With Vidar's Chair crest rail now completed, it's time for the back splats. The first step is to determine the angles for the tenon shoulders. Above, I'm test-fitting a story stick to match the front-to-back angles that I'll use for a table saw set-up to make the cuts. I also made a story stick having the side-to-side angle for the upper tenon. The side-to-side angle for the lower tenons is simply 90°.
I then cut the bottom tenon ends at the determined angle on the table saw.
To make sure I cut the parts from the optimum location on the blanks, I made a chipboard copy of the front-to-back angle block and marked the tenon locations. With this information, I could then create the best curve for the back splats using a flexible curve as a guide.
After making the curved line, I then traced that onto the side of one of the part blanks. The curve springs upward perpendicular to the top of the lumbar rail and will terminate nearly perpendicular to the underside of the crest rail. The grain lines in these blanks are such that the grain nearly follows the curve of the final part. This makes planning the grain graphics a bit more challenging.
To test out the grain graphics before committing to the tenon locations, I referenced the end grain patterns on each end and the curved layout line on the side to estimate the location of the grain lines on the face of the final, shaped part.
It looks something like what you see below. What I'd consider an ideal grain graphic on this chair would be something that followed the bold dashed line down the curving mid-line. The solid bold line is what I estimated I would achieve with this part. The lighter lines indicate what I'm guessing the overall grain pattern will look like when completed.
Next, I cut the lower tenon shoulders on the table saw and then proceeded to hand cut the tenon cheeks and sides.
The photo below shows the back splat blanks after fitting the lower tenons into the lumbar rail mortises. The clamps are there to make sure the lumbar rail isn't damaged during the fitting process.
Okay, that was the easy part. Now it gets a bit more interesting. Perhaps you should go grab a cup of hot coffee and some nourishment, then come back and relax as I walk you through the next few steps.
Below, you can see the results of the set-up for locating the upper tenons. First, I made two floating tenons. Their length was determined by the mortise depth plus 1/4". The 1/4" is arbitrary; it just provides enough room to work without getting too long and unwieldy. Next, I made two spacers the thickness of which matched the floating tenon length. I bored a hole through the spacer and inserted an extra long dowel to hold the crest rail in place.
With the crest rail in place and the back splat blanks in their proper position, I traced the ends of the floating tenons. This is a rather critical step, since the upper and lower tenons are not in alignment with each other by any measure. They're skewed on all axes.
After the trace, I then made a more formal layout of the tenons. If you look really closely, you may notice that I shifted the layout in the side-to-side direction to compensate for the length of the floating tenon I used for measuring and tracing. Remember it is skewed and this layout represents the portion of the tenon that will occupy the deepest part of the mortise.
More shoulder cutting on the table saw and hand sawing of the cheeks and sides. This is followed by fitting the tenons to the crest rail.
Now we're ready to cut out the shapes.
Below are the two back splats freshly cut on the band saw.
The first test fit is a critical one. This confirms that the back splats are the correct length. In this case, I've made the back splats just less than 1/16" too long. This allows for fitting; more on that later.
Now it's time for shaping with the block plane and spokeshave.
The grain graphics are close to what I had predicted; a nice gentle curve.
I have the parts shaped to approximate equal thickness over their entire length and width. Now I need to soften the whole back splat with a gently curving profile. For that, I start with some chamfering reference lines.
This shaping is done entirely with the spokeshave.
I use both a flat-bottomed spokeshave and another with a convex bottom.
After chamfering, I proceeded to soften the curved profile and round the edges.
With the shaping and surface preparation completed, I move on with fitting the tenon shoulders to the lumbar rail using a chisel.
Using tape and a pen, I mark the locations needing to be shaved away. Each row of marks is another pass with the chisel.
After fitting the bottom shoulders, I was now faced with the most interesting fitting. Below, I need to get the crest rail to fit evenly on both back splats as well as on both legs; all at the same time.
Above, you saw how I used a marking pen to guide my chisel. Below are the pieces of marked-up tape that were used to fit the upper tenons. It's a bit mind boggling to look at all of them at once in the photo, but one shaving at a time and the time passes quickly as the fit gradually comes together.
After the fitting, well, it's glue-up time. First, I glued-up the two lumbar rail joints. Then, a few hours later I glued-up the four crest rail joints.
You can probably see how I used the off-cuts from the lumbar rail, crest rail and back splats to help support the parts, keep them from bending out of alignment during clamping, as well as provide surfaces for the clamps to apply pressure.
Next on the agenda? Arms. The last two wooden parts for this chair.
Even though I left out a number of details for the making of the back splats, this has got to be a record-breakingly long blog post... even for me. If you're still awake and have read this far, you're either a caring relative, or just plain crazy about woodworking. Or both! Either way, thanks for tuning in. There will be more next week as I mortise, shape and fit the arms to the front and rear legs.
Hej då and happy shavings!