15x15 mm junction 3 axis for square round tubes M5

15x15 mm system for square round tubes from DIY store
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Inside Dimension = 15.5 * 15.5 mm
Nut Size M5
Nut internally

Easily printable, very stable
0.8 mm shell and 25% filling enough

For aluminum hollow section profile is the weak point.
Tighten the screw only slightly.