Bow Broke..:-( Help me understand what went wrong

[arachnid]

Hi Guys.

I finished a KennyM 60" longbow, came out 46#@28".
I had a 18" long riser, 20" long powerlam and 8" ling tip wedges.
Took it out today for some test shots it shot very well. Until I draw it and as I reached full draw- BOOM.... the top limb broke. No warning.
No the thing is, it seems like the back glass peeled of and remained intact. I don`t have a clue as to what went wrong and I`d like to know so I won`t make the same mistakes again. Here are some pics... Please tell me what you think.

Thanks.

   

     

   

   

   

   

   

   

[arachnid]

If more pics are needed, let me know

[Wolftrail]

Looks like a De-lam failure to me. I hate it when they BREAK..!  As long as ya dont get hurt.
   

[arachnid]

Originally posted by Wolftrail:
Looks like a De-lam failure to me. I hate it when they BREAK..!  As long as ya dont get hurt.
     

What can be the cause of it? And do I avoid it?

[arachnid]

Is it salvageable?
Can I make a new limb?

[Pat B]

Looks to me like the core wood gave loose for some reason. Curly woods are violated when sawn but I would have thought the glass and glue would have held it together. You can see how the wood separated along the grain. Whether it was material failure or manufacturer failure I can't say.

[wood carver 2]

The glue around the fade had bonded. Everywhere else, it just let go. It's like the wood was contaminated with oil or wax. Was the epoxy from a fresh can? Is it possible that the glass had mold   release on it?
Those are some possibilities that come to mind.
Sorry to see that. It was a nice bow.
Dave.

[arachnid]

Originally posted by wood carver 2:
The glue around the fade had bonded. Everywhere else, it just let go. It's like the wood was contaminated with oil or wax. Was the epoxy from a fresh can? Is it possible that the glass had mold   release on it?
Those are some possibilities that come to mind.
Sorry to see that. It was a nice bow.
Dave.

The glass was all good. The glue wasn't a new can, it was actually the end of a can... I didn't measure the amounts, I just eyeballed it, can that be the reason?

About the oilly wood- I didn't wipe it with aceton (I've read the it releses more oils) , I sanded it and then made grooves with a hacksaw blade to give it more gluing surface.

Don't know why it gave....

Well, is there a way to make a new limb and salvage this bow?

[kennym]

Hey Dor, hate that for you.

It is really hard to say what caused it, once the delam starts, there are a lot of things that happen that might have held forever til the stresses get changed by the thing that started the prob showed up. Hope that made sense!

To me it looks like the glass started coming loose at the tip and then tore the wood lams toward the riser.

Can't really say tho.

As far as making another limb and putting it on this bow, there were some guys at one time doing a half form and doing one limb at a time on one pieces, but I never tried it.

How many lams in there between the glass?

[arachnid]

Kenny, I had 2 lams- one parallel and one tapered. Does the number of lams really matter?

I took another look at the fades, and I noticed something on the lower fade (the one that didn`t break). Eventhough I had a pretty good glue line, once I bend the limb, it the lams rise. So something wasn`t going well...

That`s the lower fade when the limb is not bent
   

That`s the fade when the the limb is bent. | gap appears between the powerlam and the core lam
   

Don`t know if that caused the bow to fail or if it`s a result of the bow faliure...

[arachnid]

Can the reason be a too short fade-out?

[mwosborn]

Sorry to see that!  Gaps between lams are not good.  What kind of epoxy?  Did you use a hose on your form?

As a general rule I think the the fade should be about 1/16" at 1" from the end...
   
How many pounds did you air the hose up to??  I am not a big fan of going over 40 lbs...  Did you have enough resin on there and get plenty of squeeze out??
  The time you waste fixing that one, you could build a new one...  Maybe give it to a guy with one arm...  

I know it did not fail there, But IMHO I would stay away from glue joints in the fades... I like a solid piece of wood there...  It is a very high stress area...

[Stagmitis]

It looks like you may have issues with your form and clamping pressures. I see another gap up the Belly ramp...  Did you make the Lams yourself, if so what grit paper..Did you use compressed air or a brass brush to clean the lams well after using a hacksaw blade? Check for moisture in the wood? Did you use smooth-on?

[arachnid]

Originally posted by Stagmitis:
It looks like you may have issues with your form and clamping pressures. I see another gap up the Belly ramp...  Did you make the Lams yourself, if so what grit paper..Did you use compressed air or a brass brush to clean the lams well after using a hacksaw blade? Check for moisture in the wood? Did you use smooth-on?

I don`t have a air hose, I use clamps. I make dry runs and check the glue lines (especially the fades) before I glue it up.
I make the lams myself with an attachment to my oscillating spindle sander, using an 80 grit sanding sleeve (I can`t find a lower grit). After I use the hacksaw blade I sand and clean the lams so theres no chips and dust.
I use smooth on.

A german guy on facebook (bowyer`s corner group) said the problem was my fade outs being too short. Possible?

[kennym]

Shredd may have hit on something with the accents into the fades, look at how the black bends right at the joint on belly.

But it may be the fades are a bit blunt too. 1/16" at 1", and 1/8" plus a little at 2 (.140" and yes I measure with dial calipers when grinding them) is what I always try to get.

[arachnid]

Originally posted by kennym:
Shredd may have hit on something with the accents into the fades, look at how the black bends right at the joint on belly.

But it may be the fades are a bit blunt too. 1/16" at 1", and 1/8" plus a little at 2 (.140" and yes I measure with dial calipers when grinding them) is what I always try to get.

Well, at least I learned something so this failure wasn't for nothing. Guess it's just the learning curve... Too bad it cost so much time and money...

While were at it.. Is there any other important things I need to know? Mistakes usually made? Things I should avoid? Important measurements I need to know (like the fade length and thickness)?

[Crooked Stic]

What kind of core wood? It looks curly to me. If you use curly grain for much more than veneers you are asking for trouble.

[arachnid]

Originally posted by Crooked Stic:
What kind of core wood? It looks curly to me. If you use curly grain for much more than veneers you are asking for trouble.

It's Maple. I use 2 lams, one parallel and one tapered.. No veneers.
I thought that only all wood needs straight grain.. Thought glass lam bows can get away with any grain

[mwosborn]

I don't think your core wood is the cause of the failure.  However, it may have contributed to the failure once the glue began to let go.  We are all just speculating here as to the cause or the 'why did this happen".  During the shot, there are a lot of forces acting on the bow stressing wood, glass, and bonds.  Based on the pictures I have seen, I am betting on the bonds between the lams and glass being the cause.