Need a primer, help with Router Bits for Plywood Dados

[Hal Steele]

I am looking at several projects that will need dado joints in 3/4" plywood. Easy task for most of you but I am struggling. I realize that 3/4" plywood ain't all it's supposed to be so to speak and that I need undersize router bits for the job. However, the plywood varies in thickness as do the bits and none seem to match up. I find it in 23/32nds, 43/64ths, .709mm, etc. I find router bits in widths that vary from these...3/4", 45/64th, etc. Having a hard time matching the two.

Am I over thinking this? Is 1/64th close enough? How do you all match up router bit to plywood thickness? Close and then a second pass with a router or table saw? Any help appreciated!

[Mel Fulks]

What I would do is use single flute 1/2" bits. Two cuts for each dado , but they cut faster than two flute bits. Some of the single flute bits have too much metal at cutting end...and look almost like two flute bits. Try to get 1/2" (cut) bits that are
made with less diameter at the bottom. Much better chip clearance .

[Paul F Franklin]

If I'm doing a shop cabinet or the like, I just use a slightly oversize bit, whatever it may be. For furniture or other project where I want really nice joinery, I use the two pass method.

Another option is to use an undersized bit to create an undersized dado, and then cut a shallow dado on the mating piece (usually on the underside, or both to create a tongue) that is a nice fit.

One more option: use a stacked dado set on the table saw to cut the dado exactly to desire width.

[Jonathan Flachman]

You don't need to buy an exact size router bit. You'll be forever trying to match bits to different types of 3/4" sheet goods. I use something like this jig:
https://www.woodmagazine.com/woodworking-plans/routing/exact-width-dado-jig

But instead of a guide bushing, I use a top bearing 1/2" pattern bit. That made my jig simplier - just two pieces of 3/4 material for the two fences and and two for the end pieces. Super simple, I never worry about fit on 3/4" sheet goods any more.

[Hal Steele]

Thanks guys! So I'm NOT going crazy and my google search and purchase skills aren't off. I appreciate the help and the "why didn't I think of that?" jig plan.

[Mike Kees]

Twenty eight years ago or so I was in my 4th and final year of apprenticeship . Or instructor showed us how to make "router boards" to make dados. Just a piece of 1/4'' plywood with a straight hardwood piece screwed on one edge. The plywood needs to stick out more than half of the width of your router base from the hardwood strip.Then just install a straight cut bit in your router and cut the edge off,basically just like a tracksaw works. Now you simply line the edge up on your mark and cut your dado,flip around and do the other side. We even marked the other edge with a scrap piece of plywood being used for shelf held against the side we had just cut. Works excellent. About a year ago I made a jig out of two of these" boards" with the same idea as the wood mag version above. I used it on a project with 52 cubicles on a wall 10 '6'' high and 20 feet long. Biggest difference with my version to what has been described in posts above is this one does not need bits with bearings or collars.

[Hal Steele]

Thanks Mike. I get it and everything makes sense.

[Jim Becker]

For grooves that need to fit snug for critical structural reasons, the two pass method using narrower tooling is the only way to go since it can be made to match the specific sheet stock you have. I do have a set of Infinity bits that are "plywood sized" but as time marches on, they have proven to be "close" but not necessarily accurate for today's product. I always check the stock with calipers and adjust my method accordingly.

[Hal Steele]

For grooves that need to fit snug for critical structural reasons, the two pass method using narrower tooling is the only way to go since it can be made to match the specific sheet stock you have. I do have a set of Infinity bits that are "plywood sized" but as time marches on, they have proven to be "close" but not necessarily accurate for today's product. I always check the stock with calipers and adjust my method accordingly.

Thanks Jim. Sounds silly to those of you that deal with this regularly which I have not up until now. I was very surprised at the varying widths of different 3/4" plywood from various manufacturers. Then when I measure at my Big Box stores I find that even the stated measurement varies. To learn that nope, that's the way it is, here is how you deal with it is a relief of sorts.

[Frank Drackman]

This is exactly what I do. Super simple. My homemade jig will not win any awards for how it looks but I love it.

One other tip. If you are making mirror pieces (left & right side of cabinet) that need matching dados in plywood either cut the dado before you rip the sides or clamp them together and make the dado in both pieces at the same time. No chance of them being at slightly different places.


You don't need to buy an exact size router bit. You'll be forever trying to match bits to different types of 3/4" sheet goods. I use something like this jig:
https://www.woodmagazine.com/woodworking-plans/routing/exact-width-dado-jig

But instead of a guide bushing, I use a top bearing 1/2" pattern bit. That made my jig simplier - just two pieces of 3/4 material for the two fences and and two for the end pieces. Super simple, I never worry about fit on 3/4" sheet goods any more.

[glenn bradley]

Another vote for 2 passes with a smaller bit. My jig has the material clamp built in but follows the same design as many. You set the jig with the stock that will insert into the dado.

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Line the preset jig up on the mark and route the dado. Not as quick as a dado stack but, used at different times. I have a better time bringing the tool to the work when dadoing smaller parts. The jig also allows for stopped cuts and mortise slots.

[Ralph Okonieski]

My approach is similar to Glenn’s method although I do not have a jig. I use two “clamps”, similar to the blue handled one in Glenn’s photos. Position a piece of wood exact same thickness between the clamps and tighten clamps. A template router bit (has bearing) having smaller diameter than the opening rides on the inside edges of the “clamps”. Two passes and done. Works well.

Cutting both sides (left and right) of things like bookshelves or cabinets reduces (almost eliminates) chance of cutting one at wrong location. This way you are pretty sure they are located correctly.

[Hal Steele]

Thanks to all. I asked for a dado primer and got that and more.

[Jim Becker]

Thanks Jim. Sounds silly to those of you that deal with this regularly which I have not up until now. I was very surprised at the varying widths of different 3/4" plywood from various manufacturers. Then when I measure at my Big Box stores I find that even the stated measurement varies. To learn that nope, that's the way it is, here is how you deal with it is a relief of sorts.

"Technically", most sheet goods are manufactured in metric size. (MDF is often an exception) The thickness still varies because of the quality of the veneers and manufacturing, but even the "best stuff" is still going to have a little bit of variability. I will say that, in general, the veneer plywood I buy from Industrial Plywood has been pretty consistent in thickness...pretty much all North American manufactured and the veneer thickness is pretty good, too.

[Bruce Wrenn]

For dados, I use a 1/2" reverse helix bit from HerSaf ( no longer in business.) It's a screw on cutter with two cutting edges. Being reverse helix, means it press fibers down when cutting. Custom make base plates for router, which means the router is centered within 0.002". Still mark dado jig as to which side is the cord on router motor. To make it glide along dado jig, rub base plate with a dryer sheet.

[Jim Becker]

Down spiral (reverse helix) definitely help keep clean edges, but one must be careful using them, too. They push the chips down into the groove and if you don't keep things moving, excessive heat can become an issue including risk of combustion. Multiple passes with cleanout between is a good practice for this.

[glenn bradley]

I use two “clamps”, similar to the blue handled one in Glenn’s photos. Position a piece of wood exact same thickness between the clamps and tighten clamps. A template router bit (has bearing) having smaller diameter than the opening rides on the inside edges of the “clamps”.

Now that's clever Ralph; kudos on that idea.