Doctor Strange dado or: how I learned to stop worrying and love the calipers

[jonmulzer]

How deep does everyone here cut their dadoes? On an example ~3/4” plywood carcass where you are cutting dadoes for shelves? If you asked most woodworkers their answer would most likely be Ό” with a few people saying 3/8”. That is the general rule I have always heard, 1/3-1/2 of material thickness. Well I have a confession to make……

My name is Jonathan and I don’t care how deep my dadoes and rabbets are. There, I said it. I feel better. Whew! *wipes sweat from brow* (the more experienced among us can probably tune out now, they likely already know what I am about to say)

I have virtually every setup gadget ever made for table saw and router this side of the very expensive Angle Wright. I rarely use them anymore. They gather dust in a drawer in my workbench. I ceased to care how much material I am removing from my carcass sides. It is the least important measurement on my plywood carcasses and realizing that has saved me time and frustration in my projects.

Now, this is not to say that I just grab my outside dado cutters and a chipper or two for the table saw, toss them on in haphazard fashion, give the handle a few cranks and let her rip. That is far from the truth. I just finally realized one day that the amount of material being removed was far less important than the amount of material that is left, for most casework.

It really does not matter if your dado is Ό” in depth, 3/16” in depth or whatever. Somewhere between 1/8” and Ό” is sufficient for most all of our purposes, and that is the size of mine. Dadoes ranging in depth from 1/8” and Ό” when I am working with ~3/4” plywood are perfectly fine. I used the squiggly line because it is never Ύ”. Plywood generally ranges between 11/16” and Ύ”. At least the stuff I buy is. I know everyone speaks poorly of big box sheet stock but my local Lowe’s usually carries pretty good stuff. The Home Depots here in the Indianapolis area carry sheet goods that probably would not even make good firewood though. The one problem with it is variations in thickness. It is usually around 11/16”, sometimes a smidgen more.

I have gone through all of this long-windedness to let you know that I cut my dadoes to a depth that makes sizing my other parts the easiest. So in ~3/4” plywood I cut them to a depth that leaves me with exactly ½” of material remaining. I also use no measuring tools to measure the height of the cutters above the table saw surface. Crank it up until it is 1/8” or so above by just eyeballing it and cut a test dado in a scrap piece from the same sheet and then take my calipers and measure from the bottom of the dado to the opposing, outside face and since it should be greater than my magic ½” I crank it up a bit and recut. Rinse, lather, repeat until you have exactly ½” of material remaining and then cut all of your dadoes possible with this width of stack.

If you do not have calipers you can still get extremely close by using two pieces of scrap. Cut your test dadoes on both pieces and then place them back to back with the dadoes facing out and measure the combined thickness of both remaining pieces together and shoot for 1” exactly. By using two pieces you automatically account for and lessen the effect of cumulative error. Not as precise as calipers, but it should work plenty well for machining wood.

So, some of you may be asking why not just measure the plywood, subtract ½” and set your cutter height to that? Well, it comes back to cumulative error and not wanting to overcomplicate things. I do it this way because it is easy, simple, fast and it works. If you measure, measure, do the math, measure and then cut, you leave yourself open to cumulative error and you are making it a lot harder than it needs to be. You are also buying expensive setup tools that are never where you remember laying them down at last and then wasting time while looking for them and then by the time you do find them I have already cut my dadoes and started glue-up (as soon as I find where I set my glue down at last…….I really need to clean up my shop also, lol).

The next question is why am I being so anal-retentive about leaving exactly ½” of material? It makes your entire remaining math SO MUCH EASIER! You also should not care how deep the dadoes are by now! We all work off of a set of dimensions right? Or at the very least we have a set of outside dimensions in mind and some plan for scale and placement of the rest. Let us think about a simple bookshelf for a moment. We want to make it 30” wide. If you use my method when it comes time to cut your fixed shelves then you know they should be precisely 29” in length. If you cut them Ό” in depth you will probably end up with something approximately 29-3/32”, if you can even measure that closely. I on the other hand did not even have to go search for my tape measure that is not where I remember laying it down when I went to go look for the glue….. That is also assuming that you set your cutter height to exactly Ό”. Any amount you were off by you can double it and that would be your cumulative error. Those errors will all show up at the end when the bookshelf you wanted to be 30” in width is now 29-7/8” or 30-1/16” and that throws off every measurement from then on. It also could end up like a bookshelf I built once for myself where I messes up and cut my last piece to 28-1/16” instead of 29-1/16” and had to re-cut all my shelves and ended up with a bookshelf that was narrower than planned by exactly 1”.

This method works even better on drawers because by the time you end up measuring for them often times little errors have added up or you may have had to “adapt along the way” (compensate for mistakes in other words) and now your drawers are an oddball size. Drawer bottoms can also be a pain to measure for. Now that I quit caring how deep my grooves are also I leave myself with Ό” of material on my ~1/2” plywood drawer stock and when I measure my drawer I subtract ½” and cut my bottoms (I usually take off an additional 1/16” to make them easier to slide in and since they are plywood I also glue them into place.

