A large number of RP machines fall into the classification, all axes mechanical (Fig. 9.10).
There is no predominant technology in use to effect the mechanical motions in RP machines. A wide variety of the technologies in general use for positioning in industry are in evidence in RP machines, with a focus on electrical actuation (as opposed to hydraulic or pneumatic actuation). Often a machine uses different approaches for its different axes. For example, the recoater in a laser photolithography machine might be cable-driven from a capstan, while the vertical axis is driven by a ball screw. In most cases, control is effected using either a stepper motor or a servo motor with a rotary encoder on the motor, but some cases also exist of linear encoding on a slide. In general, research organizations tend to buy integrated motion assemblies for their RP machines, but commercial organizations often buy the machine elements and do custom integration. For some vendors, this approach is dictated by cost; for others it is dictated by performance considerations.
An important subset of the all-axes-mechanical RP machines have the feature in common that they deliver material. The device used to deliver material is generally a critical part of these technologies and often (perhaps always) embodies proprietary developments by the machine builder.
The class of machines that construct from sheet material is a subset of the all-axes-mechanical type. While it is possible to have a sheet machine that images optically, the nature and speed of the devices used to cut the outline of the sheet suggests that an all-mechanical approach is probably more suitable. Most of the cutting devices in use or under consideration are either fairly massive or have contact force interaction with the sheet. The exception is laser cutting, but in this case, cutting speeds are slow compared to imaging in a stereolithography vat, and mechanical actuation has some advantages in this regime of operation.
Table 9.3 summarizes the interesting contrast that may be drawn between two different approaches to building with paper as the feedstock, as practiced in the Helisys LOM process and in Kira's SAHP (selective adhesive and hot press) process, which represents a highlight in new machine configurations in Japan.
LOM can make quite large parts from a roll of paper as feedstock. In LOM the adhesive is preapplied to the roll and a new layer is added to the stack by hot roll lamination. A laser is used to cut the sheet. The use of the laser restricts this process to a lab or shop floor. The strength of the SAHP machine is that it assembles commercially available imaging tools in an office environment. The machine works from cut-sheet paper and selectively applies the adhesive with a laser printer. After hot press lamination, the sheet is cut with a knife plotter using a carbide knife. The chief weakness of the SAHP machine is that it is limited in size by the need to purchase a commercial laser printer, and Kira designers state that they have no intention of going beyond the size of their current machine. Minor limitations include the need to periodically replace the knife, and the fact that the paper used is sensitive to moisture.
Both LOM and SAHP make a cut that is nominally perpendicular to the sheet material. Another point of departure is to cut the sheet with beveled edges so as to reduce the stair-stepping effect. Work at Case Western Reserve in the United States is exploring 4-axis cutting with a laser. Most of this work has been done with tape-cast ceramic sheet (sheet made of particles held together by a polymer binder). In this case, the sheet is cut and then stacked; it would be difficult to imagine how bevel-cutting could be accomplished by stacking and then cutting, since the cut would have to terminate immediately after the first layer, and the long cut involved in a bevel cut would be difficult to control with the necessary accuracy. Work that started at MIT and continues at Rensselaer Polytechnic Institute is exploring alternate methods of 4-axis cutting of metal sheet, including laser, water jet, and machining techniques.
Professor Nakagawa at the University of Tokyo intends to build a metal lamination machine for large tooling. On the question of 2-axis versus 4-axis cutting he acknowledged the issues associated with cutting sheet with very small bevel angles; however, he did not indicate which approach he is pursuing.