Primitive Archer
Main Discussion Area => Bows => Topic started by: DC on March 03, 2017, 07:47:43 pm
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I just spent a couple of hours trying to get a "perfect" "Z" joint. I used carbon paper and feeler gauges and I can still see light through it in a few spots. Come on now, be honest, can you guys get a splice so tight that you can see "no" light through it?
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Never had a perfect one. Need room for glue right?
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Honestly - yes!
however if you simply steam the ends of your splice for 10 minutes then fit together and clamp evenly until well cooled....you too will have 'no light'!
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I'm with Bryce ! Resorcinal filled the joints nicely - Bob.
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I've never had a perfect one. I've had one so bad that I put a couple of thin shims in it. Then glued it with Unibond. Problem solved. ;)
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IMO a gap big enough to slip paper into is ok, a playing card gap isn't. I always have a little hole at the end of the points but I don't worry about that. It makes a difference where the gap is too, some areas will close up fine with the firm clamping pressure.
I've had a 100# warbow hold up fine with Z splices that weren't "perfect" but were good.
Del
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If your using epoxy you need a gap or the joint will be starved of glue
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I respectfully disagree. The amount of glue doing the work is a very small, thin film, ideally. Do a thought experiment: if you placed two surfaces an inch apart and filled the gap with the best glue possible and let it cure completely, would you trust it?
If I understand correctly, the slow curing epoxies are strong because the have time to soak into the wood surfaces and increase the amount of wood that becomes bonded. That, and good glues are less brittle than cheap or quick drying epoxy. Gaps are just weak spots. If you are worried, let the glue soak in a little before joining the parts.
I should tell you that my experience comes from building model airplanes, where holding things together is always a good idea. I will admit I have never spliced a bow stave, but the principle is the same.
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I use epoxy and I steam the joint. The fit I'm referring to is before steam. I did another one and this turned out better right out of the bandsaw. Before these two I was following John Strunks chapter in TBB? He suggested clamping the two staves together and cutting both at once. I could never get a close fit. Then it occurred to me, if you make a slight curve in your cut when they are clamped together, when you fit them together the curves will go in opposite directions and the slight curve becomes a wide gap. These last two I laid out and cut individually and the results were way better. There's a 50/50 chance one of my cuts will be straight. This is not slagging John Strunk, I'll bet he can cut a straight line so he can get away with cutting both at once.
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Here's one after about 3 hours. I can see a bit of light through it but a .004 gauge won't go though anywhere. I had to see if I could do it. The straight cut in the center started out at 4", it's 3" now. That's a bit of fiddling. The one in the background was worse. It started out as two 36" billets. ;D ;D ;D(joke) Something I'm wondering about is how you lay out your splices to allow for working the handle later. If you make the splice the width of the stave you rasp away a lot of glue surface when you finish the handle. If you make the splice say an inch wide(like mine) then the angled part of the splice runs quite a way down the bow. If possible could you post some pictures of splices. Either finished or in the rough
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A splice is very strong and won't come apart if you file away some of the glue line. I have spliced around 100 sets of billets, even added new a new limb to an existing spliced bow and never had a failure.
Here is a bow that just got a new limb, pretty narrow but the bow held up just fine.
(http://img.photobucket.com/albums/v181/ekrewson/bow%20making/newcopperheadlimb.jpg) (http://smg.photobucket.com/user/ekrewson/media/bow%20making/newcopperheadlimb.jpg.html)
I just about always glue on a piece to build up the back of a handle which adds a bunch more glued surface to the splice.
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If your using epoxy you need a gap or the joint will be starved of glue
That isn't true. if it were how come none of my splices have failed? I always make them so I can see no light (or at least that is my aim!).
'Epoxy' is such a wide term that covers a whole multitude of quite different glues, there is no rule for them all.
A starved glue joint is something I hear a lot of but the forces required to actually starve a joint of glue are very high. As Jeff says a correctly cured glueline is very thin indeed. Some epoxies can fill small gaps. Resourcinol isn't actually great at filling gaps, I spoke to the man who developed that glue back in the day when they made planes with it. He said to always get the mating surfaces fit as close as possible. He said that if you needed it to fill gaps then you should add sawdust to it.
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Well, here's a little unscientific experiment:
I had two approx. 1/4" lams of osage that the planar I was using tore some pretty sizable chunks out of right in the middle of the boards. They were toast as far as bow making goes. So, as a bit of curiosity because of this thread, I glued them together in a less-than-perfect way using 1:1 Smooth-On (it's a 3" splice):
(http://i.imgur.com/ji90Lz6.jpg)
And proceeded to bend the board until it broke:
https://youtu.be/CpKVim_Npuk
It's kind of hard to see how much I bent it from the angle of the camera, but it was bent a bit before it failed. Now, take this for what its worth, as I'm not going to go through the effort of making a solid splice on crap boards and I'm not going to bend a good board that is not spliced together (in order to make it more "scientific", that's what would need to happen). I'm also not saying "this would have been good enough!!", I'm just presenting what I recorded.
