Dado sizing for finished plywood

[Stephen McBride]

Hello!

I tried searching and found a lot of great and helpful info for cutting dados into plywood. But I'm still at a loss for one question.

I'm building a closet storage system and I'm going to use dados for the storage cubbies. I'm going to be using 3/4" plywood all around and while the main frame will be painted, I'm going to be staining the shelves which will be fit into the dados.

When choosing the dado router bit, should I add any size for the finish on the plywood which will include some polyurethane?

Seems like a 3/4" would work if I need to compensate, but I'm wondering if a undersized plywood bit would also work for a tighter fit.

[Kent A Bathurst]

Stephen - welcome to the asylum:

First - understand that "3/4" plywood" may not be dead-nuts .750" May be 23/32 = .719", or roughly 30-thouu under. There are other thicknesses out there also, so you need to be sure what size dog you're running with, and size your router bit accordingly.

Second - sounds like you intend to prefinish the shelves - That's what I do. I recommend it. However - glue won't stick to the finish, so you need to tape off the bits that will hide in the dado if you're planning on glue to hold it together.

Third - check the specs on your poly, but I think you'll find that the dry-film thickness is under 2 mil [thou] per side after 2 - 3 coats, so 3 - 4 thousandths total both sides. You ain't dealing with that level of precision here. If you don't tape off the edges per previous note, don't worry about compensating for the film thickness - just be sure you have the correct router bit.

[Edward Weber]

I agree with Kent
You really need to know if you plywood is 3/4", 23/32" or 19mm before you start

[Rick Potter]

Look on YouTube for videos of simple router guides to make dado's. It's easy to make one for the specific ply you are using, or go fancy and make an adjustable model. These jigs use a smaller bit (like 1/2"), and you go up one side and down the other to get the exact size dado.

There are dozens of examples there.

[Rod Sheridan]

To the best of my knowledge all North American plywood is metric, so you won’t find 3/4” plywood except by normal variation in sanded thickness.

I would shim a dado cutter to provide the correct size groove, and as others have said, you need to mask off glue surfaces before finishing

Regards, Rod

[Lee Schierer]

As others have said measure first, cut once and mask the glue surface.

[Bradley Gray]

I did a project like this a few years ago. The 25 sheets of ply (from the same unit) varied a lot. I ended up running the shelves on the table saw on end to fit the dados.

[Jim Becker]

Honestly, you need to actually measure the material with a caliper to get the exact thickness before you setup for your dados. Don't try and find "a bit" that is that size...use a smaller bit with two passes to get there. You may even find that each sheet may be a slightly different thickness. If you are doing the finishing yourself, you can either do your fitting after cutting and finishing the components or leaving a slight allowance for the film. You want the recesses snug but not super tight.

[Ron Citerone]

https://sawdustgirl.com/locking-rabbet-drawer-joint/


This shows for a drawer but this is how I do my shelves and web frames. Cut the dados with a router or dado stack. Then cut the rabbet on the dado stack or router table by sneaking up on your stack or router table until it fits nice. Then run all your pieces. Seems like an extra step but avoids all the dado sizing problems. I also find it is way stronger.

[Cameron Wood]

No, the thickness of the finish will not change the thickness of the plywood enough to matter.

Sanding will likely have made a bigger but still inconsequential difference.

[Stephen McBride]

Amazing. Thank you all for all the responses and tips!

The wife is getting impatient and doesn’t understand that it takes time to get something that looks nice ;)

With that in mind, I’m gonna see about making a simple jig and doing two passes with a 1/2” straight bit versus a 23/32” on a straight edge.

Thanks again!

[Cary Falk]

The other way to do it(but may take longer) is to cut the dado with one pass which is narrower than the plywood and then rabbit the other piece to the exact size of the dado.

[Kevin Jenness]

I quit using dados for carcass assembly when I got a biscuit joiner almost 40 years ago. Unless you are using ply with a calibrated core thickness typically varies +/-.015" within a lift, making consistent tight joints unlikely at best. For an accurate integral joint I prefer a tongue and groove joint to a dado.

[Kent A Bathurst]

The wife is getting impatient …….

With that in mind, I’m gonna see about making a simple jig and doing two passes with a 1/2…,!

Stephen, Stephen, Stephen …..

You’re showing up the side. This confluence of .events gives you broad plausibility for new tools

Don’t laugh in the face of The Tool Gods.

[Richard Coers]

Remember, no two sheets are the exact same thickness. Prefinished or not. 2 coats of conversion varnish on both sides will definitely affect the fit of a dado in my shop.

[Stephen McBride]

Stephen, Stephen, Stephen …..

You’re showing up the side. This confluence of .events gives you broad plausibility for new tools

Don’t laugh in the face of The Tool Gods.

I’m still in the early phases of getting tools. Just had my first project where all I had to buy was lumber a few weeks ago and she’s beginning to see the light of actually having the tools.

Router and bits are gonna be the buy for this one :)

[Mike Wilkins]

Rick Potter hit the nail on the head. If you make an adjustable routing jig you can size the dadoes to the exact thickness of the plywood. Use a 1/2" bit with bearing-run it up the left side and back down the right side. Just did it with hardwood and the fit was perfect.
Or you can just jump ahead of the line and get the Woodpeckers dado jig if your wallet can handle it.

[Bob Cooper]

For me it’s far easier to cut a rabbet on the shelf (slightly undersized) and a dado on the side. That way the fir does not have to be exact

[Rich Engelhardt]

Look on YouTube for videos of simple router guides to make dado's. It's easy to make one for the specific ply you are using, or go fancy and make an adjustable model. These jigs use a smaller bit (like 1/2"), and you go up one side and down the other to get the exact size dado.

There are dozens of examples there.This right here...
As a matter of fact - way back in 2007, I was searching for this exact jig and one of the links was to Sawmill Creek.
The funny thing is - I still haven't gotten around to making the dumb thing!

A simple search for "dado jig" on Youtube will provide a bunch of different jigs.


A "kerfmaker" is another method that can be employed.

I forgot to add - - I'd size the jig to use a 3/8" bit instead of a 1/2" bit. That way it can be used for 1/2" material also.

[Kevin Jenness]

For me it’s far easier to cut a rabbet on the shelf (slightly undersized) and a dado on the side. That way the fir does not have to be exact

If you are going to make a dado and rabbet (t&g) why not make it fit correctly? The joint will be stronger and the shelf located accurately without visible gaps. The key is to gauge the tongue thickness off the uncut face, thus avoiding inaccuracy caused by varying sheet thickness. This can be done on a router table with a rabbeting bit above the tongue or on a shaper, but not with a dado blade in a tablesaw (unless the shelf is stood on edge).

[Rick Potter]

Welcome to the wonderful world of woodworking. Lots of opinions, and all are viable.

Just wondering if you have a table saw yet? That is often the best tool for dado's, but since you referenced router bits, I didn't mention it in my previous post.

If you don't yet have one, be sure to get one that takes a dado stack.

[Thomas McCurnin]

+1 for Rick's comment.

A table saw is easier and more accurate to make dados, because the blades can be shimmed with thin shims for a perfect fit. There are plastic shims available as well to supplement whatever shims were provided with the dado set. I would also make test cuts with the plywood to insure a perfect fit.