBN and wBN-cBN tools in continuous finish hard turning of cold work tool steel

The paper presents the results of comparative study of performance of cutting tools made of ceramic-bound, binderless cBN, and wBN-cBN tool materials. The tool performance was assessed by tool wear-resistance, values of cutting forces, parameters of machined surface quality, and the state of sub-surface layer generated in continuous turning of hardened cold work tool steel. The tests were carried out under conditions of high speed machining (v _c = 120–180 m/min) both with and without a coolant. The best tool performance by the above-mentioned criteria is provided by a low-cBN material with ceramic binder.     

Machining of high performance workpiece materials with CBN coated cutting tools

The machining of high performance workpiece materials requires significantly harder cutting materials. In hard machining, the early tool wear occurs due to high process forces and temperatures. The hardest known material is the diamond, but steel materials cannot be machined with diamond tools because of the reactivity of iron with carbon. Cubic boron nitride (cBN) is the second hardest of all known materials. The supply of such PcBN indexable inserts, which are only geometrically simple and available, requires several work procedures and is cost-intensive. The development of a cBN coating for cutting tools, combine the advantages of a thin film system and of cBN. Flexible cemented carbide tools, in respect to the geometry can be coated. The cBN films with a thickness of up to 2 µm on cemented carbide substrates show excellent mechanical and physical properties. This paper describes the results of the machining of various workpiece materials in turning and milling operations regarding the tool life, resultant cutting force components and workpiece surface roughness. In turning tests of Inconel 718 and milling tests of chrome steel the high potential of cBN coatings for dry machining was proven. The results of the experiments were compared with common used tool coatings for the hard machining. Additionally, the wear mechanisms adhesion, abrasion, surface fatigue and tribo-oxidation were researched in model wear experiments.     

Ein neues Werkstoffkonzept zur trockenen Bearbeitung mineralischer Materialien

A new material concept for machining of mineral materials For the machining of mineral materials like rock, concrete and asphalt ultra hard cutting‐tools such as diamond tools are used. During the use diamond tools are cooled with water to remove the heat and to prevent an early deterioration of the diamonds. Without water cooling the diamonds at the cutting edge as well as in lower levels are damaged. For ecological and economic reasons a dry machining of mineral materials is of great interest. The consumption of coolant and the pollution of occupied buildings by alkaline water would be decreased. But specific diamonds tools are necessary to realise a dry machining. The Institute of Materials Engineering pursues a novel material concept for diamond impregnated composites to protect diamonds in deeper layers. Materials with a very low thermal conductivity are inserted in the diamond‐composites to protect the diamonds against heat and to reduce the deterioration of diamonds in lower levels. Cobalt and bronze (CuSn 85/15) with particle sizes of 45–90 μm and < 40 μm, diamonds with particle sizes of 300–450 μm and alumina with particle sizes of 350–500 μm, 150–210 μm and < 70 μm were used. The diamond‐alumina‐composites were powder metallurgically produced and were examined by light‐ and electron microscopy and digital image analysis.     

Analysis of hard turning process:thermal aspects

In manufacturing sector,hard turning has emerged as a vital machining process for cutting hardened steels.Besides many advantages of hard turning operations,one has to implement to achieve close tolerances in terms of surface finish,high product quality,reduced machining time,low operating cost and environmental friendly characteristics.In the study,three dimensional(3D) computer aided engineering(CAE) based simulation of hard turning by using commercial software DEFORM 3D has been compared to the experimental results of stresses,temperatures and tool forces in machining of AISI D3 and AISI H13 steel using mixed ceramic inserts(CC6050).In the following analysis,orthogonal cutting models are proposed,considering several processing parameters such as cutting speed,feed and depth of cut.An exhaustive friction modelling at the tool-work interface is carried out.Work material flow around the cutting edge is carefully modelled with adaptive re-meshing simulation capability of DEFORM 3D.The process simulations are performed at constant feed rate(0.075 mm/r) and cutting speed(155 m/min),and analysis is focused on stresses,forces and temperatures generated during the process of machining.Close agreement is observed between the CAE simulation and experimental values.     

Effects of cryogenic cooling on surface layer alterations in machining of AISI 52100 steels

Abstract

Microstructural phase transformations, commonly known as white layer formation in hard turned steel components, have in recent times become an interesting research topic in machining as they are related to the surface integrity and functional performance of components. Three main theories have been proposed to justify the mechanisms of white layer formation: (1) rapid heating and quenching; (2) severe plastic deformation; and (3) surface reaction with the environment. Coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on the machined surface alterations during machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride tool inserts with varying initial work material hardness, tool shape, cutting speed and feedrate. Optical and scanning electron microscopes (SEM) were used to analyse the affected layer in the machined subsurface, while X-ray diffraction technique was utilised to investigate the microstructural phase composition. The experimental results prove that the microstructural phase changes are heavily influenced by the cutting process parameters and the use of cryogenic cooling, in some cases leading to the total removal of martensite.     

Formulation and performance evaluation of a novel coconut oil–based metalworking fluid

Metalworking fluid (MWF) supplies a film of lubricant to abate friction, acts as a cooling media to rebate induced heat, and prevents metal pick-ups by flushing away the chips. Hence a liquid used as a cutting fluid reduces wear on the tool, reduces the energy consumption, and produces a better surface quality on the work piece. This paper describes the formulation of a novel water-soluble MWF and its performance evaluation during straight turning and end milling experiments carried out with AISI 304 stainless steel, mild steel, and cast iron as work piece materials. The MWF was prepared by mixing water with white coconut oil as the base oil and food-grade additives as surfactants. Viscosity, pH value, and biodegradability were measured and compared with a commercially available non-vegetable oil–based MWF. The surface roughness and tool surface temperature were measured throughout the machining experiments, and better performances were observed with the coconut oil–based MWF. Tool tip geometry and flank wear for straight turning machining operation were identified by observing scanning electron microscope (SEM) images.     

