oh bother!

[stuckinthemud]

So, this is a low-stressed ELB, 40# at 24", 73" ntn.  Its a billet bow, v-splice with a cap-piece. I went for a compass-bend but it is moving too much in the centre and burst the cap when I took it to full brace; no biggy, but the split ran off the cap and into the main body of the bow. So, if I make a new cap-piece and glue the split in the bow with runny cyano will that be OK., or should I rasp out the split and then put on a new cap.  Should the cap be  made from a curved piece of timber so I don't violate the grain on it?  The last one, the one that failed, was from straight grain but the centre is working really hard.

Thanks,

[DC]

I tried to bend a V splice as an experiment a while back and it failed. I don't think splices like bending. That said, maybe a longer cap might help. Or plane it flat and boo back it.

[Del the cat]

But on the plus side without that cap piece it prob would have exploded. Maybe make the cap longer with a gentle fade out?
Del

[stuckinthemud]

Looks like I phrased it badly, the v- splice is really solid. The cap is what failed but when I rasped it down I found a tiny void right where it failed, so, I'll just have to try again but I'm going to use epoxy next time.  The cap was 8 inches long, 1/4 thick at the centre and feathered out very gently, the void was a fraction of a mm  deep

[mikekeswick]

The tiller on elbs should be elliptical
Look how much the limbs taper, thinner wood bends further.

[stuckinthemud]

Yes, I was aiming for elliptical but I messed up the taper and ended up with the handle moving too much.  Now its at brace I was hoping to soften the outer thirds but I'm going to end up about 20# under-weight.  Still, never done an elb before, or built a billet-bow, so its all a good learning experience.

[WillS]

You don't need a "cap" over the splice.  I've seen a 160lb billet bow with a Z splice which bends through the handle.  No belly or backing cap to support it.  If you have faith in the splice don't add something else which can fail 

[simson]

I have no clue what you mean with "cap"
Do you have a pic?

[stuckinthemud]

Only the picture at the top of the thread but if you look carefully you can see the glue line and colour change where I planed off the crown on top of the splice and glued in a laminate to tidy up everything

[stuckinthemud]

Right, so I am now on cap-piece number 4.  The current back laminate (cap piece) is apple (Malus) and at 12" long;  its 1/4" thick at the centre, fades out to a feather edge and was glued up "by the book" - degreased, scored, with a resin based glue (cascamite), clamped every two inches.  The other caps were also apple but shorter and glued with two part epoxy, cascamite and cyano.  They have all failed, even after I changed the profile of the bow to an elipse by laminating on a belly lam.

This one lasted 4 draws to 24" before the cap laminate pulled off the bow.  Is apple a poor glueing timber, is it too stiff compared to the bow (blackthorn) - should I try a different wood, or should I cut my losses, literally, and make it into a take-down bow?

[Del the cat]

I try to make splice areas non bending. Don't really know what else to suggest, the only things I do that are slightly out of the ordinary:-
1. I use Highland Spring water to mix the glue, dunno if it makes a difference, but our tap water here is hard.
2. I apply glue to both surfaces despite the instruction saying only apply to one.
3. I use twice as much clamping/strapping as I think I need.
4. I allow overnight curing before any cleaning up and a full 24 hours before stressing the joint.
PS. make sure the glue isn't out of it's shelf life it shouldn't start to gel for about 2 or 3 hours.
Del

[willie]

perhaps a post mortem of the joint failures?
did the wood fail or the glue?
which wood and what is the consistency of the remains?

[stuckinthemud]

 Thanks Willie, I took the joint apart, the glue-line isn't as good as I had thought, maybe I should have wriggled it round to seat it better before I put on the clamps. 



Perhaps I should have one more attempt with this laminate - clean it up and get a better fit. If not then I could go with a different timber - I've got some shorter lengths of mulberry and some grotty bits of yew with decent sap-wood I could try.  The first cap to fail was blackthorn so I'm not sure that would be a good place to go and I don't want to cut any of my decent staves down by a foot.

[PatM]

It's always going to lift if the handle moves. It's like a growth ring sitting up there unsupported.

[willie]

.25 thick by 12 long is a 1:24 ratio. you should be able to glue that successfully. a good epoxy will help if the fit is less than perfect, but with any joint, a better fit helps. gluing can be an skill of it's own that needs practice, like anything else