Standard and thin kerf circular-saw blades are great for ripping and cross-cutting material, but what do you do when you want to create a wide groove, dado or rabbet in a piece of material? You could reposition your table saw’s fence repeatedly as you make cut after cut, but that’s frustrating and slow, even if you just need one dado cut. It’s also inaccurate. Another option would be to use a router with a straight bit, but this option also has many downfalls.
The most consistent, fast and accurate way to machine grooves is with a dado set. A dado set can be set to any width between 1/4″ and 29/32″ and can then be used with a table saw to make smooth, chip-free grooves, dados and rabbets in solid wood and sheet goods with a single pass. A dado set can also be used when cutting some joints. Tenons, for example, can have their shoulders and cheeks cut to size with a dado set. Half-lap joints can also be cut with a similar setup.
Though a “wobble blade” can be used in a similar fashion as a dado set, they are not nearly as accurate, don’t cut as cleanly and can be less safe, so I will not be covering them at length here. A wobble blade is very similar to a regular rip blade, except it has a multi-piece hub at its center, which holds the blade on an angle when the hub is rotated. The more rotation of the hub, the wider the blade will cut.
Two regular looking blades are positioned on outside of the set. Though these blades look normal at first glance, most of their teeth are ground on an angle in one direction, and must be placed so their teeth are angled outward. Their outer faces are often marked with “THIS SIDE OUT” to ensure they are positioned properly. The teeth on these blades act like crosscut teeth and score the wood fibres for a clean cut. Generally speaking, if you use just the two outer blades you will end up with a 1/4″ wide kerf.
To cut a kerf wider than 1/4″, chippers need to be added between the outer blades. Chippers look much different than regular blades, and are added or removed to obtain the desired width. As with all blades, it’s crucial that the chippers be installed in the correct direction. If there is smoking during the cut or burning of the joint afterwards, that’s a strong sign you may have one of the chippers installed incorrectly. Each chipper often has either two or four teeth, and might also have chip-limiting features. Though some dado sets have numerous chippers of the same width, chippers can come in differing widths too. This allows you to more easily select the width of kerf you need to cut.
Spacers can also be used between the chippers and outer blades to fine-tune the overall width of the kerf. Blades in a dado set are usually 8″ in diameter, though 6″ is the second most common, followed by 10″. If you have an underpowered saw you might want to consider a 6″ diameter blade.
Some table saws will come with shorter arbors and dado sets cannot be mounted on them. If you want to be able to use a dado blade, check the arbor length before purchasing a table saw. If a saw you already own has a short arbor, you may only be able to use the outer blades with one or two of the chippers.
If you have a smaller saw, or one with less than about 2HP, you might be slightly underpowered when it comes to machining wider and deeper cuts with a dado set in harder materials. A 6″ diameter dado set will require less power to run, taking smaller cuts will help the situation and a slower feed rate will also tip the scales if your favour. A smaller set is also a bit less expensive. An 8″ diameter set will dull slower, though, and because of its faster rim speed you can safely feed material past an 8″ diameter blade a bit faster. One thing to note: it’s extremely rare that you need the extra cutting height an 8″ dado set offers, so that shouldn’t be a deciding factor.
Using a dado set can be dangerous if it’s not done properly. This is true for any tool, but because dado sets are removing more material than most woodworking machines that are hand-fed, the chance for kickback can be greater. However, if a dado set is used with care, respect and knowledge, it can be an important and efficient tool to have in your woodworking arsenal.
Dado sets are generally more dangerous when the user is trying to remove too much wood with one pass. Taking a light pass is one of the best ways to prevent an accident, though adjusting the feed rate can also play a part in safely using a dado set. Following all the best practices regarding table saw usage can also reduce injury due to kickback. This means not standing directly behind the blade, using a guard and being familiar with the operation you’re undertaking.
While you’re setting up your dado set ensure all the teeth are not touching, and are evenly spaced around blade’s circumference. Because the faces of both outer blades and chippers come into close contact with one another, it’s also important that there is no debris between these surfaces. As was mentioned earlier, the “points” of the outer blades must be positioned on the outside of the set so they score the workpiece, and the chippers must also be facing the correct way.
Since the gap in your standard throat plate will not be wide enough to allow the dado set to pass through it, you will have to make one specifically for this purpose. If you would like, you can make one throat plate for cutting wider grooves and one for cutting narrower grooves.
Once the blades are set up, and the throat plate is in place and level with the table saw’s surface, you should always do a test cut to ensure the width of the dado set is acceptable. This is especially true when using man-made sheet goods, as they are almost never their stated thickness.
When cutting grooves or dados, you can set your table saw’s fence then proceed with the cuts. Cutting rabbets, which are located on the very edge of a workpiece, are different, though. Because you don’t want to set the fence flush with the inner face of a dado set, and cause damage to your blade or fence, attach a 3/4″ plywood sacrificial fence to your rip fence with clamps. Ensure the clamps don’t interfere with the travel of the workpiece. With the dado blade dropped slightly below the table saw’s surface you can adjust your fence to the correct position. Turn your saw on and raise your dado set into the sacrificial fence; just be sure the blade doesn’t cut into your rip fence. You can now run the workpiece against the sacrificial fence and create perfect rabbets.
Though most people use dado sets on their table saw, they could also be installed on a radial arm saw. Again, if you do this please be sure to follow all best practices, as the blade will only want to pull forward that much quicker.
There’s nothing saying a dado set can’t be used at an angle to create angled dados, grooves and rabbets. Install it in your table saw as normal, and other than creating a dedicated throat plate for angles, proceed as you normally would.
With the use of a dedicated sled you can make perfectly square dados across workpieces like bookshelf gables or cabinet parts.
Rob is a studio furniture maker and the editor at Canadian Woodworking & Home Improvement.