# Jig for Cutting Exact Width Dadoes



## tomp913 (Mar 7, 2014)

Like the router table, my big bench (Kreg frame with maple top) has an open hole underneath that needs to provide some organized storage (a piece of plywood with "stuff" piled willy-nilly on top doesn't count). So I'm going to combine various ideas that I've seen and make some storage. It will have two levels; some full width drawers which open from both sides on the lower and a collection of shallow drawers on the upper level. The upper cabinet will need dadoes cut in the top and bottom for the dividers and I need a way of accurately cutting tight dadoes to suit the Sande plywood I use for shop cabinets. Looking for a quick and easy way to do it, I'm going to combine a couple of ideas - a T-square router guide and a method I just saw (in an old edition of Wood Magazine?). I made the T-square and will take some photos of it in action to explain better.








Checking the assembly for square. I glued wooden edges on the contact faces, hoping that they would hold up better.









Added a brace on the back to stiffen it up. This also give a handy clamping area, just reach across from the outside and clamp the brace and workpiece.









Pocket screws to attach the brace to the beam.









The completed T-square.

How it works. Rather than just guide the base along the edge of the beam, a 1/2" spacer - assuming that a 1/2" bit will be used to cut the 3/4" wide dadoes - is placed against the beam and the first pass made running the edge of the base along the face of the spacer. The second pass is made by removing the 1/2" spacer, adding in a scrap of the material being dadoed into the part, and making a second pass which gives an exact fit to the material. I'm planning to bore a shallow hole, slightly deeper that the depth of the dadoes being cut, in the base, slightly outboard od the wood edge banding. This will give the router bit somewhere to sit when starting the cut, and will also give an exact cut line indicator so all I'll have to do is line up the edge of the groove with my layout mark. This concept works because the router is moved for the second pass by an amount equal to the difference in thickness between the 1/2" spacer and the material being used. The good thing - it will work for different thickness of 3/4" plywood, and the cut line indicator stays in the same position regardless.


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## Stick486 (Jan 4, 2013)

when you do it, you really do it well...
nice set up Tom..


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## DesertRatTom (Jul 3, 2012)

The thick bar on the T square really opens up using it with bearing guided bits. Very nice. I don't trust speed squares and use a large draftsman's square instead on jigs.


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## tomp913 (Mar 7, 2014)

DesertRatTom said:


> The thick bar on the T square really opens up using it with bearing guided bits. Very nice. I don't trust speed squares and use a large draftsman's square instead on jigs.


Not exactly a Speed Square https://tsoproducts.com/tso-products-precision-system-triangles/mtr-18-precision-system-triangle/ Guaranteed square within .001" over the 18" length.

I know what you mean about Speed Squares though. I had one that was giving me inconsistent results - of the three indicating surfaces, only one was pretty square to the opposite side. Took it to a local machine shop and they skimmed the three surfaces perpendicular to the third, and showed me the check on their CMM when I cam to pick it up. They were a little slow at the time, and the foremen give the job to one of the machinists to do on his down time - cost me a whole $25, one of the best bargains I've run across. The photos show the check against my Woodpecker 1281 - in the last two photos, both squares are hooked over the edge of my saw table and the photos show zero gap at the end. It's good to go now.


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## hawkeye10 (Jul 28, 2015)

Great idea Tom. I don't see how you guys come up with such good ideas. The woodworking gods just didn't bless me. :crying:


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## tomp913 (Mar 7, 2014)

hawkeye10 said:


> Great idea Tom. I don't see how you guys come up with such good ideas. The woodworking gods just didn't bless me. :crying:


Thanks Don, but I just combined a couple of different ideas. The router T-square has been around for years, I had one back in the day when 3/4" plywood was 3/4" thick so quick and easy - lay out where you needed the dado, line up the slot in the edge and you were good to go.. I've got a couple of 23/32" bits now (and an 18 mm bit) and you would think that one of them could be used to cut a dado for the current plywood, but they cut slightly different width grooves, and not all plywood fit either of them. That's why I thought the idea of using the spacer was such a great one - I can set the T-square up with the 1/2" bit and spacer - the initial cut stays the same and gives me an alignment mark - and a piece of the plywood I'm working with can be used to finish off the groove to the required width. I figure that I can always stick a strip of blue masking tape on the back of the second spacer if the dado needs to be a tad wider. I'd thought about making a test board with a series of grooves, each cut with a different undersize bit, and testing the plywood in the grooves for the best fit, but that might require making several T-squares. Not that they're that hard to make, but it becomes a storage problem after a while.


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## DesertRatTom (Jul 3, 2012)

I have a discontinued Dado Wiz, which is for making exact width dados using a straight edge clamp as a guide. You insert a sample of the material and that is exactly the width you get. The router rides on top of it and you use a 3/8ths or half inch spiral bit. If you want to put glue in the dado, you need to add a couple of sheets of paper to the sample piece--it's that accurate. Too bad it failed as a product. Someone is making a version of it, but I don't think it's as complete as the original. Looks like it went out of business about 2013. 

Today, a good shop made exact fit dado jig and a bearing guided mortising bit are probably your best bit although there are a couple of commercial jigs still out there.


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## tomp913 (Mar 7, 2014)

DesertRatTom said:


> I have a discontinued Dado Wiz, which is for making exact width dados using a straight edge clamp as a guide. You insert a sample of the material and that is exactly the width you get. The router rides on top of it and you use a 3/8ths or half inch spiral bit. If you want to put glue in the dado, you need to add a couple of sheets of paper to the sample piece--it's that accurate. Too bad it failed as a product. Someone is making a version of it, but I don't think it's as complete as the original. Looks like it went out of business about 2013.
> 
> Today, a good shop made exact fit dado jig and a bearing guided mortising bit are probably your best bit although there are a couple of commercial jigs still out there.


I actually have one of those too, used a couple of times - and both times the knobs must have loosened up and the dado started to widen on its own. I believe that it's a pretty good design, and maybe I should give it a second chance. It actually has a means to "tweak" the width, via the adjustment screw - I have misplaced the instructions, but managed to find them on the internet so could get by with that. Although the instructions don't cover it, the package came with a strip of self-adhesive UHMW tape that I realized was to be attached to the surfaces which contact the bar and cut down on friction. I put that on yesterday, should probably give it another test. When I dug it out the other day, the clamps that are attached to the bottom of the bar were with it so there is really no reason not to give it another test - I never felt that the cam clamp gave a positive lock on the wood which is why I bought the under-mount clamps.

I have plans for an adjustable jig, using a straight bit and guide bushing which has always seemed like a better design to me, but I kind of like the simpler T-Square design, especially now that I can adjust the groove width using it.


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