Three dimensional rapid prototyping processes, also called layered manufact
uring or solid freeform fabrication (SFF) promise designers the ability to
automatically fabricate complex shapes. SFF processes were invented with th
e assumption that designers would submit complete part models for automated
planning and manufacturing. This planning process is normally based on som
e form of "decomposition, " for example, slicing into layers. Especially fo
r newer, more complex SFF processes, there are several disadvantages to thi
s approach, primarily that decomposition is difficult and does not reliably
produce good process plans. Furthermore, it is hard for the designer to ge
t feedback on the manufacturability of his design, and today's decompositio
n systems are not fully automated. This paper presents an alternative appro
ach, "design by composition," where users build designs from ''primitives''
that include high-level manufacturing plans. When the user combines two pr
imitives with a Boolean operation, software will automatically generate a m
anufacturing plan for the new design from the plans for the source primitiv
es. In contrast to the decomposition method, design by composition offers s
everal benefits to designers, primarily access to manufacturability feedbac
k during design-time, a greater degree of automation, the ability to create
designs with embedded components (such as sensors, electronic circuits, be
arings, and shafts), and enhanced control over manufacturing plans. These a
dvantages make design by composition a more attractive approach to SFF proc
essing, especially for designers who are new to these processes. [S1050-047
2(00)01701-3].