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Finessing Dado Slots to 6mm

9K views 37 replies 15 participants last post by  RickKr 
#1 ·
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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#4 ·
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
 
#3 ·
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 .....:nerd2:
 
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#6 ·
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/woodworker-ii-all-purpose-saw-blade-for-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/woodworker-ii-all-purpose-saw-blade-for-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/abusing-dado-blade-set-box-finger-joints-194689/
 
#9 ·
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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#10 ·
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.

Charley
 
#12 ·
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.
 
#30 · (Edited)
And as Woodnthings noted, plywood thicknesses vary widely. In fact, plywood is wood and therefore subject to the same rules that all wood is subject to, particularly that it takes up and loses moisture from the air and resultantly swells and shrinks across its grain, which causes the thickness to change. That’s on top of the fact that the factories are using thinner plies and plies of differing species and multiple thicknesses.

So the takeaway is that a fixed width cutter will not work in all situations and the solution must be adaptable. “Packard’s” method is similar to what I use with a triple-chip blade that is at least 1/2 the width of the desired dado. If you just cut all the workpieces in gang passes then you’ll only have to make 2 setups that give the exact desired result: The first face is cut on all pieces, the fence is adjusted, then cut the remaining faces.

No fussing with the blades/cutters, very fast with perfect repeatability, and no added cost for added cutters. Keep it simple and think beyond the notion that things must be done in a single shot.

— Bradley
 
#13 ·
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.
 
#14 ·
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
 
#20 ·
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.
 
#21 ·
The teeth on my DeWalt dado set have alternating angled and flat teeth on the outside blades. All of the angled teeth are in the same direction, towards the outside of the blade. I have not tried one by itself to see what the bottom profile is like, but I expect it would be relatively flat with only minor bat ear-ing. I don't really need to find out as the bottom profile using the standard Sawstop combo blade is satisfactory for my purposes.

Rick
 

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#22 ·
I still think my post #10 of using an 1/8" FTG blade and the Microjig DADO Stop would do the best for you, but it will take 2 saw passes for each dado. Since the DADO Stop rides on the fence it would be easy to increment the position for doing many shelf dados along a board. Since the DADO Stop gets used as a positioning stop and the board being cut is not touching it as the cut is made as you push the board into the cut using the miter gauge, there is no chance of binding or kick-back due to this. You just move the board against the DADO Stop for the first pass and then push it forward to make the cut, then move the board to the second position of the DADO Stop and make the second cut to complete the narrow dado. Once both cuts have been made, move the fence and DADO Stop to the next position for another dado and repeat the process.

Charley
 
#24 ·
It Works Pretty Much as I Hoped

Been working on this for a couple days, off and on. First, I changed how the LS was mounted on the table saw (first three photos).

I ran three trials on scraps. The first pass was made using just the combo blade that came with the saw, incrementing the LS fence by 2-3/16" using selected positions on the scale. The slots measured 0.135".


Then, I shifted the fence by 0.100". Because I am still learning how this LS Positioner works, I used a 2" travel dial indicator and moved the fence referencing the indicator. Then I moved the scale so the selected locations were again under the hairline of the clamp. Then make the series of cuts, incrementing the fence position by the scale makers. Most came out quite close (0.235"-.0237"to my target of 0.236". The 6mm plywood fit in these quite nicely. Three were oversized and I was baffled as to why. I know this is woodworking and things are not as neat, tidy and precise as can be done in metal machining, but I didn't like the variability and the oversized slots were just too sloppy. Then I noticed that the screw mounting the fence to the LS ram on the right side was loose. Tightened that prior to the third trial.

Third trial turned out pretty good, again 0.235"-0.237", except for the last one, which was undersized (?). Previously, I had noticed some larger chunks of sawdust, chips really. Sawdust between the stock and the fence could cause the slot to be undersized. Even though that last slot was undersized, it was still usable, just a bit too tight.


