An apparatus for bottom-up fabrication of three dimensional objects, the apparatus comprising: a vat for a photosensitive polymer, a floor of the vat including a first floor portion of a first thickness and a second floor portion of a second thickness, the second thickness being less than the first thickness, the first floor portion including a working surface arranged such that, in use, light incident on the working surface interacts with the photosensitive polymer at the working surface to fabricate a portion of the three dimensional object; a build platform capable of being inserted into the vat, the build platform having a planar surface; an elevator mechanism capable of adjusting the separation between the working surface of the vat and the planar surface of the build platform; and a rotation mechanism capable of varying the relative rotational position of the vat relative to the build platform from a first position to a second position, the relative rotation being about an axis which is normal to the working surface of the vat, wherein, in use, at the first position, the working surface is located under the build platform, such that a layer of uncured photosensitive polymer exists between the working surface and the portion of the three dimensional object that is fabricated at the working surface in the first position, and at the second position, the second floor portion is positioned below the build platform, such that there is increased depth of the uncured photosensitive polymer between the portion of the three dimensional object and the second floor portion of the vat. a vat for a photosensitive polymer, a floor of the vat including a first floor portion of a first thickness and a second floor portion of a second thickness, the second thickness being less than the first thickness, the first floor portion including a working surface arranged such that, in use, light incident on the working surface interacts with the photosensitive polymer at the working surface to fabricate a portion of the three dimensional object; a build platform capable of being inserted into the vat, the build platform having a planar surface; an elevator mechanism capable of adjusting the separation between the working surface of the vat and the planar surface of the build platform; and a rotation mechanism capable of varying the relative rotational position of the vat relative to the build platform from a first position to a second position, the relative rotation being about an axis which is normal to the working surface of the vat, wherein, in use, at the first position, the working surface is located under the build platform, such that a layer of uncured photosensitive polymer exists between the working surface and the portion of the three dimensional object that is fabricated at the working surface in the first position, and at the second position, the second floor portion is positioned below the build platform, such that there is increased depth of the uncured photosensitive polymer between the portion of the three dimensional object and the second floor portion of the vat. at the first position, the working surface is located under the build platform, such that a layer of uncured photosensitive polymer exists between the working surface and the portion of the three dimensional object that is fabricated at the working surface in the first position, and at the second position, the second floor portion is positioned below the build platform, such that there is increased depth of the uncured photosensitive polymer between the portion of the three dimensional object and the second floor portion of the vat.