Performance Improvement of End Milling Using Graphite as a Solid Lubricant

In any machining operation, the use of coolants is essential to dissipate heat generated during machining and hence to improve productivity, machinability, etc. However, the use of cutting fluids in machining operations may seriously degrade the quality of environment. New cutting techniques are to be investigated to alleviate the problems associated with wet machining. To overcome some of the problems, an attempt has been made to use graphite as a solid lubricant. This paper deals with an investigation on using graphite as a solid lubricant to reduce the heat generated at the milling zone. An experimental setup has been developed to direct graphite powder continuously onto the workpiece and tool interface at the required flow rate. Experimental studies have been conducted to see the effect of tool geometry (radial rake angle and nose radius) and cutting conditions (cutting speed and feed rate) on the machining responses such as cutting forces, specific energy, and surface finish in solid lubricant assisted machining using four fluted solid coated carbide end mill cutters. Results indicate that there is a considerable improvement in the performance of milling AISI 1045 steel using graphite as a solid lubricant when compared with machining using cutting fluids in terms of specific energy requirements, cutting force, and surface finish.     

High-speed machining of martensitic stainless steel using PcBN

The performance of PcBN cutting tool during its application in the mass production of components made from AISI 440B stainless steel has been considered. The experimental tests have been performed at cutting speed ranging between 350–500 m/min at dry cutting conditions. The machining operations that have been explored included facing, turning, grooving and boring and the 3D topography of the machined surface are presented. The results show that good surface finish similar to grinding and dimensional accuracy can be achieved with PcBN tools.     

Multi-attribute optimization of end milling epoxy granite composites using TOPSIS

Epoxy granite composites are identified and recognized as better materials for machine tool applications due to inherent damping properties. However, end milling of these composites has not been explored much. Milling of epoxy granite composites presents a number of problems, namely, cutting forces and surface roughness appear during machining. This research work focuses on end milling of epoxy granite composite specimens using high-speed steel end mill cutter by varying the cutting conditions such as spindle speed and feed with a uniform depth of cut and selection of optimal machining parameters. The experimental runs of 27 different trials were carried out and three different attributes such as thrust force, tangential force, and surface roughness were analyzed. This research work presents a sequential procedure for machining parameters selection. Selection of optimal machining parameters is done on the basis of Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method.     

Optimization of cryo-treated EDM variables using TOPSIS-based TLBO algorithm

In order to machine hard and high-strength-to-weight ratio materials, electrical discharge machining (EDM) process is extensively used in aerospace, automobile and other industrial applications. However, high erosion of tool and improper selection of machining variables have emerged as a major obstruction to achieve productivity in this direction. High erosion of tool not only enhances the cost of machining but also increases the machining time by causing interruption during machining. Therefore, proper selection of machining variables and tool material life are the two vital aspects for the tool engineers working in EDM. In view of this, the present work proposes an extensive experimental investigation and optimization of machining variables of cryogenically treated brass tool materials on machining competences of Inconel 718 workpiece. The study primarily highlights the outcome of cryogenically treated soaking duration of tools along with other important process variables, viz. discharge current, open-circuit voltage, pulse-on time, duty factor and flushing pressure, on the performance measures such as electrode wear ratio (EWR), surface roughness and radial over-cut. The study revealed that soaking duration in deep cryo-treatment of the electrode is a significant variable to achieve improved machining characteristics. The performance measures are converted into equivalent single performance measure by calculating the relative closeness coefficient by the techniques for order preferences by similarity to ideal solution (TOPSIS) approach. Finally, a novel teaching–learning-based optimization (TLBO) algorithm has been proposed to find the optimal level of machining variables for the performance measures. The optimal levels of cutting variables obtained through the algorithm are validated through confirmation test, predicting an error of 2.171 percentages between the computational and experimental results. The predicted result suggests that the proposed model can be used to select the ideal process states to achieve productivity for the cryo-treated EDM.     

Optimizing the performance of high speed steel circular saw blades machining cupro 107, Inconel 600L and Nimonic PK31 nickel based alloys

Nickel based alloys are machined by methods similar to those used to cut ferrous materials, however there are additional process requirements due to the poor machinability of these alloys. The current paper reports on work undertaken to optimize the cutting conditions for high speed steel circular saw blades machining materials from three of the principal categories of nickel based alloy.Techniques have been developed and verified that simulate the cutting characteristics of multi-point cutting tools by testing blade segments that contain representative teeth. The cutting behaviour of high speed steel circular saw blades have been simulated in this manner. Materials from three of the principal classifications of nickel based alloy; Cupro 107, Inconel 600L and Nimonic PK31, have been machined over a range of cutting feeds and speeds. Cutting and thrust forces were measured and the performance criteria, specific cutting energy (Esp) evaluated. Optimized cutting conditions for each material were determined from curves of Esp against feed rate at the selected cutting speeds.In an area of high product and material costs, the information contained within this paper will be of interest to the manufacturing engineer and end user when appraising the suitability of high speed steel circular saw blades as a tool for machining these materials.Inconel 600L and Nimonic PK31 are registered trademarks of the Inco Family of Companies.