Hard Turning, Tool Life, and Surface Quality
Justify hard turning by examining cutting conditions
By Ty G. Dawson, Thomas R. Kurfess, The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA
Engineers want materials with long service lives, and processes for shaping them into finished products with tight geometric tolerances and excellent surface finish. Hardened steel is one such material, used particularly in the automotive industry for components such as bearings, gears, shafts, and cams. Soft steel must be hardened to increase the strength and wear resistance of parts made from this material. Traditionally, finished surfaces must be ground on near-net-shape, hardened-steel parts.
However, the grinding process itself may require several machine tools and several setups to finish all component surfaces. Because grinding can be a slow process with low material-removal rates, there has been a determined search for replacement processes.
Cutting edge condition of new tool
Enter hard turning. Recent improvements in machine tool rigidity and the development of ceramic and CBN cutting tools allow the machining of hardened steel. Even though hard turning consists of small depths of cut and feed rates, material removal rates can be much higher than those achieved by grinding, depending on the application. Estimates of reduced machining time are as high as 60%. This reduction can make it possible to employ flexible manufacturing systems and reduced batch sizes, both of which are growing more important in industry. Aside from decreases in machining time, reducing the number of required machine tools may be a result of the increased flexibility of hard turning compared to grinding. Using fewer machines would reduce part-handling costs and the cost associated with having multiple operators and machine setups. Hard turning can also be done without coolant.
Crater wear after 20 passes
It seems obvious that hard turning is an attractive replacement for many grinding operations, but implementation in industry remains relatively uncommon, particularly for critical surfaces. Several issues remain unaddressed. For one, hard turning can influence the workpiece surface microstructure by generating undesirable residual stress patterns and overhardened surface zones, known as white layers. The cause of these residual stress patterns and white layers and their effect on parts are not fully understood. Also, because cutting tools required for hard turning are much more expensive than tools for conventional turning, tool life influences the economics when users seek to replace grinding operations with hard turning.
To become a realistic replacement for many grinding operations, hard turning must prove it can create equivalent finished surfaces. It must hold geometric tolerances and produce undamaged surfaces that conform to surface roughness requirements. Under ideal conditions, part geometry and surface finish comparable to those generated by grinding can be hard-turned with extremely rigid machine tools and new cutting insert materials. With less rigid machines and improper cutting conditions, however, tool wear is excessive and eliminates any cost savings. Worn cutting tools also increase the amount of surface damage observed on hard-turned components.
Flank Wear
White layer and residual tensile stresses might be expected to reduce fatigue life. But research comparing fatigue life of hard-turned and ground surfaces found the hard-turned surfaces have increased life despite the existence of brittle white layers. Compressive stresses found on hard-turned surfaces also improve fatigue lives. Because the effects of white layer on the resulting component life are not well understood, industry remains reluctant to produce critical surfaces by a hard turning process that may contain white layers. More research is needed to determine how cutting parameters affect tool wear and surface integrity.
Flank wear after 40 passes
As this article is prepared, we've tested thirteen different cutting conditions. They span the range of recommended conditions for the cutting tools used, and match reasonable conditions selected in past research and by industry. Cutting speed, feed rates, and radial depths of cut were varied for four different tool materials, made by two CBN insert suppliers. We machined 52100 tube stock hardened to approximately RC62. This hardness was maintained throughout the thickness of the tube, as it was possible to quench from both the OD and ID.
The test conditions contained five subgroups. Divisions are based primarily on the cutting tool material, although the first two subgroups are separated by radial depth of cut. The second grouping is the only set of tests run at the larger radial depth of cut.
Cutting tools for all experiments were CBN-tipped cutting inserts that, in combination with the toolholder, provided -5º side and back rake angles, a 0.794-mm nose radius, and a 20º edge chamfer 0.102-mm wide. Cutting forces were recorded during each test pass using a piezoelectric dynamometer. Tool inspections were made with a standard optical microscope and a scanning electron microscope.
