Advanced process planning for subtractive rapid prototyping

Abstract

This paper presents process planning methods for Subtractive Rapid Prototyping, which deals with multiple setup operations and the related issues of stock material management. Subtractive Rapid Prototyping (SRP) borrows from additive rapid prototyping technologies by using 2½D layer based toolpath processing; however, it is limited by tool accessibility. To counter the accessibility problem, SRP systems (such as desktop milling machines) employ a rotary fourth axis to provide more complete surface coverage. However, layer-based removal processing from different rotary positions can be inefficient due to double-coverage of certain volumes. This paper presents a method that employs STL models of the in-process stock material generated from slices of the part along the rotation axis. The developed algorithms intend to improve the efficiency and reliability of these multiple layer-based removal steps for rapid manufacturing. IntroductionSubtractive Rapid Prototyping (SRP) is a considerably lesser known and utilized form of rapid prototyping technology, mainly due to continued challenges in the pre-process engineering and setup planning required. Subtractive operations in general afford excellent accuracy and repeatability, which is why it is more often utilized as an additional function of a hybrid-type rapid prototyping system that includes both additive and subtractive operations. Since the early days of RP technology, with systems such as Laminated Object Manufacturing (LOM), subtractive means have been employed as part of a solution alongside the conventional additive layer-based approaches. Although perhaps not purely subtractive, in LOM, a laser was used to cut the profile and surrounding support structures of each layer, after the layer of paper is added. In this manner, the additive approach enabled improved geometric capability while the laser cutting offered reasonable shaping accuracy. Later, the Sanders Modelmaker system, now used heavily in jewelry and dental manufacturing, utilized a machining process to accurately mill each deposited layer to a precise thickness. In a research project, Shape Deposition Manufacturing (SDM) used 5-axis machining in conjunction with a variety of deposition approaches to create both parts and molds. More recently, systems such as LAMP and Ultrasonic Consolidation (UC) continue to use subtractive means in an iterative manner to improve accuracy and surface finish. Desktop milling machines suitable for rapid prototyping have been marketed for lower-end materials in single or multisided machining operations, while lower-end, but more user-friendly SRP software systems such as Millit and Deskproto have attempted to facilitate the NC programming for these applications. To date, subtractive processes continue to be utilized in a few RP systems, while SRP-only systems have had limited success. The overwhelming success of RP technologies to date can be attributed almost solely to the additive-only machines; however, there are niche applications where an SRP system would b...

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