Speckle method evaluation of the roughness of electrodischarge machined surfaces

This paper describes the applicability of the speckle method to evaluate th e roughness of surfaces produced by non-traditional machining processes suc h as EDM, ECM or USM, where material removal occurs randomly. The method is based on the digital correlation of two speckle images produced by interfe rence phenomena arising when a coherent light beam is incident on a rough s urface. The basic principle of the speckle pattern correlation is presented and the theoretical analysis based on a hypothesis concerning the morpholo gy of the machined surface is reported. An experiment has been set-up in or der to assess the feasibility of applying the speckle method to evaluate th e roughness of machined surfaces. In the experimental tests, two speckle pa tterns produced from the same rough surface under two different illuminatio n conditions have been correlated. The two different conditions have been o btained by varying the angle of incidence of coherent light on the surface being analysed. The roughness of electrodischarge machined surfaces, as mea sured by the speckle method, is found to be in good agreement with that of a stylus instrument.