In the Down to Earth Woodworker’s most recent October 2013 Wood News column, he discussed the Dado Depth Torture Test, and in this follow-up blog entry, he has the results of his testing.
The Down To Earth Woodworker responds to a reader’s question, “Why do you make your dados 1/4” deep in 3/4” plywood?”
Pursuant to a reader’s question, I tested three dado depths (1/4”, 3/8” and 1/2”) in 3/4” plywood to see what depth provided the strongest joint in each of three potential force directions.
The test demonstrated that to some extent, it is all about the plywood. So-called “furniture grade” plywood with few plies and many internal defects can fail in spectacular ways. I suspect that the same tests in Baltic Birch or some other fine grade of multi-ply plywood might have turned out differently. But “furniture grade” is the type of plywood most of us use for shop cabinets, kitchen cabinets, and bookshelves.
In the shear test, both the 3/8” and 1/2” depth dadoes failed as a result of a split in the plywood between the plies, starting at the dado wall. The deeper dado weakened the plywood and the downward pressure on the test piece provided the leverage to split the plywood. The 1/4” depth dado was the strongest.
In the tear test, the 3/8” dado was the strongest, with the 1/4” deep dado coming in a fairly close second. I suspect that in this test the deeper dado helped… to a point. As the dado got deeper, there was more gluing surface on the sidewalls of the dado. But at 1/2”, the dado was simply too deep and the plywood surrounding the dado was weakened to the point that it overcame any advantage provided by the additional sidewall gluing surface.
In the pull test, as the dados got deeper the joint became weaker because there was not enough plywood behind the joint to hold it rigid. The flexing in the weakened plywood allowed the sidewalls of the dado to give way, and the rest was, as they say, history. Again, the 1/4” deep dado was the strongest.
Check out the video below for more details!