Robin Stevenson

Assessment of machining models: Progress report

Abstract Progress in developing and assessing predictive modeling of machining processes has been hindered by the extremely localized nonlinear physical phenomena that occur in machining and the many different types of models ranging from theoretical to empirical. The difficulty in assessing models has been cited by industry as the major barrier to use of modern machining models. Current practice in industry is to machine and change tools conservatively, or to conduct costly empirical studies for a limited selection of tools and coolants. The Assessment of Machining Models project will assess the ability of modern machining models to predict the outputs of machining processes based upon a consistent, well measured calibration data set. The data set is nearly complete and is to be used in benchmarking the predictive capability of machining models in blind tests. This paper presents the project motivation, goals, and representative calibration data set results. The next steps in the effort include release of the calibration data, solicitation and collection of predictions, and evaluation and reporting of results.

Application of an Internally Consistent Material Model to Determine the Effect of Tool Edge Geometry in Orthogonal Machining

It is well known that the edge geometry of a cutting tool affects the forces measured in metal cutting. Two experimental methods have been suggested in the past to extract the ploughing (noncutting) component from the total measured force: (1) the extrapolation approach and (2) the dwell force technique. This study reports the behavior of zinc during orthogonal machining using tools of controlled edge radius. Application of both the extrapolation and dwell approaches showed that neither produces an analysis that yields a material response consistent with the known behavior of zinc. Further analysis shows that the edge geometry modifies the shear zone of the material and thereby modifies the forces. When analyzed this way, the measured force data yield the expected material response without requiring recourse to an additional ploughing component.

The mechanical behavior of zinc during machining

It is well known that a nonzero force is obtained when cutting forces measured at different feed rates but otherwise constant cutting conditions are extrapolated to zero feed rate. In the literature, this nonzero intercept has been attributed to a ploughing effect associated with the finite sharpness of the cutting tool. However, the standard extrapolation method does not account for other variables such as strain, strain rate and temperature which also vary with feed rate and influence the work material flow stress. In this paper, the apparent flow stresses measured in high and low speed machining tests on zinc are compared with the flow stresses measured in compression tests. The results show that the flow stress measured in cutting is consistent with that measured in compression when all deformation variables are properly accounted for and that, contrary to the results obtained using the extrapolation approach, the ploughing force is negligible.

A New One-Sided Joining Process for Aluminum Alloys: Friction Stir Blind Riveting

Friction stir blind riveting is a new joining process for one-sided joining (compared with the two-sided access required for, for example, self-piercing riveting) of aluminum alloys, which eliminates the need to predrill a hole for rivet insertion. A blind rivet rotating at high speed is brought into contact with the workpieces, thereby generating frictional heat between the rivet and the workpiece, which softens the workpiece material and enables the rivet to be driven into the workpieces under reduced force. Once fully inserted, the blind rivet is upset using the internal mandrel (as in a conventional blind riveting process) to fasten the workpieces together. Our study showed that friction stir blind riveting process can be carried out over a wide range of operating parameters. The resulting joints show consistent strength under tensile load with minimal influence of changes in operating parameters. The robustness of the process against variations in operating conditions shows that the process can be carried out without high-end equipment and without requiring precise initial setup. It also suggests that the process is feasible for rapid joint fabrication in volume production. Further study revealed superior static and fatigue strength from the friction stir blind riveting process, when compared with conventional spot welding, which suggests potential for reduction in the number of joints required in a structure.

The measurement of parasitic forces in orthogonal cutting

Abstract Not all the forces measured during a cutting operation contribute to chip formation. Some fraction of the forces are parasitic forces such as ploughing or flank forces, which make no contribution to the chip formation process. It its desirable to measure these forces so that the mechanics of the cutting process can be interpreted correctly. However, a possibly more important reason is that parasitic forces are known to increase with worn tools. Thus if the parasitic force can be measured directly, or extracted from the overall measured forces it may be useful for in-process tool condition monitoring provided appropriate calibration of the relationship between the parasitic force and cutting tool dullness has been performed. A method for measuring the parasitic forces in orthogonal cutting is proposed and shown to permit calculating workpiece material properties which are consistent with those measured using other techniques. Another technique for evaluating parasitic forces which was previously shown to yield inaccurate results for zinc was re-evaluated for Delrin® and again resulted in incorrect results.

