Compound-Angle Joinery

The side-rail to rear-leg joint on a Chippendale chair—and many

other types of chairs—must account both for the trapezoid-shaped

seat and for the cant angle of the leg, making it necessary to cut

and use a compound-angle tenon.

B Y W I L L N E P T U N E

F or me, chairs are easily the most sat-isfying projects to build, but stu-dents often are puzzled by the

compound-angle joinery between the legsand seat rails. I learned how to draft, layout and cut these joints when I was a furni-ture-making student years ago, and now Iteach it at North Bennet Street School.Once you answer two critical questions—"Where do the layout lines come from?"and "How do I get the layout lines on thewood?"—you'll see that cutting these jointsisn't all that hard. What's more, once youunderstand how to cut compound-angle

joinery, cutting joinery with a single anglebecomes simple.

Recently, I built a set of Chippendalechairs. Most Chippendale chairs—and a lotof other styles of chairs—have rear legs thatcant inward as they go toward the floor butfront legs that are perpendicular to thefloor line. Although this design lends a re-fined sense of upward motion to a chair, italso introduces a fussy situation when itcomes to joining the rail to the back leg. Toallow for the cant of the legs and the trape-zoidal shape of the seat, most of the timeyou'll have to cut compound-angle tenonsbetween the legs and seat rails.

It is tempting to angle the mortises, in ei-ther the plan or elevation, to simplify thetenon problem. In the first case, the mor-tise would angle in the plan view at the

In the high style. Madeby Steve Brown, this heart-back Hepplewhite designhas curved seat rails andmore complex shapedlegs, but the leg-to-railjoinery is the same as theChippendale chairabove.

Careful tenon layout is the keyto cutting and mastering this intimidating joint

PLAN-VIEW SECTION REAR-VIEW SECTION

I teach students to lay out this joint with only two partial draw-ings—a plan (overhead) view at the bottom edge of the side railand a front elevation view. This article will show you that sim-ple drawings are all you need to know to cut this joint.

Though the joinery looks intimidating, the draw-ings make it easy to transfer the layout lines tothe rail. Once the layout lines are in place, it's simply a matter ofcutting the joint—by handsaw, bandsaw or other means.

seat-frame trapezoidal angle. In the secondcase, the mortise could be cut square to theback rail in front elevation to correct for thecant angle. Both of these moves force youto shorten the back rail tenon, whichwould weaken this critical joint.

Both historically and for chair making to-day, I think compound-angle tenons repre-sent the best possible technical solution tothis problem. Once you have a system forlaying out these joints, cutting them is notthat difficult.

Draw simple elevation and plan viewsNo matter what style chair you're building,there are two angles to consider: the cantof the leg, seen in a front elevation, and theseat-frame trapezoidal angle, seen in a plan(overhead) view. Start by doing a partial

drafting job, just enough to get the infor-mation you need for layout.

First draw the leg from a front view andshow the mortise. The mortise in the rearleg should be as far to the outside of the legas possible without sacrificing the thick-ness of the mortise walls. The mortises canbe cut square and slightly short in length,then chiseled to the correct angle at the topand bottom, making the mortise a parallel-ogram. Cutting a mortise in the shape of aparallelogram not only helps you registerthe rail, because it makes the rail's top andbottom edges parallel to the floor line, butit also makes the through-tenon look bet-ter from the back of the chair.

Transferring information from the eleva-tion, draw the sections of the leg at the bot-tom of the rail. Then you can draw the side

In any style. Made ofcurly maple, Mary

Conlan's Chippendalechair of simpler form

is built using the sameleg-to-rail joinery as a

more flashy, high-style chair.

VIEW FROMBACK OF CHAIR

L A Y O U T AND CUTTHE MORTISE

Set the mortise to the outside of the legas far as possible, taking care to seethat the outer mortise wall is at least

in. thick for strength. Lay out and cutthe square mortise parallel to the sideof the leg. Then chop the top and bottomof the mortise parallel to the floor line,making the mortise a parallelogram.The rail joins squarely to a flat section ofthe leg; cut a wind to keep it flush.

rail and its angle. Notice that the side railmust be thick enough to allow wood forthe top outside corner as well as the bot-tom inside corner, as seen in the elevationdrawing on p. 61. I also like to have extrarail thickness to allow for a shoulder at thebottom inside corner.

