A method for producing three-dimensional objects layer by layer using a powdery material which can be solidified by irradiating it with at least two electron beams, said method comprises a pre-heating step, wherein the pre-heating step comprises the sub-step of scanning a pre-heating powder layer area (100) by scanning a first electron beam in a first region (I) and by scanning a second electron beam in a second region (II) distributed over the pre-heating powder layer area (100), wherein consecutively scanned paths are separated by, at least, a security distance (ΔY), said sub-step further comprising the step of synchronising the preheating of said first and second electron beams when simultaneously preheating said powder material within said first and second regions respectively, so that said first and second electron beams are always separated to each other with at least a minimum security distance (ΔX).