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 (351, 352) emanating from at least two electron beam sources (301, 302), said method comprises the steps of: pre-heating the powdery material in a homogeneous manner to elevate the powdery material to a predetermined temperature interval ΔT, which temperature interval ΔT is below the melting temperature of the powdery material, and fusing together the powdery material, wherein the pre-heating step comprises sub-steps of: scanning a pre-heating powder layer area by scanning a first electron beam in a first region along paths (11.1 - 15.20) and by scanning a second electron beam in a second region along paths (II1.1 - 115.20) distributed over the pre-heating powder layer area, wherein consecutively scanned paths (IM.N, I(M+1).N) and (IIM.N, II(M+1).N) are separated by, at least, a security distance (ΔY), said security distance (ΔY) being adapted to prevent a pre-heated powder in said first and second regions (I, II) respectively to reach a maximum charge density, above which discharge will occur, from said consecutively scanned paths, and synchronising the preheating of said first and second electron beams (351, 352) when simultaneously preheating said powdery material within said first and second regions (I, II) respectively, so that said first and second electron beams (351, 352) are always separated from each other by at least a minimum security distance (Δ, ΔX), said minimum security distance (Δ, ΔX) being adapted to prevent a sum of a charge distribution in the powdery material from reaching a critical point at which powder particles start to repel each other. pre-heating the powdery material in a homogeneous manner to elevate the powdery material to a predetermined temperature interval ΔT, which temperature interval ΔT is below the melting temperature of the powdery material, and fusing together the powdery material, wherein the pre-heating step comprises sub-steps of: scanning a pre-heating powder layer area by scanning a first electron beam in a first region along paths (11.1 - 15.20) and by scanning a second electron beam in a second region along paths (II1.1 - 115.20) distributed over the pre-heating powder layer area, wherein consecutively scanned paths (IM.N, I(M+1).N) and (IIM.N, II(M+1).N) are separated by, at least, a security distance (ΔY), said security distance (ΔY) being adapted to prevent a pre-heated powder in said first and second regions (I, II) respectively to reach a maximum charge density, above which discharge will occur, from said consecutively scanned paths, and synchronising the preheating of said first and second electron beams (351, 352) when simultaneously preheating said powdery material within said first and second regions (I, II) respectively, so that said first and second electron beams (351, 352) are always separated from each other by at least a minimum security distance (Δ, ΔX), said minimum security distance (Δ, ΔX) being adapted to prevent a sum of a charge distribution in the powdery material from reaching a critical point at which powder particles start to repel each other. scanning a pre-heating powder layer area by scanning a first electron beam in a first region along paths (11.1 - 15.20) and by scanning a second electron beam in a second region along paths (II1.1 - 115.20) distributed over the pre-heating powder layer area, wherein consecutively scanned paths (IM.N, I(M+1).N) and (IIM.N, II(M+1).N) are separated by, at least, a security distance (ΔY), said security distance (ΔY) being adapted to prevent a pre-heated powder in said first and second regions (I, II) respectively to reach a maximum charge density, above which discharge will occur, from said consecutively scanned paths, and synchronising the preheating of said first and second electron beams (351, 352) when simultaneously preheating said powdery material within said first and second regions (I, II) respectively, so that said first and second electron beams (351, 352) are always separated from each other by at least a minimum security distance (Δ, ΔX), said minimum security distance (Δ, ΔX) being adapted to prevent a sum of a charge distribution in the powdery material from reaching a critical point at which powder particles start to repel each other.