  This paper presents a fixturing strategy for regrasping that does not require
a physical fixture. To regrasp an object in a gripper, a robot pushes the
object against external contact/s in the environment such that the external
contact keeps the object stationary while the fingers slide over the object. We
call this manipulation technique fixtureless fixturing. Exploiting the
mechanics of pushing, we characterize a convex polyhedral set of pushes that
results in fixtureless fixturing. These pushes are robust against uncertainty
in the object inertia, grasping force, and the friction at the contacts. We
propose a sampling-based planner that uses the sets of robust pushes to rapidly
build a tree of reachable grasps. A path in this tree is a pushing strategy,
possibly involving pushes from different sides, to regrasp the object. We
demonstrate the experimental validity and robustness of the proposed
manipulation technique with different regrasp examples on a manipulation
platform. Such a fast and flexible regrasp planner facilitates versatile and
flexible automation solutions.
