Now that I have cut the slat mortises in the rear legs it’s time to fit the slats to the mortises. At this point the rear legs are still square in cross section. The reason for this is that I need square reference surfaces in order to align the legs in a jig that will hold them in the same position that they will occupy in the finished chair. This will allow me to measure the distance between the legs at each mortise as well as measure the angle for the shoulder of the slat tenon. Then I can transfer those measurements to a slat blank and layout the shape of the slat.
To begin I need to build a jig that will hold the rear legs at the correct angle of rotation, the correct distance apart, and at the correct splay angle. In the finished chair the slats will rotate the rear legs so the angle of the slat tenons and the angle of rotation of the rear legs are identical. A good starting point for this angle is 25°, which generally makes a comfortable back rest. In my case I have decided on 28° instead, which will make the distance between the very tops of the rear legs a little greater. This is a design decision based on the visual appearance of the completed rear panel.
There are several known dimensions that will allow me to design and build the holding jig. An actual size drawing is needed — which I can do by hand or with a program such as SketchUp. I begin by drawing a 1-5/8″ diameter circle to represent the cross section of one rear leg. Next I need to draw a second circle to represent the opposite leg, but it must be the correct distance from the first leg. I will be basing this on the distance between the legs at the seat rung. To calculate this distance I need to know the overall length of the rear seat rung (14-3/4″) and the length of each tenon (1-1/4″). Then it’s a simple matter of subtracting the two tenon lengths from the overall length to arrive at the distance between the two rear legs (12-1/4″) at the seat rung. Finally I draw a 1-5/8″ square that is rotated 28° (to match the angle of the slat tenons) around each round leg — this square represents a cross section of the rear leg as it is now.
Starting with the drawing above, I add the blocks that will support and rotate the rear legs as shown below in red. Then I can measure the distance between the center point of each pair of support blocks (13-7/32″) which will be the measurement I transfer to the holding jig in order to place the support blocks the correct distance apart on the jig.
Not only does the holding jig have to rotate the legs and hold them the correct distance apart, but it also has to account for their side-to-side splay. In the illustration below, on the left, is a view of the rear panel as seen from behind. You’ll notice that, from the seat level down, the rear legs splay outwards side-to-side. Again, using known dimensions, I can calculate the angle of splay using a method known as rise-and-run. Rise-and-run angles are not expressed in degrees (°). Rather the angle is expressed as a specific amount of rise over a specific amount of run. For calculating the side-to-side splay I use the known rung lengths and the distance between them on the leg. The calculation goes like this: Lower Rung (15-1/2″) – Seat Rung (14-3/4″) = 3/4″ ÷ 2 = Rise (3/8″). I get the distance between the rungs (4-5/8″) from the story stick. So, the angle of side-to-side splay of the rear legs can be expressed as 3/8″ rise over 4-5/8″ run.
An interesting thing about rise-and-run is that any angle can be expressed as an infinite number of rise-and-run combinations. For example 3/8″ rise over 4-5/8″ run can also be expressed as 3/4″ rise over 9-1/4″ run — each number is simply doubled. This same angle can also expressed as 1″ rise over 12″ run. All of these rise-and-run combinations describe exactly the same angle as shown in illustration below. This method can be very useful when applied to building jigs for holding parts at specific angles or to calculate the drilling angles for the rung mortises.
Now I have all the information I need to build the jig that will hold the legs in the position they will occupy in the finished chair. I’ll begin by making the support blocks. First I mill two blocks 1-7/8″ wide x 2″ tall x 12″ long as shown in the photo on the left. I marked the first 28° angle cut on the end of each of these blocks. I have found that the table saw is the easiest way for me to make this cut. I have a saw with a left tilting blade. I set the blade at 28° and the fence at 5/8″ to make the first cut. The results are shown in the photo on the right. Next I reset the blade to 90° and the fence to 1/2″. I flip the remaining pieces to be cut over (to the position shown on the right in the right-hand photo) and make the second cut. I always use two push sticks to make both of these cuts safely.
Here is the final result — four identical support blocks cut at a 28° angle. I’ll use the two cut-offs, shown behind the support blocks, to make a jig to measure the distance and angle between the two rear legs at each mortise.
Here is the base of the holding jig. The length is 21-7/32″ which allows 4″ on either side of the center point of each pair of support blocks. I purposely made the height 9-1/4″ so that it would match the length of the run that I measured for the side-to-side splay of the rear legs.
The first step is to line up a support block on each angled line and screw them in place from below the base as well as from above as shown here.
Next I place another support block tight against each existing block and screw them in place as well.
Here is another view of the completed holding jig along with the measuring jig. Each arm of the measuring jig is 2″ wide x 10-1/2″ long x 1/4″ thick. There is a 5/16″ x 8″ slot running down the center and a 45° chamfer at the solid end. The two arms are held together with a 1/4″ carriage bolt, a fender washer and a wing nut.
To use the jigs first place the holding jig on top of the work bench. Mark the legs with the location of the rear seat rung and slip each one under a pair of support blocks until the seat rung mark aligns with the top of the jig. Clamp as shown to hold the legs firmly in place. This set up holds the legs in exactly the same position they will occupy in the finished chair.
Now I am ready to measure the distance between the legs at each mortise location as well as the angle for the slat tenon shoulder. I start by loosening up the wing nut and placing the end of one arm in line with a mortise. Then I place the end of the other arm in line with the opposite mortise and tighten the wing nut. I can now transfer these measurements to a slat blank to begin the process of laying out the shape of the slat.
One of the things I really like about this process is that it gives students a clear idea of how all the parts of the rear panel interact. Once this is understood, it is fairly simple to experiment with changes to the design. For instance I was not happy with the distance between the top of the rear legs and wanted them to be about an inch further apart. Knowing how all the parts interact I knew I could make several possible design changes to achieve that result. The most obvious would be to make the rear rungs an inch longer, but that change would also make the seat wider at the back as well as alter all the drilling angles for the side rung mortises. I settled on altering the rotation of the legs from 25° to 28° which meant I also had to change the angle of the slat tenons to 28°. This makes the back rest slightly more curved but it is still quite comfortable. And it leaves intact the design of the chair from the seat level down.
In the next post I will describe in detail the process of using these jigs to measure the distance between the legs at each mortise as well as the angle for the shoulders of the slat tenons. Then I will transfer those measurements to a slat blank, layout the shape, and finally cut out the finished slats.
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