Super Glue Strength

[Willyou]

Yesterday I was working with a walnut plank making bed side rails. The 4 corners of the bed will have knock-down connections using bed bolts. As I was doing a test fit, an un-noticed crack in one rail opened up for about 6" from the end. The crack was so narrow, there was no way to get regular yellow glue into it effectively. I ended up opening the crack as much as possible without making it worse and used thin super glue to saturate the interior surfaces. It wicked its way throughout the crack and came out the other side. I clamped it shut and let it set overnight. The crack is invisible (always was), but I am wondering about how strong the glued crack will be. I have used super glue lots of times for small stuff, but never where the strength of the joint was very important.

[MSRiverdog]

My main use for super glues for wood is as a mix for sawdust to use as a filler or for very minor splits and chips, my luck with super glues has been poor unless the surfaces mate perfectly and there's no movement/readjusting and I also use activator whenever I can. It has saved me on a few projects though.

[SteveS]

cyanoacrylates have very poor peel strength so if you have any out of plane loads it will likely fail.

[Bibliophile 13]

Not as strong as yellow glue, certainly. I've had mixed results with gluing cracks together, especially in walnut.

In situations like that, I would usually place a clamp down the board a ways to ensure the split doesn't open up further. Then I would drive a small wooden wedge into the crack to open it up just enough to force some yellow glue down into it. Finally I pull the wedge out and clamp up the glue joint. It usually works.

[Willyou]

out of plane loads it will likely fail.

Please define "out of plane loads".

[AHill]

Take a look at the most recent issue of Fine Woodworking.  They did a test of joint strength using super glue.  For the most part, the thicker CA's had better strength.  The author used a M&T joint (more like a bridal joint), and for the thicker CA's, the joint broke in the wood and not along the glue line.

[barnowl]

Yesterday I was working with a walnut plank making bed side rails. The 4 corners of the bed will have knock-down connections using bed bolts. As I was doing a test fit, an un-noticed crack in one rail opened up for about 6" from the end. The crack was so narrow, there was no way to get regular yellow glue into it effectively. I ended up opening the crack as much as possible without making it worse and used thin super glue to saturate the interior surfaces. It wicked its way throughout the crack and came out the other side. I clamped it shut and let it set overnight. The crack is invisible (always was), but I am wondering about how strong the glued crack will be. I have used super glue lots of times for small stuff, but never where the strength of the joint was very important.

Can you glue an 8" long piece of 1/2" plywood to the back side that won't show?

It would insure the integrity of your repair.

[Willyou]

Can you glue an 8" long piece of 1/2" plywood to the back side that won't show?

No. The "back side" is where the mattress and box springs will go. No room. But thanks for the suggestion.

Here is what one assembled corner looks like:



The crack runs with the grain just below the top bolt.

The good thing is that the thin CA was able to wick all the way into the crack and completely saturate both sides. Also, the mating surfaces are irregular broken surfaces rather than smooth cut. So, there will be some interlocking to resist shear. However, I guess there is some doubt about CA's strength, especially against the pressure of the tightened bed bolt. So, to get some insurance, I managed to insert an oak dowel at a slight angle across and perpendicular to the crack and adjacent to the bolt hole. Hopefully, this will provide enough strength.



Thanks for all the comments.

[FrankAtl]

Excellent fix!

[mike4244]

Next time you need to force glue in a small crack try this. Dab yellow or whatever glue you want to use on the crack line. Then place a vacuum cleaner hose under the crack. The suction will pull the glue thru the crack.
mike