Question on dado depth

[garybushey]

I am going to be building some cabinets this weekend (if everything goes according to plan). I plan on using 3/4" plywood for the sides and 1/4" plywood for the top, bottom, dividers, and back. How deep of a dado should I cut for the divider? It will be halfway up (4') if that matters.

I heard the dado should be 1/3 of the thickness of the plywood but I am not sure if that is the thickness of the side or the divider. Any ideas?

Thanks

 

[zapdafish]

I don't know that I would use 1/4" ply for bottom and top, even for divider. If they are hanging cabs, the bottom and divider will act as shelves. Depending on the width, they probably won't take much weight before they start to sag. I haven't built many cabs but I always used 3/4" for the carcass, sides, top and bottom. For the divider, I use the same thickness as the adjustable shelves I put in there. So far its been 3/4", hehe.

Can't help with dado depth, I always kind of eyeballed it. Got tired of having to keep adjusting the dado blades to go with various ply manufacturers. Then I went with the router and "plywood" bits and they never fit right either. Now I do pocketholes and couldn't be happier.

 

[jhreed]

Is the 1/4" plywood to be used for the shelves? If so, I would not do that. I would used 3/4" for the top, bottom, and shelves. 1/4" deep should be fine for the dado. I would only use the 1/4" for the back.
James

 

[Joe Scharle]

First, 3/4" plywood is not 3/4" thick. Usually 23/32" or even thinner.
Second, a 1/4" (1/3) dado in '3/4"' plywood, results in an effective thickness of 1/2" unless the inserting member fills the dado tightly and replaces the lost material.

When I use pocket hole joinery, I make a shallow (1/8") dado for registration and creep prevention.
When I use dado joinery, I cut a dado 1/3 with a dado jig that makes the cut the exact width as the thickness of the inserting member.

As for 1/4" plywood; it won't support it's own weight, so I use it for backs (vertical orientation) or tops (not seen and supported underneath).

 

[garybushey]

Thanks for all the great tips. Guess I will be running out to grab another sheet of 3/4" plywood.

 

[Ray Martin]

I generally make the depth of the dado 1/3 to 1/2 of the thickness of the material that it is being plowed in to. As to the thickness of the material, you didn't say how wide or how deep the shelves would be; the concern is that 1/4 ply is not very substantial. For a small unit, you might consider 1/2" ply for shelves and 3/4" for any thing larger. I'd probably save the 1/4" ply for the back and have that in a rabbet. I'd also suggest 3/4" material for the carcass, except on a very small unit.

The structural integrity of the shelves is also affected by how they are installed. A dado in the sides of the box helps. Using mechanical fasteners through the back panel and into the backs of the shelves also helps. Another way to strengthen or stiffen a shelf is to add a band or support to the front lip. You may want to conceal the ply edge on the front of the shelves and can do both cosmetic and structural work at the same time. Rather than put something like a tape across the grain, consider a thicker band. For example, on a shelf that is 3/4" ply, use stock that is 1" to 1 1/4" thick, with a rabbet cut into it. Glue this to the front of the shelf and you have the added shelf strength as well as a place to do an edge treatment, such as a slight round-over.

Good luck with the project. And don't forget the photos.

 

[Mt. Gomer]

Second, a 1/4" (1/3) dado in '3/4"' plywood, results in an effective thickness of 1/2" unless the inserting member fills the dado tightly and replaces the lost material.

Huh? :icon_scra Lost me here. Can you explain this for me? Thanks!

 

[Bas]

Huh? :icon_scra Lost me here. Can you explain this for me? Thanks!

One third of 3/4" is 1/4". After you've made a 1/4" dado in a 3/4" board, you're left with a wall thickness of 1/2". Obviously, a 1/2" board is not as strong as a 3/4" board of the same material.

If the mating piece is glued in real tight, that compensates for that weakness. If the fit is somewhat loose/ sloppy, then the effective strength remains at 1/2".

At least, that's how I read it

 

[Mt. Gomer]

One third of 3/4" is 1/4". After you've made a 1/4" dado in a 3/4" board, you're left with a wall thickness of 1/2". Obviously, a 1/2" board is not as strong as a 3/4" board of the same material.

If the mating piece is glued in real tight, that compensates for that weakness. If the fit is somewhat loose/ sloppy, then the effective strength remains at 1/2".

At least, that's how I read it

Ahhh, I got the 1/3 of 3/4 part, but I was thinking he meant 1/2 was the effective width in terms of the amount of support for the shelf (which didn't add up in my head), not the structural stability of the wall of the case... Got it now. Thanks!

 

[Jeff]

A trick that I've read about and tried is pretty handy with plywood shelves; it takes some of the guesswork out of getting a snug fitting dado/shelf fit resulting from the "variable" thickness of ply and it's cheaper than buying a set of ply router bits.

Cut the dado to the desired depth, but slightly narrower than the true thickness of the ply (router or dado set). Then cut a rabbet on the underside of the ply which matches the dado depth and sneak, sneak, sneak up on the rabbet thickness for a snug fit (get it dialed in using a piece of scrap ply). Then run all of your ply with that exact setup (same day, same time, etc.).

Glue only the back 1/4-1/3 of the ply shelf into the dado. The rabbet imparts some structural stability and the visual effect isn't bad either. The front face of the ply can be edge banded with solid wood and trimmed flush before cutting the rabbets. ccasion1

Good luck and show us some pics as you progress.