Machining thin-walled parts is generally cumbersome due to their low structural rigidity. Thus, to better understand the dynamic behavior of thin-walled parts during machining, various engineers and researchers in the field of metal cutting employ the Finite Element Method (FEM) due to its ability to highlight the physics involved in chip formation and the range of force generated in the cutting zone. The results of numerical simulations are evaluated using comparison with experimental data. In this paper, we study the effect of feed rate as well as the thickness of the wall part made of 42CrMo4 steel alloy on the cutting forces and workpiece displacements both experimentally and numerically during roughing and finishing turning process. The numerical study is based on the development of a three-dimensional (3D) Finite Element Model (FEM) in Abaqus/Explicit frame. In the model, the workpiece material is governed by a behavior law of Johnson-Cook. The detachment of the chip is simulated by a ductile fracture law also of Johnson-Cook. Numerical and experimental results show that the cutting forces and the quality of the machined surface depend not only on the choice of cutting parameters but also on the dynamic behavior of thin-walled parts due to their low rigidity and low structural damping during of the machining operation. Indeed, cutting forces are proportional to the feed rate and inversely proportional to the thickness of the part. The largest displacements recorded on the part are mainly along the direction of the tangential component of the cutting force. The flexibility of the part generates instability in the cutting process, but the frequencies of the vibrations are higher than the frequency of rotation of the part.