These are Drop-in T-Nuts. In this build we use 2020 and 3030 Drop-in T-Nuts. They fit inside the slots of the V-Slot 2020 and the T-Slot 3030 Aluminum profiles (respectively).

When a bolt is screwed through a plate/connector and into a Drop-in T-Nut, the Nut rotates inside the slot and rises, until it eventually grabs the inside of the profile, in a position perpendicular to the slot. In this position, the bolt is securely tightened.

This picture shows an insecurely tightened bolt. As you can see, the Drop-in T-Nut is fully fastened but in a position that is NOT perpendicular to the slot. If this happens, loosen the bolt and re-tighten.

To make the assembly process easier, this guide will often ask you to mount Drop-in T-Nuts on screws before mounting them on a profile slot. When you do this, ensure you don't over-tighten the screws into the T-Nuts, otherwise they won't have room to rotate inside the slot, creating the problem described above.