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.
Step 1: Tools Required
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.
Step 2: Create the Insert
You need to create a method for holding the insert and forcing it into position. Because I had a long fastener, I chose to thread two nuts onto the rod and use thread sealer to hold them with the flats aligned.
Step 3: Clamp the Part
Because you are dealing with hot parts it is necessary for everything to be fixed in place while heating and inserting the metal parts. I used the two different styles of clamps, because I did not have a bench vise.
Step 4: Heat the Insert
3D printers use filament material that is thermoforming. That means that a printed part can be reheated and reformed. Because of this you need to be very careful that you do not actually heat the printed part, or it will distort. By heating only the metal part to be inserted, it melts the plastic as it is being inserted. This causes the plastic to reform it and bond to the metal part. This creates a very strong fastener.
The metal part does not need to be heated for very long. With a 1500 watt gun, it only took about 15 seconds to get the nuts hot enough to reform the plastic.
Step 5: Form the Plastic (Pull the Insert Into Place)
Immediately after heating the metal part, pull or push it to insert it into position. Use constant slow movements to get it in its final position. Plan your movement, so you don't go too far. It is difficult to reform the material if you push to far, and in some cases, it may be impossible to correct a mistake.
Step 6: Clean Up Excess Material and Finish the Part
After the plastic has cooled you will need to remove displaced plastic. I used a hand file for this step to get good control. If you use power tools, be sure not to heat up the plastic or your inserted bolt will probably move.