failed jointAs woodworkers, we often seem to be obsessed by how strong joints are, and in many ways this is good. Of course we don't want people to be hurt sitting on a chair that could collapse, but in many cases the joints are many times stronger than actually needed. This is in part because of the way we need to make them in order for them to be secure.

In the associated video, I put together a variety of joints, all of them with Red Oak, just to see how well each kind of joint holds up. All of the joints were end grain to long grain, with the exception of the lap joint (which I will talk about later). End grain to long grain are the hardest joints because end grain does not glue well to long grain, well at least with much strength, so other means of fastening must be adapted.

In order to be fair with each joint, all the end grain pieces are 3 inches wide. This was selected for a couple of reasons, first of all it would accommodate the largest wood biscuit commercially available; the other reason is that by using 3" viewers could use the info to associate it with both 2" or 4". I just don't have time to run all the tests of both 2" and 4" material, so 3" seemed like a good compromise.

The lap joint was slightly smaller because I felt it was unfair to have 3-1/2 inches of long grain glued to long grain so it is slightly smaller at 2-1/2 inches ...

I set the joints up in order of how strong I felt they would be and for the most part I was correct, again with the exception of the lap joint.

The other joint that I paid special attention to was the biscuit joint. When I glued up the biscuit joint, I made sure I had glue spread in both the upper and lower slots and even spread glue right on the biscuit, both sides and upper and lower - AND I also glued the matching wood around the biscuit, so this joint probably had more attention paid to it than any other.

You will see that I re-did the  mortise and tenon and the dowel joints because both of them pulled out when I did these tests. I had let the glue stand for 2 days but clearly that was not enough so wanted to re-do these with glue set longer to see how they would fair.

I found it quite interesting that in all 4 of the dowel and M&T joints whether the joints pulled out or the wood failed as in the 2 latter tests, the pounds of pressure was nearly identical for all of them.

Here are the results ...
 

1 Butt Joint 150 Pounds  
2 Biscuit Joint 230 Pounds  
3 Pocket Hole Joint (3 screws) 475 Pounds  
4 Lap Joint 700 Pounds  
5 Dowel Joint 3/4" x 4 dowels 660 Pounds 680 Pounds
6 Mortise & Tenon 3/8 x 2-3/4 680 Pounds 680 Pounds
7 Dowel 1/4" x 4 dowels 480 pounds  

 

 There are other joints I could also have tested but in the end, I felt this a good representation for what most woodworkers would be using, and that is would sever the needs of those who want to know what sorts of strength they can expect.

 

Of course there are always variables and the results I got are just that ... results that I got. Using different species of wood, different cuts of wood could alter results either way. These are only guidelines of what to expect.

Copyright - Colin Knecht
woodworkweb.com

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