"Perfectly Flat" Dados?

by ‎04-08-2010 10:41 PM - edited ‎04-09-2010 08:05 PM

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 a deep hidden meaning or major point to this article, 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.




on ‎04-15-2010 05:32 PM

I use a wobble blade and it gives me flatter and trues dado than the stacked set I have and is much easier to set to plywood sizes.


by rwybeaker
on ‎04-16-2010 09:55 AM

Two words will cure these dados, even from the HF set:


Router Plane





And instead of worrying about cutting an exact width to match plywood or any other material (which will have some variation in thickness) try a housed dado instead.



by DrByte
on ‎04-30-2010 05:46 AM

Actually it all the above mentioned problems can be solved with one word:  ROUTER!

by WBD
on ‎04-30-2010 09:14 PM

I have to ask - other than truth in advertising, is there any reason to need perfectly flat dadoes? I cannot think of any time I've left them exposed - certainly not with plywood - are they that common in other situations? I'm not trying to be argumentative - I am really asking what situations leave them exposed?

on ‎05-01-2010 01:01 PM

I don't typically leave an entire exposed dado open (except on the outfeed table of my table saw!), but there have been times when the end profile of the dado is visible even with a filler piece installed. 


None of the decent dado sets I've used would be a problem, but I can envision some of the really cheap sets being problematic.