A Machining Allowance is necessary if the optimisation result is manufactured with Additive Manufacturing processes. After printing the part all functional surfaces are post processed and therefore enough material needs to be available. So the optimisation actually needs to produce different outputs for different requirements of the model - once a norm geometry and once a printing geometry.
Picture Norm Geometry vs MA Geometry
The optimisation is carried out and calculated without taking the Machining Allowance into consideration. It is added after the calculation process is done but before any smoothing algorithm has run over the geometry to guarantee a design without any transition problems between these different geometries.
The Machining Allowance needs to be added to all functional surfaces. The picture below shows an example of the GD-Bracket.
GD Bracket Model Browser with MA
Touching faces should always be selected in one Machining Allowance because only then the Machining Allowance is one connected volume instead of just several offsets of the faces. The picture below shows the difference of the selections:
One MA vs. several for connecting faces.
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