proper pem nut usage

I love what I have been able to do with PEM nuts, but the little guys did not exactly come with instructions!

In most cases I have simply been drilling a hole sized correctly for the screw, and either adding a dab of glue or simply letting the backing sit there freely. However the glue is imperfect, and sometimes I do not want the threads falling away after screw removal.

Is the intent to drill a size larger, so that the first depth wise half of the PEM nut can eat its way in? Or are the teeth simply supposed to do there job on the regular sized hole, and I should be setting them in place with more pressure? If drilling larger, is there a guide or chart to show how much larger?

Finally, will usage differ between materials, say fiberglass and aluminum?

Note that I eventually found the official guide for PEM nuts but it was focused on air ducts and concrete. I wanted to see what rocket builders do.

I use PEMs all the time, but then again, I'm a mech designer in Sheet metal. All my PEMs get pressed in with a press, with proper dies, pressures, etc.. At the back of each PEM 'line' catalog, they have installation guides, and they spec the proper holes sizes for the PEM hardware in the details..

They do have a line for fiberglass (circuit board applications) as well as plastics..

What exactly are you doing, as there might be another solution...

Thanks, much appreciated. Typically I use them on the inside of a fiberglass body tube, either to back rail buttons or connect a coupler without relying on threads in fiberglass. Recently I did try them on the inside of an Aeropack minimum diameter retainer, and it seems like I have the right idea but the teeth should press in farther, as they would in a larger diameter hole.

Even if I had a press it would probably be too hard or impossible to use on the inside of fiberglass tubing!

Scroll down a bit on this link.

https://www.apogeerockets.com/Building_Supplies/Misc_Hardware?zenid=2535052af71fcb2b912f3b38d040b72e

Eric,

I use them all the time in fiberglass airframes, without any sort of adhesive. With the proper size hole, I have never had one fall out or come loose.

One important thing that has helped me: Using brad-point bits in fiberglass. The hole will not be irregular as with a normal drill bit. Additionally, using wood backing when drilling the hole to eliminate frayed fibers around the hole, etc.

If I don't have the manufacturer's recommended size I measure the OD and then grab the numbered drill
bit the next size up. You can use a cap head screw and washer to pull the nut into the hole and it won't come back out. You shouldn't need any adhesive to keep them
in place.

I use PEM nuts and drill the outer hole sized to the screw, and an inner hole sized for the PEM nut but 1/64 smaller. I then put a nut on the screw before inserting into the nut and toghten as much as possible, thus seating the grippers into the av bay (remove nut, lol). I then use epoxy on the nut as extra security; you don't want to be removing the screw and then have the nut come loose before the screw is completely removed.

hball55 said:

I use PEM nuts and drill the outer hole sized to the screw, and an inner hole sized for the PEM nut but 1/64 smaller. I then put a nut on the screw before inserting into the nut and toghten as much as possible, thus seating the grippers into the av bay (remove nut, lol). I then use epoxy on the nut as extra security; you don't want to be removing the screw and then have the nut come loose before the screw is completely removed.

Click to expand...

Can you post a picture of how you used the PEM nuts?

I first saw this technique in Wayco's L3 build thread and have used it repeatedly with great success from the NC to MD retainer, simply use a screw, nut, and washer to pull the insert into place.
Search PEM drill guides, here the one I use from wiki - https://en.wikipedia.org/wiki/Drill_bit_sizes#Drill_Bit_Conversion_Table. I like a #6 or #8 for soft metals and plastics, sourced from McMaster @https://www.mcmaster.com/#pem-style-nuts/=117hm8h.