Saturday, February 13, 2010

Tréteau XXIII

You've arrived at the 23rd post as this build rolls onward. Previous posts in the series, along with other topics, are to be found in the 'Blog Archive' to the right of the page.

Just so everyone is not lost as to what part I am working on at this moment, today I will be showing the cut out for the short side braces at the opposite end of the sawhorse from the previous post - these braces I mean:


These braces are different than the ones I cut out in the previous post - notice how they attach, in the photo above, to the right-hand side leg. That leg is rotated to be aligned with the long plan axis, so the braces on the short side have overlapping slices, or barbes, to maintain a flat surface right across to the post arrises. That flat plane is important in such things as a roof surface, or large hopper, where planks would be fitted to the outside surface of the prism.

These barbes however, do not exactly simplify the cut out. I started with the most improbable looking one, which has a sliver of wood about 7" long on front. First I used my router to define as much of the tenon as I could:



Then I ran my circular saw in as far as I could to roughly define the barbe:


I followed up with a careful handsaw rip along the face of the barbe:


Then, as the French would say, "off with the head!":


Which left me with this:


I then started in on the chisel work to clean the barbe up:


The barb is really thin - less than 0.25" at its root end and tapering off to nothing, so I used the top of the sawhorse to support it while I shaved it down to the line:


The barbe is so thin and delicate that I have no option to use the kerfing technique to fit it, so I pare close to the line and hope for the best.

Next it was time to cut the 90˚ abutment at the bottom of the tenon using my azebiki:


When it was getting pretty close, I started checking it for fit:


It came out pretty well:


A view of the underside of the barbe as it fits against the leg:


I then went to work on the other three tenons - here's the trimming of the barbe on the upper side tenon for the same leg (DP):


And here I'm hacking away at the lower tenon on the opposite side, connecting to leg CM:


At last I could fit it all together:


This one will need a minor amount of trimming to get a tight fit all around, especially at the lower left connection, but it's close enough for the time-being.

Observation: those barbes are frankly a little absurd. In the case of the sawhorse, their position makes them vulnerable to damage - it would be easy to catch a pant leg on the corner and rip it or crack it. Even while I'm working on such parts I have to be very careful not to damage the thin edge and the point. As I can't in good conscience glue the barbe to the face of the leg, I will likely trim them back somewhat, to leave an exposed edge on the thing about 0.125" thick. Given the choice, it makes sense to me to design so as to eliminate the use of such contrivances.

I'm still a little puzzled why the French would choose to employ the barbe, when there are other options. It is useful to know how to make such connections, and it does relieve the carpenter of re-shaping the post, however if I were to make this sort of piece again, I would use the Japanese method of re-shaping the legs on all four faces so that no barbes would be needed and the joinery would be simpler to cut, with cleanly centered tenons, thus open to more possibilities in terms of the joints themselves, and would be more robust as well. Also, I think it makes sense to avoid, in the case of the sawhorse at least, bringing the braces to connect to the legs at the same height. If the heights were staggered, the short side braces could be done with through tenons, and could thus be wedged or pinned, or w.h.y..

I've been percolating through my head, as I toss and turn at night (like last night) a design for a new sort of joiner's or cabinetmaker's bench, one employing irregular splayed legs and the French bracing, and maybe a suspended tray underneath. It would be far stronger than the usual type with plumb legs and bolted stretchers, particularly in regards to resisting racking from the sideways loads imparted at times by planing and general pushing and pulling of objects attached to the bench top. I might start playing around with drawing this out soon. I have some ideas a cookin'!

I hope you enjoyed your visit to the Carpentry Way today. Comments/questions always welcome.

--> Go to post XXIV

5 comments:

  1. What a fascinating website! On the subject of bench leg design, I have long thought it odd that the standard "western" design uses all the worst possible structural elements. Perhaps one reason western benches tend to the massive is because of their poor structural design. I'll be interested to see your ideas!

    ReplyDelete
  2. Hi Julian,

    it is my suspicion that the leg proportions of a western bench are at least partially driven by the thickness of the top. With a thick top, as seems common, the legs have to scale up to look appropriate. Otherwise, yes, you are quite right, the legs are massive really, far thicker that they need to be for any likely load to be borne on top of the bench.

    And maybe, as you say, the legs are that thick in order to be adequately stiff a connection for the stretchers, which are usually bolted on or serves as a spacer for a tie rod. It is not an especially stout or clever way to reinforce the bench from racking forces.

    I suppose that the top of the bench needs to be thick enough so that when one does some chopping with a chisel on some piece of work, the top does not vibrate too much. Also, the top needs to be a certain thickness in relation to the bench clamps that are attached to its edge, and the bench dogs that are used through the top. Now how thick is the question? 1"? 1.5"? 2"? 2.5"? 3"?

    I'm thinking that a frame and panel top, with stout crosspieces, would allow the top as a whole to lose a little mass and still be adequate to soak up vibration. If the frame were 2.5"~3.0" thick, and the panel was 0.75"~1.0" thick, with 2" thick crosspieces affixed underneath with sliding dovetails, say....???

    ~Chris

    ReplyDelete
  3. 1.75" tops are too thin for my taste even when stiffened up with a front apron of 1.5"x4" rock maple. I have started to grow from the desire for the big heavy bench for joinery to more of a mobile set- up. Assembly tables need big flat space, joinery on needs support for the work at hand. Saw horses and a narrower( but thick) slab top with maybe a loose tool tray full length in the back that's what I likes these days. Bench and assembly need to be two separate surfaces, tools on a third separate surface to keep them off the bench would be next best too. But tool tray works o.k..
    I have been wanting a 3'x7' wood veneer solid core fire door for a layout table slab. They are pretty flat and cheap, ready to go for about $50~75

    Happy Valentines Day you big Lug. Omedeto gozaimasu on the Freedom Saw Horse. Blog is great keep it up.
    Don't wear out that snow shovel! Post a pic of the white stuff? Can't remeber what it looks like...
    C on da BIG

    ReplyDelete
  4. Hi C (on da BIG),

    some good observations there. I think having a separate assembly and joinery bench makes a lot of sense.

    I used to have a planing beam of California Redwood, about 14" x 4" - it was sable and fairly light to move about, and I liked that.

    I guess I'm looking for ways to apply these new French layout techniques for braces to my repertoire generally, and i cabinet bench seemed like a possible application. I have no immediate plans to build one, but like to explore the design possibilities through drawing all the same.

    Happy Valentines to you too!

    ~Chris

    ReplyDelete
  5. ...perhaps doing a quick check for typing errors would be of benefit...

    grr

    ReplyDelete

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