Rabbets, dados and grooves
Whether it is a rabbet to hold a cabinet back, a dado used to house shelves in a bookcase or a groove for a drawer bottom, you can mill these joints quickly and accurately with a router. These joints are all variations on the same theme. Essentially they are butt joints – you are joining end grain to face grain. Gluing end grain to face grain will not produce a very strong joint, so for additional strength and support it is best to reinforce this type of joint by housing it in a slot. In preparing these joints you are removing a square or rectangular cross section from the face of one board to house the edge or side or end of another board.
Two factors make this housed joint stronger than a straight surface to surface butt joint. First, it increases the available glue surface and second, because one board is housed within another, there is physical support. In the case of a bookshelf, the glue will hold the piece together, but when you load up the shelf with your favourite woodworking books the load will transfer directly to the sides of the bookcase, which in turn transfers the load directly to the floor. Of course, the thickness of the shelf, the width of the span and the load on the shelf will determine the amount of deflection in the shelf.
Rabbet – a notch cut with or across the grain on the edge of a board with the two sides 90º to each other.
Dado – a square or rectangular slot that runs across the grain.
Groove – a square or rectangular slot that that runs with the grain.
Dados, grooves and rabbets
Although these three joints may at first glance seem very similar, they are in fact different, and the approaches used to cut them will not be the same. Before we get to the techniques, there are two dimensions that you will need to be concerned with as you cut these joints: width and depth. Rabbets are somewhat different – we’ll look at them shortly.
Because of their similarity, dados and grooves are often referred to as ‘slots’. The difference between the two is that a dado is milled across grain, while a groove is milled with the grain. The slot can run the full width of a board, it can be stopped on one end, or it can be stopped on both ends, in which case it essentially becomes a mortise. When deciding on the depth of the slot, use guidelines similar to that of mortise and tenon joints. When you mill a slot in a board, cut it approximately 1/3 the thickness of the board. You want to have sufficient material under the joint, as in the case of a bookcase, to support the weight of the loaded shelf, but not so deep that you risk weakening the receiving piece. The width of the slot will usually be the same thickness as the material that will be housed in the slot. In some instances you may wish to mill a shoulder on the material (e.g. a tenon) that will be housed in the slot; this serves to cover the edges of the slot, making for a cleaner appearance.
A rabbet is cut on the end or face (edge) of a board – the extruding piece is the tongue. This joint is frequently used for simple box joinery where joint strength is not critical, to attach backs to cabinets, and for drawer assembly. The width of a rabbet is typically determined by the thickness of the piece that it will receive, or the depth of the slot into which it will be inserted. If you are making a cabinet with a 3/8″ thick back and 3/4″ sides, then the rabbet would be ⅜”wide. In this case, to give sufficient surface area for gluing or screwing the back into place, the depth of the rabbet should be 1/2 to 1/3 the thickness of the side piece.
There are two common approaches to cutting a slot to the proper width, and which one you chose will depend on the project at hand and the material you are working with. You can cut the slot to exactly fit the material it will house, or you can cut the slot with a standard size cutter and then mill the stock to thickness for a perfect fit.
Most woodworkers use standard widths in their construction – 1/4″, 1/2″, 3/4″. However, most plywood comes in nonstandard thicknesses – 15/64″, 31/64″, and 23/64″. Unless you use specialty ‘plywood bits’, (leevalley.com), ground to these nonstandard thicknesses, you’ll need to tweak the slot to fit the plywood. If you use a lot of plywood in your projects then these bits are a good investment. If you only use dados and grooves with plywood occasionally it is more cost effective to make the cuts in several passes until you have achieved a perfect fit. When using solid wood, if the design and construction process permit, you may be able to size your material for a perfect fit after cutting the slot.
As you work through your project and begin to look at the methods used to cut these joints using either a hand-held or table-mounted router, the differences will start to appear. Grooves are typically close to the edge of stock, and this makes it an ideal situation to use a table-mounted router with a fence. If your cut is close to the edge of the stock you will be able to use a slot-cutting bit to mill the groove with the material running vertically against the fence. The new four wing cutters, (freud.ca), provide a super smooth cut and superior chip clearing ability. If the groove is more than an inch or so from the edge of the material you will need to switch to a straight or spiral bit, and run the material horizontally on the table instead. Be sure to anticipate where the bit will emerge from the material and keep your hands away from that area. Because the grain and the cut run the same direction there will be little or no blow-out as the bit leaves the cut. The router table excels at cutting grooves and the easiest are those that are open at both ends. When milling a groove that must stop part way along the stock, use a stop block, or place a mark on the top of the stock and at the point where the groove ends; feed the material through until the two marks line up, and then turn the router off. When the bit has stopped rotating, lift the piece off the table. If you are routing a groove with two stopped ends (a mortise) then you will need to lower the work onto a spinning bit to start the cut.
Dados are often at a distance from the end of the board. Unless you have a very wide and reasonably short work piece, you won’t be able to safely rout this with a fence on a table; you’ll need to build a jig to guide a hand held router. In most cases a simple T-square guide is effective. Ensure that you make provisions to back up the cut as the bit exits the stock to prevent tearing-out the edge next to the exit.
If you are working with material that doesn’t correspond exactly in thickness to the width of your cutter then you’ll need to make the cuts in several passes and sneak up on the perfect fit. When possible, I prefer to cut my dados and grooves to size with a spiral Onsrud bit, which makes a very smooth cut. I’ve found that most of my dados and grooves are for stock ⅝” thick and narrower. To cut these I use a solid carbide 3/8″ bit.
