John A. Bailey

Friction in metal machining—Mechanical aspects

Abstract A review is presented concerning the mechanical aspects of friction at the land and rake faces of a cutting tool when machining under orthogonal conditions. Methods are described for determining the mean frictional and normal forces on these faces and calculating the mean coefficients of friction. Considerable confusion exists concerning the nature of the frictional conditions in the region of the tool land to the point where definitive conclusions cannot be drawn. However, it is suggested that sliding and subsurface plastic deformation may occur simultaneously. When machining under dry, unlubricated conditions regions of sticking friction where the frictional stress is constant and sliding friction where the coefficient of friction is constant can exist simultaneously on the tool rake face. Theories which predict a linear relationship of the form ( φ ) = A − B ( β − α ) between the angle parameters used to describe the geometry of the cutting process cannot be used to interpret much of the available experimental data. The apparent linear relationships observed in some previous experimental work are thought to be due partly to the method of presentation of the results. It is suggested that a more significant combination of the angle parameters is the difference between the shear angle φ and rake angle a, that is (φ−α). Two parameters which can be used to describe the frictional conditions at the tool rake face are the mean frictional stress and mean normal stress which can vary independently of each other. It is concluded that the mean angle of friction β is insufficient in itself to describe the frictional conditions on the tool rake face. The effects of changes in cutting conditions on the mean coefficient of friction at the tool rake face are examined and the results interpreted in terms of the associated changes in the mean normal and mean frictional stresses in that region. The lubricating action of cutting fluids is discussed briefly.

The wear characteristics of some cemented tungsten carbides in machining particleboard

This work is focused on developing an understanding of the wear mechanisms of cemented tungsten carbide tools in machining particleboard. Cutting experiments were conducted on several grades of cemented tungsten carbide tools using a high speed lathe, and their wear characteristics were determined. It was found that wear occurred predominantly on the clearance face of the tools for most grades tested. It was also found that the amount of wear after the same cutting distance correlates well with the bulk hardness of the tool material. The amount of wear generally decreased with an increase in hardness, a decrease in grain size and a decrease in binder content of the cutting tool material. Examination of the worn surfaces inside a scanning electron microscope showed that the cutting edge was worn by preferential removal of the metal binder phase from between the tungsten carbide grains. It is suggested that removal of the binder weakens the bond between the tungsten carbide grains leading to their mechanical removal from the clearance face. It is concluded that the main wear mechanism for cemented tungsten carbide tools in machining particleboard is the removal of the binder phase by plastic flow and micro-abrasion, which is followed by fragmentation and dislodging of the WC grains.

Surface damage during machining of annealed 18% nickel maraging steel Part 1 — Unlubricated conditions

Abstract The effect of cutting speed and tool wear land length on the surface damage produced during the machining of annealed 18% nickel maraging steel under lubricated conditions was determined. Machined test pieces were examined with a scanning electron microscope and an optical microscope. Surface roughness was determined with a profilometer. The results of the investigation show that during machining fine scale surface damage in a variety of forms is produced over the range of cutting conditions used. It is suggested that the fine scale surface damage is associated with the phenomena of continuous chip formation and interaction between the tool nose region and freshly machined workpiece surface. Comparisons are made between the characteristics of surfaces machined under lubricated and unlubricated conditions. The differences are interpreted in terms of the changes produced in the frictional conditions and stress distributions at the contact surfaces between the chip, tool and workpiece by application of a lubricant. It is shown that scanning electron microscopy is more indicative of the true condition of the surface than surface roughness measurements.

Residual Stress Distribution in Machining Annealed 18 Percent Nickel Maraging Steel

A novel electrolytic etching technique is used to determine the residual stress distribution in the machining of annealed 18 percent nickel maraging steel. Ring shaped specimens were machined under unlubricated orthogonal conditions with carbide cutting tools having wear lands of 0.125, 0.25, and 0.5 mm length at cutting speeds ranging between 0.05 and 1.60 ms−1 . The results of the investigation show that the residual stresses are tensile at the machined surface and decrease with an increase in depth beneath the machined surface. The maximum (near surface) residual stress and depth of the severely stressed region increase with an increase in cutting speed and tool wear land length. The results are interpreted in terms of the variations in the amount of surface region deformation produced by changes in cutting conditions.

