The elbow needs small R extrusion, which means that the concave radius should be smaller than the pipe radius in the design of elbow fixture.

Because when the ridge is bent, the inner wall of the center line of the pipe shrinks and the outer side is stretched, so that the extruded metal on the inner side will shift outward, and the bent pipe becomes round. Therefore, when designing the hole shape of the fixture, not only should the outermost radius of the pit be slightly smaller than the radius of the pipe, but also the most concave part of the concave surface should avoid the outward displacement of the pipe meat.