Bow limb broke. Help!

[WilltheArcher]

Hi guys - I'm making my first bow, a 3-piece takedown longbow. Limbs are an ash/ipe laminate. I've strung it and pulled it back (just a little bit for a while as the limbs got used to stretching, then to full draw) many times. Unfortunately today during the first big test at a range, one of the laminate limbs broke as I was stringing the bow. Here are a couple pictures of the breakage.






checking the tiller the day before.


It looks like only the ash laminate broke but I haven't had a chance to unbolt to check. The hole pattern is .75" from the bottom of the limb, then 1" between the two bolt holes. Bolts are quarter inch.

I laminated it together using titebond III wood glue and a ton of clamps. Here's a picture of the lamination: http://i.imgur.com/hmYdKiO.jpg

Anyone have any idea what I did wrong and what I can do better next time? I've got 5 days till I lose access to my woodshop so I'm kinda in a crisis crunch. All advice appreciated, THANKS!

Edit: The limbs are 28.5 inches long. They're 1-1/4" wide at the riser and taper to 5/8" at the end. The riser bed is 3" long. I checked and only the ash lam broke. The ipe below is relatively unscathed, except for an edge splinter about 8" up.

[Gordon]

Are you certain you made all the surfaces perfectly flat before gluing them? When you use TB for laminating there is no room for error.

[Peacebow_Coos]

Now that's a frankenstein!  What is that on the tips?  There was just a thread posted by Fiddler 49 about field wrapping a bow that dealt with having wedges in the limb area that contacts the riser.  That may help.  I've never made one but I'd say something funky is going on with the fit to the riser, and depending on the unbraced shape your tiller may be a touch stiff in the outers?

[Del the cat]

The bolts should bear down onto a plate the full width of the limb.
Those two washers are concentrating the stress, which would be ok on a woven material like carbon or glass fabric... but it's just going to make wood split along the grain.
Mechanically it was an accident waiting to happen.
Hope that makes sense.
Here's a pic of how I joined a limb to a riser... it was glued, but it dovetailed in along the whole width of the limb, and that's the key.
Del

[WilltheArcher]

The bolts should bear down onto a plate the full width of the limb.
Those two washers are concentrating the stress, which would be ok on a woven material like carbon or glass fabric... but it's just going to make wood split along the grain.
Mechanically it was an accident waiting to happen.
Hope that makes sense.
Here's a pic of how I joined a limb to a riser... it was glued, but it dovetailed in along the whole width of the limb, and that's the key.
Del

Del - thank you, and yes that makes perfect sense. Ironically I was already planning on replacing those washers with a much larger washer plate that spanned very nearly the width of the limb. Picture below. Do you think this will be adequate to relieve the stress concentration?

My friend and I were debating whether it delaminated then split or split and that caused the delam. You think it was the split at the fastener, right? I'm worried I did something wrong with my lamination process, so that'd be bit of a relief.

Are you certain you made all the surfaces perfectly flat before gluing them? When you use TB for laminating there is no room for error.

Gordon, I am planning on making another laminate. Is there a better material than TB? Maybe some sort of epoxy? I am certain the limb surfaces were flat. I used a wood planar to get the lams to thickness and then lightly sanded both.

[Del the cat]

Just make it full width, don't mess with anything less.
The plate can be glued to the limb or left loose and clamped by the bolts.
I'd suggest something like 3/16" inch plate.... there is a lot of leverage...
The limb is effectively trying to pivot along the riser edge where I put the yellow arrow on the right of the pic so that's only about 3" from the lower bolt... the rest of the limb is say 24" long... so that's 8:1 leverage ...
Say 40# pull on the end of the limb, that gives 320# on that bolt !!! ... and you wonder why the wood split?
That's why they don't make pry bars out of wood
Del

[bow101]

Look at this post I hope it helps..
http://www.primitivearcher.com/smf/index.php/topic,52745.0.html

[WilltheArcher]

Just make it full width, don't mess with anything less.
The plate can be glued to the limb or left loose and clamped by the bolts.
I'd suggest something like 3/16" inch plate.... there is a lot of leverage...
The limb is effectively trying to pivot along the riser edge where I put the yellow arrow on the right of the pic so that's only about 3" from the lower bolt... the rest of the limb is say 24" long... so that's 8:1 leverage ...
Say 40# pull on the end of the limb, that gives 320# on that bolt !!! ... and you wonder why the wood split?
That's why they don't make pry bars out of wood
Del

Del - really appreciate the speedy responses. I've got five days to make a bow that works! Doesn't have to be perfect, or last for super long, just needs to work (for a class).