Wear of some cemented tungsten carbide tools in machining oak

Abstract An examination of several cemented tungsten carbide tools that were used in cutting oak containing a high moisture content (green) is described in this paper. It is shown that wear occurs through the process of continuous tool nose rounding. It is proposed that the wear mechanism involves the preferential removal of the binder (cobalt) through chemical reaction with extractives present in the oak (tannins), followed by mechanical loss of tungsten carbide grains when the strength of the remaining bond between them and the binder is insufficient to resist the action of the shear forces generated by motion of the chip and the freshly machined workpiece surface over the faces of the tool. It is also proposed that there is an absence of adhesive and abrasive wear but that some wear could occur through tool edge chipping.

On surface damage during machining of AISI 4340 STEEL

Abstract The effect of cutting speed and wear land length on the surface damage produced during machining of quenched and tempered AISI 4340 steel under dry, orthogonal conditions was determined. Machined test pieces were examined with a scanning electron and optical microscope. Surface roughness was determined with a profilometer. The results of the investigation show that during machining considerable surface damage is produced; the intensity of which decreases with an increase in cutting speed and wear land length. It was found that the surface damage existed in a wide variety of forms which included chatter marks perpendicular to the direction of relative work-tool motion, long straight grooves parallel to the direction of work-tool motion, large cavities, workpiece debris, tool debris, plastic deformation, cracks, microcracks and voids. The results are interpreted in terms of the type of chip produced during machining and the interaction between the tool nose region and work piece. It is shown that scanning electron microscopy is more indicative of the true condition of the surface than surface roughness measurements.

Determination of subsurface plastic strain in machining using an embossed grid

Abstract A technique is described for embossing an orthogonal array of lines (grid) on prepared metal surfaces. Workpieces of oxygen-free high conductivity copper and 18% nickel maraging steel were machined orthogonally under dry unlubricated conditions and the plastic strain distribution in the surface region was determined from the distortion of the array. It is shown that appreciable plastic strain can be produced, the distribution of which is complex. It is suggested that such strains may be important in machining because they can affect the residual stress distribution which, in turn, can have a controlling influence on the behavior of components in service with respect to their resistance to fatigue, creep and stress corrosion cracking.

Comparison of the wear resistance of various grades of cemented carbides that may find application in wood machining

Abstract A laboratory test is described in which specimens of rectangular cemented carbide tool inserts of a standard size are allowed to slide against a rapidly rotating fiberboard disc in either the presence or the absence of a mist spray of a dilute organic acid (tannic acid or acetic acid) to simulate the cutting of green wood and cured wood respectively. It is shown that the worn surfaces of cemented carbide tools used in (field) service are remarkably similar to the worn surfaces of specimens used in the laboratory (simulation) tests. Extensive results are presented that show quantitatively the progressive wear of a wide range of cemented carbides as a function of time for sliding under wet and dry conditions. It is shown that wear depends on the type and amount of binder present in the cemented carbide and on the nature of the environment. Materials with Co-Cr and Ni-Cr binders containing significant amounts of chromium showed the greatest resistance to wear.

Effect of strain-rate and temperature on the resistance to torsional deformation of several aluminum alloys

Abstract The production of stress-strain curves for Al-1100, Al-6061, and Al-2017 over a wide range of temperature and strain-rate using a test based on the free plastic torsional straining of hollow cylinders is described. The shapes of the curves are interpreted in terms of the competing tendencies towards work hardening and thermal softening. The relationship between stress, strain-rate and temperature is found to be similar to that observed for creep. Calculated values of the activation energies for plastic flow are similar to those reported in the literature for the deformation of high-purity aluminum, and indicate that the deformation process is diffusion controlled.

Comparison of the wear resistance of selected steels and cemented carbide cutting tool materials in machining wood

Abstract An experimental investigation is described where specimens of selected steels and cemented carbides are tested to simulate cutting green wood and cured wood. Extensive results are presented that show quantitatively the progressive wear of several Stellites, steels and cemented carbides as a function of time for sliding under wet and dry conditions. A simple theoretical analysis of tool wear that applies to cutting green wood with cemented carbide tools is described. The analysis, which indicates the important parameters in the wear process, is used to predict the effect of carbide particle size on wear rate. Comparisons are made between the predicted and experimentally determined wear rates for two groups of cemented carbide materials. Good agreement is found between experimental measurements and theoretical predictions. It is shown that wear depends on carbide particle size. Superior wear resistance of cemented carbides is attributed to the high hardness and low chemical reactivity of the carbide phase. The improved wear resistance of the Stellites is attributed to the low reactivity of the matrix.