Researchers Advances Manufacturing Technology
NAMRC 34, the annual gathering of North America's manufacturing research community, highlighted work done by manufacturing's academic elite
By Brian J. Hogan
By Robert Aronson
The annual meeting of the North American Manufacturing Research Institute of the Society of Manufacturing Engineers, NAMRC 34, was held in Milwaukee May 23-26 at Marquette University. An international forum on manufacturing research, NAMRC is described by SME as "an annual conference of the international community of researchers whose works contribute to the furthering of manufacturing technology."
The keynote speaker at NAMRC 34, John Gurda, presented the talk Made in Milwaukee: Our Manufacturing Heritage. Gurda is the author of 15 books on Milwaukee, and in his presentation he discussed the rise and decline of manufacturing in the Milwaukee area. Among the talk's highlights:
Transportation was the earliest industry because of the city's deep natural harbor on Lake Michigan. Beginning in the early 1800s, Milwaukee was a major shipping point, because water travel was the dominant way of moving products and basic materials.
The first major change was the switch from just shipping to manufacturing, a trend that would last for decades. It was the era of the entrepreneur and small-shop operator who could cultivate and develop new ideas for a growing country. At the same time there was a strong immigration into the area, chiefly by Germans, many of whom were trained craftsmen. The result was the manufacture of products tied to area resources. In all cases, the engineering content of products was beginning to emerge.
At the same time, small specialty machine shops with one or several ambitious craftsmen began a number of America's well-known companies. They included Harnischfeger cranes, Chain Belt, Allen Bradley, Allis Chalmers, Kearney and Trecker, and, of course, Harley-Davidson. Despite the linking of Milwaukee to beer, during that time, it was more a matter of gears than beer production.
Manufacturing the material for WWII increased Milwaukee's manufacturing base even more, with some plants working 10-hr shifts, seven days a week.
However, the manufacturing scene began to change in the '50s. The day of the small enterprise was fading. As foreign competition grew and new industries became dominant in other areas, Milwaukee's manufacturing base began to contract.
Today, this manufacturing center, as many other former manufacturing giants in the US, has embraced service industries such as health care, data processing, finance, and insurance. In its peak manufacturing years, 57% of the Milwaukee area workforce was in manufacturing, Today it's around 7%.
In his opening remarks, NAMRI/SME President Ralph A. Resnick commented on the sobering message from Gurda that manufacturing matters in the success of a city or a nation, and further that science is critical to manufacturing. He noted that to survive in a global economy it is necessary not only to be inventive, but to be able to transfer that information to stimulate the economy. It's important that our nation should not only provide leading-edge discoveries, but have industrial champions that will get new technology onto the factory floor. NAMRC has the task of recognizing those developments during the time of research and overseeing the transition.
The Founder's Lecture is designed to recognize members of the original group of NAMRC founders. For the 34th session, the talk was delivered by Betzalel Avitzur of Lehigh University (Bethlehem, PA) on the subject, Road Map for Tube Making: from Tube Sinking to Tube Drawing with Floating Plugs.
In his talk, Avitzur, who is an expert on tube manufacture, reviewed the history of tube making beginning with the Egyptians, who manufactured simple tubes chiefly for jewelry.
Almost 200 researchers saw 80 papers presented during NAMRC 34 in Milwaukee. The following excerpts from selected papers illustrate the nature and depth of the research work now being done on the science and technology of manufacturing.
Researchers Sathyan Subbiah, Thomas Newton, and Shreyes N. Melkote of the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology (Atlanta) presented a report entitled Tool Life and White Layer Formation in Interrupted Hard Turning with Binderless CBN Tool. Their experiments were performed on 52100 bearing steel hardened to RC 58 using two different cutting tools; a Kennametal grade KD120 PCBN tool (more than 90% CBN) with a metallic binder, and a binderless CBN insert provided by Shinya Uesaka of Sumitomo Electric Industries. Longitudinal turning was done dry on a Hardinge T42P lathe. Flank and crater wear were monitored using an optical microscope (Nikon Micophot FXL) and measured using a white light interferometer (Zygo Newview 200).
The performance of a binderless CBN tool in interrupted or continuous hard turning has not been reported in scientific literature, according to the researchers. They set out to study this performance, and developed two new parameters to be used to characterize interruptions in machining. The first, interruption ratio (IR) equals uncut distance/cut distance. When a tool enters a sudden cut (e.g. a weld bead), the IR ratio is small. Zero IR means the tool is never out of cut. The second parameter, interruptions per unit length of cut (IL) is defined as the number of interruptions/length of cut. In longitudinal turning, the length of cut is the circumference of the cylindrical material being cut. Parameters IR and IL are used by the research team to characterize the severity of interruptions in hard turning.
