Sunday, November 2, 2014

Bottom Half of the Rocker; Or, How I Learned to Stopped Worrying and Love the Mortise

After what seems like months (it's actually only been one month), the bottom of the chair is cut and dry fit!  The hardest part?  The tapered mortises.  Contrary to how easy I made it out to be in my mortise and tenon post, getting a good tapered mortise and close fitting tenon turned out to be much more difficult than I expected.

I'm very glad I started with the legs on the bottom of the chair, as I made a scary mistake on (what else) my first mortise. Rather than cutting the mortise, leaving it, and shaping the tenon in the leg to fit, I got the great idea to try and do both together. Tenon doesn't fit quite right? Modify the mortise! Still doesn't fit? Maybe take a bit off the tenon. And a bit more. A little off the mortise, maybe an angle change too.

By the time I was done with it, the mortise was way too wide, and the tenon way too small. Woodworking mistake: not enough wood left.

Four legs, nicely fit into their new mortises.
From here, I had two possible options. Either I fill the mortise and try again, or scrap the leg and turn a new one, trying to craft a tenon to fit the existing mortise. To test the former option, I attempted to create a mortise in scrap wood that would fit the tenon I already had. Wasn't going to happen, the angle on the tenon was already too shallow to correctly work. That leaves the latter option, scrap the leg.

This option was met with better results. I was able to make use of previously discarded legs (so discarded due to knots), and shaped a tenon to fit the mortise. The resulting angle was even right! Whew, disaster averted! (I may still end up crafting a new leg, depending on how bad those knots are to work around. But at least I know it's possible.)

The overall lesson I took away was this: make the mortise first, and go easy with the tapered reamer. Ream a little bit, only enough to get a good angle on the mortise. Then, leave the mortise alone. Shape the tenon to fit the mortise, and when it's nice and tight, call it a day. This is the method I used for the remaining mortises, and it all went together nice and quick.

I managed to develop a nice method for turning the tenons too.  First, I start with the toe of the tenon. I set my caliper to 1/2" (or key it off another tenon), and use my pairing chisel to size the toe to fit. Then, I reset my caliper to the width of the reamer at about 3/4" up, as that guarantees the angle I'll get will be right. At the 3/4" mark on my wood, I use the pairing chisel again to pair down to the wider width. Once I have both the toe and heel, I use the pairing chisel to pair down the remainder and produce a nice flat taper from the heel to the toe, being careful not to take any more wood off either. This gives me a very reliable, repeatable tenon that fits easily into my reamed mortises.

(Very) dry test of the rockers.
The next thing I wanted to do was dry-test the rockers. Obviously without the front, rear, and side stretchers, the whole works is a bit unstable. But with some care and a bit of duct tape, I was able to do a test of the rockers. The whole thing racked quite a bit, but it rocks, is comfortable to get onto and off of, and best of all, seems to be just the right dimensions. To attach the rockers here, I used my bandsaw to cut a hole in some scrap wood wide enough to fit around the legs, then used duct tape to tape those to the rockers. The guides keep the bottom of the legs on the rockers.

From here, it's a matter of marking on the rockers where the mortises need to be. From the plan, the mortises are perpendicular to the top surface of the rockers (and you can see in the picture that that's really true). So, clamping the rockers down, I drill a 3/4" mortise straight in at my lines, and carefully ream out a bit to form the taper. Then back on the lathe, I create the tapered tenon to fit.

There's still quite a bit of racking at this stage, so the next thing to do is attach the support stretchers. Before I do this though, it's a good idea to finish the shape of the legs. Otherwise, the mortise may be too shallow (and I might get tear out as I turn over a mortise). Over to the lathe I go!

Carefully cutting the bead in the leg.  Skew chisels are hard!
The supports run parallel to the seat, so I need to cut mortises into the sides of the legs at the proper angles. Rather than doing this from the plan, I use the real chair as a reference. With the legs fully seated into the bottom of the chair, I use my angle gauge to measure the angle between the seat and the legs. Holding that angle against the legs, I can draw a perfectly parallel line for the stretchers.

I find referencing off the real work piece makes things much more accurate and aesthetically pleasing than always using measurements. With all measurements, there's always some amount of error that creeps in, and before you know it, what worked on paper doesn't fit at all! But, using the real wood as a reference, you can fine tune any fit perfectly (and adapt to any mistakes along the way, not that I ever make mistakes...). So for me, that old adage "measure twice, cut once" applies to me only in the first few cuts.  After that, everything is relative.

Drilling out the mortise for the front stretcher.
Having my line, I then take each leg and clamp them down so the line is visible. I keep my angle gauge and place it next to the leg so I can line up the angle of my drilling. I also use a mirror, aligned perpendicular to the length of the leg, to ensure I'm dead-center on the leg. From there, I carefully drill straight down, using tape on the drill bit to measure my depth to a bit over 3/4". Some careful reaming and I have a nice tapered mortise in the legs. For this mortise, I used a 1/2" forstner bit rather than a traditional drill bit to minimize tear out on the round surface of the legs. Over to the lathe, I turn the stretcher tenons to fit, then finish the shape to a nice gentle swell in the middle.

The pieces for the side stretchers were cut too long, so I had to shorten them (not an easy task after you've already rough turned them). To do that, I cut the piece to the right size on my bandsaw, then worked with the lathe by hand to position the piece back to center. I'm not sure you can ever really get it perfect this way, but if it's close enough, you can smooth out the wobbles and return the piece to perfect round and center. Then it's just a matter of re-forming the tapered tenon.

Add to that, one of the stretchers had a bad area on it, and broke as I was rough fitting it. Better to have that happen now then when we're trying to sit on it!

Putting the pieces together, I found some of the angles weren't quite right. I carefully used the reamer to widen the mortise in the direction I needed to adjust the angle, and quickly got a nice tight fit at the correct angles. The chair bottom is now much more stable, and looks beautiful! It'll be even more stable once glued, but that comes later.

The biggest takeaway I have from this exercise is that tapered mortise and tenon joints are actually not all that bad once you get the hang of it. And, though I had read that the reamer could be used to adjust the angle, I didn't realize just how easy that really is! So, though daunting, if you start with the mortise and make the tenon to fit, and carefully ream to the right angle, it's hard to get this joint wrong. ■

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