July 2008 Vol. 141 No. 1

Manufacturing Researchers Review Developments

Advances in manufacturing technology demonstrate, once again, the vitality of manufacturing in North America and all the industrialized nations

Brian J. Hogan, Editor and Mark Stratton, Member and Industry Relations Manager

To order a hardcopy reproduction of this article, click here. To purchase digital reprints or reproduction licenses, please contact the resource center at service@sme.org or call (800) 733-4763.

The 36th North American Manufacturing Research Conference (NAMRC) was held in Monterrey, Mexico, May 20–23, and featured papers by authors and co-authors from 11 countries. The event was hosted by the Tecnológico de Monterrey, a private institution founded and funded by individuals and industry, at one of the largest of their 33 campuses.

Samuel Peña, LL.M, MPA, director of foreign investment of the Mexican Secretariat of Economic Development (SEDEC), was the keynote speaker on Wednesday, May 21. He said there is movement in Mexico from a manufacturing-technology-based economy to a knowledge-based economy. "Mexico is only behind China and India in foreign direct investment. Mexico is the third largest in the world among developing countries, and Mexico is still developing economically. Mexico is shifting into a knowledge-based economy focusing on aeronautical and aerospace, biotech, nanotech, and R&D centers," Peña observed.

At NAMRC 36 a tradition of recognition was continued with awards for outstanding papers and best student research presentations, and the S.M. Wu Research Implementation award. The S.M. Wu Research Implementation award was received by a General Motors Corp. research team for work first presented at NAMRC in 1993, in a paper entitled Camshaft Grinding Using Coated Abrasive Belts.

Whirlpool, Sony, and Proeza executives convened an industry panel on Thursday evening entitled Regional Perspective on Global Engineering and Manufacturing. Industry panelists cited a big gap in students' ability to apply knowledge in developing products. Mauricio Durán, innovation director, Proeza Group (Monterrey), remarked: "Students don't know how to design and produce a product for the customer. They need to learn what the customer needs, and know the impact of choices on technology, knowledge and customer."

NAMRC also featured a tour of Nemak, a producer of aluminum cylinder heads, engine blocks, and automotive components, and workshops on cutting tools by Sandvik and laser cutting by Amada.

The following excerpts illustrate the type of manufacturing research now being carried out by members of SME's North American Manufacturing Research Institution (NAMRI).

Researchers from the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology (Atlanta) and Lockheed Martin Corp. (Marietta, GA) collaborated on a paper entitled An Experimental Study of Interfacial Burr Formation in Drilling of Stacked Aluminum Sheets.

Interfacial burr formation is a common problem when drilling stacked sheets of material. The burrs must be removed by destacking and deburring operations. If the burrs can be eliminated, it may be possible to eliminate those steps.

The researchers looked specifically at the influence of drill geometry and process parameters on burr formation when drilling stacks of 2024-T351 and 7075-T651 aluminum.

It is not known how burr formation in the interface between two or more stacked sheets in hole-drilling operation will differ from drilling through a single sheet. During the experiments, helix angle, point angle, coating type, and point type were the elements of drill geometry that were examined. Point type factors included standard point, split point, and step drill. All drills used were size #10 (0.1935" or 4.91 mm diam). Drill wear was excluded as a factor by using new drills in each run of the experiment. Feeds and speeds selected were based on reported burr formation research in the drilling of aluminum.

The method of clamping stacked sheets can have an effect on burr formation. One of the two types of clamps used was a squeeze-action hand clamp (De-Sta-Co model 424). The other clamp was a plier-operated spring-loaded hole clamp. Top sheet material was always 2024-T351, because it's more ductile than 7075-T651, and will presumably be the worst-case skin material in terms of burr formation. The bottom sheet was either material.

Two types of measurements were made. The first was to measure the post-drilling separation of the two sheets. Micrometer readings of total sheet thickness near the hole before and after the drilling operation were taken, and the difference was the post-drilling separation. The second method was to study each sheet on an optical comparator. Each burr was measured from four different radial positions around the hole's circumference. This second approach was used as the main source of burr height data for further analysis.

Optical comparator measurements indicated that frame entry burrs were some 15% smaller than skin exit burrs. The most significant factors in interfacial burr formation were drill-point angle, clamp type, and clamping distance. Combining a 118° point angle with a hole location near either a hand clamp or hole clamp reduced interfacial burr heights. Also, smaller feeds produced small skin exit burrs, while using a step drill reduced the frame entry burr.

