Working by layers 

Various auxiliary lines will be needed in order to draw the part. Since these auxiliary lines must not appear in the final drawing, they will be in a different layer from the one used for the finished model.

Create the layers called "part1", "union" and delete the layer "Layer0"
Choose "part1" as the activated 'layer to use' with a <Double-click>. From now on, all the entities created will belong to this layer.