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Thread: Finessing Dado Slots to 6mm

  1. #1

    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).


  2. #2
    I use a blade that has a flat grind such as a ripping blade. It takes a little patience when cutting to avoid tear out, but leaves a nice flat bottom with clean corners.

  3. #3
    Join Date
    Apr 2013
    Okotoks AB
    What Johnny said. But not all ripping blade are flat top grind. I have a Freud rip FTG rip blade & the top of the cut is perfectly flat. I use it mostly for box joints. To avoid tearout, use a zero clearance insert.

  4. #4
    Join Date
    Mar 2003
    Upland CA
    Never used an LS positioner, but the way I do it is to make a spacer the size you need to move the fence that little bit. Cut, move fence cut, repeat.

    Somewhere in the archives is a simple jig I made for my Unifence to do this, but I have no idea where. It made it into Woodsmith.
    Rick Potter

    DIY journeyman,
    FWW wannabe.
    AKA Village Idiot.

  5. #5
    Join Date
    Mar 2003
    San Francisco, CA
    Forrest makes a dado stack which will cut that 6 mm dado.

  6. #6
    Join Date
    Oct 2013
    Northwest Indiana
    Since you have the router table and the Incra about a 6 mm bit in the router table? Most bit manufacturers do "plywood" bits that are sized for what you are doing. The LS gives at least 17" of travel which puts you in the middle of a 34" board (farther if you mount farther back). Easily repeatable, and with a spiral (upcut if i'm thinking right?) bit you'd have a great bottom. Cut the grooves in a wide panel then (16.125" x 14", then rip them apart (the extra .125") and you're guaranteed to have the dadoes parallel. If the thin panel will slide in and out to hold the trays--you may actually need the full 1/4" anyway to allow easy clearance.
    Good luck.

  7. #7
    My vote goes for making an adjustable dado jig for you to use with a handheld router.

    My concern is that the Baltic Birch might not be precisely 6mm, so the jig I propose can be sized to whatever it is using a piece of the actual material you are using. I saw this years ago in a book by Bill Hylton called Woodworking with the Router. Here is a link to a webpage that provides someone else's overview of the same jig:

    Unlike some jigs that use guide bushings or pattern bits, this one relies on the router base, so you can use a simple straight bit less than the width of your dado. In your case I would use a 3/16 bit and build the jig to this bit which means you can make a dado a minimum of 3/16 and as wide as you like depending on the travel of the adjustable fence.

    Lay out your dadoes on one side panel, and use that panel to transfer lines to the others, taking care when placing the jig to make your cuts. I'm not sure how wide your side support panels are, but you could size the jig to cut the dado into two opposing sides in one pass guaranteeing that they will be lined up.

    I hope this helps.


  8. #8
    Join Date
    Dec 2006
    Toronto Ontario
    Unfortunately the veneered panel is often less than 6mm after sanding.

    I bought a 4 to 7.5mm Grover for this application.

    Your local tooling supplier will supply any size blade you want, they could supply you with an adjustable set for exactly what you’re doing.....Regards, Rod.

  9. #9
    Join Date
    Sep 2016
    Modesto, CA, USA
    I would buy two blades that are just a little thicker together then the 6mm needed. Mount them backwards in the saw and touch a fine grindstone to the sides until the stack thickness is correct. If I read you correct you only need to take off 0.06 inches in width. Or buy two steel blades and reset the teeth. inward.

  10. #10
    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.
    Incra LS on Sawstop JSS 06-09-19 640.JPG
    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.
    6mm Slot 2 06-09-19 640.JPG
    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.


  11. #11
    Join Date
    Dec 2007
    Kansas City
    I would cut them a little undersized and use a side rabbet plane to sneak up on the fit.

  12. #12
    Join Date
    Nov 2013
    Leland, NC
    If a person has the bucks (about $150) a digital fence on the TS is a great addition. I have been using one for about 6 years. Has completely changed the way I work in the shop.

    Doing dadoes is now easy with either a dado blade for wider dadoes or a saw blade for smaller ones.

    When I put a different blade in the saw the very first thing I do is measure the width of a small scrap piece. Then I more or less rip it in half (this cut does not have to be accurate at all). Then I put the two pieces together and measure again. Subtract the second measurement from the first and I now know how wide the blade cuts. I put this number on a sticky note. (I do not cut this test piece in half with a dado blade, I just measure the width of the dado it cut).

    Time to cut a dado. My blade is removing .098 per pass from the test above. I need to cut a .236 dado.

    First cut removes that .098. I press the button that puts the readout in incremental mode. It now reads "0".

    Subtract the .098 from .236 leaves .138 to be removed. All that remains to be done is move the fence over until the readout says .138 or something real close to that, it's woodworking folks, not a rocket ship.

    Gone are the days of endless test cuts with a stacked dado blade, fiddling, tapping, nudging, etc.

    What is also gone are the days of making "extra pieces" in case something goes wrong later on and a new piece is needed. The saw can easily be reset to any dimension needed. Life is good in the digital age.

    I have been trying to come up with a digital fence for the RAS that does not break the bank. I prefer to do crosscutting on the RAS over the TS.

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