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Dec 01, 2009, 03:35 PM
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rdeis's Avatar
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Carbon Joiner construction


I might have need to make myself some bent square stick joiners.

I have one good one to copy that appear to be a wood core surrounded by carbon. It's roughly 1/2" square and ~6" long, bent by 5 degrees or so in the middle.

Making a mold of the original is simple enough- but how should I lay up the part once the mold is made? Bent laminations are easy, but don't seem like the best way...
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Dec 01, 2009, 07:04 PM
Yep, Naza-controlled Tricopter
tonyestep's Avatar
When I've made a part like that, I've wrapped the core in 3/4 oz glass, then laid up strips of tow to the desired thickness all around, then one more layer of 3/4 glass on top. The glass serves only to bind the surface -- the carbon takes all the loads. Others who know composites better than me may have better suggestions.
Dec 01, 2009, 11:42 PM
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rogerflies's Avatar
Seems like the ideal thing would be to have the carbon fibers pressed outwardly against the inside of a closed mold. That would give you maximum density of carbon where it does the most good.

The mold could be a simple two-piece arrangement. The bottom would have a trough cut to the desired finished size. The top would bolt down to close the mold. The ends would be left open, and the mold would be a little longer than the finished part.

You'd lay in the wetted glass cloth, leaving enough to fold over the carbon just before you close the mold. Then lay in the wetted carbon tow.

Put a length of rubber tubing in the center of the layup with brass tubing over the part that's outside the mold. Finish filling the mold with wetted tow, then fold the glass over, and close the mold.

Now inflate the tubing to apply outward pressure on the tow. That'll force the excess epoxy out the ends of the mold, and the carbon will be packed tight in every direction.

After the epoxy is cured, deflate and remove the tube, and cut off the messy ends off the joiner.

You might want to bake the part after the tube is removed but before the joiner is removed from the mold and cut to final length.

I admit I have ZERO experience with carbon fiber, but it seems like a workable plan.

Roger
Dec 03, 2009, 03:53 PM
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rogerflies's Avatar
It just occurred to me that you might not have a compressor to apply pressure to the tube. If that's the case, you could run both ends of the tube outside a vacuum bag to get the same effect.

I'm surprised someone with experience with using carbon fiber hasn't chimed in with some suggestions on how to do make joiners.

Roger
Dec 03, 2009, 06:08 PM
Registered User
There was a similar thread on how to make Carbon Joiners on the SASS (Seattle Areas Soaring Society) mailing list back in 2004. It looks like you have already gotten the info you need in the posts above, but just in case it may help, here are the posts and pictures from the SASS thread by Russ Young who was building a Genie and by Jim Murphy.

----------------------
From: Lynn & Jim Murphy [mailto:[email protected]]
Sent: Tuesday, March 29, 2005 2:05 PM
To: [email protected]
Subject: Re: Wing joiners

CF joiners:

A couple of questions...first, are the joiners round or square ? and do they have the dihedral angle built into the joiner or are the just a straight rectangle shape or a round piece.

Either way, you would have to build a mold to make the desired joiner. i usually make a model of the joiner rod out of balsa being careful to make the exact shape you wish to have it become when it is molder into a carbon fiber part.

If the joiner rod is round you may be able to make the model or plug out of brass tubing that you can buy at the hobby shop.

There is some carbon fiber joiner rods sold by aerospace composites that are round and have a 3 degree dihedral angle build into the part.

I think they are 3/8 of an inch and 1/2 inch in diameter. they may have a bigger variety then that, i would have to check my catalog.

If you are interested in going any further i can send you pictures of my mold. they are very crude and easy to fabricate. just a few pieces of plywood glued to the shape that you wish to mold.

The next process after the mold is to cut up a bunch of carbon tow and lay it length wise in the mold and pour epoxy over the tow, agitating it with a stick, making sure that the tow is properly saturated with epoxy.

It sounds like a lot of work, but it is not harder than builder something like an aileron or rudder. it is just something new to try.

- jim

----------------------------
I just waxed everything with paste floor wax....tops, bottoms and sides of
wooden mold (I should have waxed the tops of the screws better on the last
one) and used a clear piece of 1/8" plastic I had laying around (waxed) and
clamped it down to be the top of the mold after I had layed and squished the
CF fibers and epoxy into the mold.

