Riser Disaster! Twice!! Help!!!

[Erwincm]

Good evening guys. I've been struggling to construct my first laminated riser design and need your help to figure out what I'm doing wrong. I've had two bondline failures in a row and I'm starting to despair :-(

   

The first riser failure was with a honey locust - cherry combo and happened during final sanding. I noticed a hairline separation and was able to pull the pieces apart by hand. This particular bow used a strip of .05 thick solid copper as an accent strip between the wood pieces and the copper was thoroughly scored prior to bonding. The wood pieces were shaped using a custom laser template of my own design and a Robo-Sander with roughest grit (60 grit I think). I used Smooth-On purchased about 14 months ago and a series if large C-Clamps to fixture the pieces together. I cured at room temp for 2 days before removing the clamps and bandsawing the riser to rough shape. When I separated the pieces, it seemed like there was much more glue on the outer riser wood than the copper accent strip so I assumed either copper was a poor material to bond to or my glue had expired.

Determined not to give up without a fight, I ordered new Smooth-On and some black & red fiberglass strips to use as accent strips. I followed the same process as above only this time my riser was constructed of quilted maple and African Blackwood. I took the glued block in to the woodshop today and starting milling it down to square all the faces when I noticed another hairline crack. Again, I was able to pull the riser apart by hand :-( This time the glue adhered just fine to the fiberglass strips but did not adhere at all to the African Blackwood even though it had visible grooves from the Robosander.

Do you guys have any idea what I could be doing wrong? I'm starting to lose hope...

Thanks,
Charles
Madison, AL

[LookMomNoSights]

Oh man that sucks.....your shaping looks absolutely beautiful!!!!
The copper thing seems to be a little weird....in regards to adhesion.  Picturing it in my mind looks cool....but the idea of bonding it in a riser gives me the willies.  But you had the same problem with the fiberglass strip!  Isn't African black wood oily or have an oil/waxy property to it that requires extra attention before trying to get it to bond?  Im interested to see what the seasoned guys have to say.....   Im sure you will work the bugs out.  And when you do,  oh man..... looks like you'll have some great looking risers!  Ill be following this.    

[wood carver 2]

I can see where the epoxy might not bond wood to metal, but wood to wood shouldn't be a problem.
Oils in tropical wood can cause issues. When in doubt, I resort to doing the same glue up using common American woods only. If it works fine, then, chances are, you have something going on with the tropical stuff.
Dave.

[wood carver 2]

BTW, I recently had a padauk/bamboo bow limb delaminate in the same way. It turned out I neglected to clean off the sanding dust thoroughly enough.
Dave.

[rmorris]

When you made your template how much offset did you give yourself for the two different radius? If you don't have enough or too much offset for your accent material you can have problems such as the ones you are having. Looks like the glue failure may be where the two pieces of wood are too big to flex and smooth on is not a great gap filler.

The first picture looks like there may be a fairly large glue joint on the left side of the copper while the second picture looks better.

[Krasus]

With the exotic oily wood,I give them a bath in acatone.  The oil will literally run out of the wood. It's pretty cool. Then you can use your glue or epoxy. I actually use titebond mrk3 on my risers.  It has a flash point over 200deg so when you do your main glue up the heat isn't a issue. It holds good. Never had one pop on me.

[mikkekeswick]

I recommend against using acetone on wood it simply lifts excess oils to the surface. There is quite a lot written about this subject on various forums if you do a bit of searching. The consensus is that it's a bad idea.
However for composite materials it does a good job. Just give it a good half hour to fully evaporate.
If you read Smooth-On's data sheet for EA40 it recommends sanding to 120 grit. Curing temps are 2 hours at 65 degs celcius, 2 hours at 75 degs and finally 2 hours at 85 degs. Then to leave the part to cool for 6 hours before removing clamps. That is their recommended procedure. Basically the bond strength improves when cured with heat.
Clearly virtually nobody heats their parts so much BUT that is Smooth-On's own recommended procedure....so it probably best to follow it as closely as possible.
If you go onto their website you can download the data sheet. I always do this with any new glue - the way I see it is i've put all that time into the project and the last thing I want is glue issues and the beat people to ask are the manufacturers.
I know from a knife maker friend of mine that copper is a pain to glue properly, exactly why I don't know but I could find out what he uses now.
Before making your next riser sort out your glue-up procedure and glue up some scrap pieces first. Once cured take them to a vice and have at 'em with hammers, spanners etc!!! Nothing like confidence in your work!

[BigJim]

I would think that the copper was a bad choice..but have no experience there.

The picture looks like wenge and not African blackwood...but pics can be deceiving.

The oils in exotic woods have less to do with adhesion issues then they are blamed for. These woods are not able to be kiln dried and are difficult to determine dryness without special equipment and or knowledge of storage and age.

Wenge is kiln dried, but even super dry wood exposed to a damp or wet environment will soon become too "wet" to glue.

Woods with MC higher than 8-9 % are not good candidates for glue. If you purchased your wood from a typical wood supplier, it is likely that you received it very or partly wet.

These are just my speculations though.
Bigjim

[bamboo]

IMO--the near [over 50%] butt joint would be too much to ask of the glue without the copper

though your design is striking!!!truly beautiful!

 you might try to incorporate a full length "I"beam into your riser-leaving the same look on the outside--although I have moved away from non-wood accents as down the road the always seem to move or glue lines fail----

  good luck!

