Can I tap that? (Fiberglass)

Marty Gingras

Mid-range Anarchist
For what it's worth, tactics might suggest that you start w/small diameter attempts (e.g., tap it) saving the larger-diameter attempts if the others fail.  Might save you having holes to fill.

 
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Quickstep192

Anarchist
827
157
Chesapeake
I think the problem with rivnuts in fiberglass is that rivnuts depend on the material they’re in not compressing. In fiberglass, the mushroom that forms on the back of the rivnut can crush the fiberglass and deform it. A washer would improve that situation, but if you can get a washer on there, you can get a nut on there. 

 

Gabe_nyc

Member
270
22
Bayside
I think the problem with rivnuts in fiberglass is that rivnuts depend on the material they’re in not compressing. In fiberglass, the mushroom that forms on the back of the rivnut can crush the fiberglass and deform it. A washer would improve that situation, but if you can get a washer on there, you can get a nut on there. 
I see your point and agree that in a structural situation this might be an issue to consider.

In this case the 3mm thickness was not thick enough to use a sheetmetal thread or to tap. Also, I was replacing a pop-rivet with a RivNut. The hole was a bit larger but basically similar.

I’m now wondering if, when used in a crushable material like FG, if steel RivNuts would be better than aluminum, or maybe at least a better quality of aluminum ones ?

 

Quickstep192

Anarchist
827
157
Chesapeake
I think the more difficult it is to deform the rivnut into the mushroom shape, the more likely it is that the rivnut will crush the fiberglass, so I’d think steel would be worse than aluminum. That said, the aluminum is much more likely to corrode. 

 

Steve

Anarchist
563
77
duluth, mn
it is not about strength to hold on a plastic cover but make sure you counter sink the gelcoat to help prevent the gelcoat from cracking. tapping is better because it cuts the treading, removes the material and the sheet metal screw cracks and displaces the material which almost always cracks the gelcoat.
I agree with all said, stay away from sheet metal screws in fiberglass.

 

Quickstep192

Anarchist
827
157
Chesapeake
Just a follow up on this. 
 

I threaded 6 holes in glass a bit over 1/4” thick and it worked like a charm. I frequently backed out the tap and cleaned it and the hole using a can of compressed air. The threads really held. I was a little surprised it works so well and I’m surprised that manufacturers use sheet metal screws because they completely suck.  
 

 

Marty Gingras

Mid-range Anarchist
Just a follow up on this. 
 

I threaded 6 holes in glass a bit over 1/4” thick and it worked like a charm. I frequently backed out the tap and cleaned it and the hole using a can of compressed air. The threads really held. I was a little surprised it works so well and I’m surprised that manufacturers use sheet metal screws because they completely suck.  
 
Glad you followed-up.

Sheet metal screw are so fast, but putting a tap in a variable-speed drill really speeds use of machine screws.  Works great in aluminum and 'glass.

 
That flange at the top limits mounting options 

a standard helicoil is more versatile 
Spot face, counter bore or dimple enough to submerge the top flange.

These work very well for thinner panels. 

Rivnut material must be appropriate for the application of course. 

Get the correct tool for installation,  the SS rivnuts can be a bear to pull...by hand.

The air puller is for cheaters!.... :)

 
Geez, anyone else use tee nuts? Never used rivnuts, but they look interesting. Looks like they are a pain to remove, where you have to be careful NOT to remove tee nuts. 

 

SloopJonB

Super Anarchist
68,601
12,297
Great Wet North
I've used these "nut-serts" in wood for panels that occasionally need to be removed - berth tops, engine box sides etc.

They work great and are simple to install.

00N1025-25mm-flanged-nut-1-4-20-quick-connect-f-02.jpg


T-Nuts are a bit different and not nearly as secure IME.

s-l500.jpg


 
Been thinking about this for a bit.  I didn't work in helicopters but worked around them in the Army, and cracking was always a problem and I picked up some techniques that worked on my motorcycles.  My crew chief friends were forever drilling relief holes to stop cracking in fiber and plexiglass, then applying epoxy or epoxy and cloth depending on whether the application was an area somebody needed to look through.  I've fixed problems in motorcycle fairings and plexiglass this way.  Motorcycle manufacturers like to use little rubber washers to cushion more fragile its mounted to a rigid frame... 

For this application I would think hard about not tapping the glass, and getting some squishy rubber washers.  If it doesn't have to be water tight I'd put a squishy washer on both sides of the panel.  Boats are *flexy* in places we don't expect. The through fastener can be a bolt or screw, the point is to cushion the part that may be subject to cracking.  Go ahead, shoot down the idea before I try it on my own boat's planned exterior engine panel cover... 

 

Crankit

New member
Depending on how often the cover has to come off: threads in fibreglass will only last so many removal cycles.

I thought of rivnuts first, you can get them in stainless so deterioration of the nut shouldn't be an issue. But it depends on the thickness of the laminate whether they're a good idea. I like the helicoil idea, if the panel is thick enough. That would be a fairly permanent solution.

Good luck!

 




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