1. A computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause the processor to perform a method comprising: obtaining a data file that includes print instructions for causing a three dimensional printer to print a three dimensional object, wherein the data file describes external surfaces of the object; determining, for a given voxel size, a voxel structure having a size and a shape to be subsumed within the external surfaces of the object; outputting user instructions for manually assembling the voxel structure and positioning the voxel structure into the three dimensional printer; and printing material onto the assembled voxel structure to form an object that subsumes the voxel structure.