When people cut a very little depth of cut, as occurs in grinding, cutting force is not the major force. In grinding, given the high friction between the tool and the cutting material, the pincipal mechanical action is the abrasion, not plastic deformation. For this reason, radial force is greater than cutting force. Although you mean micromilling, not grinding, but both processes have a common point: the size effect.

When people cut a very little depth of cut, as occurs in grinding, cutting force is not the major force. In grinding, given the high friction between the tool and the cutting material, the pincipal mechanical action is the abrasion, not plastic deformation. For this reason, radial force is greater than cutting force. Although you mean micromilling, not grinding, but both processes have a common point: the size effect.

Size effect is predominant in micro machining. As we keep on decreasing depth of cut, we are reducing the number of defects that the cutting tool comes in contact with. Hence, specific cutting energy increases drastically. As rightly described by @Daniel, removal of material by grinding is less (by cutting force), but the non-cutting forces (viz ploughing forces, heat energy etc.) can be seen as sparks.

If we take the micro machining as in grinding operation and the same is discussed here by Mr Daniel also, reason for higher thrust force is due to the cutting geometry of abrasive particles in Grinding Wheel,where cutting elements are irregular (i.e.cutting points are at random), whereas in most of other processes it is regular geometry, governed by particular tool signature and perform proper shearing action.

Dear Hoshiyar Singh sir,

Thank you for the feedback. We can use same cutting tool geometry for micro-machining as that for conventional machining. Yes, as Dr. Daniel rightly pointed out, the cutting action in micro-machining is an abrasion action rather than a plastic deformation.