On the face of it, an open mortise and tenon is kind of a "show-off joint — it's completely exposed so you can see exactly how the whole thing fits together. It also looks like a fairly easy joint to cut.
But looks can be deceiving. Since the both parts of this joint (the mortise and the tenon) are exposed, it's all too obvious when they don't fit right on the money. To get that kind of fit, I follow two rules.
First, I cut several extra pieces to use for trial or "set-up" cuts. Arid second, I don't use a ruler. The way to get the best results for this joint is to gauge each cut with the one before it.
Note: For what follows, the joint for the frames of the Jewelry Case (see page 20) will be used as an example. The first step is cut the rails (horizontal pieces) and stiles (vertical pieces) for the frames to a final width of IK*. As for the length, each piece is cut to the fmal outside dimensions you want for the frame.
After cutting the rails and stiles to size, arrange them in their final position (to form the frame) and mark an "X" on the face side and inside edge of all pieces.
Since these pieces form a frame to hold a panel, a groove must be cut on the inside edge of each piece. This groove is V* wide, V" deep and centered on the thickness of all pieces.
Although this cut can be made with a dado set, it's difficult to be sure the groove is exactly centered. Instead, I cut the grooves with a regular saw blade, using a two-pass procedure that ensures the groove will be centered.
To do this, place the "X" side of each piece against the fence, and make this first pass on all the rails and stiles, see Fig. 1. Then for the second pass, turn each piece around so the other side is against the fence (so the "X" side is out).
Note: Since I was working with '^«"-thick stock, and wanted a '//-wide groove. I did use a ruler for this first series of cuts to set the fence from the inside of the saw blade. 0Yie" minus Va for the groove leaves a total of or %2" on each side.)
Both the mortise and the tenon are cut standing on end. To support the pieces for these cuts, I used a shop-made jig that fits over the fence of the table saw, see Fig. 2. This jig is just four pieces of %" plywood glued and nailed together. It works best if the middle support piece is positioned slightly above the fence, and the bottom edges of the sides rest on the saw's table.
One other thing about this jig. The support arm must be exactly perpendicular to the table. And the nails or screws used to hold it in place should all be higher than the maximum saw blade height.
Now the mortise and tenon joint can be cut. And you can argue either way as to which should be cut first, the mortise or the tenon. It really doesn't make much difference, but I usually start with the mortise.
To set the depth of cut (height of the saw-blade), use the groove in one of the stiles as a gauge, see Fig. 3. In effect, this height is the maximum width of the tenon, so the mortise can be equal to or less than this height, but not greater.
Next, clamp the rail in the jig and use the panel groove to set the width of cut for the mortise, see Fig. 4. Once again you have a choice. You can set the blade so it's exactly on the edge of the groove, or a little wider (as shown). However, the mortise cannot be cut narrower than the groove or it will show on the end of the rail.)
Finally, cut the mortise by making the first pass with the "X" side of the rail against the jig. Then unclamp the rail and turn it around so the "X" side is out, and make another pass, Fig. 5.
clean up mortise. After making these tw?o passes on all pieces there may be a tiny sliver left in the center of the mortise. Adjust the fence to make one final pass on all pieces to remove this sliver.
Also, since saw blades don't make a flat-bottomed cut, the bottom of the mortise must be cleaned-up with a chisel.
At last, the tenons can be cut, using a two-step procedure. The first step is to make a shoulder cut to set the length of the tenon.
shoulder cuts. Since the length of the tenon must equal the width of the rail, use one of the rails to gauge the distance between the fence and the outside of the saw blade. Then, use the mortise to gauge the depth of cut, see Fig. 6. As the shoulder cuts are made, push the stile against the fence, and guide it through the blade with the miter gauge, see Fig. 7.
trim faces. After the shoulder cuts are made, the second step is to trim the faces of the tenon down to final thickness. These cuts are made with the rail standing on end and clamped to the jig.
To set up this cut, raise the saw- blade to the top edge of the shoulder cut. Then clamp a trial piece in the jig and adjust the fence until the inside of the saw blade is in line with the bottom of the shoulder cut, see Fig. 8. Make a tiial cut to trim off one face of the tenon. Then flip it around and trim off the other face.
If you're lucky, the tenon will fit perfectly in the mortise. If you're like me, you'll have to adjust the setting and make another trial cut until it does fit.
When you've got the right setting, go ahead and make these cuts in all the stiles. Then, since the tenon is a little wider than the groove, there will be two little "fins" that need to be trimmed off with a chisel, see Fig. 9.
That should do it. You should have a perfect open and mortise joint, with grooves exactly centered on each piece.
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