"Perfectly Flat" Dados?

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Blog entry by knotscott posted 09-15-2010 03:01 AM 13260 reads 1 time favorited 11 comments Add to Favorites Watch

This borders on a “knitpick” IMO, but for the sake of clarity and truth in advertising, I think it deserves to be addressed. I frequently read phrases like “perfectly flat bottoms” and “absolutely perfect” in regards to grooves and dados cut from stacked dado sets. No doubt, some are better than others, and the very best are really, really good, but nearly all the stacked dado sets I know of use beveled teeth on the outside cutters to help minimize tearout (Freud, Forrest, Infinity, Ridge Carbide, Systimatic, Oshlun, CMT, Amana, etc). In order to be of any benefit, the beveled teeth must protrude slightly above the flat teeth of the inside chippers. The protruding beveled teeth leave tiny grooves at the outside of the cut…a trait often called “bat ears”. The better sets tend to stagger some flat teeth in between the beveled teeth to minimize the depth of the bat ears, but they’re there. It’s minor enough that a lot of people don’t even realize it, but it’s not realistic when manufacturers or owners claim that these sets leave truly flat bottoms, when indeed they don’t. It’s likely that there are sets that don’t have any beveled teeth on the outside cutters and use all flat top ground teeth, which will leave a flat bottom, but those sets will also tend to exhibit more tearout in cross grain cuts than the better sets that use beveled teeth. The other option for truly flat bottoms is from a router bit, but there are other drawbacks to that method as well… there’s never a free lunch!

There’s really not an important point to this post, but just wanted to point out the reality of the results that most of us get from even our premium stacked dado sets.

This graphic from “Woodhelp” illustrates the “bat ears” effect and the cause:

Here’s another pic from Forrest’s website that shows the correct orientation of the cutters…it also shows the bat ears left by the beveled teeth:

Here’s another gross example from a cheap Harbor Freight dado set…ignoring the poor linearity across the bottom, you can clearly see the large bat ears in each corner.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

11 comments so far

View PurpLev's profile


8554 posts in 4332 days

#1 posted 09-15-2010 03:08 AM

I only have one thing to say:

Stopped Dadoes

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View RonPeters's profile


713 posts in 3564 days

#2 posted 09-15-2010 03:24 AM

I have a $100 Freud Diablo dado set and it’s not exactly ‘flat’ either – poor linearity. I also have a cheap $30 Harbor Frieght set, and each blade is a little different size.

I figure the glue has to go somewhere, maybe this is where?

-- “Once more unto the breach, dear friends...” Henry V - Act III, Scene I

View Joe Watson's profile

Joe Watson

316 posts in 4230 days

#3 posted 09-15-2010 04:11 AM

you need a router plane. you cut the dado then use the router plane to flatten the bottom of the dado.

-- Got Wood?

View swirt's profile


4944 posts in 3655 days

#4 posted 09-15-2010 04:40 AM

What Joe said, or better yet a dado plane in the width you commonly use for dados. They are simple and work great.

You can can find a lot of old wooden rabbet planes around. Some are in not great, but useable shape. The odds of finding one in exactly the width you need are slim. However if you find one that is a bit over-sized you can bring it to an exact fit for your dado set with a belt sander. It would work well at cleaning up the bottom of your dado with only one or two passes.

-- Galootish log blog,

View canadianchips's profile


2632 posts in 3680 days

#5 posted 09-15-2010 04:41 AM

I use a #39 stanley plane. It has a scewed blade. Does a remarkable job cleaning up dado’s.

-- "My mission in life - make everyone smile !"

View RonPeters's profile


713 posts in 3564 days

#6 posted 09-15-2010 05:02 AM

A router plane? Now you tell me! ;-)

I just did a quick search and found this helpful site

also in my new Lee Valley & Veritas catalog I find on p73 a router plane for $155 with fence. Very nice.

Searched for the #39 stanley plane and they vary in price (about half of the Veritas) at Handplane Central…

I’ll have to investigate further… Thanks!

-- “Once more unto the breach, dear friends...” Henry V - Act III, Scene I

View TopamaxSurvivor's profile


18954 posts in 4359 days

#7 posted 09-15-2010 05:03 AM

Geez, there sure are a lot of subtle things to watch out for in these premium tools!! :-(( thanks for the post Knottscott.

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Eagle1's profile


2066 posts in 3748 days

#8 posted 09-15-2010 11:29 AM

After reading your post. I went out to the shop and looked at my Freud Diablo Pro set. The out side blades, every other tooth is flat. And the chipper’s are flat also. I would possible check out someother Freud sets and check them out. I don’t know if the pro set is any different or not.

-- Tim, Missouri ....Inside every older person is a younger person wondering what the heck happened

View RonPeters's profile


713 posts in 3564 days

#9 posted 09-15-2010 05:47 PM

My set is the same as yours. Every other tooth…

-- “Once more unto the breach, dear friends...” Henry V - Act III, Scene I

View Jerry's profile


3388 posts in 2331 days

#10 posted 12-22-2013 07:05 PM

I bought these

This probably does not qualify as a true dado set, but technically, box joints are very short dados. You could use these to cut a dado of any length and they DO cut perfectly flat dados, but only in a 1/4” or 3/8” width.

I’m not sure this is useful information for the topic, but I thought I’d offer it anyways.

-- There are good ships and there are wood ships, the ships that sail the sea, but the best ships are friendships and may they always be.

View knotscott's profile


8369 posts in 4059 days

#11 posted 12-22-2013 11:38 PM

That’s a great box joint set, and they will indeed cut perfectly flat dados. The downside of not having beveled teeth on the outside cutters (the ones that leave the bat ears) is that they’re more prone to tearout on cross grain cuts. No issues with the grain, like for box joints, but going across the grain would likely yield some unsightly tearout. Stacked dado sets and box joint cutters are essentially two different cutting tools with different strengths and weaknesses, but there’s definitely some overlap in their capabilities.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

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