I've added a honeycomb pattern version! Because Hexagons are Bestagons. 

This is a pretty self explanatory device that we're printing, but to give you a little backstory:

The brass inserts on my Neptune 2 bed leveling knobs are wearing out, and I've been looking for an excuse to replace them anyways, the smooth rounded shape of them just feels too slippery for fine adjustment.

Thus came to me the idea to use a captive M4 locknut inside of a gear-shaped knob!

The locknut because it's steel, and less prone to wiggling itself loose over time.

The gear because not only do I love gears, but also it allows for minute adjustments with excellent grip!

You require M4 locknuts for this project.

I have made the dimensions of the inner hole such that the nut drops in easily, and has exactly the space needed, no more and no less, be it height or width.

INSTRUCTIONS:

There are several ways you can print this part and have your locknuts inserted:

 #1 : Glue.

I've included separate files for the top and bottom parts of the knob, so you could print both parts separately. This is the least technically involved method, and thus the easiest to accomplish.

 #2 : Babysit it. (manual pause)

One possibility is that when you slice the full file, you find the layer at which it switched between the hex groove and a circle, and then come to your printer around the time they swap, manually pause the print (if your printer is capable of it) and then plop in the nut before going to press resume.

This is what I initially did on my Neptune 2 because option #3 simply does not work on the normal firmware.

 #3 : Insert a pause-print script at the specific layer-height

Basically, if you're using cura, click on ‘Extensions’ then ‘Post-processing’ then  ‘Modify G-Code’.

In there, click ‘Add a script’ and and select ‘Pause at height’ from the dropdown menu.

I suggest you change the ‘Pause at’ parameter to ‘Layer’ instead, as you can easily visualize exactly which layer you will need to pause at in your slicer.

Then fill out the rest of the parameters and have fun with your print!

You'll just have to plop in the nut when the printer pauses, and then press resume! Voilà!

The only downside is that some printers, including the Neptune 2 refuse to respond to this script unless you implement some firmware changes or use octoprint.

 #4 : Z-Offset double-printing.

For those of us that want some automation in this process, but can't use the pause-print commands, this is what I've ended up doing:

I've separated the gear knob into two pieces, and sliced the bottom part.
Then I've used  ‘Extensions’, ‘Post-processing’, ‘Modify G-Code’, ‘Add a script’, ‘Search and Replace’

And then replaced M140 S0 with M140 S60, which makes it so that when my print is done, instead of setting the heated bed to 0°C, it sets it to 60°C instead (this is my PLA heated bed temperature, change it to fit yours) which allows my part to remain firmly attached while I prepare the second print.

For the second part (the top part of the print) you will need to download the ‘Z-Offset’ addon from the marketplace if you are using Cura. This allows you to enable the Z Offset parameter for your printer, which we will be using shortly.

Find out how tall your first print is (Multiply number of layers by print layer height and take into account any offset you might use in your bed leveling in your calculations)

Then change the Z Offset parameter to that value +1 layer.

Slice it (and make sure you have no build plate adhesion method selected) and then print it while the first part is still firmly attached to the buildplate.

Your Printer will start the second print off where the first one ended (if you correctly calculated the height) and voilà! Captive nut in a fully printed part.