Mortise Tenon

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A mortise and tenon joint falls into that category of "inventions that civilizations are built on." You know, like the wheel, the lever, and the inclined plane.

Okay, maybe I'm exaggerating a bit, but in the woodworking shop, there isn't anyjoint that's as versatile and strong as the mortise and tenon.

VERSATILE. The mortise and tenon typically joins pieces at right angles. So it's a simple solution for many woodworking applications. Like when you're building a frame. (In this issue I used mortise and tenon on both the hall mirror and the back of the porch swing.) Or when connecting two legs with a stretcher (like a table or chair).

And for special applications, the mortise and tenon has a number of variations. For instance, there are through, twin, and keyed mortise and tenons. But these don't require new skills. They simply apply the basicjoint to a new situation.

MORTISE

I always drill the mortise first. Then cut the tenon to fit. A mortise is limited by the size of your drill bits. But you can sneak up on the size of the tenon with the table saw. And it's easier to shave a tenon smaller than it is to chop a mortise bigger.

But even though Iarill the mortise first, I lay it out based on the finished size of the tenon. And since a tenon that's centered on the thickness of a workpiece is easier to cut, I usually center the mortise as well.

SET FENCE. A centered mortise is a sim-

1a piece of scrap to center the bit. Place the scrap against the fence and drill a shallow hole. Then flip it around to see if the second hole matches the first.

MECHANICAL STRENGTH. Besides being versatile, the mortise and tenon is strong. The obvious reason for this is mechanical. The tenon fits into the mortise like a handle in the head of a hammer.

To maximize this strength, I follow the "one-third rule." The thickness of the tenon is one-third the thickness of the mortised pie operation on the drill press. Start by adding a fence so the workpiece is roughly centered under the bit. Then set the depth of the hole. I always drill the hole a little deeper than the finished length of the tenon. This way, the shoulders of the tenon pull tight against the mortised workpiece.

Shop Tip: To set the bit dead center, start with a piece of scrap that's the same thickness as your workpiece, see Step 1 below. Drill a shallow hole. Then flip the test piece around and drill a second hole in the same

2 First drill the end holes and remove the waste with overlapping holes. To clean the sides, lower bit in W increments and slide piece back and forth, holding it firmly.

workpiece. This way, each side of the mortise is as thick as the tenon. There's no "weak link."

GLUE STRENGTH. Mechanical strength isn't the only thing a mortise and tenon has going for it. It's a very strong glue joint as well. The cheeks of the tenon create large face grain surfaces that contact the face grain sides of the mortise. As long as the mating pieces fit snug, the glue joint will be strong.

easy TO MAKE. For as strong as it is, a mortise and tenonjoint is easy to make. All it requires is a drill press and a table saw. The hardest part is cutting multiple mortise and tenonjoints all identical so they make a square frame.

The trick here is to follow a certain procedure and to use standard settings to minimize any variations between the joints. Fences, stop blocks, and featherboardsalso come in handy to make the procedure more like production work.

location. If necessary, adjust the fence and repeat the procedure. When the two holes align, the bit will be perfectly centered.

DRILL MORTISE. To drill the mortise, start with the end holes, see Step 2. Note: I drill these holesjust inside the lay-out lines.

The next step is to remove the waste between the end holes and clean the sides, see Step 2 and the box on the next page.

CLEAN MORTISE. When the mortise has been drilled, all that's left is to square up the corners with a chisel, see Step 3.

3 If you want a mortise with square corners, start by chopping straight down at the ends. Then clean up the corners by paring down on the sides of the mortise.

TENON

With the mortises drilled, it's time to cut the tenons. Cutting them on the table saw allows you to sneak up on the final size of the tenon. The miter gauge with an auxiliary fence helps support the workpiece and prevent chipout, and the rip fence acts as a stop for cutting the tenon to desired length.

TWO STEPS. Unlike the mortise, the tenon is cut in two steps. First the cheeks of the tenon are formed, see Steps 1-4. Then when a corner of the tenon fits into the mortise, the next step is to cut the tenon to width.

PROCEDURE. The procedure for both of these steps is the same. First, set the height of the blade, see Steps 1 and 5.

Next make two test cuts, flipping the piece between passes. Shop Tip: I test the cuts only on the very end ofthe workpiece, see Steps 2 and 5. This way, if the cuts are off, they can be corrected without having to make another workpiece.

When each test cut fits, the rest of the tenon can be cut, see Steps 4 and 5.

ASSEMBLY TIPS. If youhave problems get ting the joint together, there are some things you can do, see Step 6. First to allow the tenon to slip more easily into the mortise, I chamfer the ends of the tenon.

Also, if there are gaps between the pieces when they're assembled, you can often get a tighter fit by undercutting the shoulders of the tenon with a chisel.

Finally, to avoid any messy squeeze-out, I apply glue only to the cheeks of the tenon near the ends. The glue will get spread out as the tenon is pushed into the mortise. □

Iset the height of the blade, place the mortised piece flat on the table. Raise the blade so the highest tooth lines up with the side of the mortise-

Rip fence t Remove waste in a series of passes

4 Set the rip fence so the distance to the outside of the blade equals the length of the tenon. Then cut the shoulder and make repeated passes to remove the waste.

Cut Stub tenon on one end

Auxiliaiy^.J fence on miter gauge

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..-^'-V-'" Workpiece

2 Test the setup by cutting a "stub" tenon on the end of the workpiece. Then with an auxiliary fence attached to the miter gauge, make one pass for each cheek.

3 see if the stub tenon fits in the mortise. (Make sure the outside faces of both pieces are up.) If it's too tight, raise the blade slightly and test the fit again.

To cut the top and bottom edges, turn the workpiece on edge and sneak up on the cuts until the width of the tenon matches the length of the mortise.

J

Chamfer all edges----

6 To allow the tenon to slip into the mortise and provide room for excess glue, chamfer all the edges. Plus, undercut the shoulders with a chisel to ensure a tight fit.

SPIRAL END MILL BIT

A spiral end mill bit is specially designed to cut mortises. (For sources, see page 31.) Its spiral cutting edge has an "up-cut" design. So unlike Forstner bits, the chips are pulled up out of the mortise. This eliminates heat build-up and clogging. But the longer cutting edge also makes for a cleaner cut, so the sides of the mortise end up smooth.

A clean cut is also the result of higher speed. Because these bits are designed to run in a router, I set the drill press at its highest speed setting (between 3000 and 5000 RPMs).

PROCEDURE. Drilling a mortise with an end mill bit is just like drilling one with a regular drill bit. First drill the holes at each end. Then drill overlapping holes between them.

But to clean up the sides of the mortise, the end mill bit has a definite advantage. As a router bit, it's also designed to cut side to side. So you can slide the workpiece "through" the bit.

A couple words of caution. Don't clean up the entire face of the sides at one time. Instead, lower the bit W and slide it back and forth. Then lower the bit another H" and so on. Also, be sure to keep a firm grip on the workpiece when sliding it sideways. The mill bit can grab a little as it removes the waste.

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  • Maria Teresa Udinesi
    How to make a mortise in the back of a porch swing?
    8 years ago

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