  Hybrid manufacturing (HM) technologies combine additive and subtractive
manufacturing (AM/SM) capabilities in multi-modal process plans that leverage
the strengths of each. Despite the growing interest in HM technologies,
software tools for process planning have not caught up with advances in
hardware and typically impose restrictions that limit the design and
manufacturing engineers' ability to systematically explore the full design and
process planning spaces. We present a general framework for identifying AM/SM
actions that make up an HM process plan based on accessibility and support
requirements, using morphological operations that allow for arbitrary part and
tool geometries to be considered. To take advantage of multi-modality, we
define the actions to allow for temporary excessive material deposition or
removal, with an understanding that subsequent actions can correct for them,
unlike the case in unimodal (AM-only or SM-only) process plans that are
monotonic. We use this framework to generate a combinatorial space of valid,
potentially non-monotonic, process plans for a given part of arbitrary shape, a
collection of AM/SM tools of arbitrary shapes, and a set of relative rotations
(fixed for each action) between them, representing build/fixturing directions
on $3-$axis machines. Finally, we use define a simple objective function
quantifying the cost of materials and operating time in terms of
deposition/removal volumes and use a search algorithm to explore the
exponentially large space of valid process plans to find "cost-optimal"
solutions. We demonstrate the effectiveness of our method on 3D examples.
