How to rethread striped hole in thin fiberglass tube?

scanlin

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Decided to try my luck at using tiny countersunk screws to hold a 65mm fiberglass airframe to a coupler. I've been practicing (see photo of my practice tubes) but once in a while I get a screw that strips the threads out of the hole. There are only 2-3 threads per hole because the materials are so thin.

Question: Is there a good technique to fill the hole with something and then redrill and re-tap the threads?

I've tried using CA glue, aeropoxy (cured 24 hours) and 5 min JB Weld (cured for 2 hours). They all fill the hole okay and drill out ok, but when I try to use a very new (sharp) tap to make the threads (slowly), the glue/epoxy tends to crack/break and I end up with a hole with no threads (where I started).

These little screws don't have much force applied to them when inserted; light finger tight. I'm hoping there is strength in numbers (using 16 of them on one coupler, 2 rows of 8 each, because I thought it would look cool after being painted).
 

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Steve Shannon

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Threads on g10 withstand very little stress in the first place. You’re better off using an insert like a PEMnuts or even using removable rivets. I’ve even used tiny tee nuts on the inside of the coupler and filed flush with the OD of the coupler. That works well.
Once a hole is stripped in fiberglass I don’t think you can recover the original strength.
 

scanlin

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Threads on g10 withstand very little stress in the first place. You’re better off using an insert like a PEMnuts or even using removable rivets. I’ve even used tiny tee nuts on the inside of the coupler and filed flush with the OD of the coupler. That works well.
Once a hole is stripped in fiberglass I don’t think you can recover the original strength.
Thanks. I'm new to working with fiberglass. Was trying to achieve a low-profile (flush if possible) minimum drag effect. Just for aesthetics; I'm not going for any altitude records; just wanted it to look good. I also have sheer pins which stick out a tiny bit from my dual deploy airframe (annoying, but they're small).

I've never used PEM nuts before. I googled it but looks like PEM makes a few different kinds of fasteners (https://www.pemnet.com/products/product-finder). Is there one in particular that people use to join two fiberglass tubes and have as small of an external profile as possible? I do need to be able to remove them to access the electronics and such. Appreciate any pointers. Thanks!
 

rocket_troy

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Totally agree with everything Steve said. So everything I've written beyond this point comes with the caveat that I don't think this is worth pursuing.

Saying that, if you were hellbent on this, I'd 1st check your tap. Is it the right tap ie. a taper tap (starter tap). You don't want to start this with a bottoming tap.
Also, is it a good quality tap from a reputable manufacturer? There's a gaping difference between a new crappy tap and a good quality tap (even an old worn one). It normally shows up in hard materials (a good quality tap can require less than 1/10th of the torque demands of a crappy tap). I experience this all the time.
An alternative method (assuming you don't have access to a CNC mill to thread mill) would be to kinda cast the thread eg. Grab your fastener and tightly wrap a couple of wraps PTFE tape around it. Then plop some JB weld over that (or something quite thick) and then insert that into the hole carefully holding the fastener central and flush with the hole. Large amounts of Blu Tack might assist there.
Wait 12hrs then carefully unscrew the fastener.

TP
 

David Schwantz

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Make a backing plate of ply to go inside the coupler. Glue it in place, then drill hole, then you can tap the wood. But I like to epoxy a blind nut on the inside. If you choose to go with the blind nut. Make the plate then glue the blind nut in place in the plate that already has the hole drilled in it. Insert into coupler with glue then screw an oil coated screw into the blind nut through the hole in the coupler. Tighten screw to hold in place till glue is dry.
 

heada

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Also make sure you're drilling using a tap size bit for that size tap. They're very rarely a fractional size and you have to go to letter bits. You can google the tap size bit needed. Drilling the hole at the correct tap size reduces the effort to tap the hole quite a bit too.

I've also drilled the hole slightly oversize and epoxied in an aluminum sleeve and tap that instead of the fiberglass directly. The sleeve can be longer that the width of the fiberglass, tapped as desired and the filed flush to the tube.
 

rocketlabdelta

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Is there one in particular that people use to join two fiberglass tubes and have as small of an external profile as possible?

See this chart from @rfjustin for PEM nut and drill bit sizing:

That thread has a bunch of helpful information about PEM nuts.

Flat head screws give a nice, flush profile but countersunk holes in fiberglass airframes can break if overtightened. A 65mm fiberglass project I'm planning will use coupler-mounted PEM nuts and these low-profile socket head screws:


The holes in the airframe will be the size of the socket head, @JimJarvis50 style, to "pin" the coupler in place.
 

rocketlabdelta

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In theory, you could use Lumadyne inserts from Apogee Components but they're expensive if they're not out of stock:


(They're out of stock right now and sell out quickly.)
 

jahall4

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Several good recommendation here. I gravitate toward using "weld" nuts (some call them T-nuts) because they are inexpensive with lots of sizing options. Here is some video showing how I install them.
 

bjphoenix

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I think if you want to put threads in the fiberglass, you should use a big enough bolt that you can get reasonable big threads. Or you could use a small wood screw that will naturally have larger threads than a machine screw. Or you could glue in some form of metal inserts.
 

dr wogz

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you can also get 'undercut' flathead screws. the flat head 'cone' is cut to allow the countersink in a thin sheets, so you don't need a countersink in the underlying part:

 
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