First draw the line representing the out-side face of the rail blank and its angle.Here I'm assuming that the outside face ofthe rail lands flush to the top of the leg, butyou could leave a shoulder if your designcalls for it. Then draw a parallel line show-ing the bottom inside face of the rail,choosing a rail thickness that will allow foran inside shoulder of in. to in.

As a last check, draw a detail of the topsection of the leg in plan view. I draw thisas if the leg mortise runs all the way upto the top edge of the side rail. Extendthe line that represents the outsideface of the rail back through the leg tobe sure that the tenon lies within thethickness of your rail.

This construction has the side railforming a simple angle, which leaveswood sticking out from the canted legon the outside. These surfaces will bereconciled by fairing a wind into theoutside face of the rail once the join-ery has been cut. The front end of therail is left alone for the leg joints, sothe rail starts plumb at the front anddevelops a wind that becomes thecant angle of the rear leg.

To show this, draw a dotted angledline from the bottom outside cornerof the rail out toward the rail's frontend. This transfers the informationfrom the elevation onto the plan view(as in the drawing on p. 61). The planview is simplified but contains all of thecrucial points seen in the elevation. Thesetwo drawings provide the information nec-essary for laying out the joint.

Follow the drawingsto lay out the jointTo make the layout easier, I pretend themortise is extended up to the rail's topedge. Once the tongue of the tenon hasbeen cut using the method of your choice,it will be easy to shoulder down the tenonto match the real mortise (see p. 64).

Extend the lines of the mortise openingup to where the edge of the rail will land.From the bottom inside corner of the mor-tise, square up a line to the top edge of

the rail. Where these three lines cross thetop rail edge will become the source ofthe layout information.

The important thing to realize is that theinformation seen here is true only at onelocation along the rail: the plane of theshoulders (see the plan view on p. 61).

On the inside face of the rail, square aline across that shows the correct shoulderlocation, measured in from the end. HereI've left extra length for later cleanup.Then, using a bevel gauge set to the seatangle, run the shoulder lines across the topand bottom edges of the rail. These shouldthen connect with another square line, upfrom the outside face of the rail, describingthe plane of the shoulders. Your drawingshould now show the location of the tenonat this plane (see the drawing on p. 64).

One joint, many chairs.No matter what kind ofchair you're building, ifyour back legs are cantedand your seat is trape-zoidal, you'll need to usecompound-angletenons to jointhem, as wasdone withthis Chip-pendalechair by RichHeflin.

Working from the elevation drawing, seta marking gauge to x and mark this dis-tance across the top and bottom shoulderlines, measuring from the inside face of therail. From the mark on the top edge, use apair of dividers set to the distance y tomake another mark along the shoulder.The new mark on the top edge and the firstmark on the bottom edge locate the insidecheek of the tenon. From these marks,transfer the size of the mortise to locate theoutside tenon cheek.

This may sound confusing, but all you'redoing is converting the cant angle to arise/run problem. The rail width is the run,and y is the rise. The reason for the initialmarking gauge line is that it's more difficult

CONSIDER LENGTH AND SEAT ANGLE WHEN LAYING OUT TENON SHOULDERS

SECTION AT RAIL BOTTOM

While the joints atthe front of the chairare simple angles,compound-anglejoints are requiredwhere the side seatrail joins the backleg. Use simple full-sized drawings todetermine the angleof the top andbottom tenonshoulders at theback of the seat rail.Then transfermeasurementsfrom the drawingsto the rails.

1. Full-sized drawingshelp you avoid errors.Working from a full-sized plan (overhead)view, set the bevelgauge to the angle be-tween the back railarid side rail on theseat frame.

2. Marking the firstface. Set the side railinto place over thedrawing (make surethere's enough stockfor the full tenon).Make a tick mark onthe bottom inside cor-ner of the side rail, andpencil in the shoulderline on the inside face.

