X Marks the Spot

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For the past day or two I took on the challenge of drafting a carpentry drawing exercise of Bruno Sutter’s, a compagnon, detailed by Chris Hall on his blog thecarpentryway.BlogSpot.com, in a series titles X Marks the Spot.

Its basically one of those great blog series of Hall’s that seem irreducibly complex even after reading through several times, but there had to be a reason for him to present the material. As it turns out, its not terrible compared to a Mazerolle sawhorse, and teaches many important descriptive geometry concepts for a relative neophyte such as myself.

Its not worthwhile to rehash the how-to of this endeavor, Hall does a very thorough job himself. What I do want to get across is the importance of actually doing the drawing work if you want to understand how this works. I’ve made inroads with using Google Sketchup for drafting, as well as by hand, and have things I like about both.

  1. Sketchup is free, fast, and accurate (with the exception of certain functions regarding curves and circles), but doesn’t work without a computer.
  2. Hand drafting was the only option until a generation ago, and our carpentry forbearers managed much masterful work. That said, its easy to accumulate small errors in accuracy even working full scale, such as when projecting a line long distance from the edge of a footprint. While you can draft with not much more than paper, pencil, straightedge, and compass, you’ll definitely find it more manageable with stuff like a T-square, parallel rule, drafting triangles, protractor, scale ruler, and a range of hardness in pencils or line weight of pen.

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For this exercise I decided to work it out on paper, best to be familiar with both ways of working, you never know when the zombie apocalypse will take out computer functions…

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My drawing, mostly completed. Notice how hard it is to see? That’s because I’m using a very hard sharp pencil for accuracy, with the paper taped down nice and flat to the edges of the drafting board.

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The basic sequence of the drawing consists of starting with a plan view of the two crossing sticks, using the cross section to find the footprint via folding down the elevation view, and then finding where the planes of each of the faces on one stick intersect the other. By unwrapping the stick like a cardboard box layed flat the lines of intersection can be transferred, and each of the faces shown as if you were looking at them directly.IMAG2226

The drawing is to scale for the size of the pieces I wanted to use, one inch square in cross section and a little over a foot long. The advantage of drawing to scale is that the pieces can be place on the drawing and marked directly. Pretty neat, no?

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The last step of actually figuring out what bit of wood to remove presented the greatest difficulty. Hall says that you basically have to ‘intuit’ from the planes of intersection what represents the actual cut lines. My spatial skills are not quite there yet, so I made the miter cuts at the end of the sticks and held them close together in their correct orientations until I could see where one piece fit through the other and shade in between the lines.

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On the unflolded view of the drawing the planes of each of the faces of the opposing piece are marked in blue and red, with the resulting space that needs to be cut out shaded in green. Not exactly jumping out to you that these are the cut line for one square piece of wood fitting through another, but that’s what happens with stuff at slope and differing angles to plan.

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Cutout was a petite masterclass in cutting angles. I basically cut all the lines I could with a ryoba saw, and then went in with a fret saw for the bottom line that can’t be reached with a normal saw. Paring a bit with a chisel and voila! Not a piston fit, but at least it shows that I understood the drawing exercise. I had to draw this whole thing twice because I noticed my first drawing wasn’t accurate enough, lines that should have been parallel were not, and the cuts would have been all fucked up as a result.

If I wanted to do really accurate work? Computer drafting is the answer.

Splay Legged Stool

Like, say, a splay legged stool? Yup! I figured out the kinks on this one too, but it deserves much discussion of its own, so I’ll save it for next time.

4 thoughts on “X Marks the Spot”

    1. Sorry Dad, I’ll try to do a better job of introducing these concepts as I continue. I read back through my post and realized how unintelligible it was if you’re not already familiar with the concepts of descriptive geometry as a carpentry technique in complicated work.

  1. that perspective is really weird. Looks like floating…

    cool you are making the stool. I need to make one too, Julia wants to ditch the paint buckets. I have already cut the legs but forgot how to make prism form. It took me some time to realise, but this should be how I wrote down back then with a slope of 5/10 (4.96 in fact) did you already planed it to size?

    1. Yup, planed the legs to diamond cross-section last night. The easy solution to the leg cross section is in the “How to use the sashigane” book, what was that I sent it to you titled as, “Sashigane no Kata”? That was the detail that had worried me the most and it turns out that it was actually the easiest to figure out. I’ll post about it tonight after I do a bit of layout on the legs. Hall’s book on hopper joinery was a great help in understanding all the different cut angles, especially the use of Sho-Chu-Ko slope as the smallest segment of the Japanese unit triangle for the angle of the top/bottom of the mortises in the legs. “How to use the sashigane” actually shows yet another method for finding sho-chu-ko working back from the footprint of the leg in chu-ko slope foldout view. Sigh…are we becoming completely unintelligible to most people yet?

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