Glossary
Rib — definition
A thin stiffener that adds rigidity without adding thickness — the key technique for making a casting light and strong at once.
A rib moves material away from the neutral axis, where it does work, instead of placing it in the middle of the section where it only adds weight. The effect is the same as in an I-beam.
Ribs themselves have to be thinner than the wall they sit on, or they create a local accumulation of metal where they meet it. A common industry rule is to make the rib clearly thinner than the main wall and to give it draft and a radius at the root.
In sheet metal every stiffener costs an operation. In casting the rib comes with the shot. That is one of the main reasons a cast bracket can be made lighter than a welded one.