Milling a cement-based 3D printable mortar in its green state using a ball-nosed cutter

A key benefit of 3D concrete printing is the removal of the need for a mould enabling one-off and low volume production with greater variability in part geometry. The staircase effect generated by the layering is used as a feature of many applications, however for smooth surfaces and at interfaces between components, this may not always be desirable. In addition, global deformation of the part during printing may influence the accuracy of the final element. In order to compete with cast equivalents in this regard, secondary treatments such as subtractive milling and trowelling can be applied post printing. These are relatively new developments in the field of digital fabrication with concrete and there is little published work on methods and in particular, none that examine the effect of milling on the surface condition of 3D printing mortar. This paper explores these issues based on a manufacturing tooling perspective and finds that high quality surfaces can be formed, the roughness of which is affected by the size of the aggregates in the mix and the set-state of the material, which needs to be close to the solid state. In practice, this results in a ‘milling window’ which has implications for process timing and the robotic systems typically deployed for 3D concrete printing applications.