Captive nuts in fiberglass

Quickstep192

Anarchist
603
102
Chesapeake
As some of you may have seen in some of my other threads, I'm replacing the instrument panel in my J/95. I'll need to enlarge the opening and create new fastening points for the new panel. 
 

I really hate using sheet metal screws in fiberglass, especially when the glass is relatively thin. I was thinking of using rivnuts, but I suspect that the glass will crack from the compression when the mushroom on the rivnut forms. 
 

I can access the back of the opening before the panel goes in, but it's very difficult to get behind the panel to hold a nut once the panel is in place. 
 

Any suggestions for how to make a captive nut in the fiberglass that will house the panel?

 

Zonker

Super Anarchist
8,877
4,788
Canada
I've tried gluing nuts in place with a bit of epoxy with some success but I think the other suggestions of a thicker plate (you can drill and tap G10)

No to drill and tap aluminum and then leave it - or at least use Tefgel on the screws

 

LakeBoy

Super Anarchist
Since you can access the back before installing the panel, how about using washers on the rivnuts.  

Perhaps the piece you are cutting out is large enough to do some testing.

 

Justaquickone

Member
305
52
It's not a pretty solution but I've had some success with glued and glassed in S/Steel wing nuts .

Least likely to break free and spin aimlessly down the track .

Was even able to grind a little bit of the wings to reduce their profile without compromising their hold .

 
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CaptainAhab

Anarchist
794
200
South Australia
The simple way is to bond on some g10 sheet. Then use self tapers. Or if you really want tap it and use machine screws. Don't try to bond metal nuts. If you really like metal on metal, you could bond on the g10 to get your thickness then rivnut.

 

DDW

Super Anarchist
6,239
972
I think the simplest way is to glue in threaded G10 tubes. Yes I sound like a broken record on this. They can be glued into a thin panel with very limited access to the back, will be stronger than the panel, and will allow multiple - > 100 - R&Rs before they need any more attention. They are easily replaced if damaged. 

On a thin frame to which you are attaching a bezel or panel, the easiest way to do this is to buy some G10 tube of the appropriate diameter, cut them to the length you want and then drill and tap to the screw size. Scratch off the back of the panel with sandpaper around each screw hole (doesn't have to be perfect), screw the G10 tubes to the frame without the panel, fillet the back around the tube with West Six10 on your finger, wait a day, then install your panel. Really all the glue is doing is holding them from turning, but if you do a good job on the filleting it will make them stronger too.

Rivnuts might work but: SS ones have a lot of expansion force and will break out a thin panel. Aluminum less likely but can still be a problem. In either case, there is a flange on the bezel side of the frame which keeps it from fitting flush unless the bezel is spot faced or a thick gasket is used. 

 

DDW

Super Anarchist
6,239
972
Get a helper to hold the nuts.

It's not like it's going to be a frequent occurrence.
A tiny little midget helper, who is then closed into the instrument pod forever? Don't you need a door to pass in food and water to keep him alive if you ever want to get the nuts loose? And what about the stink from the potty issues? I'm not sure that is entirely practical.....

 

Quickstep192

Anarchist
603
102
Chesapeake
I have a friend who was once skinny, agile and non-claustrophobic. Now he's only one of the three. I'm currently none of the three. 
 

A couple of weeks ago I climbed in to get behind the panel and got wedged pretty solidly. I had to spend a minute to find my happy place before figuring out how to back out. 

 

allweather

Member
392
76
baltic
I think the simplest way is to glue in threaded G10 tubes.
I really would like to use something like that for a handful of things around my boat(instead of epoxy, drill and tap), but don't know if this is some kind of finished product or do I have to buy ordinary G10 tubes and tap them myself?

Because someone once explained to me how tapping such tubes cuts most of the fiber in the thread and one ends up with more or less only epoxy strength? At least in one area that would be too weak in my case.(or at least I wouldn't trust it. I guess I can just figure out how to bond a metal insert)

 

DDW

Super Anarchist
6,239
972
I don't know of a finished product. If you are well set up, you can knock them out in about 20 seconds each. If not set up then more that a few is a pain. 

If I'm, making a lot, I set up a fence on the band saw and can cut them in maybe 4 seconds each. I've made collets for the quick clamp drill press vice so I can change parts in about 3-4 seconds. I have to do two cycles, one to drill and one to tap. So drill them all, maybe 10 seconds each including the part load. Then put the tapping head in the drill press, go through again, another 15 seconds. I guess that adds up to 30 seconds each. Really well set up would be a bar pulling CNC lathe, cycle time probably less than <10 seconds and you could be eating lunch at the same time. 

On strength, this comes up repeatedly. Yes, the orientation of the cloth is not ideal, but the strength is not bad - properly done higher than the working load of the fastener. Take a perusal through this old thread, in it I describe one method of mounting hardware and do some strength tests on variations. The tests are starting about here. The highest load equipment I have fastened on my boat by threading G10 is about 7 tons working load. They have not moved in 12 years. The only ones I have had fail are #6-32 in some dodgy G10 tubing.

Now as is emphasized in that thread, you have to use your brain, if you don't have one then rent one or have someone else do the work. 

 
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