Cutting Dados & Grooves on the Table Saw
Skill Builder: Dados and grooves are one of the most basic joints you can use when constructing furniture projects. They provide a mechanical connection that can be reinforced with glue and screws, making a structure much more rigid than had it been fastened together with butt joints. While not as decorative as dovetails or finger joints, they are much easier to cut and are ideal for housing panels such as drawer bottoms.
Some woodworkers tend to use the terms dado and groove interchangeably. They are not the same things. A dado runs across the grain, a groove runs with the grain, a rabbet is cut on the edge of a board, and a slot is a recess cut in a board. Both the router and the table saw have their individual advantages when it comes to cutting dados. (We’ll be taking an in depth look at cutting dados and grooves with the router in an upcoming issue of the magazine.) With a table saw you can cut a large number of grooves or dados in sheet goods quickly and accurately using either the rip fence or a cross cut sled, as dictated by the circumstances. When cutting a stopped dado on the table saw (a dado that stops short of one edge of a board) you will have to chisel out the ramp or notch the shelf and sacrifice the strength of a tight fitting joint.
If you are only cutting a few narrow dados, then you could lay the cut out on the stock and use a regular saw blade. Make two cuts running the length of the dado defining each side and then waste away the center portion with several repeated cuts. However, because dados are such a basic woodworking joint, it makes sense to invest in a special dado blade for your table saw, particularly if you have more than a few dados to cut. Dado blades come in two basic forms, either a wobbler or a stacked dado set.
Just as the name suggests, this dado blade wobbles. The blade is mounted on a special washer that will tilt the blade on its axis. How much you twist the two halves of the washer relative to each other determines the degree of tilt and as a result the width of the dado. As the blade spins this will cause it to cut on an arc through the wood. In travelling from one side of the cut to the other, the bottom of the dado will be cut on a slight arc, which results in a bottom that is not quite flat. This isn’t much of a concern if all of the ends of the dados will be hidden by a face frame for example but looks sloppy if the ends are exposed. There is a tendency on veneered sheet stock and melamine to have some chipping on the edges. Additionally, wobble blades tend to vibrate in use more than stacked dado blades.
Stacked Dado Sets
Stacked dados are made up of two outside blades separated by a number of chipper cutters. The outside blades look like conventional saw blades, and score a clean outer cut. The inner chippers then remove the waste. To cut a dado of a certain width, mount the first outside blade on the saw arbour and begin adding the center cutters. Follow any specific instructions that come with your set, but it is best to stagger the chippers evenly. This will balance the blade a little bit more effectively and as a result the saw will run smoother with less vibration. These blades produce the best dados with just about dead flat bottoms. Because a stacked dado set is much wider than a conventional saw blade, you will need to replace the table saw’s throat insert plate with a dadoing insert plate. Most saws come with this insert. If yours did not, you can likely purchase one from your saw manufacturer, or make one yourself.
Cabinetmakers often use dados and grooves when constructing projects out of sheet goods given the ease and speed with which they can be cut on the table saw. One of the drawbacks however, is that a ¾” sheet of plywood is rarely ¾” thick. Before beginning to cut dados for any project using sheet goods, measure the actual thickness of the material you will be working with.
To adjust a stacking dado you use a set of shims that are usually supplied with the set. If shims did not come with your dado set you can buy them from most tool outlets. Plastic or brass shims will last for years. You can also buy lower priced paper shims that work just as well. Place the first outside cutter on the saw and then add enough cutters to the stack so that the final width of the stack is just slightly less than the thickness of the material that will go into the dado. Make a test cut in a piece of scrap and use a digital calliper to measure the width of the dado. With the calliper set to this width, zero the display and then measure the material itself. The calliper will display the difference between the two. Use this measurement and the calliper to put together a stack of shims of the required thickness. Place these on the stack, again observing any special instructions specific to your set, and make a test cut. If all has gone well, the material should now fit snugly into the dado. If it easily drops into the dado, it is too big, and if you need a mallet to seat it in the dado it is too tight. It should push in with only average pressure so as to provide room for glue on the sides and the bottom.
If the fit is too tight, the material being inserted will scrape all of the glue off the sidewalls of the dado and trap it at the bottom preventing the panel from fully entering the dado. Some dado sets are not adjustable for thicknesses less than ¼”. In this case, cut your dados with a standard ⅛” saw blade, and sneak up on the dado width.
Although all stacking dado sets share the same features, there can be considerable variation between them. The quality and type of carbide is one thing to consider. Some manufacturers use special ‘micro grain’ formulations, and fabricate their own carbide. Over time, pieces of carbide will eventually start to wear away from the leading edges of all blades. On micro grain carbide, the pieces that break away are smaller, and as a result the tool is able to maintain a working edge longer before needing to be taken to a sharpening service.
Using A Stacked Dado Set Safely
The first time you install a dado blade set on your saw you’ll be impressed by how little sound it makes with all of those teeth spinning at once. Just as with regular saw blades, the blade must be parallel to the guide slots and the fence or you will get splintering and burning during the cut. Because a dado does not cut all of the way through your stock you will need to remove the splitter assembly from your saw when you install a dado set. Before you install the dado blade, raise the blade to its highest position and use an engineers square to set the blade exactly 90º to the table. Remove your regular blade, along with the throat insert, and install the dado set. Then install the dado insert plate.
When you are cutting a dado in larger panels, always use push sticks or better yet, push blocks. The dado is being cut on the underside out of your sight; you don’t want any part of your hand to be in the wrong place as the blade emerges at the end of the cut. Using the rip fence to guide the cut is only safe if the pieces are wide enough in relation to their length to provide adequate bearing surface on the fence. For narrower panels use a cross cut sled for safety and accuracy. Many dado sets have anti-kickback fingers that limit the amount of material each tooth can cut, which reduces the risk of kickback caused by excessive feed pressure. When cutting rabbets always run the stock flat on the table saw rather than on the edge.
It’s good practice to use an auxiliary fence with your dado set. You can make the auxiliary fence out of solid wood or plywood. With this fence you can safely use part of the dado blade to cut a rabbet – otherwise you run the risk of having the dado blade damage the rip fence.
Before buying any dado set, confirm its compatibility with your specific model of saw. You may be restricted in size to a 6″ set if your saw lacks the capacity to run an 8″ set. As well, be sure that your arbour and nut combination provide enough length to mount the set. On some saws you may end up having to mount the chippers and one blade on the threaded portion of the shaft, depending on the width of the dado stack. – CWM