Post-drilling separation micrometer readings demonstrated similar trends to the optical comparator data, though with a lower magnitude. It was found that feed/revolution had the greatest impact on drilling thrust forces. Standardpoint drills slightly outperformed split-point drills in terms of burr height, but split points required much lower drilling thrust force. Finally, it was found that chip entrapment in the interface occurred only in cases of a single clamp located 35 mm or more from the hole location.

Characteristics of 3-D Ship Morphology and Properties in End Milling Ti-6Al-4V is the title of a paper by researchers from the Department of Mechanical Engineering at the University of Alabama (Tuscaloosa) that was delivered at NAMRC 36. Based on a design-of-experiment approach, a multiview method was provided to characterize 3-D chip morphology and properties.

A design-of-experiment based on down-milling tests was performed on a CNC milling center. The 1/2" (12.7-mm) diam cutting tool with a 50 µm nose radius is a fourflute solid-carbide end mill with a TiAlN coating. The tests were carried out with a water-soluble cutting fluid.

The 3-D view of chip formation developed by the researchers highlights the dimensions of a milled chip, specific functions of the side-cutting edge and end-cutting edge, and the relationship with process parameters such as axial and radial depth of cut. This approach fully characterizes chip morphology: top surface, free surface, back surface (tool/chip contact surface), and cross-section surface.

Results of experiments indicate that the top surface is characterized by sawtooth segmentation, which increases with cutting speed; also, the sawtooth frequency decreases with cutting speed. The free surface has two distinct sections; the major section shows less lamella structure but the corner section has more lamella structure. As for the cross-section surface, it's composed of major section and corner section with distinct sawtooth segmentation.

Characteristics of the chip microstructure indicate that the beta phase becomes less and smaller compared with the bulk chip. The beta phase is severely deformed and transformed to the alpha phase at the back side of the chip. As cutting speed increases, the deformation degree increases.

The thickness of the phase-transformed (from beta to alpha phase) layer increases with cutting speed, and phase changes in the bulk chip were not observed.

Microhardness profiles on the top and cross-sectional surfaces of the chip are characterized by higher hardness at the surface and lower hardness in the immediate subsurface followed by the stable hardness in the bulk chip. The shear band shows increased hardness, and corner section of the chip has higher hardness than that in the major section. The higher hardness on the top surface may result from a quenching effect caused by the surrounding air. Conducted cutting temperatures anneal the immediate subsurface materials due to the relatively low heat conductivity of Ti-6Al-4V. This annealing results in reduced microhardness.

One interesting finding is that average hardness in the corner section is 10.3% higher than in the major section, an effect that may be caused by more shear-induced strain hardening by the tool nose radius. Lamella structures do exhibit more frequency than in the major section, which means materials in the corner section experience more shearing.

Researchers from the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology collaborated with a researcher in the Department of System and Naval Mechatronics Engineering at the National Cheng Kun University in Tainan, Taiwan, ROC, to present the paper Achieving Machining Residual Stresses Through Model-Driven Planning of Process Parameters. Machined residual stresses are difficult to predict, due to the complex interactions between chip formation, plowing, transient stress distributions, temperature gradients, and material responses during machining.

In their paper, which makes extensive use of mathematical analysis and modeling, the researchers offer a physics-based modeling approach to quantitatively suggest the cutting condition and tool-geometry parameters according to prespecified surface residual stresses resulting from machining. Experimental data were used for model validation. The team says this outcome of this work is a method for planning process parameters and tool geometry to achieve prespecified residual stresses in machined parts.

Experimental residual stress data were used to validate the model developed by the researchers for orthogonal machining. In these tests, an uncoated tungsten carbide tool (K313) was used on an AISI 4340 steel workpiece. Width of cut was 3 mm and the surface speed was 5 m/sec. Only edge radius and depth of cut varied. Residual stresses in the workpiece or depth profiles of residual stresses were measured using an X-ray diffraction technique combined with an electropolishing method for layer removal.

Predicted residual stresses agree well with measurements. The difference in most cases is less than 30%. This error is relatively small, according to researchers, when you consider the simplifying assumptions made in the model in the context of material properties and tool/workpiece contact conditions. Also, they point out, the experimental measurement of residual stress is prone to a large margin of error. Workpiece residual stress profiles are claimed to follow the trend of the experimentally measured profiles.

