A.K. Chattopadhyay

Wear and performance of coated carbide and ceramic tools

Abstract The essential properties of modern high production cutting tools include high wear resistance, toughness, chemical stability at high temperature and under high sliding forces and a sufficiently high flow strength. It is difficult to achieve all these properties in a single tool material and techniques have been developed for coating a thin layer of a highly wear-resistant and friction-reducing material such as TiC, TiN, Ti(C,N), Al2O3 and Ti(C,N,O) onto a tough and strong substrate such as cemented carbides. The performance of such coated tools and their wear mechanisms were investigated.

Wear characteristics of ceramic cutting tools in machining steel

Abstract Some ceramic materials such as Al 2 O 3 which have a high compressive strength, hardness and chemical stability came into use industrially in the middle of this century as powder metallurgy produced indexable cutting tool inserts. These early ceramic tools were inherently weak in tension, impact and dynamic loading owing to their low tensile strength, toughness and transverse rupture strength; because of this their application was limited to medium productivity operations and work materials of softer grades. However, the technology of ceramic cutting tools has made great strides in the last few years through substantial improvements in their strength, toughness and wear resistivity. These modern ceramic tools have found wide and economic applications in high production machining of both cast iron and steels. In this paper the constructional characteristics of some modern ceramic tools and their wear behaviour and overall performances in machining mild steel at both low and high speeds are presented. The comparative results obtained are reported and discussed.

Effect of Coating Thickness on the Characteristics and Dry Machining Performance of TiN Film Deposited on Cemented Carbide Inserts Using CFUBMS

This study deals with the effect of coating thickness on the characteristics and dry machining performance of TiN film deposited on cemented carbide inserts using closed-field unbalanced magnetron sputtering (CFUBMS). The turning inserts were coated with TiN film of varying thickness ranging from 1.8 µm to 6.7 µm. The deposited coatings were characterized using grazing incidence X-ray diffraction (GIXRD), Vickers and Knoop microhardness tester, and scratch tester. The machining performance of uncoated and coated carbide inserts was evaluated in dry turning of C40 steel. The in-house developed TiN-coated carbide insert exhibited around 16 times higher tool life compared to that of the uncoated insert. Performance test clearly showed the weakness of the uncoated K-type carbide relative to its TiN coated counterpart in machining of long chipping material like C40 steel. The present investigation clearly reveals that the developed TiN-coated tool has a potential for augmenting the cutting capability of uncoated K-type carbide, normally recommended for machining of short-chipping material like cast iron, for machining of even long-chipping material like steel.

EFFECT OF CUTTING SPEED AND SURFACE CHEMISTRY OF CUTTING TOOLS ON THE FORMATION OF BUL OR BUE AND SURFACE QUALITY OF THE GENERATED SURFACE IN DRY TURNING OF AA6005 ALUMINIUM ALLOY

In the present investigation, AA6005 (ISO: AlSiMg) alloy was machined in turning operation with different cutting tools, such as uncoated cemented carbide insert, PVD TiN coated, CVD diamond coated and PCD insert, under dry environment. Effect of cutting speed was studied for each of the cutting tools with regard to the formation of built-up layer (BUL) or built-up edge (BUE). The rake surface of the tools was characterized by optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopic microanalysis. Particular emphasis was given on wear mechanism of PVD TiN coated insert, conventionally used in machining ferrous alloys, during dry turning of AA6005 alloy. It has been observed that increase of cutting speed from 200 m/min to as high as 1000 m/min could not substantially reduce formation of BUL over tool rake surface during dry machining of AA6005 alloy with uncoated or PVD TiN coated cemented carbide inserts. The potential of diamond-based tools in dry machining of aluminium alloy was also studied. Finally, the effect of cutting speed on surface finish of the workpiece machined with different cutting tools was studied during dry turning of AA6005 alloy.

On cumulative depth of touch-dressing of single layer brazed cBN wheels with regular grit distribution pattern

The present work deals with a single layer brazed type cBN grinding wheels, which have been developed in-house. Grits were actively brazed on the working surface of the wheels in a regularly distributed pattern so that it could perform grinding without loading. In general, such a wheel produces substantially high transverse roughness. This happens because of its low active grain density. A touch-dressing technique, developed in-house, has been successfully applied on these brazed wheels so that most of the grits did participate and the average roughness could be brought down to an acceptable magnitude. This work investigated the co-relation between the grit size and cumulative depth of dressing required to achieve an acceptable magnitude of transverse roughness. It was found that the required cumulative depth of dressing was dependent on the size of cBN grits. This paper also shows the effect of gradual touch-dressing on improvement of roughness of the surface ground by such new class of recently developed wheels. Experiments were conducted with microcrystalline cBN grits of three different sizes.

