S. Malkin

Grinding Mechanisms for Ceramics

Abstract The widespread utilization of high strength ceramic materials has been limited by the high cost of machining these materials by grinding. A technological basis for cost-effective ceramic machining requires a fundamental understanding of the prevailing grinding mechanisms. The present paper is intended to provide an overview of what happens during grinding as abrasive grains cut through ceramic workpiece materials. Most past research on grinding mechanisms for ceramics has followed either the “indentation fracture mechanics” approach or the “machining” approach. The indentation fracture mechanics approach likens abrasive workpiece interactions to idealized small-scale indentations. The machining approach typically involves measurement of cutting forces together with microscopic observations of grinding debris and surfaces produced. Both approaches provide important insights into the grinding mechanisms for ceramic materials.

Forces and Energy in Circular Sawing and Grinding of Granite

An investigation is reported of the forces and energy in circular sawing and grinding of gray granite. Measurements were made of the forces and power over a wide range of sawing and grinding conditions. Calculated tangential force components were found to be much different than the measured horizontal force components for sawing, but the two forces were almost identical for grinding. The location of the resultant force was proportionally further away from the bottom of the cutting zone with longer contact lengths. For sawing, the normal force per grain was nearly proportional to the calculated undeformed chip thickness. The G-ratios at different sawing rates reached a maximum value at the same intermediate undeformed chip thickness, which was attributed to a transition in the diamond wear mechanism from attrition to fracture at a critical normal force per grain. SEM observations indicated material removal mainly by brittle fracture, with some evidence of ductile plowing especially for grinding and to a lesser extent for sawing. The corresponding fracture energy was estimated to constitute a negligible portion of the total energy expenditure. About 30 percent of the sawing energy might be due to the interaction of the swarf with the applied fluid and bond matrix. Most of the energy for sawing and grinding is attributed to ductile plowing. Analogous to recent studies on grinding of ceramics and glass, the power per unit width was found to increase linearly with the generation of plowed surface area per unit width.

Temperatures and Energy Partition for Grinding with Vitrified CBN Wheels

An investigation is reported of the temperatures and energy partition for grinding with vitrified CBN wheels. Temperature distributions were measured in the subsurface of hardened bearing steel workpieces using an embedded thermocouple during grinding with a water soluble fluid at specific removal rates from 5 to 60 mm2/s. The energy partition to the workpiece and heat flux distribution within the grinding zone were estimated using temperature matching and inverse heat transfer analyses. In all cases, the maximum grinding zone temperature rise was less than 120°C. The energy partition to the workpiece was found to be only 4.0 to 8.5%. Such low energy partitions are consistent with a thermal model which takes into account conduction to the workpiece, conduction to the abrasive grains, and cooling of the workpiece by the fluid at the grinding zone.

Thermal aspects of grinding with CBN wheels

Abstract When grinding with CBN wheels, thermal damage is unlikely at the removal rates normally used in production. This decreased thermal damage with CBN wheels as compared with aluminum oxide wheels is usually attributed to the lower specific energies with CBN. Another contributing factor is the very high thermal conductivity of CBN, which enhances conduction into the grains. This beneficial effect may be offset by the sharpness of the CBN grains, which lowers the area over which conduct ion occurs. This paper presents a model of the thermal aspects of grinding which predicts the critical removal rate at which workpiece burn occurs. The effects of specific grinding energy, grain thermal conductivity, and wear flat area are explored. The model suggests that conduction into the CBN grains has the potential to increase the critical removal rate by a factor of 20 or more.

High Speed Grinding of Silicon Nitride With Electroplated Diamond Wheels, Part 1: Wear and Wheel Life

An investigation is reported on high speed grinding of silicon nitride using electroplated single-layer diamond wheels. This article is concerned with wheel wear and wheel life, and a second paper (ASME J. Manuf Sci. Eng., 122, pp. 42-50) deals with wheel topography and grinding mechanisms. It has been suggested that grinding performance may be enhanced at higher wheel speeds due to a reduction in the undeformed chip thickness. Grinding experiments were conducted at wheel speeds of 85 and 149 m/s with the same removal rate. Contrary to expectations, the faster wheel speed gave no improvements in surface finish, grinding ratio, or wheel life. Microscopic observations of the wheel surface revealed dulling of the abrasive grains by attritious wear, thereby causing a progressive increase in the forces and energy until the end of the useful life of the wheel. For all grinding conditions, a single-valued relationship was found between the wheel wear and the accumulated sliding length between the abrasive grains and the workpiece. A longer wheel life and improved grinding performance can be obtained when the operating parameters are selected so as to reduce the abrasive sliding length per unit volume of material removal.