On the second one, I put a couple of small slices of CF laminate to be the
little end pieces of the joiner. This made nice ends....the first one was a
little sparce on the epoxy and the ends were a little "pock marked"

I used pieces of Oak that I got from Home Depot for the mold. Nice and
straight and plenty hard enough.

I hope they are strong enough! But we'll find out in a couple of weeks!

-Russ Y.
------------------------------------------------

Yes, I wax the mold with partol wax. I do not know if floor wax would work. just try it on the back side of the mold or some material like your mold will be built from. some epoxy and glass on it to make a mini lay-up, let it cure and try to remove it.

If it works there...it should work on the mold

-Jim

---------------------------------------------

Here are some fuzzy pictures of my first joiner. Iím pleased overall. The ends of the joiner are a little rough, but it should work fine. The weight came out at .7 oz., the same as one of the Artemis joiners that I weighed.

I used oak for the mold. The paste floor wax worked fine.
-Russ Y.

---End of old SASS thread by Russ Young & Jim Murphy ----

-John Elliott
Dec 05, 2009, 12:48 PM
Quote:
Originally Posted by rogerflies
Seems like the ideal thing would be to have the carbon fibers pressed outwardly against the inside of a closed mold. That would give you maximum density of carbon where it does the most good.

The mold could be a simple two-piece arrangement. The bottom would have a trough cut to the desired finished size. The top would bolt down to close the mold. The ends would be left open, and the mold would be a little longer than the finished part.

You'd lay in the wetted glass cloth, leaving enough to fold over the carbon just before you close the mold. Then lay in the wetted carbon tow.

Put a length of rubber tubing in the center of the layup with brass tubing over the part that's outside the mold. Finish filling the mold with wetted tow, then fold the glass over, and close the mold.

Now inflate the tubing to apply outward pressure on the tow. That'll force the excess epoxy out the ends of the mold, and the carbon will be packed tight in every direction.

After the epoxy is cured, deflate and remove the tube, and cut off the messy ends off the joiner.

You might want to bake the part after the tube is removed but before the joiner is removed from the mold and cut to final length.

I admit I have ZERO experience with carbon fiber, but it seems like a workable plan.

Roger
the outward pressure on, or suitable compression of the wet tows is key i think...i made a mold off a steel form bent to the desired dihedral angle. i laid this out on the parting board on it's side to avoid having to make a complex parting board.

issues arise when you do not get enough compression on the tows such as voids or bubbles. if you try to fill each mold half over the top, the tows tend to migrate out onto the mold flange instead of compressing. this prevents the mold halves from properly mating. my thought is i am going to make the top half of the mold protrude into the bottom half so the act of joining the mold halves will serve to provide the necessary compression and keep the tows from escaping!

the good part is the carbon joiners are much stiffer and less than half the weight of the round steel joiners i was using.

tom
Dec 06, 2009, 11:33 AM
Registered User
T.D.'s Avatar
Here's a simple way of doing it that works well.

I made this tool about 10 years ago and it has produced 50 or so joiners with little sign of wear.

I bent a piece of heavy duty 12.5mm wide by 30mm deep/tall aluminum channel to 3.5 degrees to make a female tool half, spot welded the outer bent side, filled the inner bend gap with epoxy then sprayed a piece of cling wrap lightly with 3M77 and laid it in the tool...closed the ends of the tool channel with duct tape then made a medium stiff mix of epoxy/cabosil and poured enough into the tool to give a 2 to 3 mm thickness.

Let this cure for a few hours but still green then lay in layers of 6 or 10 ounce 'glass that has been wet out and folded over itself, press the 'glass down well with a popsicle stick or whatever, let everything cure and pull the male plug out.

Either wax the tool or use cling wrap in it then wet out carbon tows sufficient for the joiner you want to make, experiment a bit here and you'll work out how much you need. I tried bagging the layup but had much better success by squeezing the tool in a bench vice...I do it in stages, squeeze then wait a couple minutes for excess epoxy to ooze out then squeeze again etc until the carbon will not compress anymore or no more epoxy oozes out. When you get the carbon/epoxy balance correct during wet out very little epoxy will ooze out

For thicker joiners either use wider/taller channel stock or do two layups and join them side to side.

I've never had one of these joiners fail.


T.D.


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