[Troy D. Breeding]

Not sure the copper was the right idea, but the other riser looks to have a great fit on the joint and with glass and wood you shouldn't have had a problem.

That said, my next thought is on amount of glue used. Did you have a good squeese out of glue? The only time I've seen failures like this was from not using enough glue and basically having a starved glue joint.

[kennym]

I don't like the copper deal at all, but haven't done it so can't say.

The only other thing I see is the joint is too strait across the riser, almost like having a butt joint in it.

Just an observation.

Still shouldn't be able to peel it apart by hand tho,so Jim most likely has it with wet wood....


An I beam would most certainly help in there.

[Wolftrail]

Basically what kennyM says, I would be more inclined to do a cut line from end to end without to much deviation.

[LittleBen]

i think woodcarver was onto something. Try it with easy to glue domestic woods. If the problem persists, then it's not a wood problem.

[BenBow]

How long do you mix your glue? I go at least 2 minutes then let it set for at least that long to be sure it fully blends together.

[chackworth3]

I would guess either the wood is too wet or you had a dry joint.  As for the copper, I don't have any experience with it but seems like it would be hard to get a good bond with smooth-on.

[Erwincm]

Thanks for the kind comments on the aesthetics. I like to try to do things to make my bows look unique from other designs. Unfortunately, the "unique" has gotten me into some trouble here. Btw, here's a better picture of the honeylocust-cherry-copper riser from the pretty side:

 

Now let me see if I can answers some of the questions:
1. I created my Robo-Sander templates using CAD and a laser cutter with a .05 gap for the copper thickness. Eveything looked like a pretty good close fit when I clamped it up.
2. I spoke with Elmont Bingham today about the issues I'm having and he also mentioned curing at higher temp for a stronger bond. I will try that for sure next attempt!
3. I would like the source for that wood-metal epoxy. I assume it's clear like Smooth-On?
4. The picture looks like Wenge because the lighting shows a lot of contrast but it's definitely african blackwood. The wood is actually a darker and more consistent color in reality.
5. I had all the woods tested less than 8% moisture with a moisture meter before construction.
6. We had several dry days prior to bonding and the parts were bonded indoors in an air-conditioned shop so the humidity was fairly low.
7. I brushed all surfaces to be glued with a stiff clean brush before glue application and I rubbed the glue into the wood grain with finger pressure to ensure good coverage and absorption.
8. I got good glue squeeze out in the joints when I clamped it all up.
9. The I-beam idea is a good one but I don't think I can modify these risers to make it work for these parts :-(

I'm thinking about drilling a couple of blind holes into each mating half of the wood pieces and gluing in some hardened steel pins or threaded rods to hold it all together. It will be very tough to get everything lined back up but I can't give up on all this nice wood without at least a college try. It will break my heart to have to throw it in the trash heap. I can't think of any other way to try to salvage the project. I'll post some pics if I am successful. Wish me luck!

[mikkekeswick]

On your moisture meter do you have to enter the wood species or does it just give a reading on any wood? I ask because moisture meters measure mc through electrical resistance and can be notoriously inaccurate with tropicals.
I personally would cure it with 65 deg celcius minimum heat.
The problem with the copper was that it oxidises later down the line and then the oxidisation 'peels' causing a 'glue' failure. He uses rivets now!!

[Bodork]

I use smooth-on when making knives and have had excellent results bonding wood to steel and brass. The pins do the majority of the work though. I ruined some scales once after they were on the tang and decided to take the knife apart and re-do it. I was very surprised how well the epoxy held the brass liners to the steel tang without the pins. I rough the metals up with 50 grit and blow everything off with compressed air. I also drill several holes in the material where they won't be seen to make little epoxy pins. Don't have to heat it very long because that metal gets hot real fast! Keep in mind that knife handles don't have to flex.... Mike

[Mad Max]

I googled this.

Does Copper expand more when heated than wood?
EDIT
   Answer by   Rosencrantz    CONFIDENCE VOTES 2.4K
It depends on what type of wood, but overall copper does expand more than wood when heated. The main measure of predicting the amount of a material would expand under heating is the coefficient of linear thermal expansion, which is a ratio of the amount of expansion per temperature change, the higher the number, the more it will expand when heated. Copper has a coefficient of 16.6 m/(m Kelvin), while pine has a coefficient of 5 m/(m Kelvin), which suggests that copper expands more when heated than wood (at least pine).

[Sixby]

If you can hold the pieces up to sunlight or a strong light and see light with just light pressure it is not sanded close enough.I like to see no daylight anywhere when all pieces are assembled and just lightly pressured together. I pin though the part of the sight window that is going to be cut out so that I get perfect alignment as I sand. It takes a long time to get it right even if you use templates it have been my experience that I still have to hand sand and check for light.
I think that although your design looks good , pleasing to the eye that it is faulty in that your wood does not connect and you are creating a three piece riser which means that the woods are not working together like a plywood but are individualized and there is too much stress. I also believe that the different heating rates of the copper and the wood is a problem. I have experienced this using certain woods together and also using phenolic for beauty stripes. I have not had a problem at all using phenolic for the I beams or for larger areas of the riser. Go figure.
Anyway a good heavy I beam would definitely help but I still would consider changing the design so that the woods connect all the way through.

God bless, Steve