It works for rabbets also. When I rabbet for plywood backs I leave myself with ½” of material like on my shelf dadoes and then I measure my carcass, subtract 1” from the width and whatever allowances I need to make from the height if any and cut my plywood back. I do not have to take inside measurements with my tape measure that never seem to turn out quite right.

I know this was terribly long-winded and I appreciate anyone that made it this far. I am coming down with a head cold or sinus infection and for some reason I was not able to sleep and felt like writing. This was also a little tip I wanted to pass on to the newcomers so that 5-10 years down the road they were just realizing this on their own. Every set of plans out there and every woodworking show always says Ό” dado for this or Ό” groove for that. Even his Normness always says he cuts a Ό” groove for drawer bottoms and I always wonder if they aren’t more like 7/32”?

[LCHIEN]

makes sense.
Here's a tip to leave 1/2" thickness in your plywood dados without having to resort to calipers and trial and error.
Take a piece of the plywood and place next to your blade.
Set one tip of your dado blade at TDC (top, dead center) - you can approximate this by placing a tip in the center of the arbor locking wrench slot, by eyeball should be close enough.
Now raise or lower the blade to make the tip the same height of the ply, use a straightedge or better yet a scrap of wood with a straight edge, sliding over the top of the ply back and forth over the tip adjusting the height until it just ticks the your straightedge.
Now note the position of the handle on your crank, lower it exactly 6 turns to the same place.

This works on the BT3000, BT3100, 21829 etc because the elevation screw is 12 threads per inch. In my experimentation I have found it will come to about +/-.005" or less than 1/128" of the intended position this way. If anything you end up more like +0.005/ -.000" because the blade tip was a bit off TDC when you set the height, the tip position being lower by a couple of thou will make the left part a couple thou thicker.

You might want to double check the results with calipers the first time you do it but I thin it will work quite well.

[cabinetman]

The next question is why am I being so anal-retentive about leaving exactly ½” of material? It makes your entire remaining math SO MUCH EASIER!

This is an excellent post as the technique could make the process more predictable. The problem of varying thicknesses of sheet stock requires the setup like you described to get further machining accurate.

I can't dazzle you with my using fancy measuring tools or calipers, because for the most part I don't. If height or running cabinet length is critical, I may, but for the most part, knowing what is left on rabbets and dadoes, and cutting the horizontals or verticals to that 1" less measurement is simpler.

One of my habits is to do dry fitting and then measuring the overall. This will not only tell me if my machining is correct but also if the pieces are inserted completely, like bottomed out. For involved casework or long runs where small pluses or minuses could be created, I draw out on some sacrificial sheet like plywood or MDF the entire layout in plan view. The 1:1 (full size) drawing will show carcass ends with the dado, the shelf (or top or bottom) into the dado, etc. That way, when I have machined all the parts and do the dry fit, I fit the model right on top of the drawing. If it's on the lines it's a go.

Doing drawers and figuring out the depth of the rabbets or grooves I do on the table saw. For either plywood or solid wood, doing a rabbet on the sides is easy with setting the fence to 1/4" (for 1/2" thick drawer sides) and then the front and back are an easy measurement to figure 1/2" less the width the drawer should be (and deducting a 1/32" for fit). For a 1/4" slide in bottom, I don't glue in the plywood, as I like the drawer to have it removable in case it ever needs replacement. But I do make it a tight fit, as it will help square the drawer.

As for setting the saw blade height, or router bit depth of cut, I always run a sample cut.
.

[rcp612]

I feel enlightened!
Thank you for removing so much pressure from my measurements, and so many "design changes" from my future projects.
This whole explaination proves to me that we overlook the simple things in our efforts to make everything perfect.
I may have to change my signature now.

[cgallery]

Excellent points, all of 'em.

I just hope I can remember the next time I have to cuts some dados!

[Ed62]

This is a good thread. It's also eye opening. Thanks for starting it, Jon. Hope you feel better soon.

Ed

[skillybob]

This is the kind of info. I appreciate-- good job.

[jonmulzer]

Thanks fellas. I kinda thought most of the experienced people had probably figured this out long ago. I just had an "ah-ha!" moment one day when I thought of it. I used to take both of my dadoed pieces and turn them back to back and measure the remaining thickness of both pieces and then subtract that from my planned overall width to get the dimensions of my shelves. Then one day I thought, "It would be a lot easier if this just came out to an even measurement!! Well, I am machining them so I can make it come out to whatever I wish with some prior planning!" I have been doing them this way ever since.

Loring, I realize what you are saying but I can set my dado height pretty close by eye now and I usually hit my desired height within a try or two now. I could probably have it set before I could find a suitable piece of scrap most times. My dogs tend to run off with any small pieces that hit the floor.

As for the head cold, it is worse now. I spent the day at my future (grandparents-in-laws?) and their weather station was saying it was 11% humidity in there. I was about to wet a washcloth and breath through it. Hopefully being home in ~60% humidity will help get me back to where I was real soon.....