Machining 102:
Those "straight cuts" are only straight until you apply force. The end of the coupler gets loaded pretty good, forcing it into a slight angle in the direction of force application, which makes a tapered face, which forces the two pieces apart.
__________________
"¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"
|