Sunday, November 9, 2014

Top o' the Rocker to ye!

It's really coming together! I turned the last couple of days into solid shop days and managed to tear through the next few steps on the rocker: the front roll, tapered mortises on the top of the chair, and the back posts and spindles — not to mention the initial mounting of the top crest.

After having completed the bottom part of the chair, I'd become very used to the exercise of drilling out tapered mortises and spinning tenons to fit. I had already transferred the angles I needed from my model to the top of the chair seat, so drilling out these holes was a snap. I set up my mirrors and once again carefully drilled to the right angles using my angle gauge as a guide.

Before turning the tenons for the back, I decided to tackle the front roll — a part of the chair I had been putting off due to my perceived difficulty in putting it together. See, when I designed this thing, I had attached the front roll by using a thicker piece of wide wood tacked to the front of the chair. Part of it would extend the chair, the other would drop down and form the roll (once rounded). I hadn't really thought about how I'd attach it to the chair, though. Simply gluing it wouldn't work — that's a butt joint, one of the weakest glue joints you can have. And, given that this is part of the seat, I expect it will be under quite a bit of stress from legs pushing down on it. So, I had to think of something else.

What I came up with was to use a floating mortise-and-tenon joint. This is similar to a normal M&T joint, but the tenon is loose, just a piece of wood, fit into two mortises, one on each piece. It provides exactly as much strength (one of the strongest) as a normal mortise-and-tenon joint, and can be done with wood not originally cut with M&T in mind. Perfect! I decided I'd put in three tenons, one for each of the different segments I used in the seat itself. The tenons would be roughly centered in each of the three segments to maximize stability and strength.

The real challenge in this is getting the mortises on both the seat and the roll piece to line up more-or-less perfectly. To do this, I decided to make a T-shaped template jig which I'd use to drill the mortises. The top part of the T rests on the top of the seat and roll, while the long part is drilled out to pattern the mortises. I hold one face against the seat, and the opposite face against the roll. This ensures that any alignment errors I have during the layout of the mortises transfers equally to both pieces, cancelling them out. The template itself is made out of 3/4" plywood I had laying around the shop. I used my table saw to cut a 3/4" dado grove into the top of the T to receive the leg, and used two wood screws from the top to secure it in. The mortise holes in the template were drilled using a 1/2" forstner bit on my drill press.

The seat of my chair is too wide to fit under my drill press (I have a small 10" Ryobi, while the seat measures 18" front-to-back). So, I had to use my hand drill to do the cutting. I lined up my template to the front of the seat and clamped it down nice and secure. Then, with the 1/2" forstner bit in my drill, I carefully (and slowly!) drilled out the mortises using the template as a guide. Ideally, forstner bits are better used in a drill press, as they take a large amount of pressure to go into the wood. But they also produce a much cleaner, tear-out-free cut as they start, making them the perfect bit to use for this. But, in a hand drill, it goes slow, and requires a lot of pressure. Blue tape on the bit helps guide my depth (though still not exactly perfect).

Once finished, I flipped the template over and aligned it to the front roll material. Happily, the roll is thin enough to fit in my drill press, so I finished the work there. Much faster, and as I found, more accurate (the hand drill, while still fairly accurate due to the template, did leave some variation in the cuts). Not enough to worry about, as it turns out.

With the mortises rough-cut, I now needed to square them up. Turning to my chisels, I was able to do this in nothing flat. The drill bit left lots of bumpy ridges along the length of the mortises which needed to be cleaned up. A straight chop downward is enough to get those, followed by some careful paring of the sides until flat. I needed to square up either end of the mortises too, or else round my tenons, which would have involved me setting up my router table. Grabbing a chisel is much easier, so I went for that. Some careful chops back to the side, and some cleaning of the corners, and I have some nice, square tenons.

With all that done, now I needed the tenon material.  For this, I grabbed one of my walnut off-cuts and measured out three 4" x 2" x 1/2" pieces, which I cut using my bandsaw (much easier to do small cuts there than on my table saw). Initially, the pieces were a bit too big to fit in (a good problem to have!), so I carefully sanded and smoothed the pieces down until they just fit with hand pressure. Moment of truth: did I get the alignment right? Well, the pieces went together very smoothly with simple mallet pressure (exactly what you want for a nice M&T joint), and was almost perfectly aligned. Enough so that a quick hand-planing brought the tops perfectly level.

One last thing remains before calling this part finished: rounding the front roll. With it mounted on the seat (but not glued), it's plenty strong enough to work with, so I did the rounding with the piece fully attached for most of it. Initially I was going to use a saw rasp like this one to do the shaping. But then it occurred to me, my block plane would likely do this faster and cleaner. Sure enough, it worked beautifully, leaving a nice smooth surface. I still need to sand it round, but the plane took most of the material off in short time.

Having completed this, I now turned my attention to the back spindles. These went quickly, more tenon turning and careful reaming with my tapered reamer, and the spindles are all positioned at mostly the right angles. I then drilled holes into the top crest to receive the other ends of the spindles, reamed them (and cut the tenons on the opposite sides of the spindles), and reamed to fit. Getting the fit just right on this is tricky — any spindle protruding out even a little bit is enough to make all the other spindles too loose to grab on. I'm still working on getting the fit just right.

After that, all that's left are the arms and arm spindles, and carving out the seat bottom. A little finish, and this chair will be done! For the next week or so, though, I'll be focusing more on the changing table, so the rocker is going on hold. ■

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