Workpieces were prepared with interruptions of different shapes and frequencies. Binderless CBN performed better than the high-CBN insert in only one test—the case where IR is low and IL is high. Research demonstrates that IL affects flank wear in both binderless CBN and PCBN. Flank wear is characterized by groove formation by three-body-type abrasion caused by plucked-out CBN particles. At 120 m/min cutting speed binderless CBN offers no noticeable advantage over conventional CBN in tool life and surface finish. At a speed of 180 m/min, however the performance of the binderless CBN does not seem to deteriorate, unlike the PCBN tool.
Binderless CBN was found to produce thinner white layer than the high CBN tool at a cutting speed of 120 m/min. The binderless material has higher conductivity than the PCBN tool. Assuming that thermal mechanisms play a major role in white layer formation, then more of the frictional heat caused by flank wear is conducted into the binderless CBN and less enters the workpiece, reducing white layer formation. Because binderless CBN has higher thermal stability than PCBN, the heat may not damage the tool.
Researchers from three institutions, RM Arunachalam of the Mechanical Engineering Department, Sona College of Technology, Salem, Tamilnadu, India, M.A. Mannan, Mechanical Engineering Department, National University of Singapore, and Andrew Christopher Spowage, Precision Measurement Group, Singapore Institute of Manufacturing Technology, collaborated on Comparison of Surface Roughness and Residual Stresses Induced by Coated Carbide, Ceramic, and CBN Cutting Tools in High-Speed Facing of Inconel 718. Among the nickel-based heat-resistant superalloys (HSRAs), Inconel 718 is the most important and frequently used for the manufacture of aerospace gas turbine components. It's a difficult material to machine, however, because of work hardening, lower thermal conductivity, and a tendency to adhere to the cutting tool. High-speed machining using coated carbide, ceramic, and CBN cutting tools is one approach to improving productivity when machining Inconel 718. Considerable research has been done on turning and milling of this alloy with such tools; facing operations have not attracted much attention.
It's important to maintain the surface integrity of a machined component, and residual stresses (because of their contribution to premature failure of components) deserve special emphasis. The researchers paid particular attention to residual stresses and surface-roughness aspects of surface integrity in facing operations on precipitation-hardened (RC 35) Inconel 718 using coated carbides, ceramic, and CBN cutting tools. Facing operations were done on a high-rigidity CNC with constant speed capabilities, and were carried out at the respective optimum cutting conditions for the different tools.
The surface roughness of the machined surface was measured after each test using a contact-type profilometer. Residual stress distribution was measured using the X-ray diffraction technique.
There are two causes of machining-induced residual stress; thermally caused plastic deformation and mechanically caused plastic deformation. If the mechanical cause dominates, the residual stress tends to be compressive. If residual stress is thermally driven, the stress tends to be tensile. Researchers found that the PVDTiAlN-coated carbide cutting tool generated the highest value of compressive residual stress, followed by the CBN cutting tool. Multilayer CVD-coated carbide and the mixed alumina (MA) ceramic tool generated tensile residual stresses in which the maximum value of tensile residual stress was generated by the MA tools.
Although the CBN cutting tool is capable of producing a better surface finish, the PVD-TiAlN yields a finish that is good, and meets industry requirements. Also, though the cutting speed used for the CBN (150 m/min) is 2½ times that of the speed used with the PVD TiAlN-coated tool (60 m/min), material removal rate is the same, because a greater depth of cut is possible with the latter tool.
The researchers conclude that the significantly lower cost of the PVD-TiAlNcoated cutting tool (4-5% of the cost of a CBN tool) makes it a better choice for finishing operations on Inconel 718, especially when it's important to satisfy the surface integrity requirements specified for the component.
Reducing Buckling Distortion in Welded Structures Using Thermal Tensioning was the subject of a NAMRC paper given by Jun Xu and Wei Li of the Department of Mechanical Engineering, University of Washington (Seattle, WA). Welding-induced buckling distortion is a common problem when fabricating thin-walled structures. Many methods have been developed to prevent buckling distortion, but most are not desirable because they require changes in product and process designs, and involve complicated fixtures.
Welding-induced buckling distortion is basically caused by the compressive stress resulting from welding shrinkage. Once the residual stress goes beyond the structure's critical buckling load, bucking instability will occur.
Thermal tensioning is a way to preload tensile stress by imposing a preset temperature gradient over the structure to be welded. The welding zone is then prestretched to provide compensation for the compressive stress induced by the mismatched temperature distribution due to welding. After cooling, the welding shrinkage acts as a pair of external forces at the ends of the weld, causing high compressive stress at the surrounding area. Prestretching reduces the residual stress on the thin plate. Lowering the negative compressive stress around the welding zone below the critical buckling load prevents buckling distortion.
The researchers chose to study use of the thermal tensioning method to reduce buckling distortion in a boxbeam structure. They developed a FEA model to study the response of the side plates under welding and various preheating temperature conditions. Buckling formation and residual stress evolution during cooling were compared.
