Finessing Dado Slots to 6mm - Woodworking Talk - Woodworkers Forum
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post #1 of 38 Old 06-08-2019, 04:10 PM Thread Starter
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Finessing Dado Slots to 6mm

I have a need to cut quite a few dados to fit 6mm (0.236") Baltic birch plywood (BBP) shelf supports. The narrowest my dado set (DeWalt)will cut is 1/4" (DeWalt). I have an Incra LS Positioner (LS) on my router table and have made an insert for my Sawstop Jobsite saw (JSS) to allow mounting the LS on the saw.



I want to use the LS to make the dados and have figured out how to set the LS up to move over for the additional width needed, after making an initial slot using a single saw blade. I am sure I could use a standard combo blade and shift the LS for the additional width needed to get to 0.236", but the combo blade will leave those little peaks at the bottom (do they have a name?).



I am wondering if there is a way to use just one side of the dado set for cutting these slots and have the floor of the slots turn out flat. The dado set has a blade for each side of the cuts, but as stated the narrowest it will cut is 0.250" (using just those two "outside" blades). Are the inside edges of the teeth ground is such a way that they will properly cut an edge to the slot. Or, after the first slot is cut, should I put the "other" side blade on and finish the other side of the slot.



I have six dados to cut in each of six side panels (8"x14") for making shelves for holding plastic storage boxes - full of fasteners - similar to what we see at hardware stores for all their fasteners. So, with that many slots, I need a consistent way to positioning the fence, to make two passes for each slot. All of the dados can be cut using the first blade, and then the LS repositioned for the other side cut and all of them made with that setup so the blade switch only needs to be done once (in theory)



If there is another way to tackle this, I'm open to it. I have thought of using a router bit and the LS on the router table, but I do not have an undersized bit for this size slot).



Rick

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post #2 of 38 Old 06-08-2019, 04:16 PM
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The blade on the dado set wouldn't really be any different than your regular saw blade. You could just make two cuts with your regular blade. Personally I would just run the 1/4" dado blade. who is going to notice .014"
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post #3 of 38 Old 06-08-2019, 04:25 PM
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I donno if this will work .....

If you stacked/used two thin kerf 7 1/4" blades, for a circular saw and see what width kerf they made. A washer in between may add enough to get your 0.236 width .....
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post #4 of 38 Old 06-08-2019, 04:27 PM Thread Starter
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Quote:
Originally Posted by Steve Neul View Post
The blade on the dado set wouldn't really be any different than your regular saw blade. You could just make two cuts with your regular blade. Personally I would just run the 1/4" dado blade. who is going to notice .014"
Are you saying the dado blades will leave little peaks in the bottom of the slot? I though and my limited experience is that the bottom of the dado slots are flat and smooth.

I am wanting a tight fit for the self supports. The reason is these self supports are going to be only 1" wide, and not span the width across the front under each box. I do plan to glue them in place, but I do not want them drooping. I have to admit, I haven't tried the fit using the 1/4" dado set.

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post #5 of 38 Old 06-08-2019, 06:02 PM
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Quote:
Originally Posted by RickKr View Post
Are you saying the dado blades will leave little peaks in the bottom of the slot? I though and my limited experience is that the bottom of the dado slots are flat and smooth.

I am wanting a tight fit for the self supports. The reason is these self supports are going to be only 1" wide, and not span the width across the front under each box. I do plan to glue them in place, but I do not want them drooping. I have to admit, I haven't tried the fit using the 1/4" dado set.

Rick
The dado sets depend on the chippers to make a flat cut at the bottom but that narrow there is no chippers used. Still if you were to look at a wider dado done with chippers the blade cuts a v groove at the edge of the dado. You can see this in the picture.

The only way you are going to get a truly flat dado 6mm wide is to purchase a 3/16" router bit and run the cut twice. 3/16" is a fuzz less than 5mm.

If you are using glue it shouldn't matter the size difference. I frequently make cabinet doors with a 1/4" tongue and groove set and the plywood panels are running between 6 and 7mm. The only thing I can suggest is to make up a test piece using a 1/4" dado and see if it is too sloppy for you.
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post #6 of 38 Old 06-09-2019, 12:03 AM
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A while back, I was looking at the Micro-Jig Dado Stop product. It is a blade positioning jig for table saws that lets you set the two shoulders of a dado, then cut out the space in between.

https://www.microjig.com/products/dado-stop
https://www.microjig.com/products/dado-stop-pro

The problem that @RickKr noticed is that many blade profiles are not flat on top. That's because most blades are designed to make through cuts, and the blade profile doesn't matter in that case.