Tool wear not only reduces tool life, but also affects surface finish, increases cutting forces and tensile residual stress, and causes white layer surface damage. Resistance to tool wear is a function of many variables, including CBN content, grain size, binder material, tool geometry, cutting edge geometry (sharp vs. chamfered), workpiece properties, and cutting conditions. The brittle nature of ceramic and CBN tools makes them prone to chipping at the cutting edge, while abrasive wear and adhesion may cause premature tool failure. Lower-CBN-content tools with a ceramic binder are generally more resistant to abrasive wear due to increased bonding strength, while higher-CBN-content tools with metallic binders have improved fracture toughness. More work needs to be done to determine how tool material, workpiece material, tool geometry, and cutting conditions affect wear rates and life of CBN cutting tools.
Note that condition 5 is a replication of condition 1, but on the harder tube material. Conditions 3 and 4 are the only replication of identical machining conditions and same workpiece material.
For all tested conditions, tool failure was declared when an edge fracture was noticed on the tool, or when the flank wear exceeded 200µm. We selected the 200µm condition because almost all tools fractured with a flank land measurement between 150 and 200µm. Cutting time could be calculated as a measure of tool life, instead of the volume of material removed. Cutting time can be calculated using the following relationship, where V is the volume of material removed in cm3, f is the feed in cm/rev, d is the radial depth of cut in cm, s is the tangential cutting speed in cm/min, and t is the cutting time in minutes.
Tool life dramatically improvedat conditions when cutting speed was 91.4 m/min instead of 182.9 m/min. In general, increased feed reduced tool life. The effect of cutting speed was more dominant than the effect of feed rate, so to improve tool life, slower cutting speeds should be selected in combination with increased feed rates. Because material removal rate is linearly related to both feed rate and cutting speed, halving cutting speed while doubling feed rate maintains an equal removal rate. There are, however, limitations on acceptable feed rates-determined by the ability of the cutting tool to withstand increased cutting loads without fracture.
Increasing radial cutting depths also could increase removal rates, although cutting depth is often determined by the amount of stock removal required. As in the case of increased feed rates, tool life decreased with increased depth of cut. As expected, a tradeoff exists between tool life and removal rate. Optimal conditions must factor the cost benefit of decreased machining times with increased tool wear and the associated cost of increased machine downtime.
High-CBN-content tools typically have 90% or more CBN bonded with a cobalt binder material, while the low-CBN-content tools may contain only 40-60% CBN with a mixture of ceramic binder materials. The increased strength of the binder material in the low-CBN tools may explain the observed increase in tool life. Yet, low-CBN tools may not perform well for more aggressive cutting conditions or interrupted cuts, where the higher fracture toughness of high-CBN-content tools would be beneficial.
In addition to the cost effects of tool life, changes to the cutting mechanics due to tool wear and associated changes in cutting geometry are important in hard turning. This observation is particularly true because cutting tools are typically prepared with chamfered or honed edges, and the cutting geometry takes place primarily along these edges.
Because the chip-contact area occurs along the chamfered nose of the cutting tool, nominal cutting geometry can change significantly during the life of the cutting tool. To monitor changes in the nominal geometry, we used a New View 200 microscope, made by Zygo Inc. (Middlefield, CT), to produce three-dimensional images of the cutting edge. For inserts with a 20º edge chamfer and a toolholder with a negative 5º rake angle, the effective combined rake angle is 25º, assuming that cutting takes place primarily on the edge chamfer. Crater wear changes that geometry significantly, which also impacts the cutting process. While crater wear is important because it changes nominal cutting geometry, flank wear is the more typical method of quantifying the condition of a cutting tool.
Measurement of the flank land was possible using images from the Zygo microscope, an optical microscope, or SEM. With the Zygo data, two-dimensional profile sections can be obtained to easily measure the maximum length of the flank land directly from the analysis software provided with the microscope. Plotting the flank land measurements against the volume of material removed with the cutting tools shows a common pattern in the wear data.
Tool Life for All Cutting Conditions
This figure shows the tool life of each cutting condition listed in the table above, measured by the amount of material removed with the tool.
