Tips for cutting tenons

A tenon has two essential parts - the cheeks and the shoulders, see Fig.1.

CHEERS. The purpose of the cheeks is to provide a gluing surface against the sides of a mortise (a square hole or slot cut in the adjoining piece to accept the tenon).

When you cut the cheeks, you also define the thickness of the tenon. This is the critical dimension for a strong glue joint — the tenon must be thick enough to fit snugly into the mortise. But not so tight that it squeezes the glue out of the joint.

SHOULDERS. The other important parts of a tenon are the shoulders. The shoulders are designed to do a couple of things.

First, they cover up any small gaps around the mortise. And they contribute to the mechanical strength of the joint.

In its simplest form a tenon has two long shoulders which are cut on the faces of the workpiece, see Fig.1.

These shoulders define the length of the tenon and determine how deep the tenon slides into the mortise.

In addition, many woodworkers also cut short shoulders on the edge of the workpiece, see Fig.1. These short shoulders create resistance to any up and down movement of the tenoned piece.

CUTTING A TENON. There are two basic methods for cutting a tenon on the table saw — the single-pass method and the multiple-pass method, see Fig.2 and Fig.3.

SINGLE-PASS. With the single pass method, the workpiece is held vertically in a jig and passes through the saw blade to cut one cheek at a time, refer to Fig.2.

Then the workpiece is taken out of the jig to make the shoulder cuts.

By cutting tenons like this, it’s easy to produce a very smooth cheek — which makes an excellent gluing surface.

MULTIPLE-PASS. When you use the multiple-pass method on the other hand, the workpiece is laid flat on the table saw, see Fig.3. The tenon is then cut by making a series of passes over a saw blade (or dado blade).

Since the first cut is made at the shoulder line, both the shoulder and the thickness of the tenon are established with one cut.

This method requires very little set-up time and is a quick way to cut tenons, especially if you’re cutting just a few. The only problem is it can leave a rough glue surface on the cheeks.

Preparation and lay out

Even before you lay out the nine of the tenon there are a couple of things you an do to ensure a good fit.

STOCK PREPARATION. First, square up the ends and edges of all the pieces. This will prevent gaps around the shoulders of adjoining pieces.

Second. if you are cutting tenons of the same size on several pieces, make sure all the pieces are identical in thickness otherwise the thickness of the tenons will vary according to the thickness of each piece.

LAY OUT. After the stock is prepared, the next step is to lay out the tenons. 

Note: 1 always cut the mortise first, then size the tenon to fit. This way I can use the mortise as a template, see step-by-Step drawings.

ShopTip: I like to use a knife to transfer dimension because the sharp lines are more precise than a pencil line.

TRANSFER DIMENSIONS. To transfer the dimensions of the mortise to the workpiece. Start by marking the thickness of the tenon, see Step1. Then mark the width of the tenon. see Step 2.

Finally, mark the length of the tenon on the workpiece and check the depth of the mortise to make sure it will fit see step 3 and step 4.

Test fit

The secret to getting a tenon to fit snugly in a mortise is to make trial cuts on a test piece. (Note: The test

piece must be exactly the same thickness as the workpiece.)

SINGLE-PASS. To do this with the single-pass method, make the cheek cuts. Then, since the waste isn’t removed until the shoulders are cut, saw off a comer of each waste piece to check the fit, see Fig.1.

MULTIPLE PASS. If you’re using the multiple-pass method, take a few passes to create a "mini" tenon at the end, see Fig.2.

Shoulder cuts

If the workpieces have tenons on both ends (such as the rails for a frame), the distance between the shoulders is critical. If they’re not the same on every workpiece, the frame won’t be square.

STOP BLOCK. One way to cut accurate shoulders is to add a long fence to the miter gauge and use a stop block, see Fig.1. The stop positions the workpiece precisely for every cut. The fence prevents chip-out, and reduces kickback.

DONG WORKPIECE. If the Workpiece is long, clamp a stop block to the rip fence, see Fig.2. Then, butt the workpiece against the stop block and clamp it in place before making the cut.

Duplicate tenons

Here’s a quick tip when you need to cut several identical pieces with tenons on the ends (such as rails for a set of frames).

WIDE BLANK. Instead of cutting the pieces individually, start with a wide blank and cut tenons on the whole width of the blank, see drawing.

RIP STRIPS. Now just rip the blank into strips to produce pieces with identical tenons.

Stepped shoulders

A common problem when cutting tenons is that the shoulders come out uneven, see photo. This is usually caused by a rip fence that isn't 90° to the table, see Fig.1.

As you cut the long shoulders, the piece contacts the fence at the bottom, see Fig.1. But when you flip the piece on edge to cut the short shoulders, it contacts the fence higher up, see Fig.2. This pushes the piece away from the blade creating a "stepped" shoulder.

SOLUTIONS. The best way to solve this is to adjust the rip fence on your saw.

Another Way is to clamp a block to the fence, see Fig.3. This doesn't eliminate the problem, but it does minimize it.

Undercutting

The saw blade often leaves a "ridge" between the cheek and shoulder, see photo. This prevents the shoulder from fitting tight against the mating workpiece.

UNDERCUT TENON. One way to get around this is to undercut the tenon. I do this by making the shoulder cuts slightly deeper (1/32") than the cheek cuts, see Fig.1.

SHOULDERS. Another Way is to undercut the inside corner of the shoulders with a chisel, see Fig.2. Just pare away a small amount of end grain leaving a 1/16" wide border.

To do this, start by pushing the chisel straight down alongside the tenon, see Fig.2. Do this all the way around.

Then, tip the chisel at an angle to remove the waste around the shoulder of the tenon, see Fig.3.

CHAMFER ENDS. Finally, pare a slight chamfer on the ends of your tenons, see Fig.3.