Dado depth?

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Greg Guarino wrote:

A quarter will be fine in 3/4 ply if your fitting a back and framed front .
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On Wednesday, December 26, 2012 6:51:13 PM UTC-6, Greg Guarino wrote:

I usually dado to 3/8" depth in 3/4" material whether it is solid or plywood. Why? No analytical reason - that is the way I have always done it and it seems to work.
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wrote:

Why? No analytical reason - that is the way I have always done it and it seems to work.
What do you do if there are dados on both sides? I generally use 1/4" dados and get nervous if there are dados on both sides, even if they're not opposing.
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On 12/29/2012 1:33 PM, snipped-for-privacy@attt.bizz wrote:

I certainly treat plywood panels with opposing 1/4" dadoes with care until they're filled with the partitions/shelves, but it is remarkable how strong that <1/4" of plies remains.
Luckily, never had one break in all these years ... even with Murphy and the shop dummy in attendance.
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On 12/29/2012 2:33 PM, snipped-for-privacy@attt.bizz wrote:

Why? No analytical reason - that is the way I have always done it and it seems to work.

So here's the neat thing about dados, if you build them tight, you are filling the void left behind. So while you lose a little strength, most of it is gained back by filling it back in.
AGAIN AS LONG AS THEY ARE TIGHT, not sloppy loose.
So like Swingman said, just be careful while you are not all assembled. Once you are, you should be good maybe a 5% loss in strength(pulling that out of my ASS so beware).
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On Sat, 29 Dec 2012 21:03:53 -0500, tiredofspam <nospam.nospam.com> wrote:

plywood. Why? No analytical reason - that is the way I have always done it and it seems to work.

Right. The problem is splitting the dados when their being cut. A void in the wrong place can be a disaster. If you cut everything at the same time, now you're out a piece. A real bummer.
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snipped-for-privacy@attt.bizz wrote:

----------------------------------------------------- That's why epoxy and micro-balloons exist.
Lew
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On 12/29/2012 8:03 PM, tiredofspam wrote:

All of what you say makes perfect sense. However, I question the last one that you pulled out of that dark place<g>
Under some circumstances you may be correct but when you stop and think about it and consider what really happens when you use a TIGHT dado...
What I'm thinking of is along these lines.
A piece of 3/4" plywood each 5' long.
Lay it horizontal across supports either end 12" off the ground and put a 50 lbs weight in the middle. How much deflection do you see?
Now cut a 3/4" x 3/8" dado in the midpoint of the same piece and reapply the weight. What happens is just what we expect and fear: extreme deflection or it breaks due to the removal of the wood.
Now do the same thing and glue another piece of 3/4" plywood into the dado, extended vertically 12 3/8" to keep it level and apply the weight. Now how much deflection?
I think this runs true in all applications employing a dado. Properly cut and assembled, you will not lose strength, you will only gain strength. Doesn't matter in which direction the forces are applied, the dadoed assembly will always be stronger.
Unassembled? Handle like eggs<g>
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On 12/30/2012 8:44 AM, Unquestionably Confused wrote:

I think you have assumed a dado for a vertical piece. In this case I am considering a dado for a horizontal piece. And I don't believe it to be stronger, but I also don't consider it to be appreciably weaker.
In the case (you present) of that dado supporting a vertical piece, it certainly adds to the strength.
BTW an overly tight dado will weaken the structure just as much as a loose dado, perhaps even more, since you risk breaking the fibers of remaining structure left, and are no relying on glue to hold it all together, but the glue is not where the broken fibers are.
If you don't have a side rabbet plane, I would just sand or plane the piece that fits into the dado for fitting. You can control it much better.
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On 12/30/2012 11:41 AM, tiredofspam wrote:

Six of one, half dozen of the other, really. I think the point that each of us is trying to make is that once PROPERLY assembled, any weakening of the wood by the dado is negligible and, in most instances, is offset by the fact that the piece is strengthened by the dado.
Okay.. Take the side of a bookcase (what you perhaps are speaking of when you refer to the horizontal dado vs. what I stated) in that instance a dado cut in the side to insert a fixed shelf would only weaken the side with regard to lateral forces and then, only until the cabinet was assembled. Once assembled, it's strength with regard to lateral forces would be increased in every situation that I can think of and, again, PROPERLY cut dado and assembly, would have almost no measurable effect on the vertical strength. (i.e. Vertical forces pushing down on the side from the top)
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