Approximating friction cones and grasp wrench

[qwerty_1213]

Hi I'm trying to implement a grasp quality metric described by "Examples of 3D Grasp Quality Computations" by Miller and Allen. I grab an object and know the contact points and contact planes. I'm having difficulty understanding how to construct the wrench matrix at each contact point without knowing what the actual forces are at the contact points. Do you just assume they are unit vectors? Am i understanding correctly that you take the contact normal, determine some vectors on the friction cone around that normal, and use those to construct a wrench matrix such that for every contact point you would have a n by m wrench matrix where n is 6 (3 for force, 3 for torque) and m is the number of friction vectors you considered? Do i have to scale the friction cone vectors in any way?

Any help would be appreciated. P.s. I'm trying to construct the convex hull using the wrenches