If you are building anything from FDM printed parts, more than likely, you will be using 3mm Nyloc nuts to assemble the creation. At least I do. When I design the parts to be assembled, I use a 5.8mm across the flat hex hole to capture the nuts. This provides a friction fit. Getting them in can be a pain. Getting them lined up and inserted correctly is a pain. How long it takes is a bigger pain.

So, I created this little gizmo. Easy build, cheap and it works.

I present the Nyloc Nut Smasher!

Supplies

Own or have access to a 3D FDM printer.

One of these springs: https://www.amazon.com/dp/B09N6NJR3B?psc=1&ref=ppx_yo2ov_dt_b_product_details

One of these coupling nuts: https://www.amazon.com/dp/B0BG1R1LNM?psc=1&ref=ppx_yo2ov_dt_b_product_details

72mm of this M3 All-Thread: https://www.amazon.com/dp/B092QC85RV?psc=1&ref=ppx_yo2ov_dt_b_product_details

(2) M3x14 Socket Head Cap Screws

(2) M3 Nyloc Nuts.

Step 1: 3D Printed Parts

You need (1) each of these. Print them at 50%+ infill, 3 walls and 4 top and bottom layers. PLA works fine. All 3 only need 2 hours or so to print. No supports needed, just orient them on your build plate correctly.

Step 2: Assembly

Not super complicated.

Step 1. Insert 3mm Nyloc Nuts (Can't Smash them yet, but next time...) in Part "Plunger"

Step 2. Insert 3mm Coupling nut in Part "Puller"

Step 3. Cut a 72mm length of 3mm all-thread, make sure to deburr the cut ends. One good way to make sure you do not booger up the threads is to thread a plain nut past the cut line on the part of the rod you are using. Make the cut and then thread the nut over the cut end.

Step 4. Thread the all-thread into the puller assembly. Get it so that 2 or 3mm are extending past the top.

Step 5. Place the spring over the boss provided in Part "Cap".

Step 6. Place the the Puller Assembly onto the spring and hold together

Step 7. Place part "Plunger" over the all-thread and fit all of the parts together, and hold it together as a unit.

Step 8. Install the (2) M3x14 through part "Cap" and into the M3 Nylocs in part "Plunger", Tighten.

Assembly is done!!!

Step 3: Using the Nyloc Nut Smasher

Using it is a simple affair. Put the all thread through the compression side of the 3D part needing a M3 Nut Insert, Thread on a M3 Nylock nut (until it hits the nylon part), Give it a squeeze until the face of the nut is touching the part. You'll want to turn the nut so the flats of the nut and the part align. Then apply force until it is set in the part.

It works as long as your clearance is correct. Test it out and design according to your printer's tolerances. Between 5.6mm and 5.8mm works for me. Too tight and you'll snap the Nut Smasher and too loose and the nuts will not stay put.