Buckling distortion of welded structures without preheating and with 150°C preheating.
Commercial programming code (Ansys) was used to solve the coupled thermal-mechanical problem in a sequential manner. This model identified a critical preheating temperature to suppress the buckling distortion based on simulated residual stress distribution. The temperature is expected to change with plate dimensions, including thickness and width, and with the size of the preheating area. It can be easily identified with the FEA procedure developed during the study.
This critical preheating temperature was applied in a production environment. Welding experiments were done on an automated welding station placing four welds simultaneously on 9-m-long beams. Two propane torches were installed 0.15 m ahead of the welding torches to heat the centers of the box beam's side plates to the critical preheating temperature. The preheating torches and welding torches moved at the same speed to create a steady temperature field. The waviness of the buckling distortion was observed, and its depth measured using a digital dial gage.
Results indicate that the FEA model can represent the welding process fairly accurately. Waviness periods of the welded beam exactly matched the model's predictions, and the waviness depth was only slightly lower than the measured one. Differences between the model and the experimental weld results can be explained by various parameters such as initial stress conditions in the steel, lack of sufficient fixturing at one end of the beam, and other factors. The modeling procedure and understanding developed in the study is generic and can be applied to other welding-induced buckling problems.
Dae-Wook Kim and Phil Allen of the School of Engineering and Computer Science, Washington State University Vancouver, Taeksun Nam, of StressWave Inc. (Kent, WA), and Hyeon-Jae Shin of the Department of Mechatronics, Inha Technical College (Inchon, Korea) presented their report, Effect of Cold Working on Exit Burr Formation in Drilling to attendees. Before beginning the project, they appreciated that cold working has been used for many years to improve the integrity and efficiency of metallic airframe structures.
These legacy methods include split sleeve and split mandrel cold working, pin coining, and similar techniques. Each draws a rigid tool or tapered mandrel through close-tolerance starting holes, thus causing diametric enlargement of the holes, and imparting beneficial compressive residual stresses at their periphery to counteract the stress concentration created by holes. Although these processes improve fatigue life, they have drawbacks such as a high number of process steps, consumables, and expensive operating cost.
A new and patented cold-working approach, Stresswave, also called advanced cold working, uses mechanical squeezing to impart residual stresses into structures before making holes. Special indenters driven by a the raw material. The indenters are driven into the workpiece until a specific application pressure and/or displacement (determined by FEA) is reached. (Commercial FEM software, LS-Dyna, was used for the project.) A hole is made through the dimple using drilling and reaming. Advanced cold working has been shown to improve cycle life by 600% when compared to the split sleeve process.
The workpiece material used in the study was 6061T6 aluminum in three different thicknesses—3.175, 6.35, and 9.525 mm. Drilling experiments were done without coolant on a Haas Mini-Mill using HSS drills with a standard twist-drill geometry. Workpieces with and without cold working were used.
This is the drilling burr formation mechanism for cold-worked 6061T6 proposed by researchers.
Aluminum alloy burrs were found to depend upon drilling conditions. Exit burr height generally increases with decreasing drilling speed. Thickness was shown to have a relatively high contribution to exit burr height, while speed and feed did not change exit burr height significantly on cold-worked samples.
The mechanism of burr formation has been classified into three types according to the location of the initiating crack. Previous research has also demonstrated that burrs in low-alloy steel can be placed in three categories: uniform burrs (type A), transient burrs (type B), and crown burrs (type C). Type A burrs are relatively small because the final cracks occur near the hole edge, leaving only a small uniform cap along the hole's perimeter. Final cracks occur near the hole edge and in the middle of the hole to form type B burrs, which are characterized by nonuniform caps and large flakes. Finally, a crack occurs only in the middle of the hole to form type C burrs (crown burrs).
No type-C burrs were observed by researchers. Workpieces that were not heat-treated and drilled at lower feed rates formed type-B burrs, with burrs ranging to 800µm high. The burrs formed in cold-worked samples were all type A and less than 400µm high.
The following factors influence exit burr formation in coldworked samples. Average indentation size is approximately 80% of drill diam. Average indentation depth is approximately 15% of workpiece thickness for each side. Consequently, advanced cold working thins and hardens the workpiece. As the drill penetrates the workpiece, the chisel edge and cutting lips simultaneously begin cutting the workpiece because of the indentation at the top surface. When the chisel edge goes beyond the exit surface, material under the chisel edge can be extruded.
As the drill advances, the initial material fracture occurs near the center of the drill. A secondary fracture occurs as the cutting lips advance at the hole exit. As the drill travels through the hole exit, the remaining material is bent and pushed away from the workpiece. Advanced cold working not only changes the top and bottom surface contour due to mechanical squeezing, but imparts large compressive residual stresses around the indentations. The effect of these residual stresses on exit burr formation is not well known, and is the subject of continuing research.
This article was first published in the July 2006 edition of Manufacturing Engineering magazine.