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It does make me feel a little more confident in my splices. ;D ;D Thanks for that
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I always make my joints as tight as I can, I use chalk to check for high spots and cascamite is my weapon of choice, but two part epoxy is different. Two part epoxy has little or no penetration into wood fibres. Boat builders will butter a plywood joint with epoxy before applying the laminating layer of epoxy. This joint is only lightly clamped, enough to hold the laminates together, and very specifically not enough to squeeze out the epoxy, as the glue does not penetrate the fibres it relies on the fillet for its strength. In point of fact the epoxy can even be thickened with wood flour for gap filling and certain construction methods use a fillet of epoxy on the surface to strengthen a joint. I guess it reply all depends on your adhesive
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I always make my joints as tight as I can, I use chalk to check for high spots and cascamite is my weapon of choice, but two part epoxy is different. Two part epoxy has little or no penetration into wood fibres. Boat builders will butter a plywood joint with epoxy before applying the laminating layer of epoxy. This joint is only lightly clamped, enough to hold the laminates together, and very specifically not enough to squeeze out the epoxy, as the glue does not penetrate the fibres it relies on the fillet for its strength. In point of fact the epoxy can even be thickened with wood flour for gap filling and certain construction methods use a fillet of epoxy on the surface to strengthen a joint. I guess it reply all depends on your adhesive
Epoxy soaks in more than that. If you butter up laminates and let them sit, "dry" spots will emerge and need to be topped up before assembly.
Also end grain soaks up more volume and that's the part you are putting it on when splicing.
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Agreed, I am given to sweeping statements, the buttering is mostly used, I believe, for open-pore woods (I did say plywood)and does soak in, needing topping up for the laminating. So, doesn't that confirm the importance of a (thin) fillet? The laminating coat is needed because the joint has gone dry as the butter coat soaks into the fibres? Bow-woods tend to be dense, not porous, but in the end grain of a z-splice the extra glue would be more important. wouldn't it? Like Mike said, different adhesives will need slightly different approaches so, for pva, cascamite and resourcinol, really good, tight joints, tightly clamped, are important, but, with two part epoxy,while a good tight joint is also really important, a loose clamp and a thin film might be necessary. For instance, the West 105 system directions specifically state the builder must avoid starving the joint by "wetting out first...and then must ensure they apply sufficient epoxy to ensure the joint is not starved...squeezing out a small amount by clamping with the force of a firm hand-grip"
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Just one more thing. When you are test fitting to you just use hand pressure or maybe tap the end on the floor a bit?
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You run the chance of splitting if you do that. I've had snug splices split by hand pressure. Depends on the wood too of course.
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That's true, I have split Yew that way, OS not so much. I am working OS at the moment so that slipped my mind. I've got a pretty slippery mind sometimes. Sometimes a feeler gauge will go in on one side of the tang but not on the other. You know a little tap will just fix that right up. Thanks Pat
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I always make my joints as tight as I can, I use chalk to check for high spots and cascamite is my weapon of choice, but two part epoxy is different. Two part epoxy has little or no penetration into wood fibres. Boat builders will butter a plywood joint with epoxy before applying the laminating layer of epoxy. This joint is only lightly clamped, enough to hold the laminates together, and very specifically not enough to squeeze out the epoxy, as the glue does not penetrate the fibres it relies on the fillet for its strength. In point of fact the epoxy can even be thickened with wood flour for gap filling and certain construction methods use a fillet of epoxy on the surface to strengthen a joint. I guess it reply all depends on your adhesive
Have you read the bowyers bible on glues?
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One word .... epoxy.
I've never had a splice that didn't fit like crap .... I've also never had one fail .... I use smooth-on EA40
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I use, and have for many yrs devcon 2-ton slow set, never had a failure that I know of. I only use this for splices.
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I always make my joints as tight as I can, I use chalk to check for high spots and cascamite is my weapon of choice, but two part epoxy is different. Two part epoxy has little or no penetration into wood fibres. Boat builders will butter a plywood joint with epoxy before applying the laminating layer of epoxy. This joint is only lightly clamped, enough to hold the laminates together, and very specifically not enough to squeeze out the epoxy, as the glue does not penetrate the fibres it relies on the fillet for its strength. In point of fact the epoxy can even be thickened with wood flour for gap filling and certain construction methods use a fillet of epoxy on the surface to strengthen a joint. I guess it reply all depends on your adhesive
Have you read the bowyers bible on glues?
Nope, shamed to say I haven't, :-[ worked in boat building for a while, 20 years in wood-working. ;) Always learning, always happy to be taught ;D: is Cascamite a bad choice (for non-bending) splices? Just in case, I would add that depending on what I am gluing, I also use 2-part epoxy, cyano, foaming poly glue, hide glue, PVA (white or yellow) or even varnish, it all depends. >:D
My second post on two-part epoxy, I hope, clarified and evidenced my statement -speaking personally, I think you should always consider what you are sticking, what you are sticking it with and what the manufacturer's guidelines say about the suitability of the adhesive and the proper way to apply it :laugh: