Attaching PEM nuts to Fiberglass

[redheadRocketee]

Hello all!

I'm on a university competition HPR team, and for the past two years, we've had extreme difficulty getting our broached PEM nuts to stay attached to the airframe. We've tried to hold them in with Loctite 5-min, JB weld, Rocketpoxy, and similar Epoxies, and we were out of luck with all of them! I suppose it might be a sanding error.

That being said, do you guys have any tips for attaching broached PEM nuts to fiberglass, or perhaps another fastener to attach airframes to each other non-permanently?

Thanks!

 

[Titan II]

https://www.rocketryforum.com/threads/use-and-purchase-of-pem-nuts.135391/

 

[Titan II]

One possible alternative:

 

[David Schwantz]

I agree with Titan, this is what I use on mine. Depending on if it flies with Av bay or not, you can use either plastic 2-56 or steel. Only thing though, I would round all the corners and edges as to not have any recovery stuff snag on it.

 

[Worsaer]

I have had great success with PEM nuts in fiberglass, without any adhesives. When drilling the holes, what kind of bit are you using?

 

[Banzai88]

I use PEM nuts extensively in fiberglass rockets. I have a supply of standard nuts and size matched drill bits that I got from McMaster-Carr. For just a few $$ you can get pretty much a lifetime supply of nuts, drill bits, and fasteners.

I've never found that backing them with epoxy was ever 'necessary' as long as the hole is correct, and the serrated neck is seated in the material up to the collar, but I've recently been doing it just in case.

That said, before I started applying epoxy to the backs, I never had one back out or strip out.

The keys to the kingdom are a grip length that matches or is SLIGHTLY less than the thickness of your material, a properly sized hole, and the nut pulled in fully to the collar.

If you feel the need to apply epoxy, 100grit sand the SIDE of the pem nut and the area around where the nut will go. Install the nut properly. Put a small circle of tape the exact diameter of the nut on the inside where the epoxy will go (to keep epoxy out of the threads). Cover the sides of the nut with epoxy and form a fillet with the mount surface. Remove the tape and allow to cure.

 

[Bat-mite]

Consider weld nuts as an alternative. I use them. I slather the screw in Vaseline, then screw down the weld nut. I form a dome of epoxy over the entire thing. After cure id finished, a hard turn of the screwdriver breaks the screw loose from the epoxy and it can be removed easily.

 

[dr wogz]

There are a few types of PEM nuts. Are they using the expected 'CL' type nuts?

 

[caraviator]

The correct type of pem nut and size drill bit are key. McMaster has details for each pem nut and tells you what drill you need (you will need fraction, letter and numbered bits). I do glue mine not so much do to fit, but security in case I ever push to hard on the bolt.

 

[JohnCoker]

caraviator has the key: correct size drill. PEM nuts work well in G10 sheet for example:

 

[dr wogz]

I also forgot to mention, that PEM do have nuts specifically for fiberglass / G-10 / FR-4, G-12.. Printed circuit boards..

https://catalog.pemnet.com/viewitem...f2-kfs2/broaching-nuts-types-kf2-kfs2-unified

 

[redheadRocketee]

I have had great success with PEM nuts in fiberglass, without any adhesives. When drilling the holes, what kind of bit are you using?

I suppose that is where we might have been going wrong. Until just a few weeks ago, we had a very limited selection of drill bits and kind of just used what looks like it may have worked. I was sure to order a good drill bit set (HSS) to make clean holes of the right size

 

[caraviator]

Correct drill size with PEM makes all the difference!

 

[Worsaer]

Brad point drill bits cut crisp, clean holes. Once I started using them I had no problem with PEM nuts.

 

[Banzai88]

Backing the tube with something solid, or at least a piece of tape, helps, too. That and high speed and light pressure seems to cut so much better with less chance of splintering a filament wound tube.

 

[Worsaer]

Exactly! I use a hardwood closet pole, clamped on both ends in the body tube. Solid backing makes a huge difference.