A manipulator with four or six ‘fingers’ positioned symmetrically around a central axis might also serve as a sheer, able to snip along two or three planes. The cutting edges would be the edges of basal segments to which the fingers were mounted. Four-fingered manipulators would be mechanically simpler and more robust. Six-fingered manipulators would have sharper edges and therefore make more efficient sheers. In either case, to act as a sheer the fingers would have to reach past the object (stem) to be cut, so it could be grasped by their basal segments.
An alternative to the above would be a four fingered manipulator with basal segments angled at less than 90 degrees, possibly as little as 45 degrees, with space between them, and the ability to rotate into that space. In the case of 45 degree angled basal segments, each finger should be able to rotate 22.5 degrees in either direction from its rest position. This would result in both a manipulator with two sets of opposed digits, 45 degrees apart on either side, arranged along either of two axes, as well as a more effective sheer. [2012Oct13: This allows switching the plane of the gripper formed by the fingers 90 degrees through finger rotation alone, no wrist rotation needed.]
The basal segments to which I’ve been referring would appear, from anywhere along the central axis, like simple blocks of metal presenting a distinct edge. However, from the outside, with those edges hidden, they would look more like the links in multi-jointed digits that they would also be.
[2012Oct13: To be quite honest, I’ve never been able to imagine exactly how the basal sheer would work in practice, how the parts would connect and be activated. This aspect now seems far less useful than the ability to regroup fingers to form grippers, switching between perpendicular planes, by rotating each finger 45 degrees. For pruning, you’d want a gripper next to a sheer, with the whole assembly being rotatable, so you could hold onto the branch you were removing while cutting it loose, and then pull it free.]
Reposted from Cultibotics.