Many times when you print a downloaded part, the fastener holes or hex recesses are too tight. Other times, the type or size fastener that the design requires is just not available, which happens a lot here on Maui. The Item pictured here is a spindle bracket to replace the motor on a ShapeOko CNC router. Since the fasteners on the original machine were metric fasteners, the new bracket was designed the same. The problem was that the 3" long clamp bolts were not available at all. The through holes were of the proper size to be able to accept a length of 10-24 TPI all thread. That meant I could substitute a hand-made bolt assembly for the designed fasteners, but the recess for the nuts on the back side were too small to accept the 10-24 nuts. My solution was to double nut the threaded shaft and use heat to insert the nuts into the recess forming an insert that had a threaded shaft attached.
You will need to hold the piece you are modifying while you heat the 3D-printed part and press or pull the fastener into place. I used a combination of clamps so that my long threaded rods would extend below the table.That allowed me to pull them. I used a standard hobby heat gun to heat the nuts. The heated nuts would then melt the plastic as they were being pulled. For final clean up, I used a file to remove the displaced plastic from the surface. It is also a good idea to wear safety glasses, and have heat proof gloves or a pair of vise grips to hold and pull the fastener.