  This article presents a new hand architecture with three under-actuated
fingers. Each finger performs spatial movements to achieve more complex and
varied grasping than the existing planar-movement fingers. The purpose of this
hand is to grasp complex-shaped workpieces as they leave the machining centres.
Among the taxonomy of grips, cylindrical and spherical grips are often used to
grasp heavy objects. A combination of these two modes makes it possible to
capture most of the workpieces machined with 5-axis machines. However, the
change in grasping mode requires the fingers to reconfigure themselves to
perform spatial movements. This solution requires the addition of two or three
actuators to change the position of the fingers and requires sensors to
recognize the shape of the workpiece and determine the type of grasp to be
used. This article proposes to extend the notion of under-actuated fingers to
spatial movements. After a presentation of the kinematics of the fingers, the
problem of stability is discussed as well as the transmission of forces in this
mechanism. The complete approach for calculating the stability conditions is
presented from the study of Jacobian force transmission matrices. CAD
representations of the hand and its behavior in spherical and cylindrical grips
are presented.
