A method for increasing the build area when forming first and second three-dimensional articles (330a, 330b) through successive fusion of parts of a powder bed, which parts correspond to successive cross sections of the first and second three-dimensional articles (330a, 330b), the method comprising the steps of: providing a model of each of the first and second three-dimensional articles (330a, 330b); applying a first powder layer on a first build platform (314, 314a, 314b, 410, 420, 430, 440) and a second build platform (314, 314a, 314b, 410, 420, 430, 440); directing at least one deflectable electron beam (351, 352) from at least one electron beam source arranged in a first position over the first build platform (314, 314a, 314b, 410, 420, 430, 440), the directing of the at least one electron beam (351, 352) causing the first powder layer to fuse in first selected locations according to the model of the first three-dimensional article, so as to form a first part of a first cross section of the first three-dimensional article; at least one of rotating or tilting the at least one electron beam source a predetermined angle to move the at least one electron beam source from the first position to a second position over the second build platform (314, 314a, 314b, 410, 420, 430, 440), wherein, in the first position, the at least one deflectable electron beam (351,352) is deflectable between extreme positions (351a, 351b) defining a first area corresponding to the first build platform (314, 314a, 314b, 410, 420, 430, 440) and, in the second position, the at least one deflectable electron beam is deflectable between extreme positions (352a, 352b) defining a second area corresponding to the second build platform (314, 314a, 314b, 410, 420, 430, 440); and directing the at least one deflectable electron beam (351, 352) from the at least one electron beam source in the second position over the second build platform (314, 314a, 314b, 410, 420, 430, 440), the directing of the at least one electron beam (351, 352) causing the first powder layer to fuse in second selected locations on the second build platform (314, 314a, 314b, 410, 420, 430, 440) according to the model of the second three-dimensional article, so as to form a first part of a first cross section of the second three-dimensional article. providing a model of each of the first and second three-dimensional articles (330a, 330b); applying a first powder layer on a first build platform (314, 314a, 314b, 410, 420, 430, 440) and a second build platform (314, 314a, 314b, 410, 420, 430, 440); directing at least one deflectable electron beam (351, 352) from at least one electron beam source arranged in a first position over the first build platform (314, 314a, 314b, 410, 420, 430, 440), the directing of the at least one electron beam (351, 352) causing the first powder layer to fuse in first selected locations according to the model of the first three-dimensional article, so as to form a first part of a first cross section of the first three-dimensional article; at least one of rotating or tilting the at least one electron beam source a predetermined angle to move the at least one electron beam source from the first position to a second position over the second build platform (314, 314a, 314b, 410, 420, 430, 440), wherein, in the first position, the at least one deflectable electron beam (351,352) is deflectable between extreme positions (351a, 351b) defining a first area corresponding to the first build platform (314, 314a, 314b, 410, 420, 430, 440) and, in the second position, the at least one deflectable electron beam is deflectable between extreme positions (352a, 352b) defining a second area corresponding to the second build platform (314, 314a, 314b, 410, 420, 430, 440); and directing the at least one deflectable electron beam (351, 352) from the at least one electron beam source in the second position over the second build platform (314, 314a, 314b, 410, 420, 430, 440), the directing of the at least one electron beam (351, 352) causing the first powder layer to fuse in second selected locations on the second build platform (314, 314a, 314b, 410, 420, 430, 440) according to the model of the second three-dimensional article, so as to form a first part of a first cross section of the second three-dimensional article.