Influence of Grinding Parameters and Substrate Bias Voltage in Dry Surface Grinding with TiN-Coated Single Layer Galvanic cBN Wheel

According to some patented literatures, grit pull-out in single layer galvanic wheels can be arrested by a physical vapor deposited (PVD) coating on the wheels. It has been assumed, therein, the uniform high energy bombardment of TiN caused a reinforcement of the bond as well as increased the stability of the cBN layer on the core. However, an in-depth study on the performance of PVD coated grinding wheel is still lacking. The present research deals with exploring the effect of grinding parameters and substrate bias voltage on the performance of TiN coated galvanic cBN wheels compared to their uncoated counterpart in dry surface grinding of AISI 52100 hardened bearing steel. TiN was deposited at the bias voltages of 0, −60, and −90 V in an in-house PVD coating system. The coating microstructure and post-grinding condition of the wheels were observed using scanning electron microscopy (SEM) and phase detection was carried out using grazing incidence X-ray diffraction (GIXRD). With the increase in negative bias voltage, highly dense and compact structure of TiN was observed along with a reduction in column size. GIXRD indicated the formation of nickel-titanium intermetallics at the interface of TiN and nickel bond. Amongst the grinding parameters, downfeed was found to be the most influential factor on the grinding forces and specific grinding energy. Both downfeed and wheel velocity exhibited significant percentage contributions to the maximum grit depth of cut. The uncoated wheel was found to undergo many grit fractures and some pull-out during grinding. TiN deposited at 0 V bias could not prevent the fracture. However, when TiN was deposited at the bias voltages of −60 V and −90 V, such failures were noticeably restricted due to the high energy ion-bombardment and diffusion of Ti and TiN within the nickel bond and the cBN grits.

Effect of Chip Morphology during Grinding Bearing Steel Using Single Layer Brazed and Galvanic Bonded Cubic Boron Nitride (cBN) Wheel

Chip formation in grinding involves high specific energy compared to other machining processes due to high negative rake angle of grains, adverse grinding process parameters and physical–mechanical properties of the workpiece material. This causes excessive friction between chip-grit and chip-bond interfaces as there are no free flow of chip between grits and work surface. It generates high heat, which can have harmful effects on workpiece surface in case of dry grinding. But single layer brazed type (BT) or galvanic bonded (GB) cBN wheel could be the solution for the above mentioned problem, as the protrusion and gap of the grits may be monitored during manufacturing. Unlike conventional grinding, the single layer counterpart produces favorable chips during grinding bearing steel in ductile mode with lesser grinding forces and specific energy. This investigation also shows that the chips of larger volume cannot be accommodated in the intergrit spaces, which are very small in the GB wheel due to dense grit distribution and shorter grit protrusion, whereas, even with such a large chip load, the BT wheel works effectively for wider and uniform spacing and larger protrusion of the grits.

On some aspects of wheel loading

Abstract Wheel loading is one of the prime factors that determines the grinding capability of the wheel. It can lead to an increase in force and temperature to a level which causes a deterioration in the surface integrity of the work piece. A higher wheel wear rate, due to fracturing of grits or bond post, cannot be ruled out. Numerous investigations on various aspects of wheel loading have already been carried out but the results obtained are often contradictory. This work was aimed at making an integrated approach to studying the loading characteristics of grinding wheels while grinding various types of materials whose properties and behaviour vary widely in terms of hardness, ductility, oxidation and corrosion resistance.

Performance evaluation of a hard composite solid lubricant coating when dry machining of high carbon steel

ABSTRACT A novel hard composite solid lubricant coating combining TiN and MoSx has been developed using pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS). The tribological and mechanical properties together with their interdependencies with the coating microstructures have been assessed and reported elsewhere. This article evaluates the machining performance and correlates the underlying tribological aspects of different TiN-MoSx coating architectures (deposited at titanium (Ti) cathode currents of 1, 3.5, and 5 A) when dry turning AISI 1080 high-carbon steel. A comparative performance study clearly established the supremacy of the composite coating (deposited at 3.5 A Ti cathode current with ∼12 wt% of MoSx) with a hard TiN underlayer over monolayer TiN, MoSx, and other related coating architectures in terms of cutting force, tool wear, and workpiece surface roughness. The superlubricity behavior of the said composite coated tool resulted in a reduction of cutting force (by up to ∼45% compared to the uncoated tool) and exhibited a tool life of 8 min, which was eight times and more than two times longer than that of the uncoated and conventional hard TiN coated counterparts, respectively. The workpiece surface roughness, Ra, also decreased by 13 to 21% when machined with the TiN-MoSx coated tool in comparison to the uncoated cemented carbide.