How strong should a glued crack be?


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The top I am currently working on has cracked close to the top block area. The crack is about 6 cm long, on the inside and does not seem to go all the way through. I have tried to glue the crack a few times. I tested the strength of the joint by putting my thumbs on the outer side of the top approximately at the position of the crack and flexed the plate by puling the outer edges downwaeds. Unfortunately, the crack always opened again (note that I haven't fitted any cleats, yet). Usually, properly glued joints can be about as strong as the wood itself. Does this also apply to glued cracks?

 

Thanks in advance!

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The top I am currently working on has cracked close to the top block area. The crack is about 6 cm long, on the inside and does not seem to go all the way through. I have tried to glue the crack a few times. I tested the strength of the joint by putting my thumbs on the outer side of the top approximately at the position of the crack and flexed the plate by puling the outer edges downwaeds. Unfortunately, the crack always opened again (note that I haven't fitted any cleats, yet). Usually, properly glued joints can be about as strong as the wood itself. Does this also apply to glued cracks?

 

Thanks in advance!

 

What do you expect to happen ? Break somewhere else ?

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You might be right. But if it is too thick, it does not flow into the crack or gels too quickly... I have now changed my clamping strategy. We'll see how it turns out.

Maybe your clamping is too strong and you squeezes the glue from the crack?

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I assume you are using freshly made hot hide glue...Did you heat the area (with a hair dryer) before applying the glue?  Have you tried the pillar clamping method?  See link. http://hudelmayer.com/about-me/articles/hold-on-tight-the-strad-nov-2001-article-on-crack-repair/

 

It is always a good idea to glue in a cleat just above the head of the crack when you are flexing the crack to pull in the glue.

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One thing to consider is whether the crack was really clean prior to gluing.  If it was an old crack (or previously repaired crack), then it is crucial to clean out any dirt, old glue, etc. so that the hide glue can adhere directly to the bare wood.  Warm water (e.g. glue pot temp of 140F) and a very small, medium stiff artists brush works well to dissolve old glue, and also to remove dirt.  Cleaning the crack can take some time, though, depending on how much access you have to the bare wood on either side of the crack.   I'm sure you can find much more info here by searching older posts for "crack cleaning".  Good luck!

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I've had trouble this winter with glue gelling very quickly, even when the wood was well warmed. I've resorted to applying enough heat after gluing to remelt the glue thoroughly. So far it seems to be working.

Cool idea (P.I.).  Thanks :)

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One thing to consider is whether the crack was really clean prior to gluing.  If it was an old crack (or previously repaired crack), then it is crucial to clean out any dirt, old glue, etc. so that the hide glue can adhere directly to the bare wood.  Warm water (e.g. glue pot temp of 140F) and a very small, medium stiff artists brush works well to dissolve old glue, and also to remove dirt.  Cleaning the crack can take some time, though, depending on how much access you have to the bare wood on either side of the crack.   I'm sure you can find much more info here by searching older posts for "crack cleaning".  Good luck!

Jeff F. points out what I think is the most common problem with glued crack failures.  I has to be REALLY clean.  After you have glued it a few times unsuccessfully, that makes it hard to get a joint.  Harder and harder each time.  After you have cleaned it, and let it dry, take a magnifier (I mean REALLY magnify, to where it looks like the Grand Canyon)and inspect it carefully.  It will look very different than to the naked eye.  Other causes can be glue gelling too quickly and not getting into the crack all the way, also not getting good contact with the wood inside the crack well (if gelling).  Shouldn't matter about whether you put on the cleats or not, they shouldn't be holding the crack together, they are for keeping that area from flexing too much.

 

I've had trouble this winter with glue gelling very quickly, even when the wood was well warmed. I've resorted to applying enough heat after gluing to remelt the glue thoroughly. So far it seems to be working.

Try warming your shop more, or maybe using a more watery concentration than you are currently using.   That sounds dangerous if your doing a crack, good idea if you are gluing on a top.  I have like the idea of warming a joint (mainly cracks) right after glueing (but not after it gelled) like with an alcohol lamp etc.  

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Difficult to really help without "being there", but assuming the crack is free of other adhesives/dirt and that we're not dealing with a distorted plate, I'd consider two things for a start.

 

1) Glue is much stronger when made fresh, then allowed to gel, then reheated. If you're using a good clean glue, of 180 g strength or better, it should do the trick if correctly prepared.

 

2) I assume the description of "open on the inside but doesn't seem to go all the way through" may indicate that standard methods of clamping (straps, etc) may not be directing enough of the clamping force to the open side of the crack.  The pillar and wedge method, mentioned earlier in this thread, is one of the best methods I know of to direct the closing force where you need it to be.

 

post-17-0-49123900-1394968600_thumb.jpg

 

Hope this helps.

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If you have had that many goes, my guess would be that as Jefrey F and Jeff W suggest, it is a cleaning problem. Jeff W is also right about getting the workshop warm, Jeffrey H is demonstrating one of the best ways to pull a crack together eavenly and without too much presure. If all else fails try looking on Kremers web site. There is some interesting info on so called animal glues. Good luck!  

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If you have had that many goes, my guess would be that as Jefrey F and Jeff W suggest, it is a cleaning problem. Jeff W is also right about getting the workshop warm, Jeffrey H is demonstrating one of the best ways to pull a crack together eavenly and without too much presure. If all else fails try looking on Kremers web site. There is some interesting info on so called animal glues. Good luck!  

How many different Jeff's can Roger get in a post......................

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Try warming your shop more, or maybe using a more watery concentration than you are currently using.   That sounds dangerous if your doing a crack, good idea if you are gluing on a top.  I have like the idea of warming a joint (mainly cracks) right after glueing (but not after it gelled) like with an alcohol lamp etc.  

I have tried, but I simply don't possess the equipment needed to heat my garage to the desired temperature in winter. The glue is already on the thin side. Nothing wrong with re-melting gelled glue in place. DRIED glue would be a problem. I would not try to use an alcohol lamp unless it was the only possibility. I have a small electric heat gun that works just fine

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