How Do I Cut Dados?

The stacked dado cutter may be the standard for many but new woodworkers need to explore all the alternatives to make the best use of their equipment, experience and money. Click image to enlarge

There is more than one answer

Text and photos by Tom Hintz

   Cutting dados is a task all woodworkers perform regularly. Not surprisingly, questions about dados are among the most frequently asked at NewWoodworker.com. Most often, those posing the question want to know if the table saw or router is the best tool for them. The fact is, both machines are more than capable, but both have limitations that could reduce their effectiveness in a particular shop. It seems that the majority of woodworkers eventually use both, choosing the one with the best capabilities for the situation at hand.

Equipment Choice and Safety

   When shop space is limited, cutting long dados is often easier and safer when done with a hand-held router than on a table saw. Being able to clamp the workpiece in one place and moving the router along it's length rather than pushing large pieces through a table saw drastically reduces the space needed, often making the operation safer. If a good clamp-on fence or guide system is used, the resulting groove will be very accurate.

   Remember to use push blocks, push sticks or whatever other safety equipment is necessary to maintain a safe distance between your hands and the cutter. Too many woodworkers consider cutting dados to be a low-risk task. Just because the bit or cutter is buried in the stock, and is not expected to be exposed to your hands is no excuse to circumvent safety procedures. Kickbacks and wood fracturing can and do happen while cutting dados. If you are not using proper safety equipment and techniques when a kickback or fracture occurs, devastating injuries can be the instant result.

Wood Thickness

   One of the more frustrating aspects of modern woodworking is the goofy material thickness we have to deal with. The odd thickness issue is most often associated with plywood, but must be considered when working with solid wood as well.

   Buy five quarter (5/4) stock today (formerly 1 ¼"-thick) and it will actually be four quarter (4/4), or 1"-thick. Since 5/4 stock is now 4/4 thick, material once known as 4/4 stock had to be changed also and is now three quarter (3/4), or ¾"-thick.

   If this thickness philosophy were applied consistently to all wood sizes, buying what used to be ¼"-thick material could eventually mean having to give the lumberyard a 1/8"-thick piece of wood just to break even. To prevent such craziness, wood sizes formerly three quarter (3/4) ¾"-thick and below are usually between 1/32" and 3/32" under their former thicknesses. To further confuse the issue, some hardwood veneer plywood is manufactured in the standard sizes, ¾, ½ and ¼"-thick.

   All this points out the necessity of MEASURING the wood being used!

Creative Shop-Made Solutions

Jigs, like our Auto-Width Dado Jig, are popular with woodworkers to make cutting dados in various thickness materials easier, and more accurate. Click image to enlarge

   Woodworkers have devised many shop-built jigs to make cutting dados for the various wood thicknesses safer and more accurate. The Auto-Width Dado Jig, one of the most popular NewWoodworker.com Custom Plan Sets on this site, is one of those useful jigs. Some woodworkers find one or more of these jigs to be so effective that they use them to make the majority of their dados. Most of these shop-built solutions are used with a hand-held router equipped with a specific bit.

Generally, jigs are limited to dado lengths of approximately 30 inches. Longer cuts can be made with a router using various improvised clamp-on fences.

Table Saw

   Traditionally, one of the most popular dado-cutting machines is the table saw. When equipped with a properly adjusted dado blade, the table saw can repeatedly cut very accurate dados, and make lots of them in a short amount of time.

   Generally speaking, there are two types of table saw dado blades -- stacked sets and the "wobble" design. Of the two, the stacked dado design is most favored because of the flat-bottomed groove it creates. The

The stacked dado pictured is the favorite for cutting dados on the table saw. Click image to enlarge

"stack" consists of a pair of outer blades with varying numbers of "chippers" placed between them to adjust cut width. In addition, thin shims can be installed within the stack to fine-tune the cut width.
   The wobble version consists of a single blade set at an angle that "wobbles" as it spins. Wobble blades are usually adjusted by turning one side of a split hub that controls the amount of angle the blade runs at. Because these blades do in fact "wobble" during use, they tend to leave a slightly rounded bottom in the groove and are more likely to cause splintering along the sides.

