The incredible shrinking dado blade

[Todd Burch]

I'm using this (new to me) unisaw for making a house full of cabinets. I'm using paint grade 3/4" birch ply, which, is actually somewhere around 11.5/16ths thick, or some such screwy thickness.

To go fast on this task, I'm setting up my stacked dado blade to be a full 3/4" wide, (minus a sharpening or two on the blade), so the dados have a slight gap, and I position the gap where it will be least noticeable. Thank you, Mr. Caulk. Everything is getting painted.

To speed my job up even faster, for ease of measuring and laying out, I'm not cutting my dados 1/4" deep. I'm cutting them to leave 1/2" of ply behind the dado. This way, if a box is 35" wide, the tops and bottoms are 34" long. Simple to remember. While the dados will give some mechanical support, I am leveraging them more for a glue reservoir and a registration aid during assembly than anything. I am glueing and then screwing through the cabinet into the edge of the shelves and partitions.

So, I have an odd sided dado depth. I've gotten to the point that "so many cranks up, and stopping with the handle on the crank at 9 o'clock" is pretty much the right setting.

Did I mentioned I lubed up the trunnion really well when I got the saw? I did.

So I was cutting out a cabinet yesterday morning, and NOTHING was lining up right in my test mockup. I must have scratched my head for an hour and a half, and I went through MULTIPLE mock up assemblies, making sure I got this cabinet correct. Here's what it will look like: (rear view. Cubbies on the right are for wine bottles)

SketchUpScreenSnapz036.png

As you can see, this is no slouch of a cabinet when it comes to its fair share of dados and internal members.

Anyway… after 3 mock ups, I noticed something with my dado depth that seemed off. While I knew I had set it up for something in the neighborhood of < 3/16", it was WAY too shallow. Turns out, my well lubed trunnion was lowering itself, all by itself, the more the saw ran.

I guess that's why they make those locking knobs in the middle of the crank handles. Duh. I've owned my powermatic 66 since '97 and never had to lock it once to maintain blade height. However, it seems with this '98 unisaw, you pretty much gotta lock it for even maintaining the regular saw blade height!

Lesson learned.

Tip: When setting a dado depth to leave 1/2" of material (instead of cutting 1/4" deep), take two pieces of scrap, dado them, and hold them back-to-back and measure the combined thickness of two pieces instead of one. Measure for 1". Here's an illustration.

SketchUpScreenSnapz037.png

[Paolo Trevisan]

great tip, thanks.

[Matt Day]

Some good points there. I haven't had any problems with my Bridgewood cabinet saw with the blade height, but it's probably a good idea if I tighten down the handle anyway.
Thanks for the tip on the dado depth.

[scott vroom]

Tip: When setting a dado depth to leave 1/2" of material (instead of cutting 1/4" deep), take two pieces of scrap, dado them, and hold them back-to-back and measure the combined thickness of two pieces instead of one. Measure for 1". Here's an illustration.

SketchUpScreenSnapz037.png

Wouldn't it work just as well to test cut one piece and measure for 1/2"? What's the advantage of cutting 2 test pieces and measuring for 1"?

[David Kumm]

Structurally a wide dado gives up a lot of strength compared to a proper fit. As to the dado lowering, the weight of the spinning mass in comparison to a single blade is significant enough to cause your problem. Thats why you size the dado diameter by the strength of the trunnion rather than the size of the main blade. Dave

[Mark Godlesky]

Wouldn't it work just as well to test cut one piece and measure for 1/2"? What's the advantage of cutting 2 test pieces and measuring for 1"?

Using the two test pieces will double your error, making it easier to tell if you're off. In real life it probably won't matter.

[Todd Burch]

Wouldn't it work just as well to test cut one piece and measure for 1/2"? What's the advantage of cutting 2 test pieces and measuring for 1"?

My eyes need all the help they can get.

Using the two test pieces will double your error, making it easier to tell if you're off. In real life it probably won't matter.

What Mark said is exactly why I do it that way. I also use multiple scrap pieces when I need an exact fit. For instance, if I need a 3" wide piece that is spot on, I'll cut 4 pieces of scrap and measure for 12".

[Alan Schaffter]

There is always a router plane-

[Pat Barry]

Lesson learned.

