Ion beam sputtering under proper conditions can lead to the formation of regular nanopatterns on large scale areas. There has been significant effort put into understanding of the underlying physics for the nanopatterning of semiconductors. However, many of the fundamental questions concerning the pattern formation on metallic surfaces are still open. We performed a scanning probe microscopy study of the evolution of the surface morphology of Ni (001) upon low energy Ar⁺ ion beam bombardment at room temperature. In agreement with our expectations we only observe formation of ripples parallel to the ion beam direction at angles of incidence larger than 60° from the surface normal. During the initial stages of sputtering, we observe the formation of adatom clusters on the surface with a lateral size of about 8 nm. These mounds subsequently grow into ripples. The initial wavelength of the ripples coincides with the mound lateral size and further increases as a linear function of fluence. We propose a model for the pattern formation based on diffusion, local angle-dependent sputtering and noise induced by the ion beam bombardment. As suggested by the results of computer simulations based on our model, the adatom clusters act as a template for the ripple growth and the subsequent wavelength coarsening is a consequence of the kinetic roughening.