An approach to study the whirling motion of the BTA (boring trepanning association) deep hole boring system is presented by introducing the system excitation in the form of internal forces between the boring bar and the workpiece. This involves nonhomogeneous boundary conditions with homogeneous equations. The mathematical approach with the boring bar-workpiece internal cutting forces and external suppression forces will transform the problem into nonhomogeneous equations with homogenous boundary conditions. Using this approach the whirling motion of the boring bar is obtained at different points on the boring bar-workpiece assembly. External suppression forces will reduce the whirl amplitude at the same locations. Further, an experimental investigation is carried out on the BTA deep hole boring process while drilling at four cutting speeds using proximity pickups for displacement measurements.