I am often responsible for rapid prototyping parts on the job as a mechanical engineer. Regularly my designs require the use of captured nuts to joint parts together. It is possible to add a pause in your 3D print and embed nuts, but often my prints last days and run when I am not around to place nuts in the print.
In this Instuctable I will show you my technique for creating captured nuts of any size that can be placed and strongly retained after the print is complete. I have used many sizes of these in fixtures that have been through 10,000 cycles of fatigue testing at 800g's without failure or loosening. While this isn't a revolutionary idea, I have scientific testing behind the sizing of these captured nuts. I hope I can save you time, teach you a CAD trick, or inspire you to come up with a better way to hold onto your nuts!
Your 3D printer must be well calibrated dimensionally or you will need to adjust my numbers to suit your printer.
Two styles of captures
I will show you my two different styles for capturing the nuts. I call the first style "pocket style" as a hexagonal pocket is used to hold the nut. The pocket style is shown as the left part in the intro picture. As you will see this style captures the nut using detents (little bumps) in the corners of the pocket. This style works great in tension (pulling parts together) and is easiest to install the nuts. A downside is that you must have access on your part to the outer face of the pocket.
The second type is "slot style" as the right part in the intro picture illustrates. The nut in this design is retained by one detent resembling a speed bump. This style works in both tension and compression (pulling parts together and pushing them apart) and is great for hard to reach locations. You can make the slot as long as you need to reach your bolt hole location. The downside is it is a little tougher to install the nut.
To begin you must measure the width and thickness of your nut. Also measure the diameter of your bolt. Write these values down. As previously stated I am using an M6 bolt with M6 nylock nut. The nut measures 0.389 in (9.88 mm) wide by 0.2275 in (5.78 mm) thick. The diameter of the bolt is 0.231 in (5.87 mm).
For slot style captured nuts you will also need to measure the point to point maximum width of the nut as shown in the 4th picture. My M6 nut is 0.4375 in (11.11 mm) wide from point to point.