I'm feeling pretty good about this assembly and process. I think I'm ready to try putting some of these dado in my box sides. I did make several spares, so I can make a few mistakes without loosing much.

Rick
 

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#25 ·
I'm glad you finally got it!

I don't have a positioner so my approach would have been more direct. I would have stacked two thin kerf Diablo 24 T rip blades to see what width kerf they produced. If it was under .236" I would added a shim(s) to get there. If it was over .236" I would have had to use two circular saw blades 7 1/4" ... or what ever worked until I got to .236"



It's really too bad that the plywood thickness has changed and nothing works any longer. I appreciate your stickin' to it until you finally got it!

:vs_cool:
 
#26 ·
I don't have a positioner so my approach would have been more direct. I would have stacked two thin kerf Diablo 24 T rip blades to see what width kerf they produced. If it was under .236" I would added a shim(s) to get there. If it was over .236" I would have had to use two circular saw blades 7 1/4" ... or what ever worked until I got to .236"

It's really too bad that the plywood thickness has changed and nothing works any longer. I appreciate your stickin' to it until you finally got it!

:vs_cool:
Thanks. I appreciate all the feedback and ideas. If I did not have the positioner, I would have followed one of the several approaches offered. But... I do have it and have been wanting to learn more on how to use it and what else it can be useful for. I've only made one box using it but hope to make some different corner joints on these plywood boxes, even though it is overkill.

In a recent conversation with a good friend about some stresses in my life currently, she referred to me a couple times as being "methodical". I think that is what carried me through this. And there is more to come, on both fronts.

Rick
 
#27 ·
There is a very simple solution.

Get two of the Freud rip blades and put them in the saw with the teeth in the gullets of the other blade.
Typically the set of a blade is 1/64 or .397 MM. A rip blade is usually leaves a 1/8 kerf. Two back to back rip blades should make a 5.953 MM kerf. You may have to slip a sheet or two of copy paper between the two blades to get your 6 MM.
 
#29 ·
My comment regards Steve's original response without reading any other (as good as they may be). Go with this because when you add glue, glue has moister (water) which swells the wood in both mortise and tenon members of the joint. I think the .014 will be covered!
 
#35 ·
I'm not sure how thick those items are, but I sure has heck remember the code cards. Punched my Master's Thesis statistical analysis on them and had to wait for the computer department to run it overnight. We have 3x5 cards around here, but I've not used them for better than three decades. I know there won't be any code cards.

By my calculations, 0.020" of shim should do it, which I have in the DeWalt dado set. Should be easy, but ya never know. That would put the blade tip width at exactly 0.236" but it could easily cut wider, so some adjustments may be needed.

Rick
 
#33 ·
OMG Rick, please save your money and crazy drive time and don’t buy a duplicate blade. You already have all the tooling you need to successfully complete your project several times over within the travel time you are considering.

Instead, as Packard and I commented, it is far simpler and more direct to stick with one blade and shift the workpiece just enough to cut first one and then the opposite face of the dados.

Now that I’ve seen the mockups of what you’re trying to do with a series of dados up a single side piece, I’ll add that you can use this very approach to do a series of dados up the side by employing a setup jig. The idea is to cut a long stick of wood to the exact width that matches the offset of each level, then cut that stick down into the same number of individual pieces that matches the number of trays/drawers you’re supporting (these will be spacer sticks), then start by stacking all the spacer sticks between the workpiece and the fence, set the fence, make your first cut, then remove a stick to make the next shelf cut, and rinse and repeat one stick at a time until the first cut is made on all the dados.

Next, restack and reset the spacers and workpiece, then readjust the fence to cut the opposite face, then do the same cutting process for the rest of the 2nd faces.

This is probably a 60 minute job.
 
#34 ·
Scurvy,

Thanks for the encouragement and details of the process you recommend. I'll have to study what you have written because it is not coming to me with just an initial read.

