ROLE OF MACHINING IN RP

The main competition to SFF-based rapid prototyping systems is NC (numerical control) machining, used typically in small model-making shops. In comparison with current SFF processes, machining can produce parts with superior accuracy and surface finish and having a much broader range of materials, especially tooling steels. Furthermore, if only 2D drawings are available, then machining by a skilled model maker can often be executed more quickly than the time it takes to first create the 3D model required for SFF processing. Machining, therefore, remains strategically important to industry in both Europe and Japan, due in part to the relatively slow dissemination of 3D CAD in both areas. There are also concerns about the inability of current SFF processes to (1) produce parts with the accuracy and surface finish required for many engineering models, (2) build with a wide variety of engineering materials, and (3) directly produce high-quality metal parts for production tooling applications.

Completely automated CNC machining would have a significant impact on rapid prototyping. Current CNC systems, however, are not generally considered to be SFF technologies for the following reasons: they still require skillful human intervention to help plan the operations and to operate the equipment; custom fixturing and special tooling is often required; and machining has inherent geometric limitations (Fig. 2.12). However, just as SFF process performance capabilities are expected to improve, automated CNC planning capabilities are also expected to continue to improve with the proliferation of 3D CAD modeling systems. In the future, both CNC and SFF will remain important technologies for RP needs.


Fig. 2.12. Why isn't CNC machining an SFF process?

Material Removal Processes

While improving the capabilities of automated CNC planning systems is important, there is also a need to improve the machining processes themselves. There is interesting work in both Germany and Japan on machining and other material removal processes. The main application area for these processes is in rapid tool manufacturing. Tooling fabrication relaxes some geometry constraints; for example, undercut features are not normally required.

Lasercaving, which process is used by LCTec, Inc. (Germany), is being refined by the Bavarian Laser Center (BLZ). It is a material removal process used to cut cavities into metal or ceramic stock in a downward, layer-by-layer fashion. A high-power laser and oxygen source are simultaneously directed at the surface to be cut and swept across the cutting path. The heated metal oxidizes, and oxidized chips break away due to differential thermal expansion between the underlying unoxidized material and the oxide fragment (Fig. 2.13).


Fig. 2.13. Lasercaving.

The system includes a 5-axis CNC vertical milling machine and a 750 W CO2 laser. Parts are mounted on the mill and moved relative to the fixed beam. The claimed accuracy is 0.05 mm, and the material removal rate is 5 mm3/min. Overall machining times can be reduced by first roughing out material at a higher rate using the laser in a melting mode that removes material at a rate of 1,000 mm3/min. Surface roughness is about 5 µm. Primary applications BLZ is exploring are cutting cavities in tools and texturing tooling surfaces.

The JTEC/WTEC team saw examples of steel dies and leather-like textured patterns; their surface quality was excellent. These surfaces must be glass-beaded to remove oxidation; otherwise, no additional processing is required. Lasercaving can be a powerful method for building and texturing tools with fine detail and small features. In Japan, Professor Nakagawa of Tokyo University has extensive leading-edge research in the use of high-speed machining, i.e., spindle speeds of ~100,000 RPM. He is particularly interested in the use of high-speed machining for rapid tooling manufacturing.

Processes That Combine Material Addition and Removal

There are also efforts to investigate combining the benefits of material additive processes (especially that they simplify planning) with the benefits of material removal processes (that their accuracy and surface finish are superior). The primary application area has been for tooling manufacturing. The Fraunhofer IPT has developed an experimental system, called "Laser Generated RP," which uses laser welding to melt metal powder as it drops from a coaxial laser/powder distribution cone (Fig. 2.14a). Other concentric cones within the probe deliver shroud gas and fluids for cooling. The system uses either a 900 W CO2 laser or a 1,000 W Nd:YAG laser. Within the work chamber is a 2½D milling cutter to finish the walls of the metal part and improve tolerances. IPT representatives claim that this unit produces full-density parts, but they did not disclose their deposition and cutting strategies to the JTEC/WTEC team. They showed the team both thin wall and solid parts made of steel from this experimental unit. IPT plans to commercially develop this system with a die casting machine tool company over the next few years. With significant funding from its commercial partner, chances of commercializing this technique appear promising.


Fig. 2.14. Combining material addition with material removal.

In addition to his work in high-speed machining, Professor Nakagawa is developing lamination processes to build up large-scale tooling, such as forming dies for automobile bodies. Individual sections of material are shaped with CNC cutting and then stacked up to form the tool (Fig. 2.14b). However, neither the method of registration nor the joining process was disclosed to the team.

The addition/removal processes described above apparently do not incorporate support structures. Carnegie Mellon and Stanford universities are developing an addition/removal process, Shape Deposition Manufacturing (SDM), which does incorporate support structures. In SDM, a CAD model is first sliced into 3D layered structures (i.e., the outer surface of each layer maintains the 3D geometry of the original model). Layer segments are then deposited as near-net shapes and then machined to net shape before additional material is deposited (Fig. 2.15). The sequence for depositing and shaping the primary and support materials is dependent upon the local geometry; the idea is to decompose shapes into layer segments such that undercut features need not be machined, but are formed by previously shaped segments.


Fig. 2.15. Shape deposition manufacturing.

SDM can use alternative deposition sources. For one example, microcasting is a nontransferred welding process that deposits discrete, super-heated molten metal droplets in order to build up fully dense, metallurgically bonded structures. For example, stainless steel may be deposited as the primary material and copper as the sacrificial material. Other types of deposition processes being investigated include laser welding, extrusion, and 2-part epoxy mixtures.

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Published: March 1997; WTEC Hyper-Librarian