The mortise and tenon joint between the rung tenons and the legs is a critical part of what makes this chair so strong and resistant to wracking. Typically, round mortise and tenon joints (dowel joints) have a high rate of failure. However, if done right, this joint can be extremely strong and durable. The path to a good joint begins with wood selection and milling where I look for straight long grain in the board, and I mill the rung blanks parallel to the long grain. At the turning stage there are two more very important things to consider. The first is control of the moisture content of the rung tenon — in this case the rung blanks are placed in a small kiln and dried until they are bone-dry. The rungs are turned while bone-dry and kept in the kiln until assembly. After assembly the rung tenons will take on moisture and expand slightly within the mortise. The second thing to consider is the precision of the fit of the rung tenon within the mortise. The mortise is 5/8″ in diameter or 625 thousandths (.625″). When turning I aim for a tenon diameter that matches the mortise diameter plus or minus 2 thousandths. As you can imagine this produces a very tight fit. The combination of straight long grain, controlled moisture content, precision fit, and two other factors (grain orientation and good glue bond) produce a very strong joint that is highly resistant to failure.
Any lathe will work as long as it can accommodate the longest rung length. In the side chair the longest rung is 19″ and the longest turned part in any of the Boggs ladderback chairs is 25-1/2″. I always turn at a single speed so electronic variable speed is not essential, though it is important to be able to change to a slow speed for the occasional off-center turning required in some of the advanced chairs. I use a Nova 3000 lathe with an extended bed. My only complaint with this lathe is that I wish it had more mass in order to reduce vibration. As you can see I’ve added sand bags to the stand which helps a little. I’ve had lots of practice so even with the vibration I can still get a very smooth surface right off the lathe, though I believe the turning would go faster with less vibration.
I’ve added a custom tool rest that is long enough to turn any part up to about 24″ without having to move the rest. This rest is made from a 2″ piece of angle iron mounted to a wooden base and is very similar to the tool rest that Brian uses.
A notch at the bottom, right end of the rest allows me to bring the revolving center close enough to the drive center to turn parts as short as 10″ long. For anything shorter I remove the long rest and use the standard tool rest that came with the lathe. I also use the standard rest when doing the occasional off center turning required in some of the advanced chairs.
I have replaced both centers that came with the lathe with Sorby stebcentres, which I highly recommend. The head stock uses a 1/2″ drive stebcentre (left) and the tail stock uses a 1/2″ revolving stebcentre (right). These centres do a great job of holding the rung stock very firmly. The rung tenons are turned to a 5/8″ diameter so having a center with a 1/2″ diameter allows me to cut the rung blank to exact length and turn the tenons right to the edge.
I only use three turning tools to turn the rungs. A 3/8″ wide beading & parting tool is used for turning the tenons. A 3/8″ wide bedan would also work. I use a 1" roughing gouge for rough shaping the body of the rung. This is a little small for a roughing gouge, but it is what I learned on and what I am used to. Brian uses a 2″ shallow gouge for rough shaping. And finally, I use a 1″ skew chisel for a smooth, finished surface on the body of the rung.
I find that turning tools require frequent sharpening. Because of this I hollow grind all the primary bevels and then freehand hone the micro-bevels. For the roughing gouge (left) I use the Oneway Wolverine Grinding Jig with the V arm attachment to hollow grind the bevel. For the skew chisel (center) and the parting tool (right) I use the Wolverine 3″ x 5″ platform jig attachment and freehand the hollow grind. Both the skew chisel and the parting tool use the same grinding angle.
Here are the same tools with a honed micro-bevel.
And here is my sharpening set-up. On the left is a 600 grit diamond stone. I use this stone for shaping the micro-bevel. On the right is a steel plate with 3 micron diamond compound for honing the micro-bevel. Both stones are lubricated with a little camellia oil. The parting tool and the skew chisel are both honed on this pair of stones. For the roughing gouge I hone the bevel with a small diamond paddle, then remove the burr with a conical diamond hone (sandpaper on a dowel would also work).
There is one more tool that is critical to getting an accurately sized tenon. It would be virtually impossible to set a normal caliper to measure the tenon diameter to an accuracy of plus-or-minus 2 thousandths. Instead I use a wrench that has been fine tuned to measure exactly .625″ (5/8″). There are two ways to do this. In the photo below I have taken a 19/32″ wrench — these are fairly easy to find at flea markets — and filed it open to .625″. Another option is to use a 5/8″ wrench. The only problem with a 5/8″ wrench is that the opening is actually greater than 5/8″. Brian heats up the wrench and then hammers it closed until the opening is reduced to .625″. Always test the tenons created with your wrench and compare the diameter of the tenon with the diameter of the drill bit used for mortising to make sure that they match.
Light Bulb Kiln
In order to dry the rung blanks to a bone-dry state I use a small light bulb kiln. About a week before I want to turn the rungs I place them in the kiln. After that amount of time the rungs will be bone-dry, about 4% or so. After turning, the rungs are placed back in the kiln until they are needed for assembly. This kiln is a simple plywood box with insulation on the inside. It is heated to between 125°F and 140°F with a single 100 watt incandescent light bulb. I control the heat by keeping the door slightly open with a wedge. There are holes in the top that I can cover or uncover to further regulate the heat. These holes are also used to hold parts used in some of the advanced chairs that only require a single tenon end to be dried. This kiln measures approximately 18″ x 18″ x 30″ and can hold about 30 rungs.
I’ve lined the interior of the kiln with foil-faced bubble insulation. Half-inch foil-faced foam insulation will also work. There are three wooden racks for holding the rungs. Below the bottom rack and above the light bulb is a 11″ x 17″ piece of sheet metal on a metal rack which deflects the heat from the bulb and keeps it from burning the rung blanks. I monitor the temperature with an oven thermometer. I use a single 100 watt incandescent bulb. Given that these bulbs are hard to come by you might need to use two or three lower wattage bulbs. Be sure that the light sockets you use are rated for the wattage of the bulb. This is a porcelain socket which can easily handle a 100 watt bulb. The scraps of wood at the bottom of the kiln will be used for making pins for the slats.