# Jointing Principles in AMC--Part 1: Design and Preparation of Dry Joints
## 6 Preparation for Joint Production  
### Subtractive Post-Processing  
Robotic subtractive processing, as the robotic concrete printing's complementary technique, has proved its potential; hence as initially noticed, fundamentally discussing the geometries of dry concrete connections is becoming an important issue that needs to be scientifically addressed. The current TRR277 subproject (C05), through all work packages (WP1-WP4), operates the CNC-process in the Digital Building Fabrication Laboratory (DBFL), see Figure 12.  
The DBFL laboratory is unique in its conception and execution. The DBFL contains two main portals: the robotic arm with 12 axes, e.g., utilized for 3D concrete printing, and the second portal, with nine axes, e.g., as used in C05 for concrete CNC. In the initial Sections 2 and 3.1, the superiority of the CNC-milling was discussed in comparison to other methods (e.g., casting and waterjet). Nonetheless, this method also has limitations in manufacturing different geometries. The limitation has two main reasons.  
* Utilization of the rotary CNC-engines leading to typical rotating milling/sawing tools, which are only suitable for round geometries.
* Low accessibility of the CNC-arm to different sides of the geometry. Several simple joint geometries need the rotating milling tools to approach from different sides, which may easily cause a collision between the tools and the concrete specimens or the rotary engine and the clamping table.  
Figure 11: Selected joint profiles for further investigations: (**a**) smooth (**b**) triangular (**c**) arc (**d**) truncated pyramid.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Joint Profile** & **Ultimate Load** & **Joint Profile Factor** \\ \hline [-] & [kN] & [-] \\ Monolithic & 396 & 1.0 \\ Smooth & 396 & 1.0 \\ Triangular & 361 & 0.91 \\ Saw tooth & 387 & 0.97 \\ Sinusoidal & 365 & 0.92 \\ Arc & 389 & 0.98 \\ Truncated Pyramid & 380 & 0.96 \\ \hline \hline \end{tabular}
\end{table}
Table 10: Load-bearing capacity within the FE analysis regarding different joint profiles.  
The initial CNC-simulations can measure these difficulties and most of the criteria mentioned in the catalogue. The CNC-simulation is mainly essential for producing the G-codes to be sent to DBFL's central computers. The G-codes are made by EasyStone [33] (CNC-software) in these work packages. EasyStone, in addition to the dimensions and permanent details of DBFL's features, includes the type and dimensions of the available milling/sawing tools in the tool magazine that are to be applied in the G-codes and can change the tool during the CNC-process. In addition to preparing the G-codes, these simulations can be used to score some of the catalogue's criteria, mainly related to the manufacturing difficulties such as estimating the milling duration, etc.  
The CNC-technique has been utilized for around two centuries; nonetheless, robotic manufacturing dry concrete connections is a new developing technique, and the challenges before the industrial usages should be faced. The main challenges, in addition to the limited producible geometries, are:  
* High duration of the CNC-process;
* Dependency on the dissimilar properties of concrete, which may easily cause unwanted damage due to the low quality and high brittleness of this material;
* Fast ageing or abrasion of the milling tools, which in addition to increasing the CNC-costs, influences the accuracy of the geometries and difficulties in the fitting.