A well-made mortise-and-tenon joint starts with a tenon that fits snugly into its mortise and has clean, square shoulders. There are several ways to cut tenons, but one of my favorite methods uses a table saw equipped with a stacked dado blade. It's fast, accurate, and easy to repeat once the setup is dialed in.
The dado blade removes the waste from the tenon cheeks and shoulders, while its height determines the thickness of the finished tenon. An auxiliary fence attached to the miter gauge supports the workpiece during each cut, helping produce square shoulders with minimal tearout. The rip fence, used with an appropriate auxiliary setup, establishes the tenon length.
The real advantage of this method is how easily you can fine-tune the joint. A few test cuts let you sneak up on a snug fit, and a simple technique for smoothing the cheeks leaves the tenon ready for assembly.
Here's how I cut and fit traditional tenons using a dado blade.
Step 1: Set Up the Table Saw for Cutting Tenons
The basic setup consists of a wide stacked dado blade, an auxiliary fence attached to the miter gauge, and a rip fence that establishes the tenon length.
The auxiliary miter gauge fence supports the workpiece as it travels across the dado blade. It also backs up the cut, helping prevent tearout where the blade exits the wood.
The rip fence provides a consistent reference for the shoulder location. For short tenons, the original Woodsmith setup uses an auxiliary rip fence with clearance for the dado blade. This allows the end of the workpiece to register against the fence while the blade cuts the cheek.
Make sure the setup provides adequate clearance and doesn't trap loose waste between the blade and fence. Keep the workpiece firmly supported throughout each cut.
Before cutting the project parts, have a matching mortised workpiece nearby. You'll use it to check the tenon thickness and make the final blade-height adjustments.
Step 2: Cut a Test Tenon to Dial In the Fit
I always start by making a short test tenon. This can be a separate piece of scrap that's the same thickness as the project parts, or the tip of one of the actual workpieces.
Set the dado blade slightly lower than the estimated final height. Then make a cut across one face of the workpiece. Flip the piece over and make a matching cut on the opposite face to form a short tenon.
Try fitting the test tenon into the mortise. If it's too thick, raise the blade slightly and repeat the cuts. Continue making small adjustments until the tenon slides into the mortise with a snug fit.
The goal is a joint that goes together with firm hand pressure. You shouldn't have to force the tenon into place, but it shouldn't rattle around in the mortise, either.
Once the thickness is correct, leave the blade height alone while cutting the remaining cheeks.
Set the Tenon Length with the Rip Fence
Next, adjust the rip fence to establish the shoulder location and the length of the tenon.
I start with the fence positioned so the cut falls just short of the shoulder layout line. Make a test pass, check the location, and adjust the fence until the cut reaches the line.
When the shoulder position is correct, remove the remaining waste by making additional passes, working from the shoulder toward the end of the tenon.
Repeat the process on the opposite face of the workpiece. Because the blade height and fence position remain unchanged, the two cheeks should be uniform and the tenon centered on the stock thickness.
Step 3: Smooth the Tenon Cheeks
A dado blade removes waste quickly, but it can leave small ridges on the tenon cheeks. These ridges may prevent the tenon from seating smoothly in its mortise, even when the overall thickness is correct.
There's a simple way to clean them up without changing the blade-height setting.
With the workpiece supported by the miter gauge fence, move it carefully across the highest cutting point of the dado blade, using controlled side-to-side passes to shave away the ridges. Advance the workpiece slightly between passes until the cheek is smooth.
Use only a light finishing cut and maintain positive control of the workpiece. If your saw setup doesn't safely support this operation, clean up the cheeks with a shoulder plane or a sharp chisel instead.
Repeat the process on the opposite cheek, then check the fit in the mortise again. A smooth cheek makes the joint easier to assemble and gives the glue surfaces consistent contact.
Step 4: Cut the Top and Bottom Tenon Shoulders
Once the cheeks are cut and smoothed, the final step is forming the top and bottom shoulders. These cuts establish the tenon's width and complete the four-sided shoulder around the joint.
Depending on the mortise dimensions, you may need to readjust the dado blade height to remove the correct amount of material from the edges of the workpiece.
I like to start at the end of the tenon and work back toward the shoulder, removing the waste in successive passes. This keeps the remaining material supported as you approach the final shoulder cut and helps reduce chipout at the corners.
Make the last pass at the shoulder with care. A light final cut helps ensure that the shoulders are aligned and that the joint will close cleanly against the mortised workpiece.
If the dado blade leaves small ridges on these surfaces, clean them up with a suitable hand tool or a carefully controlled finishing pass.
Check the Finished Mortise-and-Tenon Joint
Before cutting the remaining tenons, fit the completed test tenon into its mortise.
Check that the cheeks fit snugly, the shoulders close tightly against the mating workpiece, and the tenon doesn't bottom out in the mortise. A little clearance at the end of the tenon ensures the shoulders can seat fully.
If the fit is correct, you can use the same settings to cut the remaining joints. Keeping the workpieces flat against the table and consistently registered against the auxiliary fence will help maintain uniform results.
With the dado blade doing most of the waste removal, the process goes quickly. And by taking the time to dial in the thickness, smooth the cheeks, and make clean shoulder cuts, you'll end up with strong, accurate tenons that are ready for assembly.
