Design Tip - 13

This design tip is the continuation of the Design Tip 12 about the screw bosses and their sink mark problems. The previous Design Tip was one improvement to reduce the risk to have sink mark, here you are another tip to minimize the sink marks.
To minimize the sink marks, a recess around the screw boss will help to reduce the sinking mass. 
In my designs I never pay so much attention on the shape of the recess. I did them from a round half torus (doughnut) to something closer to the sketch with small radius. All designs worked well.
Some days ago reading a documentation from Lanxess, they said the angle alpha has to be 30º and the depth of the recess 0.3t. I've never used formulas to design this kind of recesses, but I suppose the design guideline given by Lanxess is due to their experience, and it seems a good design. I think next time I will use these dimensions to see what happens. The 30º angle looks nice to guide the material flow to the thinner pass, the depth seems too much for my applications, so I will reduce to 0.5mm on a 2.5mm wall, but if a big screw is needed, then the boss walls will be thicker, so 0.3t could be a good number.