   Most contractor-style and cabinet saws designed for 10-inch-diameter blades can use stacked dado sets in either 6 or 8"-diameters. Benchtop saws may require specific dado blades due to shorter arbor lengths. Check the documentation that comes with your saw, or with the manufacturer, to be sure you buy a compatible dado set for your equipment. Using the wrong dado set can be extremely dangerous.

   The table saw enjoys an advantage when cutting dados on the long axis of large workpieces. Using the rip fence as a guide is easier than trying to accurately clamp a long guide for a router in place, or using an edge guide if the dado is located more than a few inches from the edge of the material.

   The down side to using the table saw for dados is setup. Getting a stacked or wobble dado blade set up to cut the correct width dado can be a time consuming task. Small variances in blade and fence alignment affect the actual width of the cut and consequently may complicate the process even more. Without accurate records detailing previous stack composition for a specific wood thickness, the trial and error process has to be repeated each time dados are required.

   Another liability of using a table saw for dado work is the amount of space required to perform the task safely. For example, cutting a dado or rabbet on the long axis of a 7-foot-long workpiece on a table saw (or router table), requires something over 16-feet of clear space to move that work piece into, across and away from the cutter. There also must be sufficient clear space to either side to be sure the workpiece does not contact an obstruction that could send it off line, possibly causing a bind at the cutter. This can be a real problem in many of our less than spacious home workshops.

Routers

Many router bit manufacturers produce plywood-specific bits, like these from Infinity Cutting Tools. Click image to enlarge

Several router bit manufacturers offer bits specifically sized to accommodate the common plywood thicknesses in use today. The "Plygroove" bits used in this story are from Infinity Cutting Tools.

   Routers equipped with bits designed for making grooves produce very flat, clean-sided dados. When guided by a good fence or edge guide system, routers can make very accurate dados.

   The only real problem with router bits is they are not adjustable. If the wood is slightly oversize due to manufacturing tolerances or swelling from humidity, the fit between the groove and wood will suffer. In these cases, adding a piece of masking tape to the fence spaces the router out enough that a second pass opens the groove and corrects the fit. If the wood thickness is substantially out-of-spec, moving the fences or using a jig may be the best way to cut appropriately sized dados in that situation.

This simple gauge block can save lots of time and money by letting you check the wood you are buying to be sure it fits the grooves your bits make. Click image to enlarge

   Woodworkers have to pay attention when buying plywood these days. I recently found three different sizes of plywood in the ¼" bin at two of the local home centers. Only the ones marked ¼" actually fit the plywood bit grooves.

   To make these shopping trips error-proof, I made a handy gauge. I cut grooves with all three plywood bits in a length of wood and then cut a small section with parts of all three grooves to use as a gauge. I take that gauge with me to the lumber dealer to make sure what I buy actually fits the bits I have available.

Dado Fit

   Regardless of the cutter used, the wood must fit the dado securely to produce a strong and visually attractive joint. Ideally, you should be able to seat the wood into the dado with light to moderate hand pressure. Having to drive it in with a mallet is too tight and just falling together too loose.

   When a dado is too tight, there is no room for glue, which substantially reduces the strength. An excessively tight fit can also trap glue at the floor of the dado, preventing the piece from seating properly.

Getting your dados to fit cleanly, without having to drive them in place is the goal. Click image to enlarge

   Dados that are too wide do not properly align the pieces and create gaps that drastically reduce the effectiveness of the glue.

   Dados are most often cut to a depth of 1/3 to ½ the thickness of the wood they are cut into. Cutting a dado deeper risks creating a weak spot in that panel. A properly fit dado need not be excessively deep to be strong and secure.

Test Cuts

   Since we know that wood thickness can vary, making test cuts is a logical way to be certain the setup is creating the fit we want. Testing allows making corrections before involving the often more expensive project wood. The time required for this testing is very small, but can save considerable amounts of wood, time and money.

   Making properly fit dados is a learned skill. The frequency with which we make this joint means it is a skill that will be often practiced. Using the proper machines and related equipment for your situation makes the process easier, more accurate and safer.

Additional Dado-Related Resources

Setting Up Stacked Dado Blades
Auto-Width Dado Jig
Infinity Cutting Tools Web Site

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