Tip: When setting a dado depth to leave 1/2" of material (instead of cutting 1/4" deep), take two pieces of scrap, dado them, and hold them back-to-back and measure the combined thickness of two pieces instead of one. Measure for 1". Here's an illustration.

SketchUpScreenSnapz037.png

The method you described here Todd is borderline genius. Why for the life of me I didn't think of this previously now that you made it so obvious makes me wonder. Thanks for sharing

[Peter Quinn]

Everyday I use the unisaw at work I thank god I bought a PM66. There isn't much I like about them except the color. Fence is decent too. The rest they can keep. The diminishing dado is something I've seen before both on a table saw and an overarm router, only on the router it gets deeper so you can't fix it with a second pass! The one problem with your method is that the thickness of the plywood varies, so you will alway have some plus or minus in the 1/2" tolerances. Probably not enough to wreck things, but with a "cut from above" method like overarm router the "leave 1/2"" thing really works.....until the router starts dropping subtly....and you don't notice it until all the parts for a whole kitchen have been run.....and you have to glue shims in every dada, tune up the router and try again. DAMHIK.

To overcome the vagaries of modern plywood and still maintain a tight dado without shims you can mill 5/8" dados in the sides then run the shelves standing up against a tall fence to leave exactly 5/8" thickness, or even 11/16" if your dado stack has thin plates, then glue the scored side to the blind side, great strength, tight joints, takes out the thickness variation from the equation.

[Todd Burch]

The method you described here Todd is borderline genius. Why for the life of me I didn't think of this previously now that you made it so obvious makes me wonder. Thanks for sharing

I've been called a lot of things, but borderline genius isn't one I hear that often! Thanks Pat.

Everyday I use the unisaw at work I thank god I bought a PM66. There isn't much I like about them except the color. Fence is decent too. The rest they can keep. The diminishing dado is something I've seen before both on a table saw and an overarm router, only on the router it gets deeper so you can't fix it with a second pass! The one problem with your method is that the thickness of the plywood varies, so you will alway have some plus or minus in the 1/2" tolerances. Probably not enough to wreck things, but with a "cut from above" method like overarm router the "leave 1/2"" thing really works.....until the router starts dropping subtly....and you don't notice it until all the parts for a whole kitchen have been run.....and you have to glue shims in every dada, tune up the router and try again. DAMHIK.

To overcome the vagaries of modern plywood and still maintain a tight dado without shims you can mill 5/8" dados in the sides then run the shelves standing up against a tall fence to leave exactly 5/8" thickness, or even 11/16" if your dado stack has thin plates, then glue the scored side to the blind side, great strength, tight joints, takes out the thickness variation from the equation.

I love my PM66 too. Hope to have it set back up in late Jan or early Feb.

I like the idea of the 5/8" (or 11/16") dado and "rabbeted mortise" you describe. Normally, I just fuss with shims (somewhere in storage, in a box, buried) on the stacked dado, but, as mentioned, these cabinets will be "good enough" as I am doing them. With my method, I only make a single pass on the female piece and do nothing with the male piece. This method would require machining both pieces, but, obviously, with real benefit. With three 2" assy screws along each joint, and glue, and a glued and stapled 1/4" ply back and 3" (glued and screwed) nailers top and bottom, I think I'll be good to go.

There is always a router plane- ...

Yes, indeed Alan. Did I mention I was in a hurry?

[Brian Holcombe]

The router plane is a godsend if you are cutting dados in solid wood and it cups slightly. Just something to keep in mind...

[Edward Oleen]

[QUOTE=David Kumm;2297797]Structurally a wide dado gives up a lot of strength compared to a proper fit. As to the dado lowering, the weight of the spinning mass in comparison to a single blade is significant enough to cause your problem. Thats why you size the dado diameter by the strength of the trunnion rather than the size of the main blade. Dave[/QUOTE

It doubles the error, thus allowing a smaller error to be detected, or increasing the accuracy of your measurement, which ever way you want to look at it.

Think of trying to measure the thickness of a single piece of paper, then consider measuring the thickness of 100 sheets, or 500 sheets, or even 1000 sheets: obviously, if your measurement is accurate to, lets say 1/64th of an inch (we don't have a nice shiny new digital caliper here) your measurement of a single sheet is pretty much meaningless, however, your measurement of 1000 sheets of paper is going to be off by 1/1000th of 1/64th of an inch.