Bolt Threads in Epoxy

[Cheechako]

Maybe you could use thread inserts and screw them together getting progressively smaller!
Or Rivnuts. Or there are many things to use: You need to ferret out what will work size wise. Or just imbed something like these in epoxy in your larger hole. The second pic is from hot rod supplier Summit.
The last pic are available from #2 inside to 3/8" inside: https://www.globalindustrial.com/g/f...serts-Themoset

[Sailorman Ed]

Well, I have learned 2 new terms today ( although I have seen both items) a well nut and a drag link socket. And I never would have thought about air coming from the fillers.
thanks CF.

[GrowleyMonster]

You can simply set a nut in epoxy. Helicoils or Chicago Screws will work good, too. I would not count too much on threads tapped into epoxy or any sort of filler material.

[oldjags]

Originally Posted by guyrj33

I've also filled the hole with epoxy and high strength filler and then inserted a well waxed bolt to form the threads in the epoxy.

That's what I've done before as well. But use a lag bolt rather than a machine threaded bolt, as the threads are much deeper and wider. I usually let the epoxy kick-off to the point that it's rubbery, and then unscrew the bolt and let it finish curing. That way you know for sure you'll be able to unscrew it in the future. Even with the wax, I worry that it'll get glued in there permanently!

[Quadrille in JB]

Practical Sailor recently did a test using epoxy and various fillers as a base for threads. Helicoils, of course are the strongest but they found that straight epoxy resin, with no fillers provided the strongest threads.
It was pretty amazing how strong the resulting threads were.

[psk125]

Got questions on epoxy? These guys wrote the book. It is now available online for free. Look up what you want to do, and do it once, the right way, the first time:

https://www.westsystem.com/the-gouge...-construction/

[Marqus]

Originally Posted by HopCar

I’d use a 1/4-20 Wellnut. Should fit perfectly in a 1/2” hole, no filler needed. Just shove the Wellnut into the hole, insert the bolt and tighten. I use small ones to install canvas snaps. They’re under a tension load and don’t pull out.

https://www.amazon.com/25-Rubber-Wel...F4-20+well+nut

Interesting, Hopcar. Not clear to me - if you just push them in, what stops them from just pulling out from even a modest pulling force, or just vibrating out ?

[Marqus]

Thanks all for your replies - just plain useful stuff. This forum has seemingly unlimited supply of great (read smart) members.

I would say that I have been cutting threads in epoxy for a while, mostly to mount electric wires, LED lights and so on, and since the duty is light those threads all doing ok. I prefer UNC threads (Imperial) because the Metric threads are finer and for this sort of thing, coarser seems better.

Oldjags idea of using a Lag Bolt to form the threads is nice - you'll get an even coarser thread. Of course lag bolts may be a bit too bulky for many situations.

Very interesting to hear (Quadrille) about Practical Sailor's test using epoxy and various fillers as a base for threads - they found that straight epoxy resin, with no fillers, provided the strongest threads. Surprising to me, but then it directly supports what Thinwater said about "air" weakening a filler mix. And Thinwater's idea of stuffing with cloth - yeah I get that that will produce a strong, hard, tappable plug (but you need firm backing behind the hole).

Regarding the various metal inserts:
Helicoil - bit of work, first have to cut a bigger thread in order to be able to insert the Helicoil to get a smaller thread.
Riv-nut (flanged) - looks good; you'd probably have to rough up the external surface so it doesn't want to twist once embedded.
Tee-nut - they won't twist, but because they are shallow they've often given me the impression of being easy to pull out with a tug.
Threaded Inserts - not sure the outer threads would cut into existing fibreglass like they would into wood, but you could bed them with wet epoxy so that could work real well.

So for my current job, I think I'll drop the Aluminum filings and try the waxed bolt into zero-filler resin method (as per Practical Sailor) for creating threads.

And I'll see if I can order a small stock of stainless flanged Riv Nuts (or ss threaded inserts) of various thread sizes (1/4, 5/16, 3/8) for future use.


Postscript: Those using wax on a bolt before sticking it in epoxy to form threads - if the epoxy hardens and for some reason the bolt remains stuck, I've found a modest bit of heat from propane on the bolt head (shield the substrate with alu foil) will often get it to release.

[DeepFrz]

West systems has an article about threads in epoxy on their website.

https://epoxyworks.com/?s=threads+in+epoxy

Better link.
https://epoxyworks.com/index.php/har...ing/#more-3674

[HopCar]

Originally Posted by Marqus

Interesting, Hopcar. Not clear to me - if you just push them in, what stops them from just pulling out from even a modest pulling force, or just vibrating out ?

They are rubber with a nut molded in. As you tighten the bolt the rubber expands and locks it in place. It works quite well if it goes all the way through the fiberglass but it also works if The fiberglass is thicker than the Wellnut is long.

[HopCar]

Here is a photo of an expanded Wellnut next to an unexpanded one.

[thinwater]

Originally Posted by Quadrille in JB

Practical Sailor recently did a test using epoxy and various fillers as a base for threads. Helicoils, of course are the strongest but they found that straight epoxy resin, with no fillers provided the strongest threads.
It was pretty amazing how strong the resulting threads were.

Actually, the winner was epoxy with glass cloth stuffed into it. Basically, filling the hole with laminate. The problem with plain epoxy is that it splits, and most fillers soften the epoxy and they strip.



See post #6.


That said, some of the suggestions for blind nuts should be very strong. I would put a waxed bolt in the nut and hang it from that, just as you hang certain types of epoxy-in concrete anchors. A little fiddly, but logically more durable if the fitting were to be removed more than once or twice. I'm going to remember that trick, though I'm not sure for what.

[Marqus]

Originally Posted by HopCar

.....but it also works if The fiberglass is thicker than the Wellnut is long.

Yes I thought the substrate had to be thin so the rubber could bulge beneath the surface, but good to know it can even grip along the side walls of a deep hole.

The nice thing about the rubber is that it should seal water out and it is hidden from UV so should last a long time. (... well, unless it soon leaches plasticiser and cracks like some rubber substitutes from a certain asian region.)

[Nicholson58]

I use gray MarineTex. Heavily filled epoxy. Duct tape the gravity drain side of a hole and fill with the stuff.

[Souzag818]

Originally Posted by psk125

Got questions on epoxy? These guys wrote the book. It is now available online for free. Look up what you want to do, and do it once, the right way, the first time:

https://www.westsystem.com/the-gouge...-construction/

The guru's...