Most general purpose and combination blades have alternating top bevel (ATB) profiles, with two angled "slicing cutters" that leave a dip in-between. Here is one example of an ATB blade that I have. Look at the tooth profile image on the right side of the web page:

https://www.forrestblades.com/woodwo...or-table-saws/

What RickKr wants is a blade with a flat top tooth, sometimes called a "raker tooth". The raker tooth cuts off the extra portion between the angled tips. Here is an example of an alternating top bevel plus raker (ATB+R) that I use. Notice the profile again. This time it is left, right, left, right, raker. That fifth tooth (the raker tooth) is flat on top:

https://www.forrestblades.com/woodwo...or-table-saws/

I wondered why more people don't buy the ATB+R profile blades. What is the penalty? When I spoke with a nice expert at Forrest, he told me that he wondered the same thing. He felt that the ATB+R did as well as ATB blades for crosscuts, but better, faster, and cooler for rip cuts because of the raker tooth.

My Freud SD208S (and earlier SD208) dado sets are configured in a similar way, with alternating bevels and flat rakers.

One issue with some of these ATB+R blades is the corners of the ATB stick up slightly higher than the raker tooth, to cut the wood fibers cleanly and avoid tearout. These are the well-known "bats ears" that leave thin score lines at the corners of the blade tips. When I used the Freud dado set in a two blade 1/4 inch configuration to cut widely spaced shoulders, and then cut out the in-between space with lots of small cuts, it left lots of score lines. Just sayin'.

Several companies make special two-blade box joint sets. Freud's box joint set has flat tops and is cut to eliminate the bat ears, but at the risk of more tear out and rougher cuts.

https://www.freudtools.com/products/SBOX8

Blades made for ripping usually make flat top cuts. The teeth are often arranged in a "triple chip grind" (TCG) configuration. Because they have fewer teeth, rip cuts are fast, but not as clean. Look at the triple chip grind pattern on the right side of this web page:

https://www.forrestblades.com/duraline/

FULL STOP

I wrote the above text and don't want to lose it because it might be helpful. I just remembered that I started a similar thread related to my own research on the same topic. Included in that thread are photos of experiments that I ran with a single blade from the dado set. See this thread, particularly the photos in post 17:

https://www.woodworkingtalk.com/f5/a...joints-194689/
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post #7 of 38 Old 06-09-2019, 12:14 AM
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In the time you typed this and read the replies you could have put one dado blade on your saw and tried it, that way you would have seen for yourself how flat the bottom is and how clean the sides are.
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post #8 of 38 Old 06-09-2019, 04:27 PM
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Quote:
Originally Posted by FrankC View Post
In the time you typed this and read the replies you could have put one dado blade on your saw and tried it, that way you would have seen for yourself how flat the bottom is and how clean the sides are.
True, but Freud says don't do it, it's dangerous. I don't know if the other dado stack manufacturers say it too.

If you click on my last link in the post above, you will see that I did it anyway and posted photos.
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post #9 of 38 Old 06-09-2019, 06:18 PM Thread Starter
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Figured It Out, At Least For Now

Thanks, All. Great discussion.



First, with the LS freshly installed on the Sawstop JSS, I had to figure how just how well it does with positioning, which I assumed would be very good. Turns out it does. Very repeatable. I did figure out that the leadscrew has some backlash (~0.005"), which has to be accounted for when moving the fence using the leadscrew dial.



Second, with the trial cuts that I was doing, I figured out the bat ears from a standard combo blade (the one that came with the JSS) are not much of a problem, so, I plan on just using this blade and not worrying about about trying to use the dado blades. That greatly simplifies things.



After making the initial cut, with a dial indicator set up on the front face of the fence, I rotated the leadscrew dial until the indicator indicated that the fence had moved 0.100". Checking that on the graduations on the leadscrew dial, it was off by about 0.001". Making a second cut at this setting produced the slot pictured above.



It measures 0.233" and the 6mm BBP fits in it snugly but not tight, just the way I want it. We'll see how that holds up with more cuts and positioning.



Rick
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post #10 of 38 Old 06-09-2019, 06:46 PM
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Freud makes a square tooth ripping blade LM72R010 that is actually 0.126" wide and I use it for making 1/8" box joints. I would think that with this blade and the Micro Jig Dado stop mentioned above that you could fine tune your dado to perfectly fit your 6 mm Baltic Birch plywood. It would take two passes, but the Dado Stop and this Freud blade should be able to give you the perfect width dado with the clean flat bottom that you are looking for.

I own both the Freud LM72R010 and a Micro Jig Dado Stop, but have never used both together. I guess it's something that I need to try some day soon. They both do a great job separately, so I can't see why this combination won't do what you need.

