A dado blade is used on the table saw when is is necessary to make a wide groove (for a dado joint), or a rabbet cut. The photo shows a modern stacked dado set. There are several type of dado blades, but it is safe to say that the stacked blade is the most popular type of dado blade. I not going into how to use the dado, just how to set it up for the proper width.
At The MakerBarn (a makerspace near Houston, TX), we have two table saws. The main table saw is a SawStop Industrial. This is a large industrial saw with the SawStop "flesh sensing" technology which will stop the blade within milli-seconds if it contacts skin. Since it is a rather involved procedure to change the blade to a dado on the SawStop (certainly not for the average makerspace user), we have a second table saw dedicated to dado use. This is an older Powermatic 66, built like a tank.
Setting-up the stacked dado to make a precise fit can be tricky, the method shown in this Instructable should make things easier.
As I understand it, in the EU, and some other countries, dado blades are actually illegal. In fact, table saws with arbors long enough to support a dado blade are not allowed. If this is the case, I would imagine there is quite a market for the older saws and table saw arbor replacements. Maybe someone from the EU could help us out on this.
Step 1: Components
The stacked dado set consists of several different components. There are two blades that look almost like normal saw blades. I refer to these as "Plates". If you examine the teeth on the plates, you will see a difference in the tooth grind for a normal blade. All of the teeth on a plate are ground and beveled to cut on one side. In a normal blade the bevel is alternated between sides. So it is important to remember to use the right-side plate on the right side of the stack and the left-side plate on the left side of the stack. Never use a plate as a normal single blade.
Step 2: Chippers and Shims
On our set, the plates are each 1/8". So if we just needed a 1/4" cut, we would only use the two plates. To make wider cuts, chippers are used. In this set, the chippers have 4 teeth. In some sets the chippers have only two teeth, in other sets the chippers look almost like normal blades. In any case, you will notice that the teeth on the chipper are a good bit wider than the body. An 1/8" chipper has a body that is precisely 1/8", but the teeth my be 5/32" wide. This is to help remove material from the bottom of the cut even when shims are used between the chippers.
This set has several 1/8" chippers, a 3/32" chipper, and a 1/16" chipper. The 3/32" chipper makes working with plywood (which is typically 1/32" undersize) much easier.
There are also several shims in three different sizes, 0.005", 0.010", and 0.020". These are places between the chippers and plates to precisely adjust the width of cut in 0.005" increments. The always go inside the stack, never outside the plates.
Step 3: Right Hand or Left Hand Arbor?
It is helpful to note whether you arbor loads from the right (as in the Powermatic in the photo) or from the left, as in a right-tilt Unisaw.
Since we will be making a prototype dado stack on the table, it is easier to start the stack with the plate that will be put on last. So in our case, I carefully lay the right-plate on the table top. Do be careful, carbide is very hard, but it can be chipped easily.
Step 4: Stack for Correct Thickness
Start stacking the chippers, ending with the remaining plate. Not that it is very important to keep the teeth away from each other. In the photo you can see how two chippers are stacked on the plate, such that the teeth do not touch.
Lay a piece of the material that will be fitted into the dado next to the stack. In this case I'm using some cedar fence picket, which is of unknown thickness. I need to cut the dado to fit this thickness closely.
Step 5: Testing the Height of the Stack
Using a small piece of wood, I can test the thickness of my stack to make it just a tiny bit thicker than the board I tend to slide into the dado (leaving room for glue).
Shims are placed between the top plate and chipper to get the height where I want it.
Step 6: Move the Stack to the Arbor
If you made the stack in the correct sequence, it's just a matter of removing the plates and chippers from the stack and placing them on the arbor. When using shims, disperse them throughout the stack so that the chippers can cleanly remove material. Also, the shims will be thinner than the threads on the arbor, so make sure the are fully in place before stacking the next chipper or plate. If they catch in the threads, they can get damaged when the arbor nut is tightened.
Step 7: Check the Stack and Tighten the Nut
When all the plates, chippers, and shims are in place, check the stack to make sure the teeth are not touching each other and that the shims are all in place.
Install the arbor washer and nut and hand tighten. Make sure at least one and a half threads are showing on the end of the arbor. If not, your stack is too thick for your arbor. You may have to remove chippers and make two passes if this is the case.
Tighten the nut as you would with a normal bade. Make sure you use a table insert designed for dado use.
Step 8: Set Depth of Cut
Setting the depth of cut is no different than you would for any other blade. NEVER attempt a through-cut with the dado.
Step 9: Check the Fit With a Test Cut
Once learning this procedure you should be able to get the proper width on the first try, but always do a test cut. In this case the goal was to produce a cut with a small gap for glue.
Step 10: Measuring the Gap
Using a spare shim I was able to measure the gap. Feeler gages would also work, but I'd have to remember where I put them. In any case, if your test dado is a bit wide, you can use the extra shims to get an idea of how much width to remove from the stack.
Happy woodworking and be careful.