Dados

[Matt Wolboldt]

So I'm starting my first set of cabinets out of 3/4" plywood. I've heard that 3/4" plywood isn't really .750", but rather some smaller value. On that advice, I picked up a plywood router bit that is supposed to cut a slightly smaller groove. Before I get started, I measured the plywood sheet and one of them stands at .760" and the other sheets are .735" *per the calipers* So I'm at a crossroads I hope you guys can help with. Should I use the plywood bit or go with my standard 3/4" bit. Thanks for the help in advance.

[Frank Drew]

Matt,

You could use only the .735" sheets for the pieces meant to enter the dados, which you'd cut at .735+, but for this very reason I sometimes cut a stub tenon on the end of the plywood, using the same face against the fence so that differences in thickness wouldn't be an issue. Having two shoulders, small as they were, also made a more rigid joint.

[Tom Hintz]

As you are discovering finding consistent thickness plywood can be a real problem. See the link below for a story I did on a technique for "standardizing" the plywood rather than the dado. It may sound like the long way around but it works well and actually has been used for a very long time, even before the modern plywood thickness goofyness.

http://www.newwoodworker.com/stndrdados.html

[Fred Hargis]

Plywood will sometimes vary even within the same piece...it can be a real crap shoot. Tom's method works well, although I usually just fight with as I go.

[Joe Scharle]

Tom's method is an excellent technique and Woodsmith has been using it in their plans for a long time as well. I, however am lazy and use this jig to size the dado to the mating end. I also have absolutely stopped buying Chinese plywood.
Canadian and American is more consistent and I haven't hit any metal in those goods.

http://www.ncwoodworker.net/pp/data/194/thumbs/Dado_Jig.JPG (http://www.ncwoodworker.net/pp/showphoto.php?photo=30616)

[frank shic]

why are you using dados on cabs? butt joints are the easiest way to go unless the ends are exposed.

[Peter Quinn]

The cabinet shop i work in mills the dado's to around .700" on the TS, then using a tall fence skims one side of the horizontal members to match the groove. Always trim at least 1/2" from the factory edge of your plywood, NEVER use the factory edge except as an initial reference for cutting the sheets, it may vary far more than the body of the sheet stock.

We us a lot of pre-finished plywood, so we always skim the side that will be blind. Skim the bottom of the bottom partition, the top of the drawer bottom divider's, etc, so the cut side can't be seen.

Also, when you make your dados, set up not to TAKE a certain amount out, but to LEAVE what you need behind! Often people think "I need a 1/4" dado", so they set the bit to 1/4" deep and go. This can be a problem on a long run of cabinets with undersized plywood. Better to set the bit so that you LEAVE 1/2" from the bottom of the dado to the back of the sheet, then add 1/2" to your interior widths. Much more consistent. We use an over arm router which makes things more consistent too.

[Howard Acheson]

To see where you really stand, rout a groove with a standard sized 3/4" bit in a section of solid or plywood. Then do the same with the undersized bit. Now fit the groove over the edge of the plywood sheets in question and see which one gives you the proper fit.

You may find the neither works well. It that case use one of a number of techiques to custom size a dado groove using long straightedges and two passes with 1/2" dado bits. Or, use the technique illustrated above.

[Frank Drew]

why are you using dados on cabs?

More work up front, but you get a stronger joint, and easier assembly.

[glenn bradley]

I use a jig like Joe shows. It can't help with inconsistencies along the length but it does give a custom fit.

[frank shic]

i used to believe that prior to the kreg corner clamps but now it's way easier to clamp up and just screw the butt joints.