Sunday, November 16, 2014

A Handplane Jig for Ladderback Chair Leg Tapering

Here's another hand tool jig. This one is more specific than the dovetail paring jig I showed in the last entry, because it's built to help with one particular step in one particular project. I use it to turn tapered square chair leg blanks into tapered octagons when I make the ladderback chair developed by J. Alexander and Drew Langsner. The dovetail jig can be used for joints of different thicknesses and widths, and I can even picture myself using it to fair up tenon shoulders. But this leg-tapering jig is so specific that I doubt it will ever be used for anything but this project.

First I'll show about making and using the jig, and then I'll explain my thinking a little bit.

Fortunately, though this jig is very specialized, it's also pretty simple, and quick to make. I found a piece of 1/2” plywood longer and wider than the leg will be. Then I cut some small stock at 45 degrees to make blocks that form a cradle for the leg blank. I glued two pairs of blocks down with their pointy ends touching each other - - they cradle the middle “cage” section of the legs where there is no taper.
Once they were securely glued in place, I laid a leg blank (already tapered) into this cradle. That guided the placement of additional pairs of blocks to support the tapered sections of the leg. The foot taper is only about 5” long, so a pair of blocks out at the very foot end suffice to hold it up. I put glue on the tapered blocks and slid them into place so they just touched the face of the leg. Each block has to go beyond the centerline of the jig, so the blocks pass alongside each other rather than confront each other.
The longer taper on the upper leg has two pairs of these by-passing blocks.
When placing the blocks under the tapered sections, I made sure not to position them where they would actually lift the leg up. Once all the cradle blocks were in place, I confirmed that a properly prepared leg blank would rest on all the blocks, rather than just a few of them.

When I dreamed this jig up, I assumed I would have to find a way to clamp the legs down while I planed, but no. A stop at either end is all it takes. The weight of the plane holds the leg blank down, the cradle of 45 degree crosspieces keeps it from rotating, and the end stops prevent it from getting pushed out the end of the jig.

Using the jig is simple. I put the tapered square blank in place, and carefully plane the center, non-tapered “cage” section. If you do this right, the flat chamfer you plane will gradually extend out into both tapers, so be aware of this. Rotate the blank from face to face until the chamfers you're making with the plane are the same size as the original four faces of the leg blank. Your goal is to make a regular octagon, with all faces the same width.

When the central non-tapered section is done, I move on to the foot tapers. Again, pay attention to be sure you end up with all the faces of the foot the same width, a regular octagon that tapers down to 1” at the end. I take care of the long taper at the top of the leg last. I tapered three back legs in about an hour with this jig.

Which raises the question: Why? Why am I using a jig to do something that a good bodger would do by eye, far more quickly? Several reasons.

The immediate reason is that I am making these chairs too slowly. I cut the tree down, rived the blanks, and surfaced the legs and rungs into squares, over a year ago. Then I got pulled away to other projects, and by the time I got back to making this chair, the wood had become much drier than what I'm used to working on with drawknife and spokeshave. So I am adapting the process I learned at Country Workshops to work with the tools I use on drier wood. The handplane cuts dry wood very accurately and leaves a beautiful surface, but isn't well suited for use at a shaving horse. This jig works much more quickly than laboriously clamping the blank in every orientation needed. Whether I would use it for green wood, I doubt.

Another reason is that I am new to making round parts with hand tools, and I am trying to learn one part of the process at a time. In order to get good results while I do it, I need to work backwards, which means starting with accurately prepared tapered octagons and rounding them with a spokeshave. Once I have that down pat I can go back one step, and move from tapered squares to tapered octagons via drawknife and spokeshave. And I will.

Jigs for hand tools? Some might question this, when generations of bodgers made these chairs with by-eye measurement and a very few tools and holding fixtures. My answer here is the same as my answer for sharpening. If we were true apprentices, who learned each aspect of every process involved in making a product through thousands of repetitions day in and out, the jig would indeed get in the way of learning. But most of us do this in our spare time, and don't have the luxury of all those thousands of repetitions under the direct supervision of a master whose livelihood was on the line. For us, a honing jig for sharpening and a tapering jig for getting a leg blank worth the labor of rounding is what allows us to come to understand what good results look and feel like, and what goes into getting them. If we use jigs like these mindfully, they can be learning tools that help us develop all our skills, rather than crutches that prevent us from going beyond mediocrity.

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