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post #11 of 38 Old 06-13-2019, 08:07 PM
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Two passes with Infinity's flat topped blade. They have two,a 4mm one and a 6.5 mm blade. They call it 5/32 & 1/4"
https://www.infinitytools.com/saw-bl...lat-top-blades

Second option, get a high speed steel 1/4" router bit and grind down the edges a little. I've done that a lot in the past to get a special width.
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post #12 of 38 Old 06-14-2019, 08:29 AM
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I make the dado slots for the backs of my cabinets with my regular table saw blade. I nudge it over a bit to make the slot wider.

I make a couple of test pieces for adjusting the fence to the exact position. This is faster and easier than setting a dado slot and I get very accurate results.
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post #13 of 38 Old 06-15-2019, 07:29 AM
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Years back Sears made a wobble dado that went from 3/16" to 3/4". I used to have one but got rid of it when I could afford a stack set. I've recently picked up one off eBay and am using it for this very purpose. Today's "1/4" inch plywood is anything but 1/4" and some projects I've done recently needed a better fitting slot without the grooved bottom. The wobble dado did perfect. On wider dado sots the wobble dado gives a rounded groove bottom but not so with the narrow settings.
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post #14 of 38 Old 06-15-2019, 01:26 PM Thread Starter
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Quote:
Originally Posted by Bob Vaughan View Post
Years back Sears made a wobble dado that went from 3/16" to 3/4". I used to have one but got rid of it when I could afford a stack set. I've recently picked up one off eBay and am using it for this very purpose. Today's "1/4" inch plywood is anything but 1/4" and some projects I've done recently needed a better fitting slot without the grooved bottom. The wobble dado did perfect. On wider dado sots the wobble dado gives a rounded groove bottom but not so with the narrow settings.
Thanks for that info. It is exactly what I was asking about. I am satisfied with using a standard combo blade and shifting it over using the LS Positioner. I would likely NOT be able to use the wobble dado anyway because my saw is a Sawstop.

I knew Baltic birch plywood is undersized but I understood it is because it is actually metric. Are you saying domestic regular plywood is now also undersized? I haven't paid any attention to that as I just haven't used much lately.

Rick

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post #15 of 38 Old 06-15-2019, 03:19 PM
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Yes and No ....

Quote:
Originally Posted by RickKr View Post
Thanks for that info. It is exactly what I was asking about. I am satisfied with using a standard combo blade and shifting it over using the LS Positioner. I would likely NOT be able to use the wobble dado anyway because my saw is a Sawstop.

I knew Baltic birch plywood is undersized but I understood it is because it is actually metric. Are you saying domestic regular plywood is now also undersized? I haven't paid any attention to that as I just haven't used much lately.

Rick

USA plywood is still the standard 4' X 8', BUT the thickness is not 3/4" or 3/8":
https://www.hunker.com/12464023/what...zes-of-plywood
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post #16 of 38 Old 06-15-2019, 03:56 PM
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Quote:
Originally Posted by Steve Neul View Post
The blade on the dado set wouldn't really be any different than your regular saw blade. You could just make two cuts with your regular blade. Personally I would just run the 1/4" dado blade. who is going to notice .014"

The only difference would be the bottom. Regular saw blades are ATB, dado blades are ground flat.
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post #17 of 38 Old 06-15-2019, 04:49 PM
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Quote:
Originally Posted by Cephus View Post
The only difference would be the bottom. Regular saw blades are ATB, dado blades are ground flat.
The chippers are ground flat but the outer blades are not. The discussion was making two cuts with just the outer blade without the chippers.
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post #18 of 38 Old 06-16-2019, 06:11 AM
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Quote:
Originally Posted by Steve Neul View Post
The chippers are ground flat but the outer blades are not. The discussion was making two cuts with just the outer blade without the chippers.
The outer blades on my Freud SD208S dado set have a five-tooth pattern: ATB-left, ATB-right, ATB-left, ATB-right, flat-top raker, ... (repeat)
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post #19 of 38 Old 06-16-2019, 07:21 AM
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Quote:
Originally Posted by Tool Agnostic View Post
The outer blades on my Freud SD208S dado set have a five-tooth pattern: ATB-left, ATB-right, ATB-left, ATB-right, flat-top raker, ... (repeat)
That would be more like a regular saw blade. Most I've seen the outer blades are ground with all the teeth to a point on the outside. This would account for the groove cut to the outside on the picture I posted.
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post #20 of 38 Old 06-16-2019, 10:25 AM
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My dado sets have grooves because the bat's ears stick up slightly beyond the flat top of the raker teeth and chippers. The bat's ears are the very top points of the ATB teeth.

According to Freud, the bat's ears score the wood for clean cuts that reduce chip out. They do leave score marks in the otherwise flat bottoms of the dado cuts.
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