Plywood carcase : dado or dowels?

[radhak]

I am building this book case with Red Oak ply. So do i dado the sides to fit the top and bottoms, or can I use dowels?

Prime consideration is strength : which is stronger, and then, which would better for avoiding racking?

[LarryG]

Prime consideration is strength : which is stronger, and then, which would better for avoiding racking?

Dado wins on all counts.

With a dado, the horizontal members (top, bottom, and shelves if they're not adjustable) are bearing on the bottom edge of the groove. With dowels, they're supported only by the cross sectional area of the dowels, and the glue joint in shear. Of course this doesn't much matter with the bottom, but a dado will still resist racking better since the end of the horizontal member is housed on at least two sides.

[jackellis]

In addition to Larry's advice...

If you attach the back of the bookcase to the back of the carcase (with fasteners, not glue), it will act like a shear wall to prevent racking. If you attached it like a drawer bottom (in a full-length dado), which I don't think is the correct way, you'd need corner bracing to prevent racking. Another option for strength is a face frame.

I've never actually built a bookcase (on the list of things to do), but I examined several store bought models we have in the house. The one with a face frame is nice and rigid. We have a European style bookcase that's held together with knock-down fasteners. It racks badly and is likely to be damaged in one of our periodic ground movement events (aka earthquakes).

[cgallery]

I agree w/ Larry.

But I recently did a test of plywood dado glue strength (nothing scientific).

We all know the strongest glue bond is long grain to long grain. What I found (in at least my test) was that the bond at the edges of the inner plys (where there was long grain) wasn't very good.

Basically, when I tore the joint apart, I was able to see that the face plys tore away from the rest of the sheet of plywood as the the glue joint failed. So in that regard, the glue was stronger than the wood.

But the joint failed at the glue line on the inner plys.

So in this case, the joint was really only as strong as the bond between the face plys and the rest of the sheet of plywood.

I'd suggest plenty of glue, and maybe even shoot some diagonal brads like Nahm does.

[scorrpio]

Definitely dado. But you can have the best of both worlds, you know.

Like assembling with dadoes, and then drilling through the sides into the shelves, driving the dowels and then either cutting them flush or rounding them. Might looks especially nice if you use contrasting wood.

Or you could drill holes in the dado bottoms beforre the assembly, so that dowels are hidden.

[Tamarack]

I have used Miller dowels after gluing as a sort of Belt AND suspenders thing.
Paul

[BrazosJake]

Rabbetted dadoes.

[radhak]

All good advice and suggestions, thanks.

It will have one fixed shelf in the middle. Am thinking dado and dowels. Visible dowels of contrasting color would be interesting.

Of course, it's gonna have face frame too. And the back is 1/4 ply on rabbets.

What's a diagonal brad?

[LarryG]

With a full back AND a face frame, you won't need the dowels unless you want them as a design element. Or maybe because you plan to park your car atop this thing.

Diagonal brad = toenailing into the dado joint from the inside corner. Primarily done as an alternative to clamping.

[radhak]

Actually, rabbetted dado might be even better. Thanks.

With a full back AND a face frame, you won't need the dowels unless you want them as a design element. Or maybe because you plan to park your car atop this thing.

Rather, a case of wanting to use my new dowelmax....
But most probably may not do that, seeing if I do the rabbetted dado, may not have enough width on the shelves for the dowels...

[JR]

Ok, I'll bite. What's a rabetted dado?

JR

[LarryG]

Rabbetted dado:

[JR]

Rabbetted dado:

Ah. Nice joint.

[eezlock]

plywood carcas....dados or dowels?

Definately dados are better. For the back panel,use rabbet joints and set it in nice and flush with the back side. eezlock

[Rand]

Wouldn't the rabbetted dado be weaker???
Since there is only 1/2 the thickness of the plywood supporting the entire weight of the shelf and it's contents?