3. Locating the topand bottom shoul-ders. Register the bev-el gauge against theline for the insideshoulder, then markthe bevels at the topand bottom of the rail.Check that your anglesmatch those in thedrawing.

4. Knife marks aremore exact. Once thetenon shoulder hasbeen correctly marked,knife-mark the lines onall sides of the rail. Theknife marks provide aspecific line to pare orshoulder-plane to.

CAREFULLY LAY OUT THE ANGLED TENON ON THE STOCK

Laying out and cutting angled tenons is a methodical process, but it's not a difficult one. Work fromsimple but accurate drawings and mark out each measurement from a single reference line on boththe top and bottom of the tenon.

1. Use a simple elevation drawing, as seen from the front of the chair,and set dividers to x—the distance from the bottom inside corner of therail to the inside corner of the tenon.

2. Set a marking gauge to the distance x between the inside face of therail and point x and scribe a line across the top and bottom shouldersfrom the inside face of the rail.

3. Set the dividers to the distance between x and y.

4. Use the divider setting from step 3 to locate point y on the top edgeof the rail, measuring from point x.

5. Set the dividers to match the mortise width on the rear leg of thechair itself.

6. From point x on the bottom of the rail, transfer the width ofthe mortise.

7. With the dividers still set to the mortise width, measure from point yto mark the tenon width at the top of the rail.

8. Tenon cheeks are marked perpendicular to the shoulder line byregistering a square against the bevel gauge—which is still set to thetrapezoidal seat angle.

9. After the top and bottom of the tenon have been marked, use astraightedge to connect the points and complete the layout.

10. After knife-marking the shoulder lines, cut the tenon and shoulderswith a backsaw, then trim to fit.

to measure from a corner using dividers.The goal here is not just to get a tenon thatfits—the rail should also land on the post atthe correct location and project at thetrapezoidal angle.

Once the base of the tenon has been lo-cated, the plan view (see p. 6l) shows thenext move. The tenon is simply square tothe shoulder. Clamp the bevel gauge to therail and square all four tenon marks out to

MARKING OUT THE WIND

the end of the rail. Once you've connectedthese lines across the end grain and knife-marked the shoulders, layout is complete,for now. Once the tenon cheeks and theside shoulder have been planed, the topshoulder can be marked out and cut. Afterfitting the tenon, mark the wood to befaired directly from the leg (see the photosat right).

Make practice cuts in scrapbefore cutting the real jointOne very direct way of cutting a com-pound-angle joint is with a handsaw. Firstthe cheeks would be sawn in the ordinaryway. The only tricky part is rememberingthat the shoulder cuts are at differentdepths on each edge. Begin sawing withthe shallow edge facing you, and avoidcutting into the tenon.

A bandsaw is good for cutting thecheeks, too. Setting the table for the cantangle (remember to keep track of lefts andrights), you can follow the cheek lines onthe top edge and the blade will follow thecant angle on the rail's end.

The tablesaw can probably get you clos-

Once the tenon has been cut and fitted, dry-fit the joint tightly and mark out the sectionof the rail that needs to be planed away. Notice that there is no material removed at thefront of the rail.

Establish layout lines. Connect the bottom lineof the wind with the bottom front corner of therail. Planing to this line gives you an even windand lets the rail meet flush at both the front andback legs.

er and thus avoid a lot of cleanup withhand tools, but the explanation is a story allby itself (for more on this technique, seeMaster Class on p. 108).

Whatever method you use, lay out withpencil first and confirm that you havethings correct. Often, the cant and seat an-gles are close enough that it's easy to grabthe wrong bevel gauge during layout. Theshoulder won't look bad, but the front legswill be way off. It's also possible to get the

lefts and rights mixed up and lay out thecorrect angle in the wrong direction. Thesemistakes make for a long day, so when indoubt, mill a practice rail and check bothyour layout and cutting method. Once thejoinery for the back end of the chair hasbeen cut, the simple angles on the frontends of the rails will seem easy.

Will Neptune teaches furniture making at NorthBennet Street School in Boston.

Take it slow. The author uses a shoulderplane to trim the cheeks, checking the tenonfrequently against the mortise until he has atight fit. He then trims to the layout lines witha shoulder plane.