Ok, I'll make plates that are the full width of the limb (1.25") and the height of the riser bed (3"). I'll let the 1/4-20 bolts clamp them down. I've got 1/8" steel plate - don't have 3/16 but this stuff is really solid so I'll hope it's enough.

Bow101 - I checked out that link, and I see how for 3pc bows they have that wedge that gives you a bit more support along the high-stress pivot area. It's smart, and unfortunately not something I'll be able to pull off in my time remaining. I hope this steel plate does enough to distribute and reduce stress concentrations in the riser bed.

Would like opinion on the lam though. I've got another 6' piece of laminated ash and ipe that I'm planning on cutting a new set of limbs out of. Friend of mine recommended I try splitting it with a chisel and redoing it with some sort of epoxy that might be stronger. Seems a little extreme to me, but we're both worried that maybe my TBIII lam wasn't strong enough. Think that can be improved, or should I focus on the steel plate?

[PatM]

 More likely to be glue failure first  and then the Ash failed in shear at the bolt when it was essentially reduced to a cable.

[WilltheArcher]

More likely to be glue failure first  and then the Ash failed in shear at the bolt when it was essentially reduced to a cable.

Hey Pat - yea that's what I'm worried about. Problem is I've got no idea how to improve the lam. Planed the wood, lightly roughed up the surface with sandpaper, then TBIII and a ton of clamps and 36 hours of dry time. Should I try to get my hands on more ipe and then lam with epoxy?

My school's woodshop closes on Friday so I've got until then to make this work. If I can't get my hands on more ipe in time I may try to make a set of limbs out of red oak, provided I can find a piece with a good grain at my local woodstore. If so, what thickness do y'all recommend? Still planning on keeping the ~30" height and 5/8" taper.

[bow101]

Just make it full width, don't mess with anything less.
The plate can be glued to the limb or left loose and clamped by the bolts.
I'd suggest something like 3/16" inch plate.... there is a lot of leverage...
The limb is effectively trying to pivot along the riser edge where I put the yellow arrow on the right of the pic so that's only about 3" from the lower bolt... the rest of the limb is say 24" long... so that's 8:1 leverage ...
Say 40# pull on the end of the limb, that gives 320# on that bolt !!! ... and you wonder why the wood split?
That's why they don't make pry bars out of wood
Del

Del - really appreciate the speedy responses. I've got five days to make a bow that works! Doesn't have to be perfect, or last for super long, just needs to work (for a class).

Ok, I'll make plates that are the full width of the limb (1.25") and the height of the riser bed (3"). I'll let the 1/4-20 bolts clamp them down. I've got 1/8" steel plate - don't have 3/16 but this stuff is really solid so I'll hope it's enough.

Bow101 - I checked out that link, and I see how for 3pc bows they have that wedge that gives you a bit more support along the high-stress pivot area. It's smart, and unfortunately not something I'll be able to pull off in my time remaining. I hope this steel plate does enough to distribute and reduce stress concentrations in the riser bed.

Would like opinion on the lam though. I've got another 6' piece of laminated ash and ipe that I'm planning on cutting a new set of limbs out of. Friend of mine recommended I try splitting it with a chisel and redoing it with some sort of epoxy that might be stronger. Seems a little extreme to me, but we're both worried that maybe my TBIII lam wasn't strong enough. Think that can be improved, or should I focus on the steel plate?

I dont know other than right it off and start again,  today as I type this message I'm trying to fix a a mistake !....will it work...?  I will find out in a couple days and post it if the bow lives.......   This bow  is a repair beyond repairs like 3 times. 

[Pat B]

Looks like you squeezed all of the glue out and left a dry glue line.

[WilltheArcher]

Looks like you squeezed all of the glue out and left a dry glue line.

Hey Pat - does that mean I clamped too much? I didn't realize that was possible, but that could explain it. It's got to be either the stress concentration like Del said, or the lamination itself. Just wish I knew which...

What is that on the tips?  There was just a thread posted by Fiddler 49 about field wrapping a bow that dealt with having wedges in the limb area that contacts the riser.  That may help.  I've never made one but I'd say something funky is going on with the fit to the riser, and depending on the unbraced shape your tiller may be a touch stiff in the outers?

Cast aluminum bowstring nocks are bolted to the tips with 6-32 screws. Maybe it's a bit fantastic, but it's pretty fun. It's funny because that's the area I was most worried about - holes, even that small, near the end of the tips. But it's slotted on so the stress is probably pretty well distributed.

[bubby]

I usually use an overlay on the attachment area to help to distribute the stress that full size washer might do the trick

[Badger]

   Did you rough sand the ipe, if you put it through a planer it might not be pourous enough.