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Casting threads in composites


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I tried wrapping a bolt with Teflon plumbers tape, JB Weld and wound on 2 mm of carbon and glass. Couldn’t spin it off. 
Probably try candle wax next but I don’t want to lose much thread. Maybe dilute with solvent or try silicone spray or????

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I cannot see that working well. What size fastener is it? I've done it but it was for a portable water tank and the threads were massive and wicked coarse. I just coated it in wax and once cured it backed right out. 

 

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9/16” bike pedal for dinghy wheels. The spine of the pedal is cut down to slide inside a tube-axle with the threads exposed. I’m hoping to demount  the wheels by unscrewing them. The left hand thread is good to keep the left wheel attached but I don’t have a left hand tap. 

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Spray the item to be cast with McLube / Dry Teflon Spray, and then layup the glass, or oversize drill hole and cast the item in epoxy. 

 

After the epoxy has set up, it should back right out with threads nicely cast.

 

Also see page 45 on https://www.westsystem.com/wp-content/uploads/Fiberglass-Manual-2015.pdf 

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Try unscrewing the bolt while the epoxy is still green - gelled, but not 100% cured. Even just a turn out and back in should break the bond enough to easily remove it once the cure is finished.

 

Otherwise a soldering iron on the head work -at least for screws. Might take a lot of heat for a bigger bolt.

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Mold release wax and a full cure always works for me. A heat cure works best as the metal expands. To apply the wax, chuck one end in the drill and spin while pinching the fastener with the waxy sponge and allow the wax to fully dry.

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Bingo. That’s the kind of technique I was looking for. The bike pedal is tricky because the threads stop before the pedal frame. I wrapped some tape but will add more next time. Otherwise it interferes when you spin it off. 

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17 hours ago, andykane said:

Try unscrewing the bolt while the epoxy is still green - gelled, but not 100% cured. Even just a turn out and back in should break the bond enough to easily remove it once the cure is finished.

 

Otherwise a soldering iron on the head work -at least for screws. Might take a lot of heat for a bigger bolt.

This. Don't let it fully cure before backing out the thread.

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You have several options. I recently did this with McLube Sailkote (which is a mold release btw), but any PTFE spray will work well and is easily to apply. The other locally sourced option is Turtle Wax. As Russel states, you need to let the epoxy fully cure before backing out the threads. Another option is to simply drill and tap the hole when complete, which works reasonably well. A final option is a threaded insert, they make some for composites: https://www.mcmaster.com/threaded-inserts/threaded-inserts-for-composites/

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On 6/30/2020 at 10:23 AM, Russell Brown said:

If you don't let it fully cure, there won't be intact and usable threads left. 

^^ This.

Also, polish the threads first. Wire brush to get them really clean, then compound and a wheel. Only takes a minute and it will help.

Those repair bushings sound smart. A bike shop would probably do it for you.

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I have had good success with molyslip. I have it for cutting threads in stainless with a tap but works great as a release if you coat the bolt and then wiping is off as much as possible.

Once the resin is cured the bolt screws out easy leaving a good thread.  Vasaline also works but get a better thread with the molyslip. I expect mould release would also work.

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You can also make a tap. I do this from time to time when I need a metric thread (I don't have metric taps and I'm too cheap to buy a set). Take a hacksaw and cut a groove in the end of the bolt that you want to make a thread for. Both sides of the bolt if you like, a little deeper than the thread depth. I don't bother tapering the tap because I only use this for soft materials (aluminum, plastic, composite etc). 

In your case, I bet you could easily saw a cut in 4 or 5 theads without losing any strength. You need a sharp edge on the cut, so a saw might be better than a grinder if you try this.  

Because it does not have a taper, you need to be very careful how you start the tap - or else resign yourself to losing the first couple of turns to wobbling and mashing of the threads. It's a bit ghetto, but it works.

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