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Main Boards => The Bowyer's Bench => Topic started by: djschwartz90 on February 01, 2015, 07:53:00 PM
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Hey everyone, I'm a relatively new bowyer (and new to the site) with a couple laminated longbows under my belt. Sorry for the length of this post, however I am having a persistent problem achieving what I feel to be an adequate bond between my glass and core wood... here is what is happening:
I just glued up a Hill style longbow two days ago. After cutting the limb profile on the bandsaw, I am able to peel the black fiberglass off of the core wood (on the narrow strips removed from the limbs) relatively easily and cleanly. The wood core laminations do not do this, they tear and break.
When I noticed that the glass could be pealed away from the core wood like this several bows ago, I began to worry about about shooting them. None of my bows have ever delaminated while shooting, however I sacrificed one of them and used a razor blade at the tip of the limb to try and pry the glass off of the core wood. Sure enough, once started, I was able to "zip" the glass off of the entire limb fairly easily, leaving a clean, smooth surface! At the time, I was using thickened West System epoxy for my laminations; thinking this was the problem, I just switched to Smooth On as I have not heard of anyone else having this problem. Unfortunately, I saw no improvement with my most recent Hill longbow, as noted above...
As for my general gluing procedures, I grind my limbs with a drum sander using 100 grit paper. I wipe down my fiberglass (the rough side) with denatured alcohol, blow it dry with compressed air, and let it sit for 10-15 minutes. I apply epoxy to both mating surfaces of each lamination, use C-clamps and wood/rubber pressure strips to hold the lams to my form, and am careful to tighten the clamps only moderately tight (as tight as I can get it with my thumb and two fingers). Based on what is occurring, it seems like I am getting a starved glue joint; I just don't see how my clamp pressure could be more than the 60-80 psi that guys using the air hose method are producing. I feel like if I use any less pressure, my glue lines will suffer.
Has anyone else had this problem? Again, I have never actually had a bow delaminate, maybe I'm just worrying about nothing, but this just doesn't seem right to me. Any suggestions would be very much appreciated, thanks!
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Here is a picture of the glass peeling away from the core wood: (https://plus.google.com/u/0/photos/103439524009386716068/albums/6111043726217914097/6111043733120644978?pid=6111043733120644978&oid=103439524009386716068)
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(http://i.imgur.com/w3uycM5s.jpg)
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Sorry, here's the pic. I'm learning lol...
(http://i.imgur.com/w3uycM5.jpg)
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I had one bow do that to me a while back, I used the microwave to heat the epoxy that time and I think I got it too hot... I mixed the epoxy and by the time I was clamping it the left over glue went solid and clumpy... After curing the bow when I took it out of the form the glass peeled off like that.
I stopped heating the epoxy in the microwave and haven't had an issue since.
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Bummer man. Hopefully someone will have some good advise for you, but that someone is not me as I have yet to make a laminate bow. I only have experience build selfbows. Hoping to do my first R/D soon so I will be interested to see what happens.
Dave
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Sure, starved glue joint. You could try to use coarser sandpaper to grind lams, I use 40 grit, and use less pressure on the clamps.
When I clamp, I use a thumb and one finger and it is nowhere near as tight as those two could make it. Perhaps half as tight as they're capable of.
Also, I don't blow off with air, I don't want to risk introducing any moisture.
How far apart are your clamps?
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What temp are you curing at.If its too high it will do that.Been there and done it.Its not starved of glue using SmoothOn.100 grit is also plenty rough for sanding.
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My clamps are 2-3" apart. And I didn't bake it, I cured it at a little higher than room temp (the heat set at 80) for 48 hours.
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Did you wash your glass with acetone? Sometimes glass can have some residual oil, dirt etc.
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The only time I had glass peel up was when I mistakenly glued up the glass smooth side down. Is there any chance that this could be the issue?
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Is your core wood the correct moisture? Glue mix correct. Stir the smooth on 5-6 mins. and it don't hurt to have more resin in the mix (white part)I never clean the wood with chemicals. Glass either unless it has oily smudges on it.
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I did wipe the glass down with denatured alcohol, but I let it dry for 10-15 min. Glued it rough side down, glue was mixed well, 1:1 by volume. As Bowjunkie said earlier, it seems like it is related to too much clamping pressure even though I thought I was going pretty light. How tight should C-clamps be when gluing up a bow?
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SmoothOn will cure at room temp but it improves a lot with heat up to 150 degrees.If you thought you were going pretty light on your clamp pressure you probably were.Cold thick SmoothOn is harder to push out.When it gets hotter and it gets thinner your clamp pressure will get even lighter and more glue will ooze out.A bow with 60# air pressure on it and is put in a hot box will squeeze out most all of the glue.Epoxy works on a molecular level.
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Yes I also doubt whether it's a starved joint. You have to really cinch those clamps up to do that.
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When you peeled the glass off did the glue stay on the wood or on the glass? Do you have a oil and moisture filter on your air compressor line?
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I vacuum the lams instead of blowing off with air. The small brush on the vacuum I wash sometimes in dishwashing soap and rinse it to make sure it is clean.. Bue--.