STUDY ON THE CORRELATION OF WORKPIECE MECHANICAL PROPERTIES FROM COMPRESSION AND CUTTING TESTS

Abstract Metal cutting tests were conducted on annealed and cold-worked 1100-grade aluminum and 33000-grade leaded brass at a sufficiently slow speed (254 mm/min) that thermal influences on workpiece material properties could be neglected. The mechanical properties of the materials were evaluated in compression using conventional low strain rate test procedures. The material behavior during metal cutting and compression was consistent when the work hardening and strain rate hardening response of these materials was considered. The values of strain rate sensitivity coefficient are in reasonable agreement with literature values. These results support previous data indicating that the material response during metal cutting is consistent with the mechanical behavior determined using conventional material test procedures.

Springback in Simple Axisymmetric Stampings

A series of experiments was conducted to measure springback (calculated from dome height measurements) in a simple, stretched sheet metal part formed using a 50.8-mm-radius hemispherical punch. Parts were formed from three materials [5182-0 aluminum, 2036-T4 aluminum, and aluminum-killed (AK) steel] using three different binder geometries (lockbead, stinger, flat), and punch penetration was varied between 1.27 and 21.6 mm, limited by failure of the aluminum sheet. The steel and 5182-0 aluminum were chosen to possess similar gage and tensile properties to highlight the effect of elastic modulus, while the 5182-0 and 2036-T4 aluminum were chosen to possess similar gage and modulus but different tensile behavior to highlight the effect of strength differences. Springback increased with increasing strength and decreasing modulus. A major finding was that, for a specific material, a primary factor influencing spring-back was the binder geometry, with the lockbead showing the least springback and least variation with punch penetration: by contrast, the stinger and flat binders exhibited much greater springback, and the magnitude of the springback was strongly influenced by the extent of punch penetration. In the worst case, the springback was more than 30 pct of punch penetration. The effect of binder region restraint on springback was evaluated by comparing the part geometry both before and after strain-free removal of the binder region by electrodischarge machining. The magnitude and sign of the binder region restraint depended on binder geometry and punch penetration but was always less than the springback observed in removing the part from the die initially.

The morphology and crystallography of diesel particulate emissions

Abstract The morphology and crystallography of particulate emissions from indirect injection diesel engines has been studied using transmission electron microscopy. Samples were collected in a diluted exhaust stream directly onto amorphous carbon films supported on electron microscope grids. Diesel particles closely resemble carbon blacks and consist of chains or clusters of quasispherical subunits. Diffraction patterns were quantified using a microdensitometer and a graphical background correction. Peak intensity was then normalized to background to eliminate exposure variations. Small clusters were found to be less crystalline than large clusters and the centers of two chain-like particulates were more crystalline than the ends. By analogy with the crystallinity studies of heat treated carbon blacks, these differences were interpreted as differences in the time/temperature histories of the particles.

High strength low carbon sheet steel by thermomechanical treatment: l. Strengthening mechanisms

Considerable attention has been focused recently on the development of higher strength automobile stampings. Traditionally increasing the strength of a stamping has been accomplished by utilizing a higher strength material with its attendant problems in springback, die wear and press loads. An alternative approach to high strength stampings based on thermomechanical (TMT) processing of a low carbon steel is described. With this approach, the contributions to strength due to work hardening during stamping and age hardening after stamping are incorporated into the overall processing scheme. The TMT process consists of heating a low carbon steel to the two phase(α + γ) region and quenching to produce a dispersion of martensite in a ferrite matrix. A second, low temperature heat treatment is then employed to improve the total elongation to acceptable levels. The processed steel is then stamped and subsequently aged at either ambient or elevated temperature to develop final strength. The relative contributions of the individual operations of the TMT process to final strength are discussed and the required heat treatment parameters are evaluated in terms of the operative strengthening mechanisms.

Improving the formability of low carbon sheet steel by control of interstitial carbon content and temperature

Several authors have shown a relationship between strain rate sensitivity and the formability of sheet metals. In this study it is shown that by cooling to subzero temperatures the strain rate sensitivity of low carbon sheet steel can be improved dramatically and that this increased strain rate sensitivity leads to improved formability. At laboratory strain rates the optimum conditions for improved formability, a temperature of ∼70 °C and an interstitial carbon content of ∼20 ppm, were determined experimentally. Optimization involves a tradeoff between strain rate sensitivity, strain hardening and fracture strain. Testing under these conditions leads to an improvement in formability of ∼13 pct.