There’s no such thing as the perfect joint. Each situation has different requirements and therefore demands a different joinery solution. So what makes a great joint? High strength, ease of machining and a nice finished look would have to play a part. The bridle joint has all of these characteristics, and more. It’s not used often and I’m not sure why.
First things first
When doing any type of mortise and tenon joint, start with the mortise first.
If you don’t already own a tenoning jig, you can make your own out of the scrap bin.
Using a dado blade and a mitre gauge, start to nibble away at the tenon.
It is essential that you clamp this joint in all three directions to make sure you get a good glue-up.
The bridle joint, also referred to as a slip joint, is one of my favourites. If I’m making a set of simple frame and panel doors I will almost always use a bridle joint to secure the four corners of the frame. Although it works well in many other situations, I find this is the best and most common use for this wonderful joint.
It is essentially a mortise-and-tenon joint connecting the ends of two work pieces. The bridle joint can be cut by hand or machine; most people, including myself, rely on the table saw to machine a bridle joint. Begin by machining the mortise or slot portion of this joint, then machine the tenon portion to fit. The tenon should end up being equal to or slightly more than one-third of the thickness of the material you are using. This ensures that both halves of the joint have enough material to produce a strong, rigid joint. I find that the easiest and fastest way to machine the slot in this joint is with a stacked dado set in my table saw and a tenon jig running in my mitre groove. The dado blade allows me to make the slot with a single pass, which is especially handy when machining many joints. The slot does not have to be the dead center of the work piece, but it should be fairly close. If you don’t have a dado set you could also use a standard ripping blade and make a number of passes to remove the waste. A large amount of material is removed when you machine the slot, so it’s important that the work piece is held securely. Since the work piece is standing on end, in a position we’re not used to, securing the piece is critical. This is where a tenon jig comes in very handy. If you don’t have a tenon jig at your disposal, a shop-made jig that runs along your rip fence can be used to hold the work piece 90° to the table saw surface. Raise the blade to the same height as the width of the mating work piece and you’re ready to make some sawdust. Well, almost ready. When making a set of doors, or almost anything really, be sure to mark the work pieces appropriately so you will know how to machine each joint properly. I like to machine door frames with the inside of the piece referencing off the tenon jig’s surface. Only half of the pieces will receive a slot, while the other half will have to wait to receive a tenon. Carefully pass the work piece over the blade. Make sure you cut all of the required slots because it is difficult to set everything up again to cut the slot in the exact same position.
With the easy half finished you can focus your attention on making a perfect-fitting tenon. I usually machine the tenon on my table saw with a dado blade, although it’s possible to use a router with a straight bit installed in your router table. Without question, you could also use the same tenon jig that helped you machine the slot. I find the dado blade easy to set up but if you have many of these joints to cut, it’s easier to work with the tenon jig. I set my mitre gauge in its slot and set the fence to the proper position in order to cut the tenons to the correct length. Setting the height of the blade is the tricky part. I like to remove the material from the underside of the tenon first, but as long as you have a system, it doesn’t matter which you machine first. You just don’t want to get confused half way through and make a mistake, so be organized and consistent. Raise the height of the blade until it is cutting a small amount of material off the side of the tenon. You will need to sneak up on the final height, so be patient. After making the first set of passes on the tenon’s cheek, check it against the slot. Raise the blade incrementally while making passes towards the proper blade height. Once you are confident with the final height, machine all of the tenon cheeks on the first side.
Now you can start to do basically the same thing to form the other cheek of the tenon. The height of the dado blade must be adjusted to produce a tenon that will fit into the slot perfectly. Since you don’t want to cut too much off with the first pass, lower the blade and once again sneak up on the final tenon thickness. You are aiming for a joint that will fit together with a medium amount of hand pressure but will not fall apart because of gravity if turned upside down. If you need to use a mallet to assemble the joint it’s much too tight. When you have the proper height set, machine the rest of the cheeks. If you do end up cutting a tenon too thin, it’s not the end of the world. Glue a shim onto the tenons cheek and when it is dry you can re-machine the tenon thickness. After all, that’s what woodworking is all about – fixing your mistakes and making them look invisible.
With the parts machined, you can get a good look at what the joint will look like when assembled. Sometimes, to produce a joint that looks a little better, and one that requires less fine-tuning when assembled, I will remove about 1/16″ from the face of the work piece with tenons on both ends. This creates a reveal between the two mating pieces and you don’t need to flush up a joint after assembly. Often when you try to make two pieces meet flush, and they are even slightly off, it looks like a mistake. By machining one piece thinner than the other the resulting reveal looks like it was made on purpose.
Once the pieces are sanded they are ready for assembly. You can assemble a door with four bridle joints, either in stages or all at once. I almost always assemble the entire frame at once, but I have to be efficient. All the clamps must be ready to go and a clear area makes the assembly go smoothly. The assembly must be clamped in all three directions in order to bring the mating surfaces together properly. Two clamps should be placed lengthwise and two widthwise so the assembly comes together snugly. Make sure these clamps are not bottoming out on a slightly protruding tenon or the assembly will not close completely. Next, four smaller clamps, each with two small wooden pressure pads to prevent marring the surface, should be placed at the corners to ensure a tight fit between the tenon cheeks and slot faces. Make sure the frame is square before you let it dry.
When dry, you can trim the overall dimensions of the frame to suit. To cheat a bit, I will often make frames about 1/8″ larger, in both length and width, than they need to be, so when assembled I can trim them to size. This produces clean, square edges and also gives you one last shot to bring the frame into square.
With a bit of practice this joint can be made quickly and simply. It also has more than enough strength for all but the most extreme situations. If you can afford the investment in a tenon jig I think it’s a wise decision. A tenon jig will assist you in making this joint, and many other more difficult joints, for years to come. It also provides a bit more safety, and that’s something we all like to have on our side. The tenon jig will allow you to make this joint quickly and easily, but remember, “quickly” can get you into trouble sometimes. While setting up to cut the final width of the tenon, a bit of patience will pay off nicely.