1、 附录 A 英文原文 Process Planning and Automation for Additive-Subtractive Solid Freeform Fabrication The demand in industry for fast, accurate renditions of designs is not new, and a whole community of specialized model makers and craftsmen has traditionally catered to this demand. This community has adop
2、ted new technology, like CNC machining, as it has become available. Nevertheless, the process of creating a model or a prototype of a design remained labor- and skill- intensive until the set of processes known collectively as Solid Free form Fabrication became feasible. The processes currently used
3、 in the SFF industry are purely additive, where material is progressively added to the part being built in the final position and shape. Newer processes coming out of the research laboratories are using engineering materials (hard metals, ceramics), and are combining addition and subtraction of mate
4、rial as a way to shape more precisely the part. A comprehensive review of the available processes can be found in Prinz, Atwood et al. 1997. Additive/Subtractive processes improve on purely additive ones in the range of materials they handle and the accuracy they provide. They are also proving to ac
5、cept more sophisticated design with multiple and graded materials in a single part Weiss, Merz et al. 1997, as well as integrating whole assemblies in one single fabrication unit. The downside to all theseimprovements is that additive/subtractive processes require a substantially more sophisticated
6、process planning and part execution control. This increased difficulty is the result of the use of CNC machining or similar material removal processes and the need to coordinate several different unit processes. The goal of this paper is to present a planning and execution framework for additive/sub
7、tractive processes, outline the issues involved in developing such an environment, and report on the progress made in this direction at the Rapid Prototyping Laboratory at Stanford University. We take the SDM process Merz, Prinz et al. 1994 developed at Stanford as the case study to apply the concep
8、ts developed in planning and execution for this class of additive/subtractive SFF processes. The first step towards automated manufacturing is to establish efficient communication between design clients and manufacturing centers. A design client can be equipped with regular CAD packages or with spec
9、ialized design software Binnard and Cutkosky 1998 where process- specific knowledge is embedded to facilitate down-stream planning tasks. On the other hand, manufacturing centers should provide manufacturability analyzers, automated process planning software and on-line execution systems. The manufa
10、cturability analyzers, for example, examine tolerance requirement of a design and verify it with their facility and process capabilities. The process planner generates sequences of process plans and associated operations and machine codes for building given parts. Execution systems read in several a
11、lternate process plans (possibly for many different parts), and determine subsequent operations and machines based on on-line job-shop configurations. Communication between designers and manufacturers can be accomplished by Internet-based process brokers Tan, Pinilla et al. 1998. These brokers recei
12、ve designs and check with available manufacturing centers for accessing turn-around time, material availability, facility capability, and dimensional accuracy. They then select manufacturers that best fit designers requirements. Figure 1 shows a framework architecture that includes the concepts outl
13、ined here. In the following sections, we will only address issues related to process planning and execution for additive/subtractive SFF processes.The first requirement for a realistic planning and execution system for any manufacturing system is to be able to interface existing CAD systems. The sup
14、plied solid models must support freeform surfaces for the sake of geometrical reasoning and path planning required for additive/subtractive processes and for the required levels of accuracy. Further development of CAD systems to be able to represent multi-material parts and graded material parts is
15、an active area of research that will have substantial impact on these processes Kumar and Dutta 1997. Aug. The required functionality for a planning system can be summarized as follows:Planning for finding a building orientation Hur and Lee 1998 has to account for the fact that additive/subtractive
16、processes can deposit and shape full 3D shapes and is not limited to thin 2D layers part shape needs to be decomposed in volumes that are readily manufacturable with the process considered. Decomposition is substantially more complex to take full advantage of the non-planar capabilities planning eac
17、h of the decomposed volumes in the two phases of the process: planning the deposition of material Kao 1998, Farouki, Koenig et al. 1995 and the machining of the final shape for each surface. In additive/subtractive SFF, geometry simplification due to decomposition avoids most of the tool interferenc
18、e, and tool access problems characteristic of path planning, offering a better chance to achieve automation. SDM and other additive/subtractive processes present a substantial increase in sophistication compared with pure additive ones regarding its execution environment. The main issues that should
19、 be considered are:SDM is a multistage process: Multistage processes require or should allow multiple processing stations and transfer of parts between stations. An industrial SDM shop needs to determine scheduling of parts and operations, floor layout, assignment of jobs to machines, etc.As soon as
20、 multiple machines are considered, the manufacture of several parts will want to take advantage of parallel processing in different stations to maximize equipment utilization. Each part can be built following several alternative sequences. The execution system should be able to take advantage of thi
21、s flexibility to optimize cost and turn-around time.The execution system should coordinate activities of machines and transfer of parts, and track and balance the state of load of each machine in the shop to achieve a smooth flow. These characteristics make the process somewhat similar to VLSI manuf
22、acturing, where an array of processes work in sequence to produce a wafer. A wafer， route travels through a variable number of machines depending on its process plan, and it is very cyclic (Lithography-Etch-Implant). In a similar fashion to VLSI manufacturing, the execution system will have to cover
23、 the handling of partially built parts and intermediate buffers.Process planning takes full 3D geometric models as inputs and outputs process description that specifies contents and sequences of operations that are necessary to produce the input parts. The contents contain machine-understandable codes for driving designated machines to perform desired