If you use a T-square guide to cut your dados, moving the guide every time as you widen the cut can lead to the possibility that the two sides of the groove will not be perfectly parallel. To keep the original reference edge in place until the cut has been completed to the full width, follow this simple process. Ensure that the bit you are using is less than the full width of the cut. Mill a strip of wood to the exact same width as the bit, and attach it to the edge of the guide. Line up the edge of this strip with your layout lines and make the cut as usual. Don’t forget to allow for the setback from the cut to the edge of the guide when setting the guide, or your slot will be in the wrong place. After the cut, remove the strip of wood, replace it with a strip that is the exact thickness of the material you will be housing in the joint, and make the cut. If you are using plywood, simply rip a strip from the edge of your sheet and use this to set the width of the cut. The result is a groove that fits your undersized plywood perfectly every time without the need for specialty bits.
Rabbets are often cut after a piece has been put together, such as for the glass in a rail and stile door or a picture frame. If the pieces are small enough they can easily be cut on a router table after assembly. For larger projects it can be better to cut them before assembling the pieces. Using a hand-held router balanced on a narrow edge to cut rabbets will leave you with less than stellar results and should only be your last resort. If you have no other option than to balance the router on edge I recommend clamping an extra strip of wood to the side of your material. This will provide additional surface area for the router to run on, making it easy to keep it perpendicular to the work piece.
Because the rabbet is cut on the edge of a board, the material can pass the cutter both vertically and horizontally; this means you can use a straight bit, spiral bit, slot cutter or rabbeting bit to make the cut. Take a look at the tooling you have at hand and the characteristics of the wood you are using to determine your best choice. If you are using a cutter without a pilot bearing you will need to use a fence to set the depth or width of cut. In most cases though, you’ll find it best to cut rabbets with a bearing guided cutter. The guide bearing runs on the uncut section of material under the rabbet and this leaves you with a clean edge and a consistent depth. These can be used on inside edges allowing you to rout into an inside corner. Afterward you will either need to round off the piece to be inserted or square off the opening. If the structure of the corner joint is likely to suffer from some chisel work then it may be safer to round the incoming parts. Rabbeting bits are available in sets that have one cutter with several interchangeable bearings. This gives you the ability to cut rabbets of several different dimensions with one cutter simply by changing the bearing and varying the projection of the bit.
Dados, grooves and rabbets are staple forms of joinery that every woodworker should feel comfortable in using. They are among the easiest joints to cut using the router and they are found in almost every project. Like all woodworking tasks, there are often many roads to the same results; consider your material, the construction process and your tooling when deciding which method to choose. Slot cutting bits are great for slots that run off the edge of the material, but will leave a ramp at the end of a stopped cut; these bits cut quickly, cleanly and are not that expensive. When your project calls for a stopped slot away from the edge of a panel then a spiral Onsrud bit is a better choice since the cut will be made with a jig and a handheld router. For cutting slots for box bottoms, nothing beats using the box-slotting bits (leevalley.com), on a router table. A modest investment in a variety of good quality bits and some time spent making a few jigs in the shop to guide the router will reward you with flawless slots and rabbets for your projects for years to come.
Stopped Grooves and Dados on the Router Table
Most times, stopped slots will be cut with a handheld router. In this case it is a simple matter of plunging the bit to depth, running it along a guide until the end of the cut and then raising the bit out of the material. When cutting a stopped slot on a router table you will need to lower the work piece onto a spinning bit. This is not as difficult as it seems although to the novice it may seem rather intimidating. When the parts I am working with are small enough to comfortably handle on top of the router table, I prefer the accuracy and repeatability that using the table and fence allows. A few simple precautions make this a safe and simple procedure. Learn to anticipate which direction the bit will pull the material. Always be sure to brace the material in every direction but one; as you move the piece you will only ever have effective control in one direction at a time.
For example, to cut a stopped groove in a stile or rail for a panel, set the distance from the fence and control the horizontal travel with two stop blocks secured to the fence. While it is possible to do this with a straight bit, a carbide spiral bit will cut the groove to full depth in one pass. A straight bit does not clear the chips very well and can sometimes clog, so it is best to make the full cut in several increments. Whichever method you choose, set the projection of the bit above the table. Run the router at a high speed; this makes it less likely that the bit will take a deep enough bite with each revolution to grab the work piece. Brace the material against the fence on one side and the end stop on one end before you lower the piece onto the cutter. When you stand in front of the router table holding the piece, it should be located between the bit and the right side end stop and fed toward the left hand stop after lowering. If you reverse this and brace the material on the left side end stop, as the material is lowered onto the bit the rotation will naturally want to pull it to the right and away from the fence. Neither of these two directions offer any support for the material, fed from right to left, the rotation of the bit will naturally push the material to the end stop first and then into the fence. With the material braced against both of these as you lower it, it will have no place to move but the one direction you want; to the left. When braced and fed properly, only light pressure is required for a perfect cut as the rotation of the bit naturally forces the material into the fence.
Here is how to make a T-square to guide the router using a couple of strips of plywood.
Before using the guide, place it on some material and rout a slot, allowing the bit to cut into the guide arm on the jig. As long as you always use the same size bit and the same router, the cuts in the cross fence will define the edges of the slot. This makes it easy to line up the jig with your layout lines. To use the guide, draw layout lines on your stock, lay the square on the material, lining up the edges of the cut in the cross piece with the layout lines.