A new model to calculate friction coefficients and shear stresses in thermal drilling

A new analytical model for thermal drilling (also known as friction drilling) has been developed. The model distinguishes itself from recent work of other investigators by improving on two aspects: (1) the new model defines material plastic flow in terms of the yield in shear rather than the yield in compression, and (2) it uses a single, variable friction coefficient instead of assuming two unrelated friction coefficients in fixed values. The time dependence of the shear stress and friction coefficient at the hole walls, which cannot be measured directly in thermal drilling, can be calculated using this model from experimentally measured values of the instantaneous thrust force and torque. Good matches between the calculated shear strengths and the handbook values for thermally drilling low carbon steel confirm the model's validity.