To easy your concern a little, I am already doing the drive to Portland, to transport my granddaughter to spend time with her grandmother. I have some time until then to consider your admonition about not buying a couple more saw blades, although, now that I have it in my head that I want to try that approach, it may be too late to dissuade me:smile2:

Rick
 
#36 ·
Scurvy,

Thanks for the encouragement and details of the process you recommend. I'll have to study what you have written because it is not coming to me with just an initial read.

Rick
Rick, to clarify a bit here:
  1. Conceptually, imagine needing your shelves to be equally spaced at 3-5/8” top surface to top surface and you want 5 shelves with the bottom of the rack/box being the lowest shelf.
  2. The shelf thickness fits within that 3-5/8”
  3. The top shelf has the same spacing and is terminated by the top of the box, so everything is all equal and symmetrical in this example
  4. Create the 5 cutting spacers from a stick of 1X4 that is cut to lengths convenient for dadoing the workpieces against the fence.
  5. In practice, you’ll stack/lay flat all of the spacers side-by-side between the fence and the blade to set the maximum offset, and running your first undersized dado cut for the first shelf against the full stack, then remove 1 spacer in order to make the 1st undersized cut in the next dado, then remove the next spacer to make the 2nd cut, et cetera until the last spacer is removed and the cut has been made. This process spares you having to reset your fence for every single cut. At this point, restack the spacers, adjust the fence for the desired shelf thickness/dado width, and repeat the above process to complete the series dados.
  6. It’s implied that prior to starting the 1st dado cut, you’ll have to decide if you want the dado width to be included within that 3-5/8” or outside it, which means the shelf thickness needs to be built into the setting of the fence, but that adjustment is performed only 2 times, at the start of the first stacking and then again at the start of the Restack. For sure, use a piece of your shelf stock as a gauge to set this.
  7. Obviously, you want to run all workpieces sharing the same dado layout and sizing all at once before removing the 1st spacer

Hopefully this helps.
 
#37 ·
What about using a thin kerf blade? Thin kerf blades will generally be 25% thinner, creating a cut of 0.091” or about 3/32”. The thinnest kerf circular saw blades currently made are 0.059” (about 1/16”) thick.-- http://circularsawblade.net/kerf
6mm converts to 15/64. 2 passes with a narrow kerf, jog the fence a bit between passes. Freud makes a narrow kerf version of their glue line.
 
#38 ·
What about using a thin kerf blade? Thin kerf blades will generally be 25% thinner, creating a cut of 0.091” or about 3/32”. The thinnest kerf circular saw blades currently made are 0.059” (about 1/16”) thick.-- http://circularsawblade.net/kerf
6mm converts to 15/64. 2 passes with a narrow kerf, jog the fence a bit between passes. Freud makes a narrow kerf version of their glue line.
DRT, where have you been, my friend :wink:?

Stacking thin kerf blades has been suggested/discussed in several posts in this thread, #3, 25, 27 31, and 35 at least. That is the approach I am currently pursuing. As I mentioned earlier, I am in Portland this weekend for other reasons, but planned to use some of my downtime to locate a couple Freud glue line blades with 0.118" kerf. I figured that would be easy, but it turns out not so. I called the three major woodworking stores in town and NONE had even on of those blades in stock (LM74R010). I ordered them online and they will be waiting for me when I return home.

I opted for the 0.118" kerf so I could gang them like a dado stack, with about a 0.020" shim between them to have a chance for a single pass cut to the desired size. I have thin shim stock in case the the shims from my dado set (in 0.005" increments) do not get it to the desired width.

It was suggested early on that I just use the dado stack for 1/4" and be done with it, that 0.014" wouldn't be noticed. That would be true and I would have done it that way IF these shelves were full width. But these shelf "supports" are only 1" wide, and do not span all the way across. I think they need to be a snug slip fit in the dados so they don't droop during the gluing.

Rick
 
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