Can you cut dadoes into 3/4" thich wood for shelves?

[JonW]

I'm going to make some sort of shelf system for some stereo gear. The shelves have to be pretty sturdy- some of the amplifiers and such weigh over 35 pounds each. So maybe 100-150 pounds total. Five shelves. About 4' high. Shelves about 24" by 20". I'm thinking of making the shelves from 1/2" thick baltic birch plywood. And the legs from 3/4" thick plywood. I haven't decided how to hold the shelves to the legs.

One thought is to craft some blocks, maybe 3" tall, 3" wide, 1.5" thick. Shape them to be pretty. And screw them to the legs. Then rest the shelves on these. Should be strong enough, I'd think. Not sure if I want to be looking at the screws, though.

Another thought is to make dado cutouts in the legs to hold the shelves. I've never tried to make dadoes before. Although I do have a router. I'm wondering if cutting half way into the legs of 3/4" thickness, will it be strong enough? Any insights are appreciated. Thanks.

[Copper]

I don't think that would be a problem since the gap would be filled with the shelf and all the weight would transfer from one piece to another. The depth of the dado should probably be about half the thickness of the legs. Keep in mind though that if the shelves are made of plywood, that they won't be exactly 3/4" thick. They will actually be 23/32" thick and you will have to rout a dado of that width. You can do this be either using a 1/2" bit and do two passes that equal the 23/32" width or you can purchase a special plywood dado bit. Holbren currently has them on sale here. If you use a 3/4" bit, the shelves won't fit snug.

[Tom Miller]

....Another thought is to make dado cutouts in the legs to hold the shelves.

That's a very standard way to do it.

But, I think you'll want the 3/4" thickness for the shelves, even moreso than the uprights. (But I'd use the 3/4" there, too.)

Check out the Sagulator to calculate shelf sagging based on material and dimensions. Keep in mind, it's only a guide.

Also, there are ways to add strength to a shelf, such as adding a thicker edge piece, etc.

Concerning router bits: the "plywood" sized bits aren't sized for baltic birch ply. I'd suggest a normal size bit, but use an undersized one and make two passes (i.e. use a 1/2" bit for a 3/4" dado).

Regards,
Tom

[LarryG]

I agree 100% with 95% of what Copper wrote ... pretty good odds. The 5% where I'd differ is that unless the shelves will be loaded unusually heavily, a 1/4" deep dado will be enough.

Two reasons for this. One, you'll be cutting through fewer plies of the plywood used for the legs; the center ply will continue full thickness, uninterrupted. Two, dados that go a third of the way through look better than dados that go halfway through (although this may not matter if you cover the plywood edges with trim).

EDIT: Agree with Tom, too. I missed the part about the 1/2" shelves.

[scorrpio]

If you want it to be strong and look good, consider sliding dovetails. Run a dovetail bit on both shelf sides on top and bottom to form a tail. Measure the tail at its base, then cut dadoes that wide in the legs. Actually, cut the dadoes a bit narrower. Finally, with same dovetail bit, work the inside of these dadoes. You might need to make a couple additional passes in order to fine-tune these channels. A sheet of sandpaper wrapped around a steel ruler will let you sand the channel insides for final fitting. The shelves should slide into place by hand or with light mallet types - not by force. Then, just glue the shelves in. You could either make them through to 'show off' the dovetail, or you could maked stopped sliding dovetail to conceal it on the front - or you could make through dovetails, but sand them flush and apply veneer to conceal the plywood edges.

[scorrpio]

That sagulator is a nifty tool. According to it, a 24x20 shelf made from 3/4" fir ply will sag only 0.01" under 35lb center load. Even with a 100lb center load, sag is only 0.03" - less than 1 millimeter. You won't even see it.

[Copper]

I agree 100% with what Larry said!

[LCHIEN]

My 2 cents:
It will sag less than Scorpio suggested, because your AV gear will not have the load centered on the shelf but actually at two points closer to the edge where its supported.

But I'm going counter to popular voting on dadoed shelves.

However, my thinking is unless you have a strong bent for aesthetics vs. strength, with 150 lbs of expensive gear in a cabinet I would not trust shelves sitting in 1/4" dados. Not enough side to side grain gluing. I would support each shelf with a small ledge, the ledge glued and nailed or screwed to the side supports. Don't rely on the shelves for structural support. And plenty of cross bracing either a back or an X to keep the cabinet from racking if someone tried to slide it sideways. That will want to break those dadoed shelves and drop that expensive AV gear.

If you want to make it pretty you can round or chamfer the front edge of the ledge where it is visible from the front of the cabinet.
I think if you build it like I suggest then 1/2" shelves will be fine if the shelves are not much wider than the components which are usually ~17" wide.

[LarryG]

Yes, I'm assuming this unit would have a back, to prevent racking.

But for the weight mentioned here, 1/4" deep dados are plenty. Kitchen wall cabinets, pantry shelves, and bookshelves routinely support similar or greater loads on four small brass or plastic pins. A plywood shelf dadoed and glued into a plywood upright across its full width ... that ain't going nowhere. The shelving units in my home library are made of 3/4" fir and the dados are all 1/4" deep. No structural problems after some 25 years, and there are some shelves sized for oversized books that are probably carrying close to 100 lbs on one shelf alone.

