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Foolproof Tenons

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Foolproof Tenons

Two blades and a rock-solid jig guarantee success.

By Tom Caspar

Mortise and tenon joinery is the heart of many classic furniture projects. It’s an incredibly strong, time-tested method of connecting boards. Making these precisely fit joints can be fussy and demanding work. But it doesn’t have to be.

There are at least a dozen ways to cut tenons.We’ve chosen a technique that delivers the goods every time, whether you’re a beginner or seasoned pro. It’s simple, safe and economical. Best of all, this technique will save you a lot of set-up time. Rather than fit tenons by trial and error for each project, you can easily reproduce the same size tenons, time after time.


What you'll need


A Tenoning Jig You can buy a commercial jig or make a better one yourself. Looking for a better mousetrap, we designed a heavy-duty jig that slides on top of a tablesaw fence. (Your fence must have parallel sides for this jig to work.)

Features:

-Large capacity. The 8-in.-tall sides are high enough to support a rail up to 36-in. long.

-Dedicated sides. No commercial jig offers this handy feature. The left side supports the edge of a rail; the right side supports the face.

-No tear-out. None.A backer board prevents annoying tear-out when you rip the tenon cheeks. It’s easy to replace the backer board when you change settings.

-Safe operation. Generous handles keep your hands out of harm’s way. An oversize block behind the backer board covers the saw blade after it makes the cut.

Click any image to view a larger version


Two Saw Blades and Spacers

Make smooth-sided tenons with a pair of matched sawblades separated by spacers.You can use the outside cutters of a dado set or two carbide-tipped 7-1/4-in. circular saw blades (about $8 each).

Two blades and spacers guarantee a perfectly sized tenon time after time.


A Zero-Clearance Insert with Splitters

Make a dedicated insert for your tablesaw so your tenon cutting is as safe as possible. Although the parts you cut will be clamped to the jig, this insert provides additional support from underneath.



Build the jig


Our jig rides on top of your tablesaw and is guided by the fence.The fit between the sides of the jig and the fence has to be just right. If it’s too tight, you’ll have to push too hard. If it’s too loose, your tenons won’t be straight. Here are some tips to make building and fitting the jig easier:


Cutting List


-Drill all the 13/64-in.-dia. clearance holes in the jig with a drill press (Detail 1, right).

-Creep up on the width of boards C,D, and E. For your jig to slide properly, the right fit is a matter of 64ths of an inch.Rip the boards slightly oversize, then clamp them between the faces of the jig and try sliding the jig along your saw’s fence.

-If your jig is too tight, shim the edges of boards C, D and E with masking tape.Don’t laugh! It’s an effective way to fine-tune the fit.

-Clamp the jig together and make sure it’s square to the top of the tablesaw before drilling any pilot holes and driving in the screws. Don’t glue the jig together.You may need to disassemble it later to adjust the fit.

Fig. A: Exploded View


Cut the corners of the jig on the tablesaw (Detail 2, below). Stop the cut at the pencil line and withdraw the board. The bottom side of the board may be overcut, but that’s OK. It’s only a jig!


Detail 1


Detail 2



Getting started

Mill all your stock straight and square, and boldly mark the face and edge of each piece. Usually the face side is the outside of a rail and the reference edge is the top of a rail.


1. Make the mortises first. It doesn’t matter whether you use a mortising machine, a router or a drill press. It’s far easier to fit a tenon into a mortise than it is to customize a mortise to fit a tenon.


2. Lay out the tenon’s cheeks directly from the mortise.This joint won’t be flush, so first you must draw a line representing the setback of the rail from the front of a leg or a stile. Draw lines across the face and edge of the rail representing the shoulders of the tenon so you can set the height of the cut on the tablesaw.


3. Install homemade spacers between the saw blades.You’ll need three different kinds of spacers to fit every size tenon (approximately 1/8 in., 1/16 in. and 1/32 in. thick). Make them the same diameter as the flange on your saw’s arbor.The best way to figure out the right mix of spacers is by trial and error, making tenons in scrap wood. Fortunately, if you save and label the spacers you should only have to go through this process once.



Cut the cheeks

One pass through your ganged-up saw blades and you’ve cut both sides of the tenon.


4. Raise the blades to match the shoulder line on your rail.


5. Clamp the rail to the tenoning jig, face side in, and adjust the tablesaw’s fence so the layout marks line up with the two saw blades. Try a pair of inexpensive 7-1/4-in. circular saw blades. They work surprisingly well.


6. Rip the cheeks. Slide the jig through the cut, then pull the jig back and unclamp the rail. Rip all your tenons. Make sure the marked face of each of your rails is the face you clamp against the jig. When you’re done, remove the double blades from the tablesaw and install a crosscut saw blade.



Cut the long shoulders

Make perfect, tight-fitting shoulders with a crosscut saw blade.


7. Cut the long shoulders. Set up your miter gauge with a fence, and clamp a stop block to it. Raise the stop block above the waste piece so the waste won’t get trapped.

Cut all the shoulders on the face sides of the rails first.Then readjust the height of the blade, if necessary, to cut all the shoulders on the back sides of the rails.

Raise the blade so it barely touches the cheek.



Oops!


My shoulders are drooping! I thought they were going to line up just right but somehow they became offset. I checked the ends of the rail with an accurate square, and wouldn’t you know it, they were off.The culprit is my miter gauge. It wobbles in the slot, so I don’t get an accurate 90-degree cut every time.And if the shoulder cuts (Photo 7) aren’t exactly square, the shoulders won’t line up at the edges.

The fix? I bought a premium miter gauge ($100 to $170) that fits tighter in the slot. Rather than toss out the rails, I re-cut both ends as little as possible and re-cut the shoulders.Now my project is a bit smaller than originally planned, but the joints are perfect.



Cut the tenon's width

Make two rip cuts on the right side of the tenoning jig. They establish the tenon width.


8. Mark the width of the tenon directly from the mortise.


9. Rip the tenon to width. First, move the fence to the left side of the saw blade. Clamp the rail to the right side of the tenoning jig and adjust the fence until the blade lines up with the marks.

Raise the blade about 1/32-in. lower than the shoulder to avoid cutting into it.

There isn’t any fussy micro-adjust on this jig. Simply nudge the fence with your fist.



Cut the short shoulders

Sawing all four shoulders flush isn’t realistic. Don’t push your luck! Finish them with a sharp chisel.


10. Rough cut the waste pieces on the tablesaw. Position the stop block so the short shoulders are proud by 1/32-in. or so. In theory, it’s possible to make this cut precisely flush, but in practice it’s darned hard. Don’t risk messing up your crisp shoulders!


11. Remove the stub waste with a chisel.This paring requires a keen edge, but if you cut too deep, it won’t show. Bevel the ends of the tenon with a file so it will easily fit in the mortise.


12. Test the fit. If your tenon is too wide to fit into the mortise, it’s easy to go back and trim off one side with the tenoning jig. If it’s too loose, glue the waste pieces back on and cut again.




This story originally appeared in American Woodworker August 2001, issue #88.

August 2001, issue #88

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