Comparison of Methods to Measure Grinding Temperatures

Experimental measurements of grinding temperatures are used to estimate the energy partition to the workpiece. In the present investigation, three different temperature measuring techniques are compared for estimating the energy partition in grinding. Grinding temperatures were measured in the workpiece subsurface under identical surface grinding conditions using an embedded thermocouple and a two color infrared detector, and on the workpiece surface using a foil/workpiece thermocouple. All three methods gave comparable temperature responses which were consistent with analytical predictions for the same energy partition.

High Speed Grinding of Silicon Nitride With Electroplated Diamond Wheels, Part 2: Wheel Topography and Grinding Mechanisms

This is the second in a series of two papers concerned with high speed grinding of silicon nitride with electroplated diamond wheels. In the first article (ASME J. Manuf. Sci. Eng., 122, pp. 32-41), it was shown that grinding of silicon nitride is accompanied by dulling of the abrasive grains and a significant increase in the grinding forces and power. High wheel speed caused more wheel wear, which was attributed to a longer accumulated sliding length between the abrasive grains and the workpiece. This second paper is concerned with the progressive change in wheel topography during grinding and how it affects the grinding process. A statistical model is developed to characterize the wheel topography during grinding in terms of active cutting grains and wear flat area. According to this model, continued grinding is accompanied by an increase in both the number of active grains and the wear flat area on the wheel surface as the wheel wears down. The measured increase in grinding forces and power was found to be proportional to the wear flat area, which implies a constant average contact pressure and friction coefficient between the wear flats and the workpiece. Increasing the wheel speed from 85 to 149 m/s significantly reduced the contact pressure, which may be attributed to a reduction of the interference angle, but had almost no effect on the attritious wear rate of the diamond abrasive. Therefore, more rapid wear of the diamond wheel at higher wheel speeds due to a longer sliding length may be offset by reduced contact pressures and lower grinding forces.

Analysis of Energy Partition in Grinding

An analysis is presented for the fraction of the energy transported as heat to the workpiece during grinding. The abrasive grains and grinding fluid in the wheel pores are considered as a thermal composite which moves relative to the grinding zone at the wheel speed. The energy partition fraction to the workpiece is modeled by setting the temperature of the workpiece surface equal to that of the composite surface at every point along the grinding zone, which allows variation of the energy partition along the grinding zone. Analytical results indicate that the energy partition fraction to the workpiece is approximately constant along the grinding zone for regular down grinding, but varies greatly along the grinding zone for regular up grinding and both up and down creep-feed grinding. The resulting temperature distributions have important implications for selecting up versus down grinding especially for creep-feed operations

CAD/CAM for Geometry and Process Analysis of Helical Groove Machining

Helical groove machining using disc-type tools, such as for drill flutes, helical gear teeth, threads, etc., is extremely complex due to the helical motion of the profiled tool along the helical cutting path on the workpiece. In the present paper, a mathematical analysis is presented as the basis for a CAD/CAM system for design and manufacture of components with helical grooves. The system helps the user design the profile of the tool or the helical groove, and thereafter analyze the subsequent machining process. An example is presented to illustrate how the system works and the results which can be obtained.

Wear of electroplated CBN grinding wheels

An investigation is reported on the wear of single-layer electroplated cubic boron nitride (CBN) grinding wheels and how the wear process affects the wheel topography and grinding behavior. Internal cylindrical and straight surface grinding experiments were conducted over a wide range of conditions on hardened bearing steel with wheels containing different abrasive grain sizes. The radial wheel wear was characterized in each case by an initial transient at a progressively decreasing rate to a steady-state wear regime at a nearly constant rate until the end of the wheel life. Wheel wear during the initial transient was found to be mainly due to pullout of the most protruding weakly held grains, and the radial wheel wear in the steady-state regime was dominated by grain fracture. The wear rate in the steady-state regime for various grinding conditions and grain sizes was found to be directly related to the undeformed chip thickness. Dulling of the grain tips by attrition and fragmentation caused an increase in the grinding power. Wheel wear was accompanied by a progressive increase in the.active grain density and a corresponding decrease in surface roughness. The surface roughness was found to depend mainly on the active grain density and is insensitive to the operating parameters.