May have to just agree to disagree.

[JonW]

Wow. You guys are great. So many replies in such a short time. Let’s see…

OK, so realize I’m quite new to woodworking. So I’m guessing something like dovetails is not really an option for me at this point. Heck, it was only today that I even figured out what a dado is. Rabbets were last week.

I was thinking of a design without a back. Basically, 5 shelves held up by 4 legs (each about 3” wide and 3/4” thick) on the side. Actually, I was going to have a curve in the front legs and have the back legs straight. The open back will allow work with all the zillions of cables there are to deal with. And generally make the thing look lighter. I don’t want something that looks like it weighs 500 pounds.

Right, the plywood thicknesses aren’t really 3/4”, 1/2” et/c. I’d need to do 2 passes. Not sure how I could get them to be really straight. I was just reading this nice link for a jig:
http://www.newwoodworker.com/rtrdadojig.html

From playing around with the big sheets, the 1/2” feels plenty strong enough to handle the weight without adding any braces or anything. For the Sagulator (tremendous name!) using a 35 lb center load and 1/2” plywood fir, 24” x 20” shelves I get 0.04” sag. I think that should be OK.

So I’ve got some votes for 1/4” dadoes and some that are deeper. And even a vote for small ledges, which was my original thought. And I guess I’ll need some back bracing even with the ledges. OK, had not thought about that. Hmmm… how to make the back braces?

Anyways, you folks are super. Keep the ideas coming. Thanks!

-Jon

[Tom Miller]

Wow. You guys are great. So many replies in such a short time. Let’s see…

What else is there to do? Most of us are at work.

So I’m guessing something like dovetails is not really an option for me at this point. Heck, it was only today that I even figured out what a dado is. Rabbets were last week.

Well, sliding dovetails aren't too hard, but, agreed, keep it simple for now.

I was thinking of a design without a back. Basically, 5 shelves held up by 4 legs (each about 3” wide and 3/4” thick) on the side.

Ahhh, I was picturing solid sides. I can see why you're thinking no back. But then we're back to worrying about racking forces. This might take a little more thought....

Regarding a dado jig: With a top-bearing pattern bit, you can make an excellent guide using your shelf stock itself to set the thickness. Kind of like this:



In the picture, the padauk (orange wood) is the workpiece. The spacing of the guide is set to the thickness of the shelf, and the bearing of the pattern bit shown rides against the plywood guide.

Regards,
Tom

[JR]

So I’ve got some votes for 1/4” dadoes and some that are deeper. And even a vote for small ledges, which was my original thought. And I guess I’ll need some back bracing even with the ledges. OK, had not thought about that. Hmmm… how to make the back braces?

I fall into the 1/4" camp. That should be plenty of bite for a shelf. The small ledges idea is an option, but I don't favor it because - well I just don't like the small ledges as much as the dadoes.

The back braces can be easy or just a bit tricky. The easy way is to just add horizontal piece to each shelf. Make it about 2-1/2" wide and just screw or nail it to the legs and along the shelf. The top of the brace should be right at the top of the shelf.

Alternatively you could put a 2-1/2" notch in each leg, 3/4" deep. This would allow the brace to fit in there, improving it's anti-racking properties. Once again the top of the shelf should be level with the top of the brace.

Oh, I also favor 3/4" shelves. It would be a nice solid-feeling piece when you're done.
JR

[Pappy]

In your scenario, I think you can get away with 1/2" for the shelving. Personally, I would go with a 1/4" dado and add a 1" to 1 1/2" face under each shelf attached with a rabbet.In the back of the shelf I would put the edging/brace above the shelf. This will stop anything from being pushed off the back and give you a area to lay excess wires/cables.

Either I missed it, or you didn't say how you were going to finish the shelves. Unless you are using very high grade cabinet plywood, the edges will need to be covered for appearance.

[JimD]

I agree with Loring on most stuff but dado depth is one of those things where we do not agree. I usually use dados even shallower than 1/4 inch. The reason is that the shear strength of the vertical member is very large and you do not need a deep dado for it to hold. Loring's arguments about glueing area are valid but I've built a lot of furniture with simple butt joints screwed together and they are still working after 20+ years with no issues. One of the key reasons I've started using dados is it makes assembly much easier.

If you think you might ever want to move the shelves, I wouldn't rigidly attach them. Make a box with movable shelves. They will sag a little more than if they were rigidly attached but perhaps not too much.

If you do not want a solid back, you still have to brace against racking. A diagonal brace across the corners would work well - especially if it is an X across both diagonals. I've had success, however, with something like a 3 or 4 inch vertical piece at the top and bottom at the back that is screwed into the sides and top or bottom. The whole purpose is to help hold the corners at right angles. There are many ways to do it, it is largely an choice of how you want it to look.

Jim

[cjupdike]

Jon,

I did just what you are wanting to do when I built the closet organizer for my wife. I used 3/4 ply for both the sides and shelves, dado'ed the shelves in 1/4" on both sides and edge banded all the parts. I've used some pieces here for nailing to the wall and they've added torsional rigidity. The dados were done using a Lee Valley plywood straight cutting bit which is sized for the nominal size of the 3/4" ply (they also have it for 1/4 and 1/2 ply as well).