Layered Manufacturing allows physical prototypes of 3D parts to be built directly from their computer models, as a stack of 2D layers. This article proposes to decompose the model to be built into minimum number of layers and then build each layer separately by performing machining operations on a given sheet of metal, using a 3-axis CNC milling machine and adhering the layers, allowing large models to be built in parallel. The decomposition of the model is carried out in such a fashion that it decomposes all the inaccessible regions in the object such as under-cut concave edges. For a given build up direction, the undercut edges, which cause a part to be inaccessible by a tool, are extracted and eliminated in two decomposition steps ensuring that the number of layers are as least as possible, thus minimizing the build time.