Friday, October 28, 2011

Flat bottom table saw cuts

If you've spent much time watching New Yankee Workshop, you've seen Norm cut a wide dado (slot) in a piece of wood by making multiple passes with the table saw, moving the wood a little to the side after each pass.

But when I tried it, I found that my saw left a V-shaped profile at the bottom of the cut, like the one on the left here:

That's because my blade, like most table saw blades, has an Alternating Top Bevel (ATB) grind.  You can see a good picture at the bottom of Rockler's blade selection page.  If you look carefully at a blade edge-on in the store, you can generally see the alternating profile, like you see on that page.

What I needed was a blade with a flat top grind.  Apparently there are blades where all the teeth have a flat (or "raker") grind, but I've only ever seen them on Dado sets.  (Ripping blades supposedly are ground this way, but I've never seen one that actually was.)

So instead of having all flat top grinds, I wanted an ATB/R (Alternating Top Bevel with Raker) blade.  The rockler page calls this a "combination" grind.  Remarkably, only two of the 10" blades they sell have this grind.

I ended up buying two blades: the Dewalt 7150PT ($36.04) and the Freud LU84R011 ($69.99).  Both are 50 tooth "combination" blades, designed to be good at both ripping and crosscutting.

The Dewalt blade also happens to be a "thin kerf" blade -- it makes cuts that are 0.100" wide, compared to the 0.125" kerf of the Freud.

Once the blades arrived, I took some test cuts in a piece of 3/4" MDF to see how they compared to my original 80-tooth Diablo ATB blade.  (Huh, turns out that's also a 0.100" thin-kerf blade.  I never noticed before!)

I cut both single-pass and multi-pass slots:

Here you can see the profiles left by the blades in single-pass cuts:

The cut on the right is my ATB Diablo blade.  It leaves the deepest V-groove.  Measuring the groove in the wood with my calipers, it looks like the bottom of the groove is about 0.016" deeper than the top.

The cut on the left is the Dewalt ATB/R blade.  Disappointingly, it also leaves a pretty significant V-profile, about 0.009".

The wider middle groove is from the Freud.  It's the flattest, but still not perfect; I see about 0.005" difference between the top and bottom.

On to the multi-pass cuts.  I should have been more careful to control the amount I moved between each pass, but the results agree pretty well with the single-pass tests.

The photo at the top of the page shows the Diablo ATB results.  Here's the slot left by the Dewalt:

And here's the slot left by the Freud:

Conclusion: I'm not particularly satisfied with the dadoes from the Dewalt blade.  The main reason I considered replacing my existing blade was to get flat bottomed channels, and the slots it leaves are bumpy enough that I'd still want to clean them out by hand with a chisel.  On the other hand, it's half the price of the Freud, and cuts just as easily.

The Freud blade is the one I'll keep on my saw (there's another table saw at work I'll use with the Dewalt).  I was hoping for completely flat slots, but these are good enough for my needs.

1 comment:

Unknown said...

Hi Lunkwill.
Only after finishing the finger joint jig for my table saw, I realized that I had ATB blade.
So I was googling about exactly same problem and arrived at this post.
I see this was from 2011, but have you got any improvements on the subject?
If you had, please let me know.
Thank you.