Results indicate most tools failed when flank wear was between 150 and 200µm. While the cause is ultimately a loading that causes brittle fracture on some portion of the cutting edge, knowing the range of flank wear that corresponds to this condition may provide a simple way to predict failure. Knowing failure is expected beyond a flank land of 150µm provides a basis for calculating the amount of material (or cutting time/cutting length) that can be safely machined with a tool at different cutting conditions. More work will help determine how cutting parameters influence constants, so we can develop a method for predicting flank wear for ranges of reasonable cutting conditions.
If hard turning is to replace finish grinding operations, it must be capable of producing surface finishes comparable to those generated by grinding. Unlike grinding, where surface finish is determined by the size, shape, hardness, and distribution of abrasive grains in the grinding wheel, hard-turned surfaces are nominally defined by the geometry of the cutting process, primarily by the cutting tool's feed rate and nose radius.
From profile data, arithmetic centerline average roughness Ra can be calculated by Equation (1) below. For conditions typical in hard turning, it can be approximated by Equation (3), where f is the feed/revolution and R is the nose radius of the cutting tool.
(1)
(3)
For most cutting conditions, experimental and theoretical Ra values match very well, except at low feed. The discrepancy at low feeds is explained by an increase in the dominance of plowing action. This explanation is reasonable, because at lower feeds, cutting depth decreases across the entire length of the cutting edge. Ideally, feed could be used to determine the quality of a finish-machined surface. This is not possible, however, due to increased plowing, which causes material to deform plastically around the nose of the cutting tool and produce larger peaks on the machined surface than theoretical calculations predict. Larger nose-radius tools would help improve finish, but plowing will always limit surface quality.
Most research on surface integrity of hard-turned steel surfaces has concluded that hard turning results in a thin rehardened surface layer of material that may be followed by an overtempered region just beneath the white layer. Due to the microstructural changes to the material, the rehardened layer looks white in an optical micrograph after chemical etching, while the tempered region appears dark, thus the terms white-etching areas and dark-etching areas.
While most research has shown the existence of white layers on hard-turned surfaces, the cause, magnitude, and effect of such layers are not well-understood. White layers are typically associated with residual tensile stress at the surface, while in some conditions white-layer surfaces exhibit residual compressive stresses. Obviously, residual stress on a surface affects the performance of a workpiece in service. There are differing results, for example, on the fatigue lives of hard-turned surfaces compared to ground surfaces.
Cutting coolant can influence the generation of white layer. Because white layer is thought to occur as the result of a phase transformation on the surface (very similar to the bulk heat treatment of steels), cutting coolant might help eliminate thermal damage by keeping the workpiece surface cool. Some reports say cutting coolant eliminates white layer, but other studies show coolant having no effect. Tool condition is also believed to be an important factor, with new tools producing undamaged surfaces, while white layer increases with increasing tool wear. The cause of this increase is not clearly defined, but may result from increased heat generated by friction between the tool and workpiece as the flank land increases. It may also result from increased plastic deformation associated with increased friction.
Surface integrity in hard turning also causes some reluctance to use hard turning as a finishing operation for critical surfaces. Despite such reports, current research shows hard turning, even with very worn cutting tools, can produce undamaged surfaces.
The proposed explanation for the existence of white layer when cutting with low-CBN-content tools and not with high-CBN-content tools is a difference in thermal conductivity, with the high-content tool's conductivity being twice that of low-content tools. There are primarily three routes for heat removal from the cutting process: the chip, the tool, and the workpiece. Assuming that most heat is removed with the chip, the remaining heat must go into the workpiece and cutting tool. If resistance to heat flow into the tool increases, the workpiece surface will get hotter, a condition that low-CBN-content tools can produce.
Hard turning offers many potential benefits when compared to grinding. To increase the use of this technology, questions about its ability to produce surfaces that meet surface finish and integrity requirements must be answered. In addition, the process must be justified economically, which requires a better understanding of tool wear patterns and life predictions.
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