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Also, make sure your denatured alcohol is not contaminated. Instead of using the compressed air, try just using a stiff bristled nylon (or similar) brush to clean your lams and glass. Try mixing the smooth on at 2 parts A: 1 part B. Make sure that your form and lams are perfectly flat.
Are you clamping directly to the glass, or do you have some sort of pressure strip?
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I was thinking the same thing.I wonder if the denatured alcohol was contaminated or if it penetrated deep enough that it just appeared to have dried.I quit years ago trying to degrease woods and glass.On oily woods you just seem to keep drawing oils up to the surface by dissolving them.Just a good sanding is all thats needed.
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I have a bit of a different take on your issue, I think the sandpaper the manufacturer is using to prep the glass is getting to dull and not etching the glass well enough. I've had several limbs come back to me from random batches the past several years where there will be pea to quarter sized spots on the limbs where the glass has started to separate from the veneer. I grind all my veneers, (except veneers I'm going to dye) with 36 grit to make sure I'm getting lots of surface for the glue to stick to. I've started hand-sanding my glass now with a piece of 60 grit to increase quality of the gluing surface. Hope that eliminates the problems in the future.
If I were you, I would go with a rougher sandpaper for your laminations than 100 grit, at least 60 grit. It would also be a good idea to rough up that glass a little more as well. my $.02 for what it's worth.
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If you want to get a higher temp on the set up without a box.
I put two shielded 100 watt light bulbs on my table next to the form. Cover with a wire basket and add a sleeping bag over everything.
Works well over night.
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120 grit is what is recommended by SmoothOn.They test their products more than we do so I would go with what they recommend.
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I have worked with epoxies for over 30 years and have found that mixing ratios are not that critical. I don't wipe my lams or glass before glue up with anything and never have a problem. Maybe I am just lucky. I would certainly like to know where your problem is if anyone has the answer.
James
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With over 30 years experience using epoxies I have found that the mixing ratios are not that critical. I don't wipe my glass or lams with anything prior to clue-up and never have a problem, Maybe I am just lucky. I would certainly be interested in a solution to your problem if anyone has the answer.
James
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Thank you guys for all your comments, so much to consider! With so much variation in sanding and general prep techniques, I think that I'm going to do some test glue ups using different prep/sanding and clamp pressure variations to see if I can determine what is causing my problems. I'll be sure to post my findings for anyone who is interested.
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Jd, if you have some scrap left try some experimenting.
One glued like normal one where you use some heat like a light bulb.
You could also test the surfaces with different sandpaper.
I'm guessing glue it self or not enough heat.
You should have to grind the glass off no peeling.
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http://www.smooth-on.com/tb/files/EA40.pdf
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100 grit is wrong for grinding laminations, use 40 grit on your drum sander.
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I don't use anything less than 40grit on my veneers and sometimes use 36 grit when grinding core material.
The only problem I ever had when building a bow was the one time I wiped my glass and veneers down with denatured alcohol. Didn't de-lam, but had some blotchy spots show up between the glass and veneer. I simply brush the veneers, core lams, and rough side of the glass clean of any loose pieces with a fine wire brush. That brush is never used for anything else.
I haven't built as many bows as some, but that was my only failure in the glue/joint department in over 100 bows.
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Something's wrong with your glue, I don't know what "thickened" means for epoxy. Even if it was starved under each clamp.. between clamps would be fine. Finger oil on glass or wood... unless you rubbed wood and glass though your hair there would be some spots the glue worked. Toss that glue and get some EA:40 Smooth On.
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100 grit is to fine, I didn't notice you mentioned this before. Back down to 60 as the smoothest, 40grit is even better. I still don't think this is your whole problem, maybe the fades where the pressure is the greatest but you have a glue problem, replace it.
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After it has pulled apart is the glue residue stuck to the wood or the glass? I have been building bows commercially for 30 years, I always wash every gluing surface with acetone it does not bring up the oils it washes them off the surface and forces them down. Blow off your surfaces, brush them off ,then take a clean white cotton cloth soaked in acetone and see what you have you'll be surprised what residue is still there.
I grind all my lambs with 36 grit and my veneers are ground one side 36 and the side to the glass up to 150 I know some bowyers that grind veneers 220 grit, it makes the wood grain come out better with a smoother grind and we have had no problems.
Smooth on should be heated most epoxys are stronger if heated.
I've said before you can take any bow apart with a butter knife . Once the glass starts stipping you can remove it pretty easily....Tim
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Tim- It's hard to tell, but I would have to say there appears to be more glue stuck to the glass than the wood. And yes, your last point is what I am really questioning - none of the bows I have built have failed or delaminated while shooting, but I am questioning whether or not the glass should be able to be stripped off of the bow with just a razor blade. So you are saying that once you start peeling the glass with a knife or something, it is normal for it to be able to be stripped off relatively easily?
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I think it does I have played with some broken bows and lifted the glass and it came off pretty easy but usally the glue was more on the wood than the glass . I have some old limbs laying around I,ll experiment....Tim
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I use 80 grit all the time. I think your wood has to much moisture. Water comes out when heated and gets trapped under glass causing problem. Ive had this problem in the past.