Researchers from the Department of Mechanical Engineering at Michigan State University (East Lansing) and the Digital Manufacturing System Center of the Korea Institute of Industrial Technology (Incheon, South Korea) brought a paper entitled Understanding Tool Wear of Multilayer Coated Carbides in Machining 1045 Steel to NAMRC 36. It discusses the rationale for the extraordinary wear resistance of multilayered (TiN/Al₂O₃/TiCN) coated carbides when turning AISI 1045 steels.

They found that the crater wear rate was drastically reduced with the exposure of the Al₂O₃ layer, and that when TiCN was exposed, the surrounding Al₂O₃ continues to protect the coating. Crater wear accelerates to expose WC, and causes catastrophic failure after the protection of the Al₂O₃ is gone.

Despite cracks on the original new inserts, coating delamination was not observed. The location of maximum temperature coincident with maximum crater wear progressed into different locations as each layer was exposed. So the wear resistance of multilayer coated carbides comes from the location of maximum crater wear progressing into multiple locations as each layer is exposed. Flank wear evolution follows a trend similar to that of crater wear.

In the machining experiments, a Super Quick Turn 200M CNC lathe from Mazak (Florence, KY) was used to dry-turn two types of 1045 steel bars: hot-rolled pearlitic HV 233 and refined pearlitic HV 175. The researchers used C6 inserts coated with TiCN, Al₂O₃, and a top layer of TiN. Machining conditions were constant at 250 m/min cutting speed, 0.3048 mm/rev feed rate, and 1.905 mm DOC. Flat-faced inserts designated ISO SNMA190612 and KC9315, provided by Kennametal (Latrobe, PA), were used. Among other points, researchers conclude that the Al₂O₃ coating is the main contributor to the crater wear resistance of TiN/Al₂O₃/TiCN coated carbide. Also, the effectiveness of multilayer coated tools arises from the fact that the location of maximum crater wear progresses into multiple locations as each layer is exposed.

A research team from the Department of Mechanical Engineering at Iowa State University (Ames, IA) presented a paper entitled CO₂ Laser/Waterjet Cutting of Polycrystalline Cubic Boron Nitride. Polycrystalline cubic boron nitride (PCBN) is difficult to machine due to its hardness (only diamond is harder) and its good chemical and thermal stability.

PCBN is available in two forms: thin layers on a cemented carbide substrate and a solid cylindrical compact produced by consolidation of boron nitride powders with binders at high pressures and temperatures. The binders are either metallic (cobalt) or ceramic (TiN and AlN). Cobalt and TiN-sintered PCBN cause the compact to be electrically conductive, which allows the blanks to be cut by EDM. But the AlN-sintered PCBN blanks are electrically insulating, and can't be cut using EDM. Waterjet cutting is slow and yields wider kerf, poor surface finish, and taper. A Q-switched Nd:YAG laser in the fundamental mode can also be used, and employs metal blow and evaporation mechanisms. Consequently, recast layer, conical and wider kerf, rough edge, and thermal damage can all cause problems.

The researchers reported on a cutting method based on a crack separation mechanism for electrically insulating PCBN blanks with AlN as the binder phase. It relies on the development of tensile stresses due to chemical transitions and thermal gradients. These stresses cause controlled separation of material through crack formation rather than by melting and evaporation. The material separation technique consists of melting and resolidification of thin surface layers of binder-containing PCBN in an oxygen-rich environment. The technique was testing with a CO₂ laser alone and a hybrid CO₂ laser/waterjet (LWJ) machining system.

Experiments were conducted on an insulating PCBN blank supplied by Diamond Innovations Inc. (Worthington, OH). A 10.6 µm wavelength CO₂ laser with rated power of 1500W (Model 820 from Spectra Physics, Mountain View, CA) was used in all experiments.

During LWJ cutting, the heated recast layer is rapidly quenched due to the waterjet. Rapid quenching leads to thermal shock and the development of additional thermal stresses. Because PCBN is a good thermal conductor, there are significant thermal gradients across the wafer thickness, and the spatial proximity of the heated material under the laser spot and quenched material under the waterjet will result in large in-plane thermal gradients. Consequently, the heated material will be subjected to compressive stresses while the quenched material will experience tensile stresses.

The material-removal mechanism was determined to be controlled fracture of PCBN wafers due to development of tensile stresses associated with high-temperature oxidation of AlN in an oxygen-rich environment and thermal gradients induced by the laser beam and waterjet.

To order a hardcopy reproduction of this article, click here. To purchase digital reprints or reproduction licenses, please contact